Q. Why do some people gain weight when they hardly eat anything, while others eat a lot and never put on a pound?

A. Two theories of weight control explain this common observation. One is the "set point" theory, which says that your body has a built-in weight regulator, acting something like the thermostat in your home. When you eat extra food, your weight regulator turns your metabolism up and you "burn off" the extra calories. When you eat less, your metabolism drops and you conserve more of the calories you take in. The set point may also work by making you feel hungry or full depending on changes in your weight.

The other explanation is the "fat cell" theory. The idea here is that the number of fat cells you have as an adult is more or less constant. They may increase in number, but probably don't decrease. When you lose weight, your fat cells shrink, but don't disappear. Instead, they lie ready to grow again, once you stop dieting and exercising. The fat cell theory also says that overweight children tend to become overweight adults.

Although many factors play a role in obesity -- heredity, behavior and environmental influences -- these two theories explain why many people, heavy and thin, have a difficult time changing their weight.

Q. I suffer from peripheral neuropathy. I've never seen anything written about this disease. Can you tell me if there is any treatment that would ease the pain and burning in my legs?

A. Peripheral neuropathy is not a disease in itself, but a disorder of nerves that lie outside (peripheral to) the brain and spinal cord. Neuropathy means nerve disorder (literally, nerve suffering) and implies that something has damaged the nerve so that it doesn't work properly.

It most commonly affects the feet and legs, with the arms next in frequency. Peripheral neuropathy can be caused by dozens of afflictions, so the key to treatment rests with diagnosing the cause. Unfortunately, in many cases no cause is obvious and treatment is by trial and error.

Among the disorders that can produce peripheral neuropathy are: pinched nerve; diabetes; excessive alcohol drinking; kidney failure; certain B vitamin deficiencies (niacin, thiamin, B6 and B12); certain poisonings (arsenic, lead, mercury and others); certain chemicals found in industry (benzene, carbon monoxide, organophosphates and others); side effects of certain drugs (nitrofurantion, isoniazid, vincristine and others).

Symptoms depend on which kind of nerve is involved. If it's a nerve that controls muscles, weakness results. If it's a nerve used for sensation, then numbness, tingling, burning or pain develops.

Treatment begins with eliminating or controlling the cause and taking medicine for symptoms, starting with pain relievers. Antidepressants like Elavil and seizure medicines like Dilantin and Tegretol sometimes help the symptoms of pain and burning you describe. If all else fails and you still have disabling pain, I recommend talking to your neurologist about getting treatment at a chronic pain clinic.

Q. I'm a 24-year-old woman who falls asleep after lunch. I first noticed this after drinking milk, so I cut back on milk. Now, I can't seem to stay awake no matter what I eat. This is not appropriate behavior for a teacher. I thought it was psychological at first, but now I'm not so sure. What can I do?

A. You need to find out whether you have simple drowsiness after eating or the sleeping disorder called narcolepsy.

There are several theories about why we get sleepy after eating heavy meals.

One is that digested foods release substances into the bloodstream that have a natural sedating effect. This explains the time-honored use of milk as a sleep aid.

Another theory is that the blood circulation is channeled to the intestines to help absorb food. This may temporarily affect the amount of blood going to our brain, making us drowsy.

And in many people, there is a natural dip in wakefulness in mid-afternoon, coinciding with the time after lunch.

If you get sleepy only after lunch, eat a lighter meal and try drinking coffee, tea or cola to get the stimulating effect of caffeine.

Narcolepsy (meaning a numbing attack or, more loosely, a sleeping fit) causes irresistible and uncontrollable urges to sleep. In its milder form, you may find yourself frequently dozing off without the usual explanations of lack of sleep, overexertion, a recent heavy meal or a boring lecture or play.

Oddly enough, many people with narcolepsy will also have something called cataplexy (meaning "to strike down"). In this condition, hearty laughter or other emotions can trigger a sudden nod of the head and buckling of the knees, sometimes making the affected person collapse, without any loss of consciousness.

Narcolepsy generally responds well to stimulant medications such as Ritalin and changes in routine (taking short naps during the day when possible). Other causes of recurrent daytime drowsiness are: psychological problems like depression; excessive use of alcohol or sedating drugs; breathing problems during sleep causing multiple unconscious awakenings during the night. underactive thyroid gland (hypothyroidism).

Two sleep labs in the area are: Georgetown University Sleep Disorders Center, 3800 Reservoir Rd. NW, Washington, D.C. 20007, 625-2020; and National Capital Sleep Center (affiliated with Suburban Hospital), 4520 East West Hwy., Suite 402, Bethesda, Md. 20814, 656-9515.

For information about narcolepsy and sleep disorders, contact the American Narcolepsy Association, P.O. Box 5846, Stanford, Calif. 94305, (415) 591-7979.