Underweight is defined as having a BMI below 18.5. The World Health Organization (WHO) created the following definitions for thinness based on BMI:

• Mild thinness – BMI between 17 and 18.99
• Moderate thinness – BMI between 16 and 16.99
• Severe thinness – BMI below 16

Although some people can be naturally thin and still healthy, in large part because of genetic factors, some might lose weight suddenly or over time for other reasons. These include

• Decreased appetite due to illnesses or diseases such as hypothyroidism, cancer, Crohn’s disease, inflammatory bowel disease, or celiac disease
• Decreased appetite because of medications taken for depression, high blood pressure, or osteoporosis (aspirin and some other nonsteroidal anti-inflammatory medications can also cause gastrointestinal side effects that can contribute to weight loss)
• Refusal to eat because of an eating disorder such as anorexia
• Problems chewing, tasting, or swallowing
• Depression, anxiety, or other psychological problems that reduce appetite
• Mobility problems that make it difficult to shop for and prepare meals
• Limited finances or lack of transportation that make food shopping and preparation difficult

Being underweight can contribute to symptoms such as lethargy (lack of energy), depression, and loose, elastic skin. It can also contribute to several negative health consequences, including the following:

• Increased risk for anemia and other nutritional deficiencies
• Increased risk of bone loss and osteoporosis
• Lowered immune function (which makes you more susceptible to and less able to fight off infections)
• Delayed wound healing
• Heart problems and blood vessel diseases
• Reduced muscle strength
• Reduced ability of your body to regulate temperature
• Increased risk of amenorrhea in women

How to Gain Weight

If you want or need to gain weight to make up for weight loss caused by illness, stress, or other causes, here are some tips to help you do so safely:

• Increase fluid intake. Drinking more low-fat milk and 100% fruit juice can provide you with valuable nutrients without filling you up.
• Add vegetable oils (like canola and olive oils), vegetable oil spreads, or salad dressing (made with unsaturated fats) to foods or use them to cook or prepare foods such as pasta, potatoes, lean meats and poultry, and salads.
• Eat nuts, seeds, and nut butters (for example, add almonds, walnuts, or sunflower seeds to salads; use pine nuts or shaved almonds to top sautéed broccoli or green beans; spread almond or peanut butter on apple slices, celery sticks, or toasted whole wheat bread).
• Add shredded cheese or grated Parmesan cheese to salads, potatoes, pasta, vegetable side dishes, and soups.
• Eat small, frequent meals. This will give you more opportunities to get key nutrients into your diet without filling you up too much.
• Do muscle-strengthening exercises twice a week, working all your major muscle groups.

Hypothyroidism, also known as underactive thyroid, is a condition in which there is too little thyroid hormone (a hormone that keeps the metabolism revved up); symptoms might include weight gain or difficulty losing weight, constipation, hair loss, dry skin, and increased sensitivity to cold temperatures.

Amenorrhea is a condition in which a woman does not or no longer menstruates; it is caused by malfunctioning of the pituitary gland, ovaries, and uterus.

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