There should also be care not to overdiagnose or overtreat young people who may watch their weight and eat sparingly. Just try to encourage eating of good-quality, lower-calorie foods and suggest a supplement program such as the one at the end of this section.

For people with problems of being underweight, a medical evaluation is usually needed, especially if the problem is recent. A general physical exam and blood chemistry test may rule out medical problems such as anemia, abnormal thyroid function, or even cancer. Mineral tests and a diet profile may assist in isolating nutritional deficiencies. Usually, though, if both parents were trim and the individual has been thin most of his or her life, no medical problems will be revealed. A faster metabolism and lower potential for fat storage are usually the explanation.

People with low weight who have difficulty gaining weight often need a combined program. Working with a psychologist, stress counselor, or hypnotherapist to deal with some of the psychological, attitudinal, and emotional factors may be helpful. Stress reduction such as through relaxation exercises, music, or meditation, can be important, especially for nervous or high-strung individuals. Learning to slow down internally and externally can improve metabolism and the assimilation of nutrients. Stopping smoking or the overuse of caffeine is very helpful at adding a few pounds, as it will often slow the metabolism, at least initially.

The other, primary focus for gaining weight is good nutrition—diet and supplements. In this area, the suggestions are the opposite of those for weight loss. Bigger portions, many meals along with extra snacks, and more healthy, easily digested, high-calorie foods are the foundation of the plan. Increasing calories is the key; an extra 500 calories per day over and above body requirements can lead to a pound a week weight gain. More yogurt and cheeses, nuts, avocados, rice, potatoes, and bread with some butter may be helpful and healthful for getting extra calories. This will also increase dietary fats somewhat; fats are more caloric than the complex carbohydrates or sweeter foods. If there is a low cardiovascular risk, with a moderate cholesterol level (175–210) and good blood pressure, even more fats, especially monounsaturated fats such as olive oil, can be consumed. Smokers are at a slightly greater risk of developing cardiovascular diseases with this higher-fat diet, though generally additional oils, seeds, butter and other milk products, and even meats, if those are tolerated, may be used to add weight. It is still wise to avoid fried foods and hydrogenated oils; an increase in vegetable fats, especially cold-pressed oils is wiser than adding more animal fats.

Often, underweight people experience symptoms of fatigue and coldness in the body. Fat helps to keep us warm, and low body fat with poor circulation will reduce vitality and warmth. Fatigue may also be related to nutritional deficiencies secondary to limited caloric diets and low intake. Vegetarians and people who eat macrobiotic diets tend to have lower weights, but these may be healthier weights if these people consume reasonable amounts of calories, protein, and vegetable oils—and mostly nutrient-rich foods. Still, these diets can more easily lead to deficient calorie intake, because they are lower in fats, and most of the foods consumed are low-calorie foods (better for weight loss). In these cases, more animal proteins can be therapeutic. Deep-water fish and organic poultry are best; but even "organic" red meat and liver may be helpful for deficient, tired, cold-type people. Low thyroid function and anemia can produce fatigue and coldness; however, hypothyroidism usually causes some weight gain also. Most often, low weight with fatigue results from inadequate nutrition. In the Chinese traditional energy system the imbalance that is associated with "weak Fire element" may lead to fatigue, low endurance, and coldness, although this symptom complex may be associated with either low or increased weight. These symptoms may also accompany anemia, a part of "weak Fire" or weak blood, and in this case more iron, as is found in the animal meats, poultry and fish, or as supplements, is the appropriate medicine. Folic acid, vitamin B12, copper, and adequate protein intake, as well as regular exercise also help to build up the blood and improve the energy, endurance, and weight.