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Fat

Fat is a type of lipid, a diverse group of organic compounds that are insoluble in water but soluble in nonpolar solvents, essential for energy storage, cellular structure, and metabolic functions in living organisms.^[1] Primarily, fats consist of triglycerides, which are esters formed from one molecule of glycerol and three fatty acid chains, typically containing 14 to 24 carbon atoms.^[1] These fatty acids vary in saturation—saturated fats have no double bonds between carbons, monounsaturated fats have one, and polyunsaturated fats have multiple—determining whether the fat is solid (like butter) or liquid (like oil) at room temperature.^[2] In the human body, fats are stored in adipose tissue, a specialized connective tissue composed mainly of adipocytes that not only reserves energy as triglycerides but also secretes hormones like leptin and adiponectin to regulate appetite and metabolism.^[3] Fats play critical roles as macronutrients in the diet, providing 9 kilocalories per gram—more than twice the energy of carbohydrates or proteins—and facilitating the absorption of fat-soluble vitamins such as A, D, E, and K.^[2] Essential fatty acids, including linoleic acid (an omega-6 polyunsaturated fat) and alpha-linolenic acid (an omega-3), cannot be synthesized by the body and must be obtained from sources like vegetable oils, nuts, seeds, and fish.^[2] Dietary fats influence health outcomes: saturated fats from animal products like meat and dairy can elevate low-density lipoprotein cholesterol levels, increasing cardiovascular risk, while unsaturated fats from sources like olive oil and fatty fish help lower these levels and support heart health.^[2] Beyond nutrition, fats form the hydrophobic tails of phospholipids in cell membranes, contributing to their fluidity and barrier properties.^[1] Adipose tissue exists in two main forms: white adipose tissue, which predominates and stores excess energy while cushioning organs and providing insulation, and brown adipose tissue, which burns fat to generate heat through thermogenesis, particularly in infants and certain adult depots.^[3] This tissue also acts as an endocrine organ, releasing adipokines that modulate insulin sensitivity, inflammation, and energy homeostasis; dysfunction in adipose tissue expansion leads to ectopic fat deposition in organs like the liver, contributing to metabolic disorders such as obesity and type 2 diabetes.^[3] Overall, fats are indispensable for physiological balance, with their composition and distribution profoundly affecting human health and disease.^[1]

Chemistry and Structure

Definition and Composition

Fats, also known as triglycerides, are a class of lipids defined as esters formed from one molecule of glycerol and three molecules of fatty acids, characterized by their hydrophobic nature that renders them insoluble in water.^[1] This hydrophobicity arises from the nonpolar hydrocarbon chains of the fatty acids, which dominate the molecule's structure and prevent interaction with polar solvents like water.^[4] While fats represent a major subgroup of lipids, the broader category of lipids also includes related compounds such as phospholipids, which feature a glycerol backbone esterified with two fatty acids and a phosphate group, and sterols like cholesterol, which possess a distinct four-ring structure without fatty acid chains.^[5] The basic molecular structure of a fat consists of a three-carbon glycerol backbone where each hydroxyl group is esterified to a fatty acid chain via dehydration synthesis, resulting in a neutral, nonpolar triacylglycerol molecule.^[6] These fatty acid chains vary in length and degree of saturation, but collectively they impart the defining properties of fats. Physically, fats exhibit melting points that increase with fatty acid chain length due to enhanced van der Waals forces and are higher for saturated chains, which pack more tightly, compared to unsaturated ones with kinks from double bonds.^[7] Additionally, fats have a density typically ranging from 0.7 to 0.95 g/cm³, less than that of water, causing them to float on aqueous surfaces.^[8] Historically, fats were first isolated from animal tissues in the 18th century primarily for practical applications such as soap-making and candle production, marking the beginning of their systematic extraction.^[9] A pivotal advancement occurred in 1811 when French chemist Michel Eugène Chevreul analyzed soaps derived from pig fat, leading to his identification of individual fatty acids and the recognition that fats are compounds of glycerol and these acids, laying foundational work in lipid chemistry.^[10]

Fatty Acids and Triglycerides

Fatty acids are long-chain carboxylic acids consisting of a hydrocarbon chain and a carboxyl group, classified primarily by the presence and number of carbon-carbon double bonds in the chain. Saturated fatty acids contain no double bonds, resulting in a straight chain structure, as exemplified by palmitic acid, denoted as C16:0, where the notation indicates 16 total carbon atoms and zero double bonds.^[11] Unsaturated fatty acids feature one or more double bonds; monounsaturated fatty acids have a single double bond, such as oleic acid (C18:1), while polyunsaturated fatty acids contain multiple double bonds, like linoleic acid (C18:2).^[11] The standard notation system specifies the total carbon chain length followed by the number of double bonds (e.g., C18:1), often with additional details on double bond positions and configurations for precision.^[12] In unsaturated fatty acids, double bonds exhibit geometric isomerism, occurring in cis or trans configurations that influence molecular shape. The cis configuration positions the hydrogen atoms on the same side of the double bond, introducing a kink in the chain that disrupts straight alignment.^[13] In contrast, the trans configuration places hydrogen atoms on opposite sides, producing a straighter chain similar to that of saturated fatty acids.^[13] This difference in shape affects chain packing and physical properties, such as melting points.^[13] Fatty acids vary by chain length and human synthesis capability. Short-chain fatty acids have fewer than six carbon atoms (e.g., butyric acid, C4:0), medium-chain fatty acids contain six to twelve carbons (e.g., caprylic acid, C8:0), and long-chain fatty acids exceed twelve carbons (e.g., stearic acid, C18:0).^[14] Essential fatty acids, such as linoleic acid (an omega-6 polyunsaturated fatty acid) and α-linolenic acid (an omega-3 polyunsaturated fatty acid), cannot be synthesized by the human body due to the absence of specific desaturase enzymes and must be obtained from the diet.^[15] Non-essential fatty acids, including most saturated and monounsaturated types like palmitic and oleic acids, can be produced endogenously from other nutrients.^[15] Triglycerides, the primary form of stored fat, form through the esterification of one glycerol molecule with three fatty acid molecules, a dehydration reaction that links the carboxyl groups of the fatty acids to the hydroxyl groups of glycerol. This process releases three water molecules and can be represented by the equation:

