Why Is Being Underweight Bad?

The national conversation about weight typically trends toward those who are overweight or obese. Approximately 1.7 percent of adults in the United States are underweight, however, according to the National Center for Health Statistics. By definition, this percentage includes those who have a body mass index, or BMI, of less than 18.5. Reasons for being underweight vary from eating disorders to biological causes, but the condition can play just as much of a role in a person's health as being overweight.

Feet on a scale (Image: daizuoxin/iStock/Getty Images)

Physical Health Effects

The consequences of a low body weight can vary based on the cause. Someone suffering from an eating disorder can encounter very different problems than a person who simply has an incredibly high metabolism. Those who are underweight and don't eat enough calories to sustain their bodies, however, might experience a number of health problems, including delayed growth and development -- particularly in children and teens -- and a weakened immune system. Such conditions boost the risk of infection and disease. Other serious health risks include bone loss, which can lead to osteoporosis; anemia, which is the result of a lack of iron; heart irregularities and blood vessel diseases; and delayed wound healing. Some warning signs that low body weight is becoming an issue include a lack of energy, depression, loss of bulk and loose, elastic skin, according to Washington Department of Social and Health Services.

Effects for Women

Being underweight has particular consequences for women, who could stop menstruating or have trouble conceiving, according to Shady Grove Fertility Clinic. When body fat is below a certain level, the body tries to conserve energy by stopping certain bodily functions such as periods, a condition known as hypothalamic amenorrhea. Even if you do have regular periods, your body could stop releasing eggs from your ovaries. Women with a BMI of at least 20 have a better chance of conceiving than women below this level, according to the BabyCenter website.

Mental Effects

Being underweight can affect your thinking process, concentration and mood, according to "Cognitive Behavior Therapy and Eating Disorders." This is particularly significant if being underweight is due to an eating disorder. You might find it difficult to switch rapidly between topics or focus on a single topic. You might be more easily irritated, inflexible in your routine or experience a heightened obsessiveness with eating, such as requiring chewing your food a certain number of times.

Gaining Weight Healthfully

If you're struggling with weight gain, first see your health care provider to rule out a biological cause that requires medical intervention. Once you're in the clear, focus on gaining weight by increasing your consumption of nutrient-dense, high-calorie foods. Options include complex carbohydrates such as brown rice and oatmeal and healthy fats, including avocados, nuts and peanut butter. If large portions of food aren't appealing to you, eat smaller meals more frequently throughout the day. Resist the urge to gorge on junk food; it might be high in calories, but it won't do anything for your health.

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