A BMI below 18.5 is classified as underweight for adults. This threshold, set by both the CDC and the World Health Organization, applies to anyone age 20 and older regardless of sex. While BMI is a simple height-to-weight ratio and not a perfect measure of health, falling below 18.5 is associated with meaningful health risks, including a shorter life expectancy and weaker bones.
How BMI Categories Work for Adults
BMI divides adult weight status into four main categories based on kilograms per square meter of height:
- Underweight: below 18.5
- Normal weight: 18.5 to 24.9
- Overweight: 25.0 to 29.9
- Obese: 30.0 and above
To put the underweight threshold in practical terms, a person who is 5’6″ would be considered underweight at roughly 114 pounds or less. Someone 5’10” would cross into underweight territory below about 129 pounds. You can calculate your own BMI by dividing your weight in pounds by your height in inches squared, then multiplying by 703.
The Threshold Is Different for Children and Teens
For children and teens ages 2 through 19, BMI isn’t compared against a fixed number. Instead, it’s measured against growth charts for kids of the same age and sex. A child is considered underweight if their BMI falls below the 5th percentile, meaning they weigh less than 95% of peers their age. This percentile-based approach accounts for the fact that healthy body composition changes dramatically during growth and puberty.
Why Being Underweight Shortens Life Expectancy
A large UK study tracking 3.6 million adults found that underweight men had a life expectancy roughly 4.3 years shorter than men in the normal BMI range, measured from age 40. For underweight women, the gap was 4.5 years. These aren’t small differences. They’re comparable to the life expectancy reductions seen with moderate obesity.
The relationship between low BMI and death risk follows a pattern researchers describe as U-shaped: both the very low and very high ends of the BMI spectrum carry elevated risk, with the lowest mortality clustered in the normal range. Below a BMI of 25, each unit decrease in BMI is associated with progressively higher mortality risk.
Health Risks Linked to Low BMI
Being underweight doesn’t just correlate with shorter lifespan in population studies. It creates specific, measurable problems in the body.
Weaker Bones
Low BMI is one of the strongest predictors of osteoporosis. A large study published in JAMA Network Open found that people who were extremely underweight during adolescence developed osteoporosis at a rate of 330 per 100,000 person-years, compared to just 79 per 100,000 among those who had been in the obese range. People who stayed underweight from adolescence into adulthood faced the highest risk of all: nearly double the osteoporosis rate of those with normal BMI, with similar increases for both men and women. Less body weight means less mechanical stress on bones, which reduces the signals that tell your skeleton to maintain its density.
Higher Infection Risk
Your immune system depends on adequate nutrition to function properly. Research shows a U-shaped pattern for infection risk as well, with both underweight and obese adults getting sick more often than those in the normal range. In children, underweight status is an especially significant risk factor for infections, partly because it often reflects broader nutritional shortfalls that impair the body’s ability to fight off pathogens.
Nutrient Deficiencies and Anemia
When your body doesn’t take in enough calories, it’s also likely missing key vitamins and minerals. Anemia, caused by insufficient iron or certain B vitamins, is one of the most common complications. If you’re underweight, a healthcare provider will typically check for nutrient deficiencies because correcting them can prevent cascading problems like fatigue, poor wound healing, and muscle weakness.
Medical Conditions That Cause Low BMI
Not everyone who is underweight simply needs to eat more. Several medical conditions drive weight below the healthy range even when appetite and intake seem adequate. An overactive thyroid gland speeds up your metabolism, increases your body’s energy expenditure, and can accelerate digestion, all of which make it difficult to maintain weight. Conditions that interfere with nutrient absorption in the gut, such as celiac disease or inflammatory bowel disease, can also keep BMI persistently low. Eating disorders, chronic infections, certain cancers, and uncontrolled diabetes are other common medical drivers. When low BMI has a medical cause, gaining weight usually requires treating the underlying condition rather than simply increasing calories.
The 18.5 Cutoff May Not Fit Everyone
The 18.5 threshold was developed using data from broad adult populations, and it doesn’t account for some important differences. Age is one of the biggest. In older adults, the relationship between BMI and health appears to shift. Research on elderly hospitalized patients has found that higher BMI within the normal range is actually associated with better survival, and that the standard cutoffs “may not be appropriate for classifying the elderly in terms of mortality risk.” For people over 65, a BMI that would be considered perfectly normal for a 30-year-old might actually signal increased vulnerability.
Body composition also matters. A naturally lean, muscular person and a person who is thin due to malnutrition can share the same BMI while having very different health profiles. BMI cannot distinguish between muscle and fat, or between someone who has always been slim and someone who has recently lost weight involuntarily. That context, whether your weight is stable or changing and whether you’re getting adequate nutrition, matters as much as the number itself.