\text{Glycerol} + 3 \text{ Fatty Acids} \rightarrow \text{Triglyceride} + 3 \text{H}_2\text{O}

The resulting triglyceride molecule features a glycerol backbone esterified at each of its three carbon positions, with the specific fatty acids determining the triglyceride's properties.^[16]^[17]

Biological Functions

Role in Cells and Membranes

Fats, particularly in the form of phospholipids, are fundamental to the structure of cell membranes, where they spontaneously assemble into a bilayer configuration. This bilayer consists of hydrophilic phosphate heads oriented toward the aqueous environments on either side of the membrane and hydrophobic fatty acid tails sequestered in the interior, forming a semi-permeable barrier that separates the cell's interior from the external milieu.^[18] The arrangement ensures membrane integrity by providing mechanical stability and compartmentalization, preventing the free diffusion of most polar molecules and ions while allowing passage of small nonpolar substances like oxygen and water.^[18] Cholesterol, a key sterol lipid, integrates into the phospholipid bilayer to modulate its physical properties, particularly fluidity and permeability. By intercalating between phospholipid molecules, cholesterol restricts the lateral movement of fatty acid chains at physiological temperatures, thereby reducing membrane fluidity and preventing excessive rigidity or leakage; this buffering effect maintains optimal membrane function across temperature variations.^[18] In eukaryotic cells, cholesterol constitutes up to 50% of the plasma membrane's lipid content, enhancing selective permeability and supporting the embedding of proteins essential for transport and signaling.^[19] Beyond structural roles, certain polyunsaturated fats like arachidonic acid serve as precursors for eicosanoids, bioactive lipid mediators that regulate cellular signaling. Arachidonic acid, released from membrane phospholipids by phospholipase A2, is metabolized via cyclooxygenase pathways to produce prostaglandins, such as prostaglandin E2, which promote inflammation by inducing vasodilation, vascular permeability, and immune cell recruitment at injury sites.^[20] These eicosanoids also influence hormone regulation; for instance, prostaglandin E2 modulates gonadotropin-releasing hormone neuron activity in the hypothalamus, affecting reproductive signaling and broader endocrine responses.^[20] In neural tissues, lipids form the myelin sheath, a multilamellar extension of glial cell membranes that wraps around axons to provide electrical insulation. Composed of over 70% lipids—including cholesterol (about 27%), galactosylceramide (23%), and plasmalogens (10%)—myelin creates a hydrophobic barrier that minimizes ion leakage and enables saltatory conduction, dramatically increasing nerve impulse velocity by up to 100-fold compared to unmyelinated fibers.^[21] This lipid-rich composition ensures the sheath's compaction and stability, protecting axons from electrical short-circuiting and supporting efficient signal transmission in the nervous system.^[21] From an evolutionary perspective, lipids played a pivotal role in the emergence of cellular life by enabling compartmentalization in protocells, primitive vesicle-like structures that concentrated prebiotic molecules. Amphiphilic lipids with 10–20 carbon chains self-assembled into membranes under low-ionic-strength conditions, such as freshwater pools, forming barriers that encapsulated nucleic acids and facilitated early metabolic reactions, including proton gradient formation for energy transduction.^[22] This lipid-driven compartmentalization was crucial for the transition from abiotic chemistry to Darwinian evolution, evolving into more complex phospholipid-based systems in modern cells.^[22]

