DEAR DR. DONOHUE: I am a 17-year-old boy, 6 feet tall and weigh only 140 pounds. No matter what I eat, I cannot gain any weight. How would you advise me to put on some pounds? – W.C.

DEAR DR. DONOHUE: A muscle magazine had an ad for 900-calorie shakes. If you drink four shakes a day, you’ll gain 14 pounds in two weeks. What would you use to do the same for yourself? – D.S.

ANSWER: D.S., I don’t want to gain two pounds a day. That’s too much weight in a short time. In another part of D.S.’s letter, he wanted a diet where 50 percent of calories come from protein. I believe the reasoning is that he wants to build muscle weight. Muscle is 75 percent water and 20 percent protein. The rest is glycogen, fat and some other materials. If 20 percent of total daily calories is protein, that’s enough protein to build muscles. Perhaps a bit more would help, but not much more.

Weight gain is a problem for some people. I suspect they have a metabolism that burns calories at a rate that is higher than it is for most of us. Even with a higher metabolic rate, weight gain is possible. W.C. and D.S. have to buy a cheap book that lists the calorie content of foods. Then for three to seven days, they have to write down every bit of food that goes into their mouth. With that information, they can average their daily calorie intake. If people want to gain two pounds a week, they have to increase their daily calorie intake by 1,000 calories a day.

They have to eat three decent meals a day, and eat larger portions than they are used to. They have to snack three to nine times a day. The snacks should be “calorie dense” foods – foods with a lot of calories in small amounts. Some examples of calorie-dense foods are: 2 tablespoons of peanut butter, with 190 calories; 1 cup of ice cream, with 340 to 560 calories; 1 ounce of peanuts (and who eats only 1 ounce – it’s slightly less than a small handful) has 166 calories; half a cup of raisins, 260 calories; a 14-ounce milkshake has 470 to 720 calories, depending on how much ice cream is in it. Weight is bound to be gained.

DEAR DR. DONOHUE: Will you please comment on my two teen daughters? They force themselves to outdo each other in holding their breath. They want to increase the duration they swim underwater since they’ve developed a passion for skin diving, and they want to get themselves ready for a skin-diving vacation.

My older daughter can hold her breath for longer than two minutes. The younger one is not far behind. I can hold my breath for a minute and a half.

The younger one can now hold her breath for four minutes. Her mom and I are getting worried. What advice would you give them? – J.H.

ANSWER: Humans take a breath every three to four seconds with good reason. We need to take in oxygen and get rid of carbon dioxide, a waste gas that comes from cell chemistry.

When we hold our breath, we create an oxygen deficiency and a carbon dioxide excess. After 40 seconds of breath-holding, most people have an all but irresistible urge to breathe again. A few people can hold their breath for two minutes. And if you want to talk about nature’s rarities, some can hold it for as long as five to seven minutes. Your daughters should not try to do that.

Hypoxic blackouts are a danger to divers who hold their breath too long. People have died doing this. I can’t give you an exact time of what’s safe for your daughters, but I would say your younger daughter has reached that point. I know they’re lovely kids, but they’re worrying me too.

DEAR DR. DONOHUE: I’m over 40 and recently went back to lifting weights. My wrists have been sore after a session a few times. Is this due to the age factor, or am I doing something wrong? – M.P.

ANSWER: At 40, it can’t be an age thing.

I’ll bet you’re overdoing it – trying to lift too much weight, too many times, without giving yourself enough rest. Stop lifting for a week or two. When you restart, begin with half the weight you’ve been using and half the repetitions you’ve been doing. Only when you can lift the weight for three sets of 12 repetitions should you increase the pounds.

READERS: Many women have written with general questions about endometriosis. The pamphlet on that topic explains its causes and treatments. To obtain a copy, write to: Dr. Donohue – No. 1105, Box 536475, Orlando, FL 32853-6475. Enclose a check or money order for $4.75 U.S./$6.75 Can. with the recipient’s printed name and address. Please allow four weeks for delivery.

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. Readers may also order health newsletters from www .rbmamall.com