I have been told by several people that drinking large amounts of grapefruit juice speeds up metabolism, burns calories and destroys nutrients. I find this hard to swallow. Is there any truth to it?

Not a juice glassful. The idea that grapefruit contains an enzyme that burns calories has been stitched on to a crazy-quilt of fad diets throughout the years. But it is just plain nonsensical. Before food enzymes could participate in body metabolism, they would need to be absorbed. Enzymes, however, are proteins, and cannot be absorbed intact. They must first be digested or broken down into amino acids, the basic building blocks from which they are created.

At that point, they lose their identity as enzymes, and therefore could not affect body metabolism. We are puzzled by those who claim that the enzymes "destroy" other nutrients, but who can offer no reasonable scientific explanation why the enzymes might do so.

Notwithstanding the false advertising by food faddists, grapefruit has many nutritional benefits. Most notably, it is a low-calorie source of vitamin C. A medium half, containing just 55 calories, provides almost the entire Recommended Dietary Allowance for an adult. It provides iron and B vitamins as well and, if you happen to choose the pink varieties, you will get vitamin A from the carotene they contain.

Frozen and bottled grapefruit juices are excellent, convenient foods. Still, you might want to treat yourself to a glass of freshly squeezed juice which, in our view, along with fresh orange juice ranks as one of nature's tastiest delights.

I have heard that a supplement called lecithin is useful in lowering blood cholesterol levels. What is lecithin, and does it work?

Lecithin is the name for a group of chemically similar compounds. They are fat-like substances, phospholipids with several different parts: glycerol (an alcohol), phosphoric acid, a nitrogen-containing unit, and fatty acids.

Although they are commonly known as a nutritional supplement, lecithins are found naturally in food, especially egg yolks, organ meats, whole grains and legumes. They are also used as emulsifiers in processed foods.

The claim that they can lower blood cholesterol and prevent, or even cure, heart disease is based on the fact that they act as emulsifiers and function in transporting fats. Several decades ago, it was reported that when given by injection, lecithin decreased cholesterol deposits on the arterial walls of some experimental animals. In later studies, when it was given orally, no such effect was observed, in all likelihood because the lecithin was digested.

It is important to note that because lecithins contain both fatty acids and glycerol, they provide calories -- 225 per ounce, to be precise. So for those who are watching their weight as well as their cholesterol levels, taking lecithins will not only be of no benefit to their serum cholesterol, it also will hamper their efforts to reduce.