Energy Storage and Adipose Tissue

Fats serve as the primary long-term energy reserve in vertebrates, stored efficiently in adipose tissue to meet metabolic demands when immediate energy sources like glucose are unavailable. This storage form provides approximately 9 kcal per gram, more than double the 4 kcal per gram yielded by carbohydrates or proteins, allowing organisms to stockpile substantial energy in a compact volume.^[23] The high energy density of fats has offered an evolutionary advantage, enabling human ancestors to survive extended periods of famine by efficiently storing surplus calories during times of abundance for later mobilization.^[24] Adipose tissue, a loose connective tissue composed mainly of adipocytes and supporting stroma, is the dedicated site for this energy storage, housing triglycerides within lipid droplets that can expand or contract based on nutritional status. White adipose tissue (WAT), the most abundant type in adults, features large, unilocular adipocytes with a single large lipid droplet occupying most of the cell volume, minimizing metabolic activity to prioritize energy conservation.^[3] These cells derive from mesenchymal precursors and accumulate in various depots to buffer excess energy intake.^[25] In contrast, brown adipose tissue (BAT) comprises smaller, multilocular adipocytes packed with numerous mitochondria and expressing uncoupling protein 1 (UCP1), which enables the tissue to oxidize stored fats for heat production through non-shivering thermogenesis rather than ATP synthesis.^[26] BAT is prominent in newborns and hibernating animals but persists in adult humans in limited depots like the supraclavicular region. A third type, beige adipose tissue, emerges as an inducible intermediate within white fat depots under stimuli such as cold exposure or β-adrenergic signaling; these cells exhibit multilocular morphology, elevated mitochondrial content, and partial thermogenic capacity akin to brown adipocytes, though they originate from a white adipocyte lineage.^[25] Hormonal signals finely tune the balance between fat storage and release in adipose tissue to maintain energy homeostasis. Insulin, secreted in response to elevated blood glucose, promotes storage by suppressing lipolysis—through dephosphorylation and inhibition of hormone-sensitive lipase (HSL)—and enhancing glucose uptake for triglyceride synthesis via activation of lipoprotein lipase and lipogenic enzymes.^[27] Mobilization occurs under fasting or stress conditions, where counter-regulatory hormones like glucagon and catecholamines stimulate adenylate cyclase, elevating cyclic AMP levels to activate protein kinase A; this phosphorylates and activates HSL, hydrolyzing triglycerides into free fatty acids and glycerol for systemic release.^[28] Adipose tissue distribution influences its accessibility and function, with subcutaneous fat forming a protective layer under the skin and visceral fat encasing abdominal organs. Sex differences arise primarily from gonadal hormones: estrogen favors subcutaneous deposition in women, particularly in gluteofemoral regions, while androgens promote visceral accumulation in men.^[29] Aging shifts this pattern, with postmenopausal women and older men experiencing increased visceral fat due to declining sex steroid levels.^[30] Overall adiposity is often quantified using body mass index (BMI), calculated as weight in kilograms divided by height in meters squared, though it provides a general rather than depot-specific measure.^[31]

Production and Sources

Biosynthesis in Organisms

De novo lipogenesis (DNL) is the primary pathway for endogenous fatty acid synthesis in organisms, converting excess carbohydrates into lipids for storage and membrane formation. This process begins in the cytosol with the carboxylation of acetyl-CoA to malonyl-CoA by acetyl-CoA carboxylase (ACC), an ATP-dependent step that provides the two-carbon building blocks for chain elongation.^[32] The malonyl-CoA is then transferred to acyl carrier protein (ACP) and iteratively elongated by fatty acid synthase (FAS), a multifunctional enzyme complex that adds two-carbon units in a series of condensation, reduction, dehydration, and further reduction reactions, primarily producing palmitate (C16:0) as the end product in animals.^[32] Further chain elongation occurs in the endoplasmic reticulum using malonyl-CoA and NADPH, extending fatty acids for incorporation into triglycerides or phospholipids.^[33] In animals, DNL predominantly occurs in the liver and adipose tissue, where it synthesizes non-essential saturated and monounsaturated fatty acids from glucose-derived acetyl-CoA under conditions of nutrient abundance.^[34] Animals cannot synthesize essential polyunsaturated fatty acids like linoleic and alpha-linolenic acid de novo, relying instead on dietary sources for these.^[34] The pathway is compartmentalized, with acetyl-CoA generated in mitochondria and transported to the cytosol via citrate shuttle mechanisms to fuel synthesis.^[33] Plants synthesize fatty acids de novo in plastids, particularly in developing seeds where triacylglycerols accumulate as storage oils, as seen in olives where oleic acid (C18:1) predominates via the stearoyl-ACP desaturase pathway.^[35] After synthesis, fatty acids are exported to the endoplasmic reticulum for desaturation by endoplasmic reticulum-localized desaturases, introducing double bonds to form unsaturated species like oleic, linoleic, and linolenic acids essential for membrane fluidity and oil quality.^[35] Seed-specific regulation enhances lipid accumulation, with enzymes like ACC and FAS upregulated during oilbody formation.^[36] Microbial organisms, such as oleaginous yeasts like Yarrowia lipolytica and Rhodotorula glutinis, perform DNL similar to animals but with higher yields, producing up to 70% of dry weight as lipids under nitrogen limitation, making them promising for biofuel production.^[37] In these microbes, FAS operates iteratively to build fatty acids, which are then esterified into triacylglycerols in lipid droplets, with genetic engineering targeting ACC and desaturases to optimize chain length and unsaturation for biodiesel.^[37] DNL is tightly regulated by dietary factors, where excess carbohydrate intake, particularly fructose, activates transcription factors like carbohydrate response element binding protein (ChREBP) to upregulate lipogenic genes such as ACC and FAS in liver and adipose.^[38] Genetic variations, including mutations in lipogenic enzymes like FAS or ACC, can impair synthesis and protect against diet-induced obesity, as demonstrated in knockout mouse models.^[34]

