Q. My fiance is so anxious for me to lose weight that he bought me a can of a diet protein powder. It comes with a bottle of Vitamin B6, which is supposed to act as a diuretic. I couldn't believe the price. An 11-day supply cost $20. I decided to send the label information, as well as the instruction booklet, to get your opinion.

A. We have found much of the advice in the booklet to be misleading, erroneous and in some cases downright harmful.

In the first place, a dieter should not restrict fluids, even for four days, as advised in the booklet. Restrictions of fluids produces a more concentrated urine and consequently can increase the likelihood of stone formation in predisposed individuals. In individuals who may have a problem with fluid retention, the usual procedure is to restrict sodium not water, to eliminate the excess.

Despite the claim, there also is no reason to believe protein powder will improve skin, hair texture or fingernails unless a protein deficiency existed in the first place (which is not likely).

It is probably true that if you take the protein powder in lowfat milk or juice for two meals a day and eat one meal of a reasonable size at night, you'll lose weight - not because the powder has any magic properties, but because you'll probably be consuming only about 1,200-1,300 calories a day. However, we feel that a diet where the 1,200 calories comes a variety of fresh and lightly processed foods is far safer, certainly more enjoyable and more likely to help build good habits to maintain your weight loss.

In short, our advice is simple: Return the powder, follow a sensible diet and save the money for a new dress when you have attained your weight goal.

Q. In discussing fiber, you have mentioned a substance called lignin. What is it?

A. Lignin has recently been described as the "old man out" among fiber components because it is the only one that is not a carbohydrate. It is a particularly tough substance (its name comes from the Latin word for wood, lignum ) that lends strength and support to the constituents of plant cell walls.

Lignin has been of particular interest to animal nutritionists for many years. While it is thought to have hide salt blinding capacity in humans, studies have been inconclusive. Part of the problem in learning about how lignin, as well as other components of dietary fiber, works is that its functioning is affected by the presence of the other constituents in the cell wall of the plants. Thus, studying a single fraction of fiber gives a distorted picture.

Our present understanding of the importance of lignin in human nutrition is far from complete.

Q. You have mentioned that antioxidants are often used in food processing. What are these substances and why are they necessary?

A. It may seem strange that a vital substance like oxygen can be the culprit in food spoilage, but it can. The natural attraction of some foods for oxygen can cause many undesirable changes. Unsaturated fats, for example, may become rancid. The oxidation of pigments and other components causes food discloration. Because oxygen is often closely associated with food, it is extremely difficult to eliminate it all during food processing and packaging, and sometimes even small amounts of oxygen are enough to cause undesirable changes.

Antioxidants have been used commercially for about 30 years. The most widely used include BHA (bufylaxed hydroxyanisole), BHT (butylaxed hydroxytoluene) and propylgallate. Citric acid, also a natural antioxidant, is often used to tie up metals that promote oxidation.

Frequently two or more of these antioxidants are used together because a combination has been found to be more effective than a single substance.