DEAR DR. BLONZ: I understand why they put vitamin D in milk, as it aids in the absorption of calcium, which is plentiful in dairy. But why do they put vitamin A in milk? What is the connection? -- R.R. San Jose, California
DEAR R.R.: Nutrients get added to foods in a couple of ways. Enrichment is the adding back of nutrients that were lost during processing, and fortification is the addition of nutrients to a food where they do not normally occur. This addition of nutrients was begun in the 1930s as a practical means to increase the nutritional value of foods, often in response to specific deficiency conditions.
Nutrients were added to foods when typical intakes of that nutrient were below desirable levels for a significant number of people. The foods chosen to be "enhanced" were those typically consumed in sufficient quantities to have the desired effect in the target populations (and not represent any danger for excessive consumption by others). The reliance was on staple foods, such as cereals, milk products and salt.
Finally, the nutrients would have to be added in a form that would be stable and could be easily absorbed. Vitamin D, and later vitamin A -- two fat-soluble substances -- were added to dairy products mainly because they were thought to be the best vehicles for these nutrients.
DEAR DR. BLONZ: I am reluctant to try shortcuts, but I wanted information on the latest product claiming to help people "lose weight without dieting or exercising." It is called chitosan, and there are videos all over the Internet and infomercials on TV. One has a person talking about taking it before a big fatty meal, and your body won't absorb the fat. -- F.S., Austin, Texas
DEAR F.S.: Chitosan is derived from chitin (KITE-un), a component of the shells of crustaceans, such as crab, shrimp and lobster. Chitosan has an ability to bind with fat in the stomach before it has a chance to be absorbed. The chitosan-bound fat travels through the digestive tract and is eventually eliminated from the body in the feces.
If there's an upside to chitosan, it's that fewer fat calories get absorbed. There are estimates that each gram of chitosan can absorb four to eight times its own weight in fat. This means that a 250-milligram capsule could prevent the absorption of 1-2 grams of dietary fat (9 to 18 calories' worth) -- assuming that the fat is in the digestive tract along with the chitosan.
The negatives are that chitosan is not selective in the fatty substances it binds to. That means that it has an equal ability to grab onto beneficial compounds such as fat-soluble vitamins, valuable phytochemicals and omega-3 fatty acids. One study found that a high intake of chitosan led to a rapid decrease in the blood level of vitamin E.
It is also plausible that chitosan could interfere with the absorption of medications where the active component is a fat-based substance. Steroids, birth control pills, hormone replacements and many cholesterol-lowering medications, for example, could be at risk.
Finally, given that the unabsorbed, chitosan-embraced fat passes into parts of the digestive tract where dietary fats rarely go, there may be side effects like discomfort, intestinal gas and diarrhea. Keep in mind that chitosan -- or any other supplement -- does little to influence healthful eating habits, which are the key to long-term success.
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