A BMI (body mass index) below 18.5 is considered underweight for adults. Both the World Health Organization and the CDC use this same cutoff, regardless of age, sex, or race. For context, that translates to about 136 pounds for someone 5’7″ or 108 pounds for someone 5’2″. But the number alone doesn’t tell the full story, and the threshold shifts for children, older adults, and certain populations.
BMI Ranges for Underweight Adults
BMI is calculated by dividing your weight in kilograms by your height in meters squared. For adults 20 and older, the CDC defines the categories as:
- Underweight: below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Obesity: 30 or higher
Within the underweight range, the WHO draws an additional line. A BMI below 17.0 is classified as “thinness,” and below 16.0 is considered an extreme threshold associated with a markedly increased risk of serious illness, poor physical performance, lethargy, and death. So while any BMI under 18.5 qualifies as underweight, the further below that line you fall, the more significant the health concern.
How It Works for Children and Teens
For kids and teenagers aged 2 to 20, the standard BMI cutoffs don’t apply. Instead, the CDC uses BMI-for-age percentiles, which compare a child’s BMI to other children of the same age and sex. A child is considered underweight if their BMI falls below the 5th percentile. This approach accounts for the fact that body composition changes dramatically during growth, making a single number meaningless across different ages.
Why the Threshold May Not Fit Everyone
BMI was originally developed from a single sample of white, middle-aged European men. That origin creates real limitations when applying it across diverse populations. For the same BMI, women carry a higher percentage of body fat than men. South Asian populations tend to have a different body composition that may warrant entirely different cutoffs. Polynesian body types also differ enough that standard thresholds can misclassify people.
Muscle mass and bone structure further complicate the picture. Someone with a naturally small frame might have a BMI of 18.0 and be perfectly healthy, while someone with more muscle mass could register well above 18.5 yet still be nutritionally depleted. BMI doesn’t distinguish between fat, muscle, and bone. It’s a screening tool, not a diagnosis.
For older adults, the standard cutoff may actually be too low. Research published in the Journal of General Internal Medicine found that among elderly hospitalized patients, those who died during follow-up had lower BMI values than survivors, even when all subjects fell within the technically “normal” range. The study’s authors concluded that BMI cutoffs developed for the general population may not be appropriate for classifying elderly people in terms of mortality risk. Some geriatric guidelines suggest that a BMI below 22 in older adults deserves closer attention.
Physical Signs Beyond the Scale
The number on your scale or your BMI calculation is one piece of data. Your body often provides others. Common physical signs associated with being underweight include persistent fatigue and weakness, dizziness or lightheadedness, low pulse and blood pressure, and hair thinning or loss. You might get sick more frequently and take longer to recover from ordinary infections.
Mood changes are also common. Irritability, apathy, and depression can all stem from not getting enough energy and nutrients. For women and girls, irregular or missing periods are a significant signal. Weight loss-related amenorrhea, the cessation of menstrual periods for more than six months, typically occurs after losing 10 to 15% of body weight in a short time frame. Earlier theories proposed a minimum body fat percentage necessary to maintain ovulation, but that idea has been complicated by the observation that some athletes with very low body fat menstruate normally. The more current understanding is that low energy availability, meaning you’re burning more than you’re consuming over time, is the primary driver of hormonal disruption.
Common Medical Causes
Being underweight isn’t always about eating too little. Several medical conditions can prevent your body from absorbing or using the calories you take in. Celiac disease damages the lining of the small intestine and interferes with nutrient absorption. Chronic inflammation of the pancreas impairs digestion. Long-lasting infections, including parasitic infections, can drain calories faster than you replace them. An overactive thyroid speeds up metabolism so dramatically that weight loss happens even with a normal appetite. Undiagnosed diabetes can cause weight loss as the body breaks down muscle and fat for energy it can’t get from glucose.
Eating disorders, particularly anorexia nervosa, are another major cause, and they frequently go undiagnosed for months or years. Overuse of laxatives also leads to nutrient loss and weight that drops below a healthy range.
Health Risks of Staying Underweight
Carrying too little weight places real strain on your body’s systems. Your immune function weakens, making you more vulnerable to infections and slower to heal. Bone density drops when the body lacks sufficient calories and nutrients, raising the risk of fractures. In women, the hormonal disruption from low energy availability reduces estrogen levels, sometimes to less than half the minimum concentration needed for normal bone maintenance and reproductive function. This creates a compounding problem: low weight leads to low estrogen, which accelerates bone loss, which increases fracture risk.
Fertility is directly affected. Difficulty getting pregnant is a recognized consequence of being underweight, driven by the same hormonal cascade that disrupts menstrual cycles. Cardiovascular function can also suffer, with low blood pressure and a slow pulse contributing to dizziness and fainting episodes.
Gaining Weight Safely
If you’re trying to move from underweight into a healthy range, the general approach is a modest calorie surplus rather than a dramatic increase. Research on body composition suggests aiming for a weight gain rate of roughly 0.25 to 0.5% of your body weight per week. For someone weighing 120 pounds, that’s about 0.3 to 0.6 pounds per week. A calorie surplus of 5 to 20% above what your body needs to maintain its current weight is a reasonable starting point.
Quality matters as much as quantity. Prioritizing protein intake, at least 1.8 grams per kilogram of body weight daily, helps ensure that weight gained includes muscle rather than just fat. Getting a minimum of 20% of your calories from fat supports hormone production, and filling in the rest with carbohydrates provides energy for daily function and any exercise you’re doing. Gaining too quickly tends to add more fat and less lean tissue, so a gradual approach produces better long-term results.
If you’ve lost weight unintentionally, or if you’ve been underweight for a long time with symptoms like hair loss, fatigue, or missed periods, investigating the underlying cause is more important than simply eating more. The right approach depends entirely on what’s driving the low weight in the first place.