QUESTION: I would love to see a fitness article that addresses the thin person who wants to gain weight (in the form of muscle mass) while maintaining a healthy diet.ANSWER: I love this question, for I, too, am a thin person who struggles to keep weight on, let alone gain it. And, like the reader, I feel a bit lonely.It is considered bad form to complain about being underweight, but it is a serious problem, especially as you age. Being thin does not, despite what advertising claims, equal being healthy. There aren't many books or businesses banking on helping someone gain weight-unless, of course, you're a bodybuilder. And no milkshakes, please. The reader and I want to gain weight while keeping and building lean muscle mass.The answers seem straightforward: Eat smarter and more often; incorporate strength-building exercise into your regimen.

Finding the right balance of nutrition and exercise is critical, says Mike Locke, director of Fitness and Sports Performance at the Bellingham (Wash.) Athletic Club."Nutritionally, we want the right balance of complex carbohydrates, proteins and fats. The right mix is very dependent on your specific metabolism. Some of us do well with higher percentages of complex carbs, lower protein and low fat, while others do much better with moderate amounts of each."As for exercise, we need to strike a balance between aerobic and resistance activity. Resistance exercise, done properly, can add muscle mass and can be performed two or three times per week using moderate weights. Think balance and use large muscles in functional ranges of motion."Too much aerobic activity can burn not only fat but also muscle. Aerobic exercise can be performed three times per week or more at moderate levels. Heart rate should be monitored to maintain effectiveness. It is also wise to have the person's body composition analyzed to determine lean and fat content."

Nancy Jerominski, a Seattle certified personal trainer at Pro-Robics, suggests doing aerobic exercise two to three times per week, but limiting the sessions, at least at first, to between 30 and 60 minutes. Vary the intensity levels. Too many people fall into the trap of simply increasing the duration."The reader's weight-training should consist of using moderately heavy loads, doing two to three sets of eight to 12 repetitions with 60- to 90-second rest periods between each set. She should reach muscular fatigue in 60 to 90 seconds. Each session should last 30 to 60 minutes and should have eight to 10 exercises that challenge major muscle groups."Muscular fatigue or failure means when form breaks down, not when she is writhing to get a final rep out. There should be no flinging, swinging, leaning or improper form-ever. If she can breeze through three sets of 12 reps without working to fatigue during at least one set, she needs to increase the weight. By working at the proper intensity, she can recruit up to 50 percent more of her body's muscle mass. This means getting lean and putting on the 'right kind of weight.'"Putting on the proper weight is an ongoing process, a function of habit, she adds."She does not need to spend endless hours in the gym. In fact, she will quit if she does. She just needs to be sure the time she spends in the gym is spent well. Despite everything she may read in the popular women's fitness magazines, women do not get 'big' from lifting with progressively heavier weights. They get strong, lean, and their muscles take on the shape they were evolved to have."

Susan Kleiner, a registered dietician and author of several books on sports nutrition, correctly theorizes that the reader and I are not as interested in food as most. She says we need to eat every two hours, which means planning ahead and often carrying food. That also means learning to eat even when you're not hungry.In her book "Power Food" (Rodale, $31.95), Kleiner includes a diet for the "hard gainer," which takes into account a person's activity level. The diet is heavy on carbohydrates (55 to 60 percent), with plenty of protein (16 to 20 percent). Even a hard gainer needs to choose mostly whole foods, like whole grains, vegetables and fruits, low-fat and non-fat dairy products, lean meats and healthy vegetable fats, she says."Sometimes, if you are an avid high-intensity exerciser, you may need to either take the foot off the accelerator or lay off altogether in order to let your body slow down. Thin people usually have high metabolic rates, and constant, hard exercise pumps it up even higher."<B><I>Richard Seven is a staff writer at The Seattle Times. Send questions on workouts, equipment or nutrition to him at: Pacific Northwest magazine, The Seattle Times, P.O. Box 70, Seattle, WA 98111.</I></B>