What Does Underweight Really Mean?
Being underweight is defined as having a body mass index (BMI) below 18.5. This is estimated to be less than the body mass needed to sustain optimal health.
Conversely, over 25 is considered overweight and over 30 is considered obese.
Use this calculatorTrusted Source to see where you fit on the BMI scale (opens in a new tab).
However, keep in mind that there are many problems with the BMI scale, which only looks at weight and height. It does not take muscle mass into account.
Some people are naturally very skinny but still healthy. Being underweight according to this scale does not necessarily mean that you have a health problem.
Being underweight is about 2–3 times more common among girls and women, compared to men. In the US, 1% of men and 2.4% of women 20 years and older are underweight (Trusted ).
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What Are the Health Consequences of Being Underweight?
Obesity is currently one of the world’s biggest health problems.
However, being underweight may be just as bad for your health. According to one study, being underweight was associated with a 140% greater risk of early death in men, and 100% in women (Trusted )
In comparison, obesity was associated with a 50% greater risk of early death, indicating that being underweight may be even worse for your health.
Another study found an increased risk of early death in underweight men, but not women, suggesting that being underweight may be worse for men.
Being underweight can also impair your immune function, raise your risk of infections, lead to osteoporosis and fractures and cause fertility problems.
What’s more, people who are underweight are much more likely to get sarcopenia (age-related muscle wasting) and may be at greater risk of dementia.
Several Things Can Cause Someone to Become Underweight
There are several medical conditions that can cause unhealthy weight loss, including:
- Eating disorders: This includes anorexia nervosa, a serious mental disorder.
- Thyroid problems: Having an overactive thyroid (hyperthyroidism) can boost metabolism and cause unhealthy weight loss.
- Celiac disease: The most severe form of gluten intolerance. Most people with celiac disease don’t know that they have it (Trusted Source).
- Diabetes: Having uncontrolled diabetes (mainly type 1) can lead to severe weight loss.
- Cancer: Cancerous tumors often burn large amounts of calories and can cause someone to lose a lot of weight.
- Infections: Certain infections can cause someone to become severely underweight. This includes parasites, tuberculosis and HIV/AIDS.
If you’re underweight, you may want to see a doctor to rule out any serious medical conditions.
This is particularly important if you have recently started losing large amounts of weight without even trying.