Why fats matter, how they’re built, and how the body moves them around
Revised 18.09.2026 for accuracy.
There are many types of lipids, or fats, in the body. Lipids form an integral part of the cellular membrane — membrane lipids include cholesterol, glycolipids, and phospholipids. Fats are also an efficient, light-weight fuel store: because they’re energy-dense and light in weight, fat is the “go-to” long-term storage fuel for a mobile species. Myelin — the fatty sheath that insulates the axons of many neurons — depends on lipids for its formation, and is essential for normal neurological conduction. Fats are also critical to bile acid synthesis and to steroid hormone production. Fats remain essential to body function.
Conditions of fat excess, however — such as atherosclerotic disease linked to high dietary saturated fat intake — can lead to significant morbidity and are a leading cause of mortality. The predominant lipid in the human body is the neutral fat known as triglyceride. Triglycerides have three fatty-acid chains attached to a glycerol backbone; fatty acids themselves are hydrocarbon chains built from carbon and hydrogen.
Saturation and the physical properties of fat
Whether a fatty acid chain is saturated or unsaturated confers relative flexibility to its carbon backbone, and this is of real importance in both health and disease. Unsaturated fatty acid chains are more flexible and malleable, while saturated fatty acid chains are more fixed and rigid. This molecular property underlies the physical properties of fats — why some are liquid at room temperature and others are not — and is part of why saturated fats are generally considered more deleterious to cardiovascular health than unsaturated fats.
A fatty-acid chain can have single or double (rarely triple) carbon–carbon bonds. A chain of only single C–C bonds is saturated — its carbons are “saturated” with hydrogen. Saturated fats are typically solid at room temperature; these “solid fats” are found predominantly in animal products and are associated with atherosclerotic cardiovascular disease. The American Heart Association recommends no more than around 13 g of saturated fat per day (roughly 6% of total calories on a 2000-calorie diet).
When one or more C–C bonds in the chain are double, the chain is unsaturated — the individual carbons are not fully saturated with hydrogen.
Palmitic acid, for instance, has no double bonds and is solid at room temperature; oleic acid has one double bond; alpha-linolenic acid has three — both of the latter are liquid at room temperature.
Triglycerides are formed by esterifying the three hydroxyl groups of glycerol with three fatty acid molecules. As a general rule, plant-based triglycerides tend to be oils and animal-based triglycerides tend to be solid fats — though this isn’t universal: tropical plant oils such as coconut and palm oil are highly saturated and solid or semi-solid at room temperature, while fish oils, despite being animal-derived, are rich in polyunsaturated fatty acids and remain liquid.
The amphipathic fatty acid
The phase state of fats is an important determinant of their behaviour in the body. Every fatty acid has a polar carboxylic acid head — polar courtesy of its oxygen atoms — attached to a long, non-polar (hydrophobic) hydrocarbon tail. It’s this amphipathic structure, not just saturation, that governs how fatty acids and their derivatives behave at interfaces with water (e.g. within membranes and micelles).
Classifying lipids
Simple lipids: – Fatty acids – Triacylglycerols, diacylglycerols, and monoacylglycerols — TAG structural diversity depends on the type of fatty acid present, the position of the ester bond on the glycerol backbone, and the degree of unsaturation – Waxes (esters of fatty acids with higher alcohols) — including sterol esters (e.g. cholesterol–fatty acid esters) and non-sterol esters (e.g. vitamin A esters)
Compound lipids (a lipid plus a non-lipid component): – Phospholipids — phosphatidic acids (e.g. lecithin, cephalins), plasmalogens, sphingolipids (e.g. sphingomyelins) – Glycolipids — carbohydrate-containing – Lipoproteins — lipids in association with protein.
Derived lipids: derivatives such as sterols and straight-chain alcohols, obtained by hydrolysis of lipids from the groups above, which still possess the general properties of lipids — solubility in organic solvents such as ether, chloroform, and acetone.
Choosing fats wisely
Because dietary fat composition affects cardiovascular risk, the type of fat matters as much as the quantity. Both monounsaturated and polyunsaturated fats are preferred over saturated fat for cardiovascular health — the evidence here is nuanced: large randomised trials and meta-analyses (including the AHA’s 2017 advisory) show that replacing saturated fat with polyunsaturated fat produces a clear, consistent reduction in cardiovascular events, while the evidence for monounsaturated fat is drawn more from epidemiological data (e.g. the Mediterranean diet literature centred on olive oil). In practice, most current guidance favours minimising saturated fat and emphasising a mix of mono- and polyunsaturated sources over any strict ranking between the two.
