DIGESTION OF FAT AND OIL

Maintenance of a healthy digestive system requires input from lipids, which include molecules such as cholesterol, appropriate saturated and polyunsaturated fatty acids, and other lesser known components such as glycosphingolipids.

Cholesterol is the precursor to bile acids, which are needed to digest and absorb long-chain fatty acids. Cholesterol is also recognized for its physiological importance in the skin and the intestine where it plays an important structural role as a component of the organ membranes. Cells lining the digestive tract are particularly rich in cholesterol.

Saturated fatty acids play their role in membrane integrity by providing appropriate fatty acids for certain parts of membrane structures. Among their various roles are the signaling activities that, for example, tell the gastrointestinal musculature when to contract. Polyunsaturated fatty acids also provide raw material for membranes, and work as precursors to the various prostaglandins, needed to maintain important functions of intestinal motility.

Glycosphingolipids are lipids with single sugar molecules attached found in cell membranes, especially in the brain. They also protect against gastrointestinal infections, especially in infants and children. Whole milk is an adequate source, especially human milk. Fat digestion of phospholipids and other lipids makes up very minor, but sometimes important, parts of the building blocks for tissues. These special lipids are usually made by the body and diet is not the major source.

Digestion of fatty acids from triglycerides is different for the regular long-chain fatty acids (14 carbons to 22 carbons) than it is for short- and medium-chain fatty acids (4 carbons to 12 carbons). Usually about 95 percent of the fat is available for digestion when the mixture of fatty acids is varied.

The digestion of regular fats and oils, which are usually long-chain triglycerides, requires bile acids as well as lipases. In adults this digestion usually starts in the small intestine and is done with the aid of lipases and bile acids. The bile acids allow the triglycerides to be properly emulsified and the lipases break the triglycerides into individual fatty acids and monoglycerides in the small intestine. When these parts are absorbed through the wall of the intestine, they are reassembled into triglycerides and carried into the body through the lymph system on chylomicrons.

Short- and medium-chain fatty acids from fats such as milk fat or coconut oil or palm kernel oil are broken off from the triglycerides without the need for bile acids. They are then shuttled directly to the liver through the portal artery without the use of chylomicrons. In the case of a meal with a large amount of lauric acid, some of this medium-chain fatty acid does travel via chylomicrons through the lymph system.

Fat digestion of cholesterol and other sterols is frequently not described accurately. Both cholesterol and other sterols do not provide any calories, and the amount that is absorbed is relatively small except in infants. Adults probably absorb only about 25 percent of the cholesterol they consume, and even less of the other sterols. Cholesterol plays a role in membrane structure as well as for production of bile acids and hormones. Other sterols are not usually part of the body’s tissues unless they are consumed in large amounts.

People ask why fat is digested more slowly than either protein or carbohydrate, and sometimes think that this means that there is a problem with digestion of fat; however, the slow digestion is really only nature’s way of maintaining an even amount of the energy distribution.

Fat digestion in infants is somewhat different from fat digestion in adults, especially if the infants are fed human milk. The digestion of fats in the infant begins in the mouth with the function of several digestive enzymes that are special to the infant. The fatty acids are broken down in order to be well digested. A special enzyme coming from the mammary gland enables most of the cholesterol from the human milk to be absorbed by the infant. Cholesterol is a very important nutrient for the infant, especially for its role in brain and other central nervous system development. The typical infant formula is greatly lacking in cholesterol and also lacks the enzyme that aids in the absorption of cholesterol.

Dietary fats, like those in butter, meat or cooking oils, are basically organic compounds composed of carbon, hydrogen, and oxygen. They consist of complex molecules and are the most highly concentrated source of energy in our daily diet. They belong to a class of substances called lipids. Unfortunately, dietary fats do not dissolve in water, as a result they are not easily broken down by fat-digesting enzymes (lipase) in the watery content of the gastrointestinal tract. Thus fats tend to take longer to digest than carbohydrates or proteins.

HOW FAT IS DIGESTED

Although a small amount of lipase is secreted by Ebner's glands on the tongue, and by the stomach, these digestive actions are not significant, as almost no real breakdown of fat occurs until the fats reach the duodenum in the form of gastric chyme.

FAT BREAKDOWN IN THE SMALL INTESTINE

Fat digestion and absorption requires that the complex fat molecules be broken down into smaller more manageable molecules. This is done by mixing the fat with the digestive enzyme lipase, which enters the duodenum from the pancreas - the main source of enzymes for digesting fats and proteins. Lipase chops up lipid molecules into fatty acid molecules and glycerol molecules.