Industrial Extraction and Processing

Industrial extraction of fats begins with sourcing from animal tissues or plant seeds and fruits, employing mechanical and chemical methods to separate lipids from other components. For animal fats, rendering is the primary technique, involving heating tissues to melt and separate fat from proteins and water. Dry rendering heats ground animal by-products, such as beef suet or pork fat, in enclosed vessels at temperatures around 115–145°C to evaporate moisture and solidify proteins, allowing fat to be drained and filtered. Wet rendering, used for higher-quality edible fats, adds steam or hot water to the material in a cooker, facilitating fat separation through centrifugation while minimizing oxidation. These processes yield tallow from beef or lard from pork, with yields typically 80–95% of available fat.^[39]^[40] Vegetable oils are extracted from oilseeds like soybeans, rapeseeds, or sunflowers using mechanical pressing followed by solvent extraction for maximum efficiency. Mechanical methods include expeller pressing, where seeds are crushed and heated to 60–100°C before being forced through a screw press, rupturing cells and squeezing out oil, often recovering 60–80% of the oil content. For higher yields, up to 99%, solvent extraction employs n-hexane, a non-polar solvent, which is percolated through flaked and cooked seeds in an extractor; the miscella (oil-solvent mixture) is then distilled to recover the oil and recycle the hexane. This combination is standard in large-scale plants, processing over 580 million metric tons of oilseeds annually worldwide as of 2025.^[41]^[42]^[43] Post-extraction, crude fats undergo refining to remove impurities and improve stability for food, cosmetic, and industrial applications. Degumming eliminates phospholipids (gums) by adding water or phosphoric acid to hydrate them, followed by centrifugation to separate the gums, preventing cloudiness and emulsion issues. Neutralization, or alkali refining, treats the oil with sodium hydroxide to saponify free fatty acids into soapstock, which is washed out, reducing acidity to below 0.1%. Bleaching adsorbs pigments, trace metals, and oxidation products using activated clay or carbon at 90–110°C under vacuum, followed by filtration for color clarity. Deodorization, the final step, involves steam distillation at 220–260°C under high vacuum to strip volatile odors, flavors, and remaining free fatty acids, yielding neutral, stable oils. These steps ensure compliance with food safety standards, with losses typically 5–15% of crude oil weight.^[44]^[45]^[46] To create semi-solid fats for products like margarine and shortenings, hydrogenation partially saturates double bonds in unsaturated vegetable oils using hydrogen gas and a nickel catalyst at 120–180°C and 1–5 bar pressure, increasing melting points and oxidative stability without fully eliminating unsaturation. This process, developed in the early 20th century, transforms liquid oils into plastic fats but has been largely phased out in favor of trans-fat-free alternatives due to health regulations. Interesterification, an alternative modification, rearranges fatty acids within or between triglycerides using chemical catalysts like sodium methoxide or enzymes such as lipases, altering physical properties like solidity and spreadability without producing trans fats. For instance, interesterifying palm stearin with vegetable oils produces stable shortenings for baking, maintaining functionality while complying with bans on partially hydrogenated oils.^[47]^[48]^[49] Processing generates byproducts like glycerol, obtained via hydrolysis of triglycerides with steam or enzymes, splitting fats into fatty acids and glycerol for use in soaps, pharmaceuticals, and biofuels. In fat splitting, high-pressure steam (260°C, 60 bar) hydrolyzes up to 99% of triglycerides, yielding crude glycerol at 10–15% of input fat weight. Sustainability challenges include deforestation linked to palm oil expansion, which converted approximately 10.5 million hectares of forest to palm oil plantations between 2001 and 2015, primarily in Indonesia and Malaysia.^[50] Certifications like the Roundtable on Sustainable Palm Oil (RSPO) address this; as of 2024, about 20% of global palm oil is RSPO-certified, enforcing no-deforestation policies and traceability, though critics note enforcement gaps in smallholder operations.^[51] The RSPO's 2025 Impact Update reports ongoing efforts to enforce no-deforestation policies across certified supply chains.^[52]^[53]^[54]

Metabolism

Digestion and Absorption

Dietary fats, primarily in the form of triglycerides, undergo initial limited hydrolysis in the stomach by gastric lipase, which cleaves short- and medium-chain fatty acids, but the majority of digestion occurs in the small intestine.^[55] Upon entering the duodenum, bile salts secreted from the gallbladder emulsify the fats, breaking them into smaller droplets to increase the surface area for enzymatic action.^[55] This emulsification is crucial as it prevents the fats from forming large aggregates that would resist enzyme access.^[56] Pancreatic lipase, released into the duodenum, then hydrolyzes the emulsified triglycerides at the sn-1 and sn-3 positions, producing 2-monoacylglycerols and free fatty acids, with colipase aiding the enzyme's attachment to the lipid-water interface.^[55] These digestion products, along with lysophospholipids and cholesterol, are solubilized by bile salts into mixed micelles—spherical structures with a hydrophilic outer layer that facilitate transport across the unstirred water layer to the enterocyte brush border.^[55] Absorption into enterocytes occurs primarily via passive diffusion for free fatty acids and monoacylglycerols, supplemented by transporters such as CD36 for long-chain fatty acids and fatty acid transport proteins (FATPs) that enhance uptake efficiency.^[55] Within the enterocytes, long-chain fatty acids and monoglycerides are re-esterified in the endoplasmic reticulum via the monoacylglycerol pathway to reform triglycerides, which are then packaged into chylomicrons with the aid of microsomal triglyceride transfer protein (MTP) and apolipoprotein B-48.^[55] Short- and medium-chain fatty acids, however, diffuse directly into the portal bloodstream and are transported to the liver without re-esterification or lipoprotein packaging. These chylomicrons are exocytosed into the lymphatic system via lacteals, entering the bloodstream through the thoracic duct to distribute lipids systemically.^[55] Efficiency of this process can be impaired by factors such as inadequate gallbladder function, which reduces bile salt availability and thus emulsification, leading to steatorrhea.^[55] Dietary fiber, particularly soluble types, can bind bile acids or directly interact with lipids, attenuating micelle formation and fat absorption.^[57] In malabsorption conditions like celiac disease, damage to the small intestinal mucosa from gluten-induced inflammation reduces the absorptive surface area and disrupts enterocyte function, hindering lipid uptake.^[58]