Composition of fatty acid types in selected foods (% of total fat):
| Food/oil | Monounsaturated | Polyunsaturated | Saturated |
|---|---|---|---|
| Butter | 30.3% | 3.7% | 66.1% |
| Margarine | 46.3% | 33.3% | 20.4% |
| Mayonnaise | 29.8% | 54.8% | 15.4% |
| Canola oil | 61.7% | 30.8% | 7.5% |
| Olive oil | 77.3% | 8.6% | 14.1% |
| Peanut oil | 48.4% | 33.6% | 18.0% |
| Safflower oil (high-oleic) | 78.5% | 15.4% | 6.2% |
| Sesame oil | 41.5% | 43.8% | 14.6% |
| Sunflower oil | 20.8% | 68.5% | 10.8% |
Table: detailed fatty-acid-composition and iodine-value reference table across vegetable oils and fats, by carbon chain length
Digestion of fats
Dietary fats are emulsified by admixing with bile salts before enzymatic digestion can proceed efficiently.
| Location | Major events | Enzyme / secretion | Details |
|---|---|---|---|
| Mouth | Minor digestion | Lingual lipase (salivary glands) | Cleaves some fatty acids from triglycerides, yielding diglycerides + free fatty acid |
| Stomach | Additional digestion | Gastric lipase (stomach) | Further cleaves fatty acids from di-/triglycerides |
| Small intestine — Phase I | Emulsification | Bile (no lipase) | Emulsifies triglycerides, diglycerides, and fatty acids into micelles |
| Small intestine — Phase II | Enzymatic digestion | Pancreatic lipase (pancreas) | Cleaves remaining fatty acids from diglycerides, yielding monoglycerides + free fatty acids |
Absorption of fats
Dietary lipids — triacylglycerols, cholesterol, cholesterol esters, and phospholipids — enter the stomach largely intact. Only triacylglycerols are acted on there: lingual and gastric lipase hydrolyse medium- and short-chain fatty acids from the sn-3 position, yielding 1,2-diacylglycerols. In the small intestine, these lipids combine with bile salts to form micelles, acted upon by intestinal and pancreatic enzymes. Short-chain free fatty acids move directly into the portal circulation and bind albumin, while the remaining lipids — glycerol, glucose, cholesterol, long-chain free fatty acids, monoacylglycerols, diacylglycerols, lysophosphatidylcholine, and other lysophospholipids — enter the intestinal mucosal cell.
Within the enterocyte’s endoplasmic reticulum, glycerol is converted to α-glycerol phosphate (also produced from glucose via glycolysis); this, together with mono- and diacylglycerols, cholesterol, and lysophospholipids, is re-esterified into TAGs, cholesterol esters, and phospholipids. The lipoprotein Apo B-48 is synthesised in the ER and Golgi apparatus; combined with the resynthesised lipids, it forms a chylomicron, which is excreted from the enterocyte into the lymphatics. Other lipoprotein fractions contribute additional apolipoproteins to circulating chylomicrons.
Transport and storage: lipoproteins
Lipids resynthesised in the enterocytes, together with fat-soluble vitamins, are collected in the cell’s endoplasmic reticulum as large fat particles, which receive a surface layer of Apo B-48 while still within the ER.
Apolipoproteins are the protein components of lipoproteins — several distinct types are recognised (commonly taught as a core set of around nine: the A, B, C, and E families), and they help stabilise lipoproteins as they circulate in the aqueous environment of the blood.
Chylomicrons are lipid particles of exogenous (dietary) origin — the form in which resynthesised lipid largely leaves the enterocyte.
Very Low Density Lipoprotein (VLDL) is synthesised and secreted by the liver, carrying endogenously produced triacylglycerol into the circulation. Low-Density Lipoprotein (LDL), by contrast, is not itself a hepatic secretion product — it’s generated in the plasma as a downstream product of VLDL catabolism: lipoprotein lipase progressively strips triglyceride from circulating VLDL, first producing intermediate-density lipoprotein (IDL), which is further remodelled (partly via hepatic lipase) into cholesteryl-ester-enriched LDL.