However, because fat does not dissolve in water, the fat molecules enter the duodenum in a congealed mass, which makes it impossible for the pancreatic lipase enzymes to attack them, since lipase is a water soluble enzyme and can only attack the surface of the fat molecules. To overcome this problem the digestive system uses a substance called bile, produced in the liver but stored in the gallbladder, which enters the duodenum via the bile duct. Bile emulsifies fats - meaning, it disperses them into small droplets which then become suspended in the watery contents of the digestive tract. Emulsification allows lipase to gain easier access to the fat molecules and thus accelerates their breakdown and digestion.

HOW FAT IS DIGESTED AND ABSORBED INTO THE BLOODSTREAM

Lipase and other digestive juices break down the fat molecules into fatty acids and types of glycerol. Absorption of fat into the body, which takes 10-15 minutes, occurs in the villi - the millions of finger-like projections which cover the walls of the small intestine. Inside each villus is a series of lymph vessels (lacteals) and blood vessels (capillaries).

The lacteals absorb the fatty acids and glycerol into the lymphatic system which eventually drains into the bloodstream. The fatty acids are transported via the bloodstream to the membranes of adipose cells or muscle cells, where they are either stored or oxidized for energy.

Since glucose rather than fat is the body's preferred source of energy, and since only about 5 percent of absorbed fat (the glycerols) can be converted into glucose, a significant proportion of digested fat is typically stored as body fat in the adipose cells.

The glycerol part is absorbed by the liver and is either converted into glucose (gluconeogenesis), and/or used to help breakdown glucose into energy (glycolysis).

SUMMARY

FAT DIGESTION

Fat digestion takes much longer than the digestion of carbohydrates and somewhat longer than the digestion of proteins. A raw salad consisting of nonstarchy vegetables can be digested within two to three hours. When free fats such as corn, sesame, peanut or other oils are added to the salad, digestion is delayed for another two or three hours.

Coating our food with free oils inhibits the natural digestive processes by preventing digestive juices access to these foods until the oils are digested. Consequently, by the time the oils or fats surrounding the other food particles are digested, the elementary carbohydrates or proteins in the vegetables have begun to ferment (carbohydrates) or putrefy (proteins) in the stomach.

FATS REQUIRE SPECIAL DIGESTION

Free fats, unlike carbohydrates, require special digestive action before absorption. This is because the end products of all digestion are carried in a water medium (that is, the blood and lymph). Free fats are not soluble or transportable in these water mediums until they undergo special changes.

TRACING FAT DIGESTION

After fats leave the stomach, they enter the duodenum of the small intestine. Their presence causes the stimulation of the gallbladder, which forces bile down into the small intestine. The bile emulsifies, all the fats in the intestines.

The emulsified fats are then split by enzymes into fatty acids and glycerol. At this point, the fats can be absorbed through the intestinal mucosa. During absorption, the fatty acids and glycerol recombine with a small amount of protein to form microscopic particles of fat called chylomicrons.

The fats in the form of chylomicrons are now soluble enough to enter lymph circulation. The fatty acids are converted to the liver to acetate or ketone bodies as an energy source for the cells.

The fat which is not used immediately for the body's energy needs is stored primarily in adipose tissue. Adipose tissue is a special kind of tissue (found mainly around the stomach, thighs and buttocks) which contains the necessary enzymes to continually produce and release new fat to meet the body's needs.

REFERENCES

1.     MI Gurr & AT James. Lipid Biochemistry: An Introduction. Chapman and Hall, London, 1971.

2.     Mary G. Enig, Ph.D. Know Your Fats: The Complete Primer for Understanding the Nutrition of Fats, Oils, and Cholesterol. Bethesda Press, Silver Spring, Maryland, 2000.

3.      Dusenbery, David B. (1996). “Life at Small Scale”, pp. 113-115. Scientific American Library, New York. ISBN 0-7167-5060-0.

4.      Dusenbery, David B. (2009). Living at Micro Scale, p. 280. Harvard University Press, Cambridge, Mass. ISBN 978-0-674-03116-6.

5.      Wooldridge K (editor) (2009). Bacterial Secreted Proteins: Secretory Mechanisms and Role in Pathogenesis. Caister Academic Press. ISBN 978-1-904455-42-4.