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.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.

DEAR DR. BLONZ: As a teacher, I am concerned about what some parents give their children for lunch at my school. When I ask the children, some say that they have these types of foods at home, as well. I have been given the go-ahead to develop a nutritional lesson plan that involves doing a blog for the parents. What are your thoughts on highly processed foods for children? -- M.L., Oakland, California

DEAR M.L.: Highly processed children's meals are not the stuff on which long-term healthy eating habits are established. It is less of a problem if we are taking about whole foods that are minimally processed, such as drying or freezing fruits and vegetables. But if sweetened snacks and fabricated "foods" tend to be the routine rather than the exception, it's time for a reexamination of priorities.

I think the blog is a great idea, but I would proceed cautiously. The idea is to enlist, not alienate. One strategy I used, when speaking to students at my son's middle school, was to scan the labels of children's favorite foods with them. I went through the ingredients, explaining which were good for them and which were not (and all the various ways of naming "sugar"). The outcome was that the children went home and started lecturing their parents on ingredients! The lesson can be eye-opening, and also quite entertaining.

A child's interest in healthy eating really should be home-grown, so to speak. In your lesson plan, you might consider sharing ideas that involve the whole family. For instance, parents can get kids involved in menu planning: Discuss options for upcoming meals, and ask for input. If there is more than one child, they can take turns helping to plan meals.

Asking children to name their favorite vegetables or fruits, and then letting them pick them out at the market, may increase the odds that they'll eat them. Parents also need to have kids help with meal preparation, whether it's making a lunchtime sandwich, helping measure ingredients or simply arranging food on a serving plate.

Whenever possible, take kids along on shopping trips; just make sure they've been fed beforehand. At the grocery store, children are on the receiving end of a tremendous amount of marketing -- cartoon-character promotions, end-aisle displays, colorful package designs, kid's-eye-level placement of snack foods -- all designed to influence them, and consequently, their parents' purchases. A full stomach can help them navigate this manipulation, as can their new knowledge from your blog.

All these efforts will have greater chances for success if they are designed to work together, and if the parents are on board. Lessons about the nutritional value of various foods, and their importance for growth and good health, can provide a foundation to establish healthful habits for life. I applaud your energy and wish you well in your efforts.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.

DEAR DR. BLONZ: A friend told me to take guarana pills for alertness, better concentration and faster learning. I've also heard that guarana is used as a cure for impotence. What can you tell me about it? -- D.P., Charlotte, North Carolina

DEAR D.P.: Guarana (Paullinia cupana) comes from the seeds of a South American climbing shrub. The claimed effects you mention, at least those associated with alertness and better concentration, relate to the fact that guarana has a high concentration of caffeine -- over twice that found in the coffee bean, the kola nut or cacao (cocoa). Coffee drinkers view their brew with a degree of reverence for its stimulating effects, but caffeine actually functions as a plant's phytochemical defense to dissuade animals from dining on its seeds.

As regards your statement concerning the other alleged use for guarana, there is no research to support the use of guarana or caffeine to treat impotence.

DEAR DR. BLONZ: I know this is the case with bananas, but do oranges get sweeter and improve in nutritional value as they age? What about color and storage? -- B.B., Baton Rouge, Louisiana

DEAR B.B.: There is no reason to "save" an orange in the hopes that it will improve in flavor or nutritional value. Some fruits, such as bananas or stone fruits, contain enzymes that remain active after harvest, and these enzymes break down complex carbohydrates into simpler, sweeter-tasting sugars. This, however, isn't the case with oranges, a fruit that is traditionally harvested at the peak of ripeness.

What that means is that when it is picked, the orange is as sweet as it's going to be. It is also the time at which the vitamin C content will be the highest. As for color, it may not always be a reliable indicator of ripeness and flavor, as perfectly ripe oranges sometimes carry a greenish tinge. The color of an orange tends to reflect the temperatures in the days prior to harvest.

Although they will keep for a few days at room temperature, it's best to keep oranges in the refrigerator. When an orange begins to shrivel, its nutrient value will have also begun to decline.

DEAR DR. BLONZ: I am hoping that you can weigh in on a debate my son and I are having. I told him that when vegetables are boiled, nutrients evaporate with the steam unless the lid is on. He has dismissed this, saying that all the nutrients stay in the cooking water. -- J.A., San Diego

DEAR J.A.: Your son is much closer to the truth. Although some nutrients are destroyed by heat, they do not evaporate and get carried away in the steam. Those nutrients that are water-soluble and heat-stable will remain intact in the cooking water. The nutrients that don't dissolve in water remain in the vegetable.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.