The four major lipoprotein classes
There are four major classes of circulating lipoprotein, each with a characteristic protein and lipid composition, distinguished by the relative proportion of lipid to protein. Their structure reflects a conglomerate of individual components arranged to minimise hydrophobic contact with water. Because lipid and protein composition is reflected in density, density — an easily measured attribute — forms the operational basis for defining the lipoprotein classes.
Human plasma lipoproteins — composition and density:
| Chylomicron | VLDL | IDL | LDL | HDL | |
|---|---|---|---|---|---|
| Density (g/mL) | <0.95 | 0.950–1.006 | 1.006–1.019 | 1.019–1.063 | 1.063–1.210 |
| Protein (% dry weight) | 2 | 7 | 15 | 20 | 40–55 |
| Triglycerides (%) | 83 | 50 | 31 | 10 | 8 |
| Free cholesterol (%) | 2 | 7 | 7 | 8 | 4 |
| Cholesteryl esters (%) | 3 | 12 | 23 | 42 | 12–20 |
| Phospholipids (%) | 7 | 20 | 22 | 22 | 22 |
| Apoprotein composition | A-I, A-II, B-48, C-I, C-II, C-III | B-100, C-I, C-II, C-III, E | B-100, C-I, C-II, C-III, E | B-100 | A-I, A-II, C-I, C-II, C-III, D, E |
Source: Mathews, van Holde & Ahern, Biochemistry, 3rd ed. (2000), Table 18.1.
The hydrophobic core of each lipoprotein particle consists of TAG and cholesteryl esters, both typically carrying unsaturated fatty acid chains — this helps maintain a liquid consistency at the core even at body temperature.
Works cited
- Advanced Nutrition and Human Metabolism, 5th Edition.
- Morvaridzadeh M, Zoubdane N, Heshmati J, Alami M, Berrougui H, Khalil A. “High-Density Lipoprotein Metabolism and Function in Cardiovascular Diseases: What about Aging and Diet Effects?” Nutrients. 2024; 16(5):653.
- American Heart Association — “Saturated Fat” (heart.org, Healthy Eating: Fats).
- Amerman, Erin C. (Ed.), “The Urinary System” — [note: this citation appears to be carried over from a different post; recommend removing or replacing with the correct lipid-metabolism source]
Why fats matter, how they’re built, and how the body moves them around
Revised 18.09.2026 for accuracy.
There are many types of lipids, or fats, in the body. Lipids form an integral part of the cellular membrane — membrane lipids include cholesterol, glycolipids, and phospholipids. Fats are also an efficient, light-weight fuel store: because they’re energy-dense and light in weight, fat is the “go-to” long-term storage fuel for a mobile species. Myelin — the fatty sheath that insulates the axons of many neurons — depends on lipids for its formation, and is essential for normal neurological conduction. Fats are also critical to bile acid synthesis and to steroid hormone production. Fats remain essential to body function.
Conditions of fat excess, however — such as atherosclerotic disease linked to high dietary saturated fat intake — can lead to significant morbidity and are a leading cause of mortality. The predominant lipid in the human body is the neutral fat known as triglyceride. Triglycerides have three fatty-acid chains attached to a glycerol backbone; fatty acids themselves are hydrocarbon chains built from carbon and hydrogen.
Saturation and the physical properties of fat
Whether a fatty acid chain is saturated or unsaturated confers relative flexibility to its carbon backbone, and this is of real importance in both health and disease. Unsaturated fatty acid chains are more flexible and malleable, while saturated fatty acid chains are more fixed and rigid. This molecular property underlies the physical properties of fats — why some are liquid at room temperature and others are not — and is part of why saturated fats are generally considered more deleterious to cardiovascular health than unsaturated fats.
A fatty-acid chain can have single or double (rarely triple) carbon–carbon bonds. A chain of only single C–C bonds is saturated — its carbons are “saturated” with hydrogen. Saturated fats are typically solid at room temperature; these “solid fats” are found predominantly in animal products and are associated with atherosclerotic cardiovascular disease. The American Heart Association recommends no more than around 13 g of saturated fat per day (roughly 6% of total calories on a 2000-calorie diet).
When one or more C–C bonds in the chain are double, the chain is unsaturated — the individual carbons are not fully saturated with hydrogen.
Palmitic acid, for instance, has no double bonds and is solid at room temperature; oleic acid has one double bond; alpha-linolenic acid has three — both of the latter are liquid at room temperature.
