Q. After studying a few diet books, I've noticed an interesting trend. Many of them discourage alcoholics from following the diets. As a recovering alcoholic, I'm curious about the reasons.

In addition, some weight-reducing programs I've seen advertised say that people with a history of alcohol abuse within the past two years should not participate in the program. Why do some diets and weight programs discourage alcoholics from taking part? Are there medical reasons, behavioral reasons, or both?

A. If you're a recovering alcoholic without any liver disease or other medical condition that your doctor feels would prohibit you from dieting, I don't see any reason that you couldn't.

Weight-reducing programs usually recommend that you stop drinking before starting with them for several reasons. First, many heavy drinkers have nutritional deficiencies to begin with, and dieting might further jeopardize their health. Second, alcohol represents "empty calories." Ounce for ounce, alcohol has almost twice the number of calories as sugar, but provides virtually no nutritional value. Last, people who can't control their drinking also might be unable to follow a weight-reducing program.

In many ways, weight control programs and Alcoholics Anonymous have a lot in common. They both use group support in trying to modify behavior and help people gain control over their lives. For this reason, having successfully recovered from alcoholism might make it easier for you to follow a weight control plan. In fact, the support you receive in a comprehensive weight control program should reinforce your efforts to maintain sobriety through programs like AA.

Whether you're a recovering alcoholic or not, I recommend a low-calorie diet that's nutritionally complete, meaning that it provides you with all the essential nutrients in a well-balanced way. Fad diets that consist of only a few items are potentially harmful, especially if you suffer from liver disease brought on by alcohol abuse.

In terms of priorities, it makes good sense to achieve sobriety before starting a weight-reducing program. Since you've already done that, there shouldn't be any obstacle preventing you from attaining your next goal.

Q. For the past six months, I have had a very painful hard lump the size of an egg, swollen and tender, in back of my knee. Occasionally, it becomes very tight and makes walking difficult unless I use crutches. Could you tell me what this could be? I'm 49.

A. It's difficult to tell without an examination, but my guess is you have a Baker's cyst.

Named after an English surgeon, this cyst is a small sac filled with joint fluid protruding from the back of the knee joint. Because the fluid can sometimes empty into the knee joint, the cyst can vary in size over time.

Baker's cysts usually develop in knees damaged by arthritis, such as osteoarthritis (degenerative joint disease) or rheumatoid arthritis. Occasionally, they develop for no apparent reason. Once a cyst is there, it may not bother you at all, or it may cause pain or trouble with walking.

At times, a Baker's cyst can rupture, or break open, causing joint fluid to drain down inside the leg around the calf muscles. This complication is very painful and is often mistaken for phlebitis, inflammation of the veins of the leg.

Your doctor may order an ultrasound or sonogram of the knee, which uses sound waves to create pictures of internal body structures. Another way to diagnose a Baker's cyst is by having an arthrogram, an X-ray of the knee joint after injection with dye.

Baker's cysts that are causing difficulty can be removed by surgery. From the amount of trouble this seems to be giving you, it sounds like surgery is worth looking into.

Jay Siwek, a family physician from Georgetown University, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington. Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071. Questions cannot be answered individually.