I feel I am being discriminated against. It seems as if every article on diet in the popular press deals with obesity. No one ever writes about problems in gaining weight. How about some advice for those of us who are forever struggling to gain a few pounds?

A It's true that many more words are directed to those people who are obese than to those who are under-weight. There are several good reasons for this. In this first place, over-weight and obesity are far more common problems in our society. Moreover, being overweight, unlike being underweight, is closely associated with an increased risk of a number of serious conditions and therefore represents a threat to health. In contrast, the thinnest people seem to live longest, have less chance of developing diabetes and are least affected by degenerative diseases of the heart, lungs and kidneys.

Nonetheless, you are quite right that those who struggle with the problem of gaining weight may feel at least as frustrated with their lack of success as the overweight individual who perennially "diets," but either fails to reach his goal or regains the unwanted pounds soon after he has lost them.

In fact, for genetically thin people who apparently have fewer fat cells, attempts to gain a significant amount of weight may be an almost futile task. Possibly the best advice we can give is to capitalize on thinness by wearing clothes and hairstyles that those with more ample figures shun.

if you are physically quite active, you can also try to reduce your activity while taking care not to reduce your food intake. That way, you may take in more calories before your appetite mechanism signals that you're no longer hungry.

Finally, you can try to increase your caloric intake by adding between-meal snacks on a regular basis. In doing this, however, we would caution you to space your snacks well between meals. That way, they won't interfere with your appetite at mealtime and defeat the purpose for which they were intended.

Q: I have an elderly friend who regularly gets shots of Vitamin B12, which she says makes her feel much better. She recommends that I do the same. Is there any reason that I should take her advice?

A: No. As with other vitamins, B12 therapy is useful only when a definite deficiency exists. Shot should not be used.

B12 deficiency can occur in two ways. Strict vegetarians, who consume no animal products and do not take B12 supplements, have low vitamin stores and are most vulnerable to a dietary deficiency. So are alcoholics who consume poor diets.

There are certain individuals who may take in adequate amounts of Vitamin B12 but lack the "intrinsic factor" necessary to absorb the vitamin. These individuals must receive life-long Vitamin B12 therapy to prevent the particular type of anemia and neurological degeneration that characterize the disease. Beyond that, however, there is simply no rationale for the regular administration of Vitamin B12 shots.

It is true that the ability to absorb Vitamin B12, which is essential to the functioning of all body cells, decreases with age. However, since it is available in a wide variety of animal foods, a well-balanced diet should still provide all you need.

Q: Our dentist says that we should eliminate all sugar and sweets. I'm trying to cooperate, but it is fairly difficult. In an effort to reduce our consumption of sucrose, I've begun using honey. Is it as harmful as sucrose in producing tooth decay?

A: At the moment, there are no precise ratings for foods and sweeteners that may prompt tooth decay, although research on this subject is underway. And there are enough facts available to guide you in keeping dental caries to a minimum.

The relationship between sweets and tooth decay is related more to how long the food stays in the mouth than to the type of sugar used. Sugars, but no starches, are capable of filtering through dental plaque. Bacterial action then digests these sugars, releasing acid that is corrosive to the enamel.

As a result, if you put honey on your cereal or toast, for example, it is likely to be more damaging than if you dissolve it in tea.

The rule for controlling tooth decay is to reduce both amount and frequency of sugar-containing foods in your diet. Sweet snacks between meals are especially harmful. In other words, a pick-me-up caramel bar at 3 p.m. is far more menancing than a cup of tea with lunch.