Triglycerides are formed by esterifying the three hydroxyl groups of glycerol with three fatty acid molecules. As a general rule, plant-based triglycerides tend to be oils and animal-based triglycerides tend to be solid fats — though this isn’t universal: tropical plant oils such as coconut and palm oil are highly saturated and solid or semi-solid at room temperature, while fish oils, despite being animal-derived, are rich in polyunsaturated fatty acids and remain liquid.
The amphipathic fatty acid
The phase state of fats is an important determinant of their behaviour in the body. Every fatty acid has a polar carboxylic acid head — polar courtesy of its oxygen atoms — attached to a long, non-polar (hydrophobic) hydrocarbon tail. It’s this amphipathic structure, not just saturation, that governs how fatty acids and their derivatives behave at interfaces with water (e.g. within membranes and micelles).
Classifying lipids
Simple lipids: – Fatty acids – Triacylglycerols, diacylglycerols, and monoacylglycerols — TAG structural diversity depends on the type of fatty acid present, the position of the ester bond on the glycerol backbone, and the degree of unsaturation – Waxes (esters of fatty acids with higher alcohols) — including sterol esters (e.g. cholesterol–fatty acid esters) and non-sterol esters (e.g. vitamin A esters)
Compound lipids (a lipid plus a non-lipid component): – Phospholipids — phosphatidic acids (e.g. lecithin, cephalins), plasmalogens, sphingolipids (e.g. sphingomyelins) – Glycolipids — carbohydrate-containing – Lipoproteins — lipids in association with protein.
Derived lipids: derivatives such as sterols and straight-chain alcohols, obtained by hydrolysis of lipids from the groups above, which still possess the general properties of lipids — solubility in organic solvents such as ether, chloroform, and acetone.
Choosing fats wisely
Because dietary fat composition affects cardiovascular risk, the type of fat matters as much as the quantity. Both monounsaturated and polyunsaturated fats are preferred over saturated fat for cardiovascular health — the evidence here is nuanced: large randomised trials and meta-analyses (including the AHA’s 2017 advisory) show that replacing saturated fat with polyunsaturated fat produces a clear, consistent reduction in cardiovascular events, while the evidence for monounsaturated fat is drawn more from epidemiological data (e.g. the Mediterranean diet literature centred on olive oil). In practice, most current guidance favours minimising saturated fat and emphasising a mix of mono- and polyunsaturated sources over any strict ranking between the two.
Composition of fatty acid types in selected foods (% of total fat):
| Food/oil | Monounsaturated | Polyunsaturated | Saturated |
|---|---|---|---|
| Butter | 30.3% | 3.7% | 66.1% |
| Margarine | 46.3% | 33.3% | 20.4% |
| Mayonnaise | 29.8% | 54.8% | 15.4% |
| Canola oil | 61.7% | 30.8% | 7.5% |
| Olive oil | 77.3% | 8.6% | 14.1% |
| Peanut oil | 48.4% | 33.6% | 18.0% |
| Safflower oil (high-oleic) | 78.5% | 15.4% | 6.2% |
| Sesame oil | 41.5% | 43.8% | 14.6% |
| Sunflower oil | 20.8% | 68.5% | 10.8% |
Table: detailed fatty-acid-composition and iodine-value reference table across vegetable oils and fats, by carbon chain length
Digestion of fats
Dietary fats are emulsified by admixing with bile salts before enzymatic digestion can proceed efficiently.
| Location | Major events | Enzyme / secretion | Details |
|---|---|---|---|
| Mouth | Minor digestion | Lingual lipase (salivary glands) | Cleaves some fatty acids from triglycerides, yielding diglycerides + free fatty acid |
| Stomach | Additional digestion | Gastric lipase (stomach) | Further cleaves fatty acids from di-/triglycerides |
| Small intestine — Phase I | Emulsification | Bile (no lipase) | Emulsifies triglycerides, diglycerides, and fatty acids into micelles |
| Small intestine — Phase II | Enzymatic digestion | Pancreatic lipase (pancreas) | Cleaves remaining fatty acids from diglycerides, yielding monoglycerides + free fatty acids |
Absorption of fats
Dietary lipids — triacylglycerols, cholesterol, cholesterol esters, and phospholipids — enter the stomach largely intact. Only triacylglycerols are acted on there: lingual and gastric lipase hydrolyse medium- and short-chain fatty acids from the sn-3 position, yielding 1,2-diacylglycerols. In the small intestine, these lipids combine with bile salts to form micelles, acted upon by intestinal and pancreatic enzymes. Short-chain free fatty acids move directly into the portal circulation and bind albumin, while the remaining lipids — glycerol, glucose, cholesterol, long-chain free fatty acids, monoacylglycerols, diacylglycerols, lysophosphatidylcholine, and other lysophospholipids — enter the intestinal mucosal cell.