Oxidation and Utilization

Fatty acids, derived from the digestion and absorption of dietary triglycerides or mobilization from adipose tissue stores, undergo oxidation primarily in mitochondria to generate energy through catabolic pathways. Lipolysis, the initial step in fat utilization, is regulated hormonally; hormones such as epinephrine bind to β-adrenergic receptors on adipocytes, activating adenylyl cyclase to increase intracellular cyclic AMP (cAMP) levels, which in turn activates protein kinase A (PKA). This leads to phosphorylation and activation of hormone-sensitive lipase (HSL) and perilipin, facilitating the hydrolysis of triglycerides into free fatty acids and glycerol for subsequent oxidation.^[59] The primary catabolic pathway for fatty acid oxidation is β-oxidation, occurring in the mitochondrial matrix after activation of fatty acids to fatty acyl-CoA in the cytosol via acyl-CoA synthetase, followed by transport into mitochondria through the carnitine shuttle system. Once inside, β-oxidation proceeds in a series of four enzymatic steps per cycle: dehydrogenation by acyl-CoA dehydrogenase (producing FADH₂), hydration by enoyl-CoA hydratase, a second dehydrogenation by 3-hydroxyacyl-CoA dehydrogenase (producing NADH), and thiolysis by β-ketothiolase, which cleaves off a two-carbon unit as acetyl-CoA, leaving a shortened acyl-CoA for the next cycle. For saturated even-chain fatty acids like palmitate (C₁₆H₃₂O₂), seven cycles of β-oxidation yield 8 molecules of acetyl-CoA, 7 FADH₂, and 7 NADH.^[60]^[61] The acetyl-CoA produced enters the citric acid cycle for further oxidation, while NADH and FADH₂ donate electrons to the electron transport chain to generate ATP. For very-long-chain fatty acids (more than 18 carbons), initial shortening occurs via β-oxidation in peroxisomes, which lacks the electron transport chain and thus produces H₂O₂ instead of FADH₂; the resulting medium-chain fatty acids are then transferred to mitochondria for complete oxidation.^[62] During prolonged fasting or low-carbohydrate states, excess acetyl-CoA from hepatic β-oxidation is diverted to ketogenesis in the liver mitochondria, where two acetyl-CoA molecules condense to form acetoacetyl-CoA, which is converted to 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) and then to acetoacetate; acetoacetate is partially reduced to β-hydroxybutyrate. These ketone bodies are released into the bloodstream and serve as an alternative fuel for extrahepatic tissues, including the brain, which cannot directly oxidize fatty acids.^[63] Complete oxidation of triglycerides yields approximately 38 kJ/g in ATP equivalents, significantly higher than carbohydrates (about 17 kJ/g), underscoring fats' role as an efficient energy reserve.^[64]

Dietary Aspects

Common Food Sources

Dietary fats are primarily sourced from animal products, plant materials, and processed foods, contributing significantly to daily intake across global cuisines.

Animal Sources

Butter, derived from cow's milk, serves as a common source of saturated fats used in cooking and baking. Lard, rendered from pork fat, provides a versatile animal fat traditionally employed in frying and pastries. Fish oils, extracted from fatty fish such as salmon, offer polyunsaturated fats, including omega-3 varieties abundant in marine sources.^[65]^[66]^[67]

Plant Sources

Olive oil, pressed from olives, is a staple monounsaturated fat in Mediterranean diets and culinary applications. Coconut oil, extracted from coconut meat, is notable for its high saturated fat content despite its plant origin. Nuts and seeds, such as walnuts and sunflower seeds, deliver polyunsaturated fats and are incorporated into snacks, salads, and trail mixes.^[68]^[69]^[70]

Processed Foods

Margarines, typically formulated from non-hydrogenated vegetable oils through processes like interesterification, function as butter substitutes in spreads and baking.^[71] Fried items, including french fries and doughnuts, absorb fats from cooking oils like canola or peanut during preparation. Baked goods, such as cookies and cakes, frequently incorporate non-hydrogenated shortenings made from palm or soybean oils for texture and shelf life.^[72]^[73] In the 2020s, global consumption trends reflect a rising dominance of plant-based oils, with soybean oil comprising about 30% of total vegetable oil production in 2025/26, driven by increased demand in food processing and biofuels.^[74] These sources yield fats that align with classifications of saturated, monounsaturated, and polyunsaturated types.

