DEAR DR. DONOHUE: I’m singing the beer-belly blues, and I would like to end the song but don’t know how. I have been doing sit-ups without making any visible progress. What is the best exercise for slimming the waist? – C.H.

ANSWER: You can’t focus on a single exercise or only on exercise. You have to pay attention to posture, too. If the lower back juts too far inward, the stomach protrudes. It takes a conscious effort, all day long, standing, walking or sitting, to flatten that inward curve. You also have to pay attention to what you eat. Invariably a protruding abdomen has too much fat, and getting rid of it through watching calories and aerobic exercise (brisk walking, jogging, biking) is essential to winning the battle.

Vary your stomach exercises. Do more than sit-ups, good as they are. I’ll give you a few examples.

The trunk curl is done lying on the floor with arms crossed over the chest and knees bent 90 degrees, feet on the floor. Curl the head and shoulders upward until the shoulder blades clear the floor, and hold that position for a count of five before lowering yourself down. Repeat the exercise 10 times – once you get the hang of it. A reverse curl is done in the same position, but raise the lower part of the body off the floor, including the buttocks. This exercise benefits the lower abdominal muscles, the ones that are weak in most people and that are a major contributor to a slack abdominal wall. A third exercise, one that involves the side abdominal muscles, is done in the same starting position with only a slight variation. Rest the left ankle on the bent right knee. Raise the head, shoulders and trunk diagonally, moving the right shoulder toward the left knee. After 10 repetitions, take a short rest, and then rest the right ankle on the bent left knee and repeat the exercise, moving the left shoulder toward the right knee.

The fitness booklet outlines a program of both aerobic exercise and stomach exercises. Readers can order a copy by writing: Dr. Donohue – No. 1301, Box 536475, Orlando, FL 32853-4675. Enclose a check or money order (no cash) for $4.50 U.S./$6.50 Can. with the recipient’s printed name and address. Please allow four weeks for delivery.

DEAR DR. DONOHUE: By the charts, I am overweight, but I maintain that I am quite fit. I fast-walk 3 miles every day. My wife says I am kidding myself. Is it really necessary to be skinny as a rail to be fit? – R.K.

ANSWER: Rail-skinniness is not a prerequisite to good health. However, leanness is usually a health benefit. Lean people who are fit are doubly benefited. The problem lies in defining what healthy leanness and fitness are.

We have to depend on weight charts and body mass index for a definition of a healthy weight.

Fitness is the ability to run, jog or briskly walk for an extended period of time without collapsing. How extended? Fifteen to 20 minutes or more is good, but 10 minutes suffices if done twice a day.

It’s been shown that overweight men who are fit by the above definition can still be considered healthy if most of their excess weight is not fat. Not so strong statements can be made for overweight women. For women, body weight close to the norm is essential for health.

DEAR DR. DONOHUE: My buddy claims that when he drinks coffee in the morning, he’s able to do much more exercise. He says his energy soars and he doesn’t tire. Is there any truth to this? If there is, how much coffee do you have to drink? – M.J.

ANSWER: Exercise physiologists have shown that drinking coffee an hour before exercise increases the time to exhaustion by 20 percent.

Caffeine slows the burning of muscle glycogen, the sugar stored in muscles and their principal fuel source. It causes the release of fatty acids that serve as a substitute fuel for glycogen.

For a person weighing 175 pounds, 2.5 cups of coffee achieves this effect.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475.