What Happens to Your Body When You're Underweight?

If you're on the thin side with a weight that's below normal for your height, your doctor may advise you to gain weight. Being underweight can endanger your health and may indicate that you're not getting all the nutrients you need to look and feel your best. The effects of being too slender depend on how far below normal your weight has fallen and how you got there. Getting enough calories from nutritious foods can help you gain some pounds and put a spring in your step.

You're considered underweight if your body mass index falls below 18.5. The body mass index, or BMI, is an estimate of fatness computed by dividing your weight in pounds by your height in inches, squared, and multiplying the total by a conversion factor of 703.

The formula reads: BMI = weight in pounds / (height in inches x height in inches) x 703.

Approximately 1.7 percent of the American adult population is underweight, according to the 2011–2012 National Health and Nutrition Examination Survey. Illness, trauma, surgery, genetics, overexercising, excessive stress, skipped meals or self-deprivation can all cause you to lose too much weight. Older adults may become underweight due to certain medications or because their appetites diminish naturally. Check with your doctor if you've lost weight for unexplained reasons though. You may have an underlying health issue, such as a problem with nutrient absorption, like Celiac disease, or an overactive thyroid.

In some cases, there's no health issue at all, and your genetic build makes you naturally thin compared to other people. Your doctor can help confirm if you just have a naturally thin and lanky build. As long as you're eating a balanced diet, getting all the nutrients and calories you need, and you have adequate energy, being slightly underweight might be OK for you.

Sometimes people are underweight because they're not getting enough nutrients needed for good health -- either from not consuming adequate amounts or from improper absorption. When you're deficient in calories and nutrients, your immune system suffers. You may have trouble fighting off infections and other illnesses. It's harder to heal from trauma or surgery if adequate nutrients aren't available to heal and rebuild tissue. Underweight older adults may be more susceptible to complications from flu and pneumonia, too.

If you're a very slender woman, your period may become irregular or halt altogether. This signals erratic hormone production and means you're probably not producing enough estrogen to support healthy bone mass, which could up your risk of developing osteoporosis.

Being underweight and nutritionally deficient may compromise what's going on inside your body at a systemic level. If your diet isn't providing you enough calories, your cardiovascular, endocrine, gastrointestinal, renal and central nervous system may all suffer. When you improve your nutrition intake, you'll feel a difference in your health -- and see one, too -- as your hair grows more lush and your complexion more radiant.

When you aren't getting adequate nutrition, it's normal to feel too fatigued and lethargic to exercise or even walk with the family or your dog. You may have little stamina for a full day of work or school, and avoid social events because they tire you out. Your self-esteem may also suffer if your size makes you feel subconscious. After putting on a little weight, you'll start to look and feel more energetic, so you can add more physical and social activity that can help stimulate your appetite.

Healthy weight gain takes time. Focus on adding quality muscle mass, not just fat, to bolster your health and strength. A calorie surplus of 250 to 500 calories per day helps you put on 1/2 to 1 pound per week. Achieve this by eating slightly larger servings at meals or adding calories to the foods you already eat. For example, cook oatmeal in whole milk instead of water to add 150 calories per cup; spread 2 tablespoons of peanut butter on your snack-time banana for an extra 190 calories; and add a 1/4 cup of shredded cheese to your dinner baked potato for about 115 calories.

A high-calorie shake eaten as a snack or before bed also boosts your calorie intake. Use quality ingredients such as fresh fruit, dry milk powder, Greek yogurt, nut butter and flax seeds to create a calorie-dense drink. Snack often on high-calorie foods, such as nuts, dried fruit and whole-milk dairy to further increase your daily calorie intake.

If you're able and your doctor approves, perform resistance training a couple times a week to encourage the development of lean muscle mass. At each session, work all the major muscle groups with four to eight repetitions of each exercise using a heavy weight. A fitness professional can help you design a program that's appropriate for you. Even if formal strength training isn't an option, daily activities, such as carrying groceries and doing laundry, can help you develop some functional muscle.