The health dangers of obesity are well known, but being underweight can compromise your health, too. Although less than 2% of U.S. adults over age 20 are underweight, (a metric used to gauge your body fat about your weight and height) under 18.5 can cause weakened immunity, fertility and hormonal problems, and nutritional deficiencies that can lead to anemia and osteoporosis. What’s more, research suggests being underweight is linked to an increased mortality risk.

Whether you’re underweight or just looking to put on some muscle, it’s important to know how to gain weight—particularly lean body mass—in a healthy way.

Reasons to Gain Weight

Medical reasons for being underweight include long-term illnesses like cancer and recovering from eating disorders and thy…