“The single most serious hidden problem in America is—amazing as it sounds—being underweight,” says David R. Reuben, author of Dr. David Reuben’s Quick Weight-Gain Program (Crown). “At least 47 million Americans are underweight and 5% of all deaths in this country are related to being underweight.”

   According to Reuben, 17% of people, who may otherwise be healthy, weigh 25% or more below their desirable weight.Why is this a problem? Because if the body is left to rely on muscle rather than fat reserves as a major source of energy, a condition known as catabolism can occur. If muscle mass decreases too much, the body can become vulnerable to a host of illnesses including organ failure and immune deficiency, as well as loss of bone density and a general diminished ability to avoid physical injury.

High-Revving Engine
   Generally, a person can be considered underweight if their body weight is 10% to 20% lower than the average weight for their height. Many factors can contribute to someone being underweight, including hereditary predisposition, thyroid or pituitary disorders and, of course, such diseases as cancer or AIDS. But many underweight people simply tend to have a higher basal metabolic rate (BMR) than others. BMR is the rate at which the body burns energy to maintain basic body functions when a person is awake but inactive.

   It’s also possible that a brain chemical called malonyl-CoA may be the skinny culprit. This chemical is involved in fatty acid production and is thought to be a key appetite regulator. A flaw in the creation of this chemical may be why very thin people often feel less hungry during the day and eat less at meals.

   Unfortunately, many underweight people tend to reach for the types of food that their well-padded counterparts try to avoid—fried foods, soft drinks, pastries, etc.—in order to add bulk. But fatty, sugary foods actually depress normal hunger signals and lead to unhealthy eating patterns, as well as cause a lack of energy and inadequate nutrition.

Eating and Pumping to Gain
   To successfully “fill out” your figure, it’s important to follow a healthy, balanced diet, including an adequate intake of fluids and essential minerals. According to Reuben, a realistic weight-gain program aims to achieve more lean muscle mass; at least half of your total daily calories should come from carbohydrates, with 30% of your caloric intake coming from fat and 10% to 15% derived from protein.

   While it isn’t necessary to obsess over ratios, you should generally aim for 0.5 grams of complete protein (such as whey) per pound of weight each day. And make sure every calorie counts. In other words, skip the empty-carb items and increase your consumption of low-fat, high-protein and high-fiber foods instead: starchy vegetables like carrots and winter squash, dense breads, nuts and nut butters, full-fat yogurt, dried fruit and bananas.

   Resistance training, in the form of weight lifting or weight-bearing aerobic exercise, is an important aid to weight gain. It’s not only crucial in developing and maintaining muscle mass, but will also help to increase energy and appetite. Weight training and exercise also prevents bone density loss and the kind of lower back and posture problems that can afflict the underweight. You don’t have to become a body builder to enjoy these benefits. In fact, working out more than three times per week may backfire on you, since your body can only build muscle when at rest and overdoing it will cause muscles to become overstressed.

Extra Helpings
   Certain supplements and nutrient-fortified drinks and energy foods can be a real boost to your weight-gain regimen. Here’s a sampling of the bulk-building supplements and how they can help.

   HMB (beta-hydroxy-beta-methylbutyrate) is a fatty acid first used as a feed additive to increase lean meat in livestock and later introduced to bodybuilders to increase muscle growth. HMB is a byproduct of the amino acid leucine produced by the body and present in trace amounts in some plants. In several studies, participants gained both strength and lean body mass in as little as three weeks when they took three grams of dietary HMB in conjunction with resistance training.

   Glutamine is an amino acid closely associated with muscle protein growth that helps the body utilize other amino acids. It also increases nitrogen retention in muscle, and maintaining a positive nitrogen balance ensures that the body receives the optimum amount of protein required for muscle growth and repair. Glutamine is the primary carrier of nitrogen—up to 35%—into the muscle cell. Part of its muscle-building action may also be due to its ability to promote the release of growth hormone.

   Whey protein, derived from milk protein, maintains nitrogen balance and promotes muscle growth. It also has the highest biological value of any protein studied to date. An added benefit is that whey protein can increase blood levels of glutathione, an antioxidant essential to a healthy immune system.

   Creatine, a natural byproduct of liver, kidney and pancreas functioning, is another muscle builder that is stored in muscle cells as free creatine or bound to a phosphate molecule as phosphocreatine. More than half a century ago, researchers discovered that creatine improves nitrogen balance and increases energy, and is associated with healthy weight gain. The most bioavailable form is creatine monohydrate.

   Conjugated linoleic acid (CLA) is a fatty acid found in low concentrations in meat and dairy products that prevents muscle loss and helps the body to maintain a healthy weight by regulating the composition and disposition of body fat.