Within the enterocyte’s endoplasmic reticulum, glycerol is converted to α-glycerol phosphate (also produced from glucose via glycolysis); this, together with mono- and diacylglycerols, cholesterol, and lysophospholipids, is re-esterified into TAGs, cholesterol esters, and phospholipids. The lipoprotein Apo B-48 is synthesised in the ER and Golgi apparatus; combined with the resynthesised lipids, it forms a chylomicron, which is excreted from the enterocyte into the lymphatics. Other lipoprotein fractions contribute additional apolipoproteins to circulating chylomicrons.
Transport and storage: lipoproteins
Lipids resynthesised in the enterocytes, together with fat-soluble vitamins, are collected in the cell’s endoplasmic reticulum as large fat particles, which receive a surface layer of Apo B-48 while still within the ER.
Apolipoproteins are the protein components of lipoproteins — several distinct types are recognised (commonly taught as a core set of around nine: the A, B, C, and E families), and they help stabilise lipoproteins as they circulate in the aqueous environment of the blood.
Chylomicrons are lipid particles of exogenous (dietary) origin — the form in which resynthesised lipid largely leaves the enterocyte.
Very Low Density Lipoprotein (VLDL) is synthesised and secreted by the liver, carrying endogenously produced triacylglycerol into the circulation. Low-Density Lipoprotein (LDL), by contrast, is not itself a hepatic secretion product — it’s generated in the plasma as a downstream product of VLDL catabolism: lipoprotein lipase progressively strips triglyceride from circulating VLDL, first producing intermediate-density lipoprotein (IDL), which is further remodelled (partly via hepatic lipase) into cholesteryl-ester-enriched LDL.
The four major lipoprotein classes
There are four major classes of circulating lipoprotein, each with a characteristic protein and lipid composition, distinguished by the relative proportion of lipid to protein. Their structure reflects a conglomerate of individual components arranged to minimise hydrophobic contact with water. Because lipid and protein composition is reflected in density, density — an easily measured attribute — forms the operational basis for defining the lipoprotein classes.
Human plasma lipoproteins — composition and density:
| Chylomicron | VLDL | IDL | LDL | HDL | |
|---|---|---|---|---|---|
| Density (g/mL) | <0.95 | 0.950–1.006 | 1.006–1.019 | 1.019–1.063 | 1.063–1.210 |
| Protein (% dry weight) | 2 | 7 | 15 | 20 | 40–55 |
| Triglycerides (%) | 83 | 50 | 31 | 10 | 8 |
| Free cholesterol (%) | 2 | 7 | 7 | 8 | 4 |
| Cholesteryl esters (%) | 3 | 12 | 23 | 42 | 12–20 |
| Phospholipids (%) | 7 | 20 | 22 | 22 | 22 |
| Apoprotein composition | A-I, A-II, B-48, C-I, C-II, C-III | B-100, C-I, C-II, C-III, E | B-100, C-I, C-II, C-III, E | B-100 | A-I, A-II, C-I, C-II, C-III, D, E |
Source: Mathews, van Holde & Ahern, Biochemistry, 3rd ed. (2000), Table 18.1.
The hydrophobic core of each lipoprotein particle consists of TAG and cholesteryl esters, both typically carrying unsaturated fatty acid chains — this helps maintain a liquid consistency at the core even at body temperature.
Works cited
- Advanced Nutrition and Human Metabolism, 5th Edition.
- Morvaridzadeh M, Zoubdane N, Heshmati J, Alami M, Berrougui H, Khalil A. “High-Density Lipoprotein Metabolism and Function in Cardiovascular Diseases: What about Aging and Diet Effects?” Nutrients. 2024; 16(5):653.
- American Heart Association — “Saturated Fat” (heart.org, Healthy Eating: Fats).
- Amerman, Erin C. (Ed.), “The Urinary System” — [note: this citation appears to be carried over from a different post; recommend removing or replacing with the correct lipid-metabolism source]