Classification of Dietary Fats

Dietary fats are primarily classified based on the degree of saturation, the configuration of double bonds, and the length of their carbon chains, which influence their physical properties, stability, and metabolic handling. Saturation refers to the presence or absence of double bonds in the hydrocarbon chain of fatty acids, the building blocks of fats. Fats with no double bonds are saturated, those with one are monounsaturated, and those with multiple are polyunsaturated, while trans fats feature a specific geometric arrangement of double bonds. Chain length categorizes fatty acids as short-, medium-, or long-chain, affecting their melting points and absorption rates.^[2] Saturated fats consist of fatty acids with straight hydrocarbon chains lacking double bonds, allowing maximum hydrogen atom attachment and resulting in a compact structure. These fats are typically solid at room temperature due to their high melting points and exhibit high stability against oxidation, making them less prone to spoilage compared to unsaturated types. Common examples include palmitic acid (C16:0) and stearic acid (C18:0), found in red meat and dairy products.^[2]^[75]^[76] Unsaturated fats contain one or more double bonds in their fatty acid chains, introducing kinks that prevent tight packing and keep them liquid at room temperature. Monounsaturated fats have a single double bond, typically in the cis configuration, as seen in oleic acid (C18:1) from avocado and olive oils; these are moderately stable to oxidation. Polyunsaturated fats feature multiple double bonds, such as in linoleic acid (C18:2) from corn oil, rendering them more fluid but highly susceptible to rancidity through oxidative breakdown, especially under exposure to heat, light, or oxygen.^[2]^[75]^[77] Trans fats are unsaturated fatty acids with at least one double bond in the trans configuration, where the carbon chains extend on opposite sides of the bond, creating a straighter shape that mimics saturated fats' solidity despite the unsaturation. They arise artificially through partial hydrogenation of vegetable oils to produce stable shortenings and margarines, though this process has been largely discontinued in many regions due to health concerns and regulations; or naturally at low levels (2-5%) in ruminant products like beef and dairy from microbial biohydrogenation in the animal's rumen.^[78]^[2]^[71] This configuration imparts unique packing properties, contributing to firmness in processed foods. Fatty acids are further classified by chain length, which determines their physical state and digestive fate: short-chain (fewer than 6 carbons), medium-chain (6-12 carbons), and long-chain (more than 12 carbons). Medium-chain triglycerides (MCTs), such as those composed of lauric acid (C12:0) in coconut oil, feature shorter chains that enable rapid absorption directly into the portal vein without incorporation into chylomicrons, contrasting with the slower, lymphatic absorption of long-chain triglycerides predominant in most dietary fats. Most dietary fatty acids have even-numbered chains ranging from 4 to 24 carbons, with longer chains generally yielding higher melting points and more solid consistencies.^[2]^[79]

Health Implications

Essential Fatty Acids

Essential fatty acids are polyunsaturated fatty acids that the human body cannot synthesize de novo and must obtain from the diet. The two primary essential fatty acids are linoleic acid (LA), an omega-6 fatty acid (18:2 n-6), and alpha-linolenic acid (ALA), an omega-3 fatty acid (18:3 n-3).^[15]^[80] The necessity of these fatty acids was first recognized in 1929 by George O. Burr and Mildred M. Burr, who demonstrated through rat studies that dietary deprivation of unsaturated fats led to a specific deficiency syndrome, distinct from other nutritional lacks, thereby establishing LA as essential.^[81] Subsequent research confirmed ALA's essentiality, as humans lack the delta-15 desaturase enzyme required for its synthesis.^[82] These fatty acids play critical roles in maintaining cell membrane fluidity, where their unsaturated bonds allow flexible phospholipid bilayers essential for cellular function and signaling.^[83] Additionally, LA and ALA serve as precursors for eicosanoid production, including prostaglandins, thromboxanes, and leukotrienes, which regulate inflammation, platelet aggregation, and vascular tone.^[15] Deficiency in essential fatty acids manifests as scaly dermatitis, impaired wound healing, growth retardation in children, alopecia, and increased infection susceptibility, often observed in cases of prolonged fat malabsorption or inadequate parenteral nutrition.^[84]^[15] In the body, LA is elongated and desaturated to form arachidonic acid (AA, 20:4 n-6), while ALA undergoes similar conversions to eicosapentaenoic acid (EPA, 20:5 n-3) and docosahexaenoic acid (DHA, 22:6 n-3); however, these pathways are inefficient, with ALA to DHA conversion rates typically below 5% in adults due to competitive inhibition by omega-6 fatty acids and limited enzyme activity.^[85]^[86] The World Health Organization, in collaboration with the Food and Agriculture Organization, recommends that linoleic acid provide 2-3% of total energy intake and alpha-linolenic acid 0.5-2% to prevent deficiency and support optimal health.^[87]

