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Side Notes

Lipid (Fat) Digestion and Metabolism

Lipid (Fat) Digestion and Metabolism

Lipids are a catch-all category for metabolic molecules that are not clearly some other kind of molecule. Vague, I know, but that’s how it is. When talking about dietary lipids, though, we restrict ourselves mostly to fatty acids and triglycerides. Triglycerides have gotten a bad rap because serum triglycerides (in the blood) are epidemiologically implicated in heart disease. We will examine a little bit how serum triglycerides are related to dietary lipid intake, but first, an overview of dietary lipids, their sources and types, and their digestion

Fatty Acids

On the left is the illustration of the plan carboxylic acid group. On the right is capric acid—a fatty acid. Note—in organic chemistry diagrams, if no element symbol is specified at the end of a line, that is assumed to be a single (or double, triple) bond to a carbon. Images in the public domain.

Fatty acids are molecules of carbon, hydrogen, and oxygen. They have a carboxylic acid group at one end of them—i.e., a carb double bound to an oxygen and single bound to another oxygen, which is bound to a hydrogen. The carboxylic acid group is often written as COOH or C=OOH, to emphasize the double bond. The last hydrogen bond is very weak and often comes apart in water, leaving the RCOO- and H+ ions free in the water, thus making these substances weak acids.1 Fatty acids distinguish themselves from other carboxylic acids with a fairly long chain of carbons extending away from the carboxylic acid group.

Triglycerides

On the left is a bare glycerol molecule, on the right, a triglyceride. Note the long chains bound to the glycerol molecule. These were fatty acids esterified onto the glycerol molecule. Public domain images.

The fat content in our food, however, comes not in the form of free fatty acids—in fact, an abundance of free fatty acids in food is part of rancidity—but as triglycerides (sometimes called tri-acyl glycerols). In these, the free fatty acids lose the hydrogen on the carboxylic acid group and instead that binds to a spot on the glycerol molecule.2 The glycerol molecule has three such spots, so when three of these fatty acids are esterified to it, it is called a triglyceride. (Mono- and di- glycerides also exist, where only one or two of the spots have fatty-acid chains, but these are comparatively rare.)

Plants and animals—indeed most organisms—prefer the triglyceride storage of excess fat. It is compact, easily broken apart when the acids are needed, dense, and electrochemically neutral (indeed, the long aliphatic chains expel water). A great deal of olive oil, for instance, is triglycerides of oleic acid.

Nomenclature

Saturated, Monounsaturated, and Polyunsaturated

When talking about fatty acids—or their glycerol esters—one encounters the terms saturated, monounsaturated, and polyunsaturated. A saturated fatty acid (SFA) is one where there are no carbon-carbon double bonds. It is “saturated” with as much hydrogen as it can while remaining a carboxylic acid. A monounsaturated fatty acid (MUFA) has a single double bond, and a polyunsaturated fatty acid (PUFA) has more than one double bond. This is important to their chemistry: More double bonds means it is more easily oxidized.

Saturated fats are ordinarily solid at room temperature. Monounsaturated fats are liquid at room temperature but start to solidify at fridge-like temperatures (below about 40 fahrenheit). Polyunsaturated fats remain liquid until below freezing ordinarily.

Hence, animal fats contain a higher concentration of saturated fats (think butter, lard, beef tallow—all solid at room temperature). Note that these fats aren’t purely saturated, in fact beef tallow is only about half saturated fat, by mass. The most common monounsaturated fat comes in the form of oleic acid from olive oil or others in Canola oil. (About 73% of olive oil by weight is made of MUFAs.) PUFAs prevail in seed oil, in general, although exceptions exist.

Finally, you may find products labeled as “hydrogenated vegetable oil,” a.k.a., Crisco. In these cases, MUFA- and PUFA-dense vegetable oils are exposed to a catalyst and hydrogen to make them solid at room temperature and therefore appropriate to use as shortening.

Colon Notation

Sometimes, one sees a colon notation with fatty acids. For instance oleic acid 18:1. The 18 means that there eighteen carbons in the aliphatic chain; the one means that there is one double bond. An 18:2 fat would have two double bonds. Thus one can tell if a fat is saturated (having a zero after the colon), monounsaturated (a one after the colon), or polyunsaturated, with this notation. With MUFAs and PUFAs, one sometimes sees more specification (no more specification is needed for SFAs): For instance oleic acid would be written as “18:1 cis-9”, saying that its double bond is present between the ninth and tenth carbons (when counting from the end opposite of the carboxylic acid group) and that that double bond is cis, rather than trans.

ω- and n- notation

Furthermore, one sometimes sees ω- or n- notation. An omega-3 or ω-3 fatty acid is an unsaturated fatty acid where the first double bond occurs at the third location from the end opposite the carboxylic acid group. In dietary concerns, one mostly hears about ω-3 and ω-6 acids, but others are relevant. For instance, oleic acid (see above) is ω-9. The n- notation serves the exact same purpose. (Oleic acid is an n-9 acid.)

Short, Medium, Long, and Very Long Chains

These names are applied to both fatty accids and their triglyceride forms. Short chains refer to carbon chains between 1 and 5 carbons long. Medium chains have 6–12 carbon atoms, long chains 13–21 carbons, and very long chains 22 carbons or more.

Digestion of Dietary Fat

Thestory of the digestion of lipids begins in the mouth, where lingual lipase begins to break down triglycerides into di- and mono-glycerides. This action continues in the stomach, where gastric lipase continues. Short-chain fatty acids may be absorbed by the stomach, but, as in the carbohydrate case, most absorption begins in the small intestine. Gastric and lingual lipase perform about 30% of hydrolysis needed to fully break down the triglycerides. The partially digested food from the stomach (“chyme”) enters the duodenum, the first part of the small intestine.

In the small intestine, chime mingles with pancreatic juice. Pancreatic juice first emulsifies the fatty content with bile salts from the liver. Then, the fat is free to mingle with a complex of pancreatic lipase and colipase, where the remaining 70% of hydrolysis takes place. Short and medium chain fatty acids may passively diffuse directly into the hepatic vein. Longer chains (which constitute most fatty acid intake), glycerol esters, cholesterol, and fat-soluble vitamins3 must first enter into micelles, which are large molecule complexes with a soapy outer layer. The outer part of this soapy layer freely disolves in water, while the inner part is lipophilic (dissolving fats).

The small intestine enterocytes absorb these micelles, repackage free glycerol, glycerol esters, and free fatty acids into triglycerides, pack the lipid contents into another carrier, the chylomicron, which serves a similar purpose of making the lipids water-soluble. These chylomicrons enter the lymphatic system, forming chyle, which then drains into general blood circulation. They do not go directly to the liver first, as with all other digestion products.

In the blood,


  • 1. When writing out the formula of organic molecules linearly (such as H3CCHOH or similar), one sometimes uses R to represent a general chain of organic atoms. In this case, R is always an alkane, alkene, or alkyne, i.e., just carbons linked to each other with single, double, or triple bonds, with the requisite number of hydrogens to fill out the molecule.
  • 2. The process where an oxygen, already single-bound to some organic compound, is single bound at the other side to another organic compound is called esterification and the resulting compound is called an ester.
  • 3. Viz., Vitamins A, D, E, and K