My friend wants to go on one of the low-carbohydrate diets that have been fashionable for years. I know it's not a good idea, but I don't have the background to convince her. Can you help me?

ANSWER - Perhaps the most tangible reason you can give her is that these diets don't promote the types of changes in habits that encourage long-term weight maintenance. Another problem with them is that, unless carefully planned, they can often be high in fat.Despite such drawbacks, it's easy to see why these diets have been attractive. In the early days of the diet, weight loss is usually considerable. Unfortunately, the dramatic results are too good to be true.

Here's how it works: The diet contains less carbohydrate than the body needs. To make up the difference, carbohydrate stores, in the form of glycogen, are taken from the liver. Glycogen is stored with as much as four times its own weight in water. As it is used, large amounts of fluid are also lost, creating that precipitous drop in the needle on the scale.

In addition, when carbohydrate intake drops below a level of 60 grams, there is a loss of minerals such as sodium and potassium that help maintain fluid balance. That, too, results in water loss. All this happens within three days or so of starting the diet and lasts for several days.

From then on, however, weight loss tapers sharply. Every deficit of 3,500 calories should lead to a pound of weight loss. But at this point or shortly thereafter, dieters tend to become unhappy with their regimen and success comes to a grinding halt. Worse yet, with a return to former habits, all the lost weight is regained - and often more besides.

Besides dissuading your friend from trying a diet that really doesn't work, perhaps you can assist her in finding a program that will help her improve both her eating and exercising habits. That way, she will not only shed pounds, but will succeed in keeping them off permanently.

QUESTION - Is there any truth to the claim that aluminum, which migrates from cookware and foil packaging, is linked to Alzheimer's disease?

ANSWER - Evidence suggests that aluminum toxicity does not cause Alzheimer's disease . The theory, which has received considerable attention in the press, stems from the finding of aluminum accumulation in the brain of individuals with AD.

In studies where aluminum was injected into the brains of rabbits, it caused pathological changes. Beyond that, patients on dialysis have developed dementia or memory loss and other impairments of brain function when the aluminum concentration of water used to prepare the dialysis solution was too high.

But these observations don't add up to a cause-and-effect relationship. Most researchers now believe that the buildup of aluminum is not the cause of the disease but rather a nonspecific effect of injury to the neurons. For one thing, under microscopic examination, the three groups in whom aluminum accumulation has been studied - individuals with AD, rabbits injected with aluminum, and patients undergoing dialysis - the cells look quite different.

Second, while aluminum buildup in victims of AD has received the publicity, it is not unique to the disease. Mineral buildup is seen in other diseases, too. Finally, population studies have failed to find a tie between aluminum exposure and AD. In short, current evidence provides no reason to throw away your aluminum cookware.

QUESTION - I'm aware that you take a consistent position against using megadoses of vitamins. But not long ago, I read an article that described people who need huge amounts of one vitamin or another to live. Was this quackery or are there exceptions?

ANSWER - Without seeing the article, we can't tell you whether it was accurate. But it is true that such cases do exist. In 1954, an infant with convulsions responded to very large doses of vitamin B-6 (also called pyroxidine). This was the first recognized case of what are termed "vitamin-responsive inborn errors of metabolism."

Since then, many other genetically linked disorders, involving a number of vitamins, have come to light. They can be related to a defect in absorption, transport or use of the vitamin. Treatment requires doses anywhere from 10 times to as much as 1,000 times the Recommended Dietary Allowances that cover normal healthy individuals.

These cases are extremely rare. And while treatment is very effective, under no circumstances should people try to diagnose and medicate themselves with excessive doses of any vitamin. Three decades after the first case of a genetic disorder requiring treatment with very large doses of pyroxidine was described, the medical literature contained the first reports of toxicity related to the use of megadoses of the very same vitamin.

1991, Washington Post Writers Group