Effects of Saturated and Unsaturated Fats

Saturated fats, found predominantly in animal products and certain tropical oils, have been shown to elevate low-density lipoprotein (LDL) cholesterol levels in the blood.^[88] This increase in LDL cholesterol contributes to the development of atherosclerosis by promoting plaque buildup in arterial walls.^[89] Recent meta-analyses indicate that reducing saturated fat intake and replacing it with unsaturated fats can lower cardiovascular disease (CVD) risk; for instance, a Cochrane review of randomized controlled trials found that such replacement over at least two years reduced combined CVD events by 21%.^[90] In contrast, unsaturated fats, including monounsaturated and polyunsaturated varieties from sources like olive oil, nuts, and fish, exert beneficial effects on lipid profiles. Monounsaturated fats improve the ratio of total cholesterol to high-density lipoprotein (HDL) cholesterol, as demonstrated in trials of the Mediterranean diet, which emphasize these fats and have shown reductions in LDL cholesterol alongside HDL increases.^[91] Polyunsaturated fats further lower serum triglyceride levels, helping to mitigate hypertriglyceridemia, a risk factor for metabolic disorders.^[92] Regarding bone health, observational data from the National Health and Nutrition Examination Survey (NHANES III) reveal that higher saturated fat intake is inversely associated with bone mineral density (BMD), particularly at the hip, potentially due to impaired calcium absorption.^[93] Conversely, unsaturated fats support bone density; for example, monounsaturated fat consumption correlates positively with bone mineral content across skeletal sites in adults, based on recent NHANES analyses.^[94] Both saturated and unsaturated fats are calorie-dense, providing approximately 9 kcal per gram, which can influence weight management if consumed excessively. However, unsaturated fats are linked to enhanced satiety through greater stimulation of cholecystokinin (CCK) release, a hormone that promotes feelings of fullness, compared to saturated fats.^[95] Trans fats, a processed variant of unsaturated fats, share some adverse effects with saturated fats but are addressed separately due to their distinct industrial origins.

Trans Fats and Rancidity

Trans fats, also known as trans fatty acids, primarily occur in artificial forms produced through the partial hydrogenation of vegetable oils, a process that adds hydrogen to unsaturated fatty acid chains to create semi-solid fats with improved stability and texture. This industrial method often results in the formation of elaidic acid, the trans isomer of oleic acid (trans-9-octadecenoic acid), which constitutes a significant portion of the trans fats in partially hydrogenated oils.^[13]^[78] Artificial trans fats have been linked to serious health risks, including a 23% increase in coronary heart disease mortality for every 2% of energy intake derived from them, due to their adverse effects on lipid profiles and inflammation.^[96] In response, the World Health Organization (WHO) in 2018 urged global elimination of industrially produced trans fats by 2023 through its REPLACE framework, a step-by-step guide to policy implementation.^[97] As of early 2025, best-practice policies in 62 countries covered 3.9 billion people (nearly half the global population), estimated to prevent a significant portion of trans fat-related deaths, with the World Health Organization recognizing four additional countries (Austria, Norway, Oman, and Denmark) in May 2025 and aiming for 90% global burden coverage by the end of 2025; the next application cycle for validation closed in August 2025, with ongoing efforts continuing.^[98]^[99] Natural trans fats, in contrast, arise endogenously in ruminant products and include conjugated linoleic acid (CLA), a group of positional and geometric isomers of linoleic acid predominantly found in dairy fats and beef. CLA, such as the cis-9, trans-11 isomer, constitutes up to 90% of natural trans fats in these sources and has been studied for potential health benefits, including anti-cancer effects observed in animal models where it inhibits tumor growth and promotes apoptosis in breast and colon cancer cells.^[100] However, human evidence remains limited, with observational studies showing associations between dietary CLA intake and reduced cancer risk but lacking robust clinical trial support for causation or efficacy as a supplement.^[101]^[102] Rancidity in fats refers to the spoilage processes that degrade quality, primarily through oxidative and hydrolytic mechanisms, leading to off-flavors, odors, and reduced nutritional value. Oxidative rancidity involves a free radical chain reaction initiated by the abstraction of a hydrogen atom from allylic positions in polyunsaturated fatty acids, particularly at double bonds, propagating via peroxyl radical formation and terminating in volatile compounds like aldehydes; this is accelerated by heat, light, oxygen, and metal catalysts.^[103]^[104] Hydrolytic rancidity, meanwhile, results from enzymatic or chemical breakdown of triglycerides into free fatty acids and glycerol, often triggered by moisture and lipases, producing soapy or rancid tastes.^[105] These processes significantly impact shelf life, especially in frying oils where repeated high-temperature exposure (above 150°C) promotes polymerization and oxidation, shortening usable life from months to days without intervention.^[106] Antioxidants such as vitamin E (tocopherols) mitigate oxidative rancidity by donating hydrogen to peroxyl radicals, interrupting the chain reaction and extending stability in applications like deep-frying.^[106] Detection of rancidity commonly employs the peroxide value (PV), which measures the concentration of hydroperoxides formed during early oxidation stages through iodometric titration, providing a quantitative indicator of initial lipid peroxidation (typically expressed in milliequivalents of peroxide per kilogram of fat).^[107]^[108] The U.S. Food and Drug Administration (FDA) requires declaration of trans fat content on nutrition labels if exceeding 0.5 grams per serving under regulations established in 2006, stemming from the 2015 determination that partially hydrogenated oils are not generally recognized as safe, with full compliance deadlines extended to January 1, 2021, to support industry reformulation.^[109]^[71]

