DEAR DR. DONOHUE: Hot temperatures make it difficult for me to exercise out of doors. Does heat put a higher strain on the body? What can be done to combat an increase in fatigue due to heat? I want to stay in shape, but I don't want to damage my heart. - S.M.B.

ANSWER: Heat puts extra strain on the body. Muscles heat up greatly over their resting temperatures during exercise. Without built-in ways to dissipate that heat, we'd parboil. In summer heat, if it's also humid, the task requires supreme efforts.Because we don't evaporate sweat as easily in hot and humid weather, we lose one very effective means of heat loss. We have to depend on additional automatic controls, like the shunting of warm blood to the skin surface for heat exchange with the atmosphere. The closer the ambient temperature is to the blood temperature the less effective this method is.

During acclimatization (about two weeks), the body learns to sweat earlier in exercise than it did before, and kidneys learn to save salt and fluids. However, even though we are learning to adapt internally, we still have to help out a little.

Drink lots of water. You don't need salt tablets, as many think, since, as I said, the body is learning to conserve salt. Oh, there may be need to add a bit more salt to food in the early days of a heat wave, but that's about it.

You must dress appropriately for exercise. Wear light colored clothing that reflects rather than absorbs sun rays. Wear clothes that permit air circulation, like fishnet tops when jogging. Change any wet clothing regularly to avoid trapping of heat on the skin.

Such precautions should remove any risk to your heart. Pay special attention to the humidity index as opposed to the simple thermometer reading.

DEAR DR. DONOHUE: Please help me. I'm desperate. I do knee-bent sit-ups every morning to help flatten my tummy. All of my exercise tapes tell me to breathe upon lifting shoulders upward towards the ceiling. Do they mean inhale or exhale upon lifting upwards? - N.G.

ANSWER: During the straining part of any exercise, the upward rise of a sit-up for example, you exhale. During the relaxation phase, you inhale. More is made of this than necessary. The real point is to breathe during any exercise and never strain and hold your breath. That creates abnormally high pressure within the chest, impeding blood return to the heart. For other tummy flatteners, see the "Waist Trimming" material. Order by writing Dr. Donohue/No.38, Box 830, Gibbstown, NJ 08027-9909, enclosing a long, stamped, self-addressed envelope and $2.

DEAR DR. DONOHUE: My 20-year-old daughter is a fitness buff. She does 50 minutes of aerobics each day, eats high-fiber foods, lots of fresh fruits and vegetables. She avoids red meat, eating chicken or fish broiled. She avoids dairy products. She recently had her cholesterol checked and it is 264. We have no history of heart disease. What else can she do? - Mrs. N.M.

ANSWER: That's pretty high cholesterol for a 20-year-old, especially in one who's doing all the right things to keep it down. I can't advise any change in lifestyle. However, I do think it is time that she looked into other means of lowering cholesterol.

I am thinking of medicines. Her doctor can outline a drug program, beginning with the ones that work with the fewest side effects.

DEAR DR. DONOHUE: I read you religiously (not on my knees though). Can you stand one more letter regarding cholesterol? I'm a female, 68, with high cholesterol and triglycerides, but in good health. I've been on Lopid, and my cholesterol has gone from 356 to 242. Is Lopid safe in light of the warnings about liver and kidney disease? I don't want cancer in my later years. - Mrs. M.S.

ANSWER: I can always stand one more letter on this important subject.

You checked certain paragraphs in the manufacturer's drug information literature on Lopid. Such literature can be scary for many patients, since all reported effects, good and bad, must be listed.

On the good side, Lopid has been found to bring about a 34 percent decrease in chances of heart attack. It lowers bad cholesterol, increases the good, and lowers triglycerides.

You cite references to liver and kidney disease. That warning applies to those who have such problems prior to use. The liver breaks down Lopid and the kidneys must eliminate it, so with those organs already damaged, there are potential problems.

Your cancer fear stems from studies among laboratory animals. This has not happened in humans, a common situation in drug studies.