Olestra and Vitamin Solubility
The solubility properties of vitamins determine how well they will be absorbed by the body. Water-soluble vitamins can easily enter the bloodstream by diffusion, since the stomach contents, extracellular fluid, and blood plasma are all aqueous solutions. Fat-soluble vitamins must be consumed together with dietary fat to be absorbed. The vitamins are first dissolved in the dietary fat. Then, bile released from the gall bladder acts like a detergent and allows the fat (with the vitamins dissolved in it) to be solubilized in micelles. (Recall the discussion of detergents and micelles from the "Membranes, Proteins, and Dialysis" experiment.) However, some newly-developed food products, unfortunately, have been found to disrupt this pathway for absorbing fat-soluble vitamins in the body.
In recent years, many types of "fat-free" foods have come into the marketplace. One such type of these foods contains artificial fats that are substituted for the natural fats and oils found in the foods. These artificial fats add no fat or calories to the diet, because they are not digested or absorbed by the body. The main artificial fat commercially in use is Olestra. Olestra is marketed under the name Olean by Proctor and Gamble, Inc. It is a synthetic sucrose ester that is not digested or absorbed by the body. How does this work? Olestra, like natural fat, has nonpolar hydrocarbon chains. But whereas fat has only three such chains attached to a glycerol molecule (and thus is known as a "triglyceride"), Olestra contains eight such chains attached to a sucrose molecule. (Refer to the figure on membrane structure in the "Membranes and Proteins" experiment for the structure of glycerol.) To digest natural fats in the body, lipase (an intestinal enzyme that breaks down lipid molecules) removes the hydrocarbon chains from the glycerol. The hydrocarbon chains are then emulsified (incorporated into micelles) with bile, and absorbed into the bloodstream. Because Olestra has so many hydrocarbon chains, there is not enough room for lipase to reach the place where they are attached to the sucrose, and so the side chains cannot be removed. Therefore, the nonpolar Olestra molecule is too large to form absorbable micelles, so it passes through the intestinal tract, undigested and unabsorbed by the body. Olestra has been approved by the FDA for use in savory snacks, such as potato chips.
Unfortunately, Olestra may not be as healthy as it first sounds. It has been shown to cause gastrointestinal symptoms including abdominal discomfort, flatulence, and changes in stool consistency. More importantly, it interferes with the absorption of fat-soluble vitamins from food when present in the small intestine at the same time as other foods. Because it is nonpolar, Olestra can dissolve fat-soluble vitamins. Hence, Olestra in the small intestine competes with fat-containing micelles in the intestine for absorption of fat-soluble vitamins. Anything the Olestra absorbs is carried out of the body with it and is therefore not available for absorption by the body. Adding more fat-soluble vitamins to food containing Olestra seems to be effective in preventing Olestra from depleting the body's supply of fat-soluble vitamins. However, long-term studies are not yet conclusive on the effects of continued ingestion of Olestra on humans.
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This page created by Matt Traverso, Washington University in St Louis.
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