Omega-3 and Omega-6 Fatty Acids

Omega-3 polyunsaturated fatty acids (PUFAs) include alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), which play key roles in modulating inflammation and supporting neurological functions.^[110] ALA, found primarily in plant sources, serves as a precursor that the body can partially convert to EPA and DHA, though conversion efficiency is limited to about 5-10% for EPA and less than 1% for DHA.^[15] EPA and DHA exhibit anti-inflammatory effects by serving as substrates for specialized pro-resolving mediators, such as resolvins, which actively resolve inflammation rather than merely suppressing it.^[111] DHA is particularly concentrated in retinal cell membranes and postsynaptic neuronal membranes, where it supports vision and cognitive processes by maintaining membrane fluidity and facilitating synaptic signaling.^[15] Primary dietary sources of EPA and DHA are fatty fish like salmon and mackerel, while algal oil provides a vegan alternative that directly supplies these long-chain omega-3s without relying on fish-derived products.^[112] Omega-6 PUFAs, such as linoleic acid (LA) and its metabolite arachidonic acid (AA), are essential for cellular structure but can promote inflammation when in excess. LA, abundant in vegetable oils like soybean and corn oil, is elongated and desaturated to form AA, which serves as a precursor for pro-inflammatory eicosanoids, including prostaglandins and leukotrienes that amplify immune responses during injury or infection.^[113] These eicosanoids contribute to the initiation of inflammation, contrasting with the resolving actions of omega-3-derived mediators.^[114] The balance between omega-6 and omega-3 intake is crucial, as modern Western diets typically exhibit an omega-6 to omega-3 ratio of approximately 15:1, far exceeding the ideal ratio of around 4:1 observed in evolutionary hunter-gatherer diets rich in wild plants and seafood.^[115] This imbalance arises from increased consumption of omega-6-rich processed foods and reduced intake of omega-3 sources, potentially exacerbating chronic low-grade inflammation.^[116] Omega-6 and omega-3 PUFAs compete for the same metabolic enzymes, particularly fatty acid desaturases encoded by FADS1 and FADS2 genes, which introduce double bonds in the conversion pathways; high omega-6 levels can thus inhibit the limited production of EPA and DHA from ALA.^[117] Recent 2024 research demonstrates that lowering the omega-6:omega-3 ratio through omega-3 supplementation reduces tender joint counts and disease activity in rheumatoid arthritis patients by enhancing anti-inflammatory lipid profiles.^[118] During pregnancy, DHA is vital for fetal neurodevelopment, accumulating in the brain and retina to support neuronal growth and visual acuity; supplementation has been shown in a 2018 Cochrane review to reduce the risk of preterm birth by 11% (RR 0.89, 95% CI 0.82-0.97), likely by stabilizing gestational membranes and mitigating inflammatory pathways.^[119]

Disease Associations and Guidelines

Dietary fats play a significant role in cardiovascular disease (CVD) risk, with saturated and trans fats elevating the likelihood of atherosclerosis and related events through increased low-density lipoprotein cholesterol levels, while unsaturated fats, particularly polyunsaturated ones including omega-3s, offer protective effects by improving lipid profiles and reducing inflammation.^[88] The American Heart Association's 2024 recommendations advise limiting saturated fat intake to less than 6% of total daily energy to mitigate CVD risk, emphasizing replacement with unsaturated fats for optimal heart health.^[120] Evidence linking dietary fats to cancer is mixed, with higher saturated fat consumption associated with increased prostate cancer risk due to potential promotion of tumor growth and inflammation, though causation remains unestablished.^[121] Omega-3 fatty acids, conversely, have been linked to a reduced risk of colorectal cancer in large cohort studies, possibly through anti-inflammatory mechanisms and modulation of cell proliferation.^[122] For breast cancer, associations with saturated fats show inconsistency across studies, with no definitive causal relationship identified in comprehensive reviews.^[121] In metabolic disorders, excessive saturated fat intake induces insulin resistance primarily through the accumulation of ceramides in tissues like muscle and liver, which impair insulin signaling pathways and promote inflammation.^[123] Polyunsaturated fats, in contrast, enhance insulin sensitivity, as demonstrated in clinical trials where their substitution for saturated fats improved glucose homeostasis and reduced markers of resistance in participants with metabolic syndrome.^[124] Current nutritional guidelines underscore a balanced approach to fats within overall dietary patterns. The USDA's Dietary Guidelines for Americans, 2025-2030 (finalized in late 2025 based on the 2024 Advisory Committee Scientific Report), prioritize whole foods rich in unsaturated fats—such as nuts, seeds, and fish—over isolated or processed fats, recommending that saturated fats constitute less than 10% of total energy while promoting patterns like the Mediterranean diet to support metabolic and cardiovascular health.^[125] For managing hypertriglyceridemia, guidelines from the American Heart Association endorse 2 to 4 grams daily of combined EPA and DHA from omega-3 sources to achieve triglyceride reductions of 20% to 50%, particularly in patients with levels exceeding 500 mg/dL.^[126] Emerging research from 2024 and 2025 highlights potential links between ultra-processed foods high in unhealthy fats and neurodegeneration, including Alzheimer's disease, where such diets may accelerate cognitive decline through oxidative stress, inflammation, and disruption of brain lipid metabolism in longitudinal cohort analyses.^[127]

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