Q. In your recent article about caffeine, you did not mention its role in exercise. Could you provideus with an update on evidence on that subject?

A.Just how caffeine might affect athletic performance is not completely understood.Nor has a consistent positive effective between caffeine and performance been demonstrated.

Any possible benefit appears to be related to the observation that it mobilizes free fatty acids for the body to use as fuel. This "spares" muscle glycogen and theoretically should increase endurance. But studies that have examined this theory have been inconsistent.

For example, caffeine taken at the level of 4 to 5 milligrams per kilogram of body weight (equivalent to an individual weighing 154 pounds drinking 2 cups of regular coffee) has been reported to increase prolonged aerobic endurance in events like marathoning in some but not all individuals. There is no evidence that it offers any advantage in aerobic events lasting less than 30 minutes.

Moreover, people vary in their ability to tolerate caffeine. Some experience a wide variety of side effects, including nausea, headache, anxiety, insomnia, restlessness, increased heart rate and fluid loss. And as one review of the subject points out, well-trained athletes already use free fatty acids efficiently, so any benefits from caffeine may be small.

Q. As part of a campaign to lose weight over the past several months, I have been eating lots of fresh fruits. Among my favorites are plums. Can you tell me something about their nutritional value?

A. Since you mention an interest in weight control, we assume that your primary concern involves the number of calories. As a general word of warning, we emphasize that the size of the fruits, and hence the number of calories they contain, can vary greatly. A fresh plum weighing a little over more than two ounces (slighly less than seven per pound) provides about 35 calories. But it is not uncommon to find plums twice that size.

The small plum would give you about 10 percent of the day's allowance for vitamin C, small amounts of the B vitamins and a little vitamin A. It is also a reasonably good source of potassium.

Q. I have a small group of friends who would like to launch a serious effort to lose some weight and keep it off. We thought a "buddy system" might be a good way to proceed. Con you suggest some printed material that might help us?

A. A new manual, the LEARN program for weight control by Dr. Kelly Brownell of the department of psychiatry at the University of Pennsylvania, provides an excellent step-by-step approach to modifying behavior.

LEARN stands for Lifestyle, Exercise, Attitudes, Relationships and Nutrition. The lessons in this manual provide accurate information in each of these areas. The author also addresses the many myths that cluster around the subject of dieting and explains why these claims cannot be true. It is a supportive and logically organized guide that is a useful "how-to" for individuals sincerely interested in losing weight once and for all.

To receive the manual, send a check for $15 to Brownell at the Department of Psychiatry, University of Pennsylvania, 133 S. 36th St, Philadelphia, Pa. 19104. The check should be made out to Dr. Brownell.

We also suggest that your group consider enlisting the services of an ADA (American Dietetic Association) registered dietitian to provide individualized guidance for your effort.

To locate such an individual, you might inquire at your state, county or local health department. An increasing number of these professionals are now listed in the Yellow Pages. Finally, the state or district dietetic association in many communities is now listed in the Yellow Pages as well, and these organizations can be helpful in providing names of dietitians in private practice.