A BMI below 18.5 is classified as underweight by both the CDC and the World Health Organization. BMI, or body mass index, is a simple ratio of your weight to your height squared, and while it’s not a perfect measure of health, it’s the standard screening tool used to flag potential nutritional concerns. Being below that 18.5 threshold doesn’t automatically mean something is wrong, but it does signal that a closer look at your overall health is worthwhile.
How Low BMI Categories Break Down
Not all underweight BMIs carry the same level of concern. The WHO uses a tiered system to distinguish mild from dangerous thinness. A BMI between 17.0 and 18.4 falls into the “mildly underweight” range. Below 17.0 is considered moderate to severe thinness. And a BMI under 16.0 is associated with a markedly increased risk of serious illness, poor physical performance, chronic fatigue, and even death. That 16.0 cutoff is treated as an extreme limit in clinical practice.
For children and teenagers, BMI works differently. Instead of fixed cutoffs, doctors use growth charts that compare a child’s BMI to others of the same age and sex. A child whose BMI falls below the 5th percentile for their age group is considered underweight. This percentile-based approach accounts for the fact that healthy body composition shifts dramatically as kids grow.
What a Low BMI Does to Your Body
When your body consistently doesn’t get enough fuel, it starts making trade-offs. Some of the most common complications of being underweight include anemia (not enough healthy red blood cells to carry oxygen), loss of bone mass, and loss of muscle mass. Your immune system also weakens, which means you get sick more often and take longer to bounce back from infections and injuries.
Bone loss is particularly concerning because it often happens silently. You won’t feel your bones thinning, but the risk of fractures climbs steadily, especially in the spine, hips, and wrists. This isn’t just a concern for older adults. Young people who stay significantly underweight for months or years can fail to build the peak bone density they’ll rely on for the rest of their lives.
Muscle loss compounds the problem. Less muscle means less strength, worse balance, and a lower resting metabolic rate, which can make it even harder to maintain a healthy weight over time. Fatigue becomes persistent because your body is running on less energy than it needs for basic functions like temperature regulation, tissue repair, and hormone production.
Common Causes of Low BMI
Some people are naturally lean and have always been on the lower end of the BMI scale without any health problems. Genetics, a fast metabolism, and high activity levels can all contribute. But when low BMI develops over time or comes with symptoms like fatigue, hair loss, feeling cold constantly, or irregular periods, there’s usually something driving it.
Medical conditions that can cause unintentional weight loss include overactive thyroid (which speeds up your metabolism beyond what your food intake can keep up with), digestive disorders that prevent your body from absorbing nutrients properly, chronic infections, diabetes, and cancer. Mental health conditions like depression and anxiety can suppress appetite for weeks or months. Eating disorders are another significant cause, and they carry their own set of serious health risks beyond low weight alone.
Medications can also play a role. Some drugs used for ADHD, depression, or chronic pain reduce appetite as a side effect, and over time that reduced intake shows up as weight loss.
Low BMI and Pregnancy
Starting a pregnancy with a BMI under 18.5 changes the weight gain targets your provider will set. The CDC recommends that underweight individuals carrying a single baby aim to gain 28 to 40 pounds during pregnancy, which is higher than the 25 to 35 pounds recommended for those starting at a normal weight. For those carrying twins, the target is 50 to 62 pounds.
These higher targets exist because the developing baby needs consistent caloric and nutritional support, and an underweight starting point means there’s less reserve to draw from. Insufficient weight gain during pregnancy is linked to preterm birth and low birth weight in the baby, so this is one area where the numbers genuinely matter.
The Limits of BMI as a Measure
BMI is useful as a quick screening tool, but it has well-known blind spots. It can’t distinguish between muscle and fat, so a lean, muscular person might register as “normal” while someone with very little muscle but more body fat might also register as “normal” despite having different health profiles. On the low end, a naturally small-framed person with a BMI of 17.8 who eats well and feels fine is in a very different situation than someone at 17.8 who has lost 15 pounds in the last three months without trying.
There are also population-level differences in how BMI relates to health. Research on Asian populations, for instance, has shown that the standard BMI cutoffs for overweight and obesity don’t align well with actual metabolic risk in those groups, with health risks appearing at lower BMIs than in European populations. While the underweight threshold of 18.5 hasn’t been formally adjusted for different ethnicities, it’s worth knowing that BMI thresholds aren’t universally precise across all body types and backgrounds.
What Gaining Weight Looks Like
If you’ve been told your BMI is too low, the path forward depends entirely on why it’s low. For someone who simply doesn’t eat enough calories, the fix is straightforward in theory: eat more calorie-dense foods, eat more frequently, and focus on foods that pack nutrition into smaller volumes (nuts, avocados, whole grains, olive oil, dairy). Adding 300 to 500 extra calories per day is typically enough to gain about a pound a week.
Strength training helps ensure that the weight you gain includes muscle, not just fat. Even modest resistance exercise two to three times a week can stimulate muscle growth when paired with adequate protein and calories. This also helps rebuild bone density over time.
When low BMI stems from a medical condition, treating the underlying cause is the priority. No amount of extra calories will help if your thyroid is burning through energy faster than you can consume it, or if a digestive condition is preventing absorption. Blood work and a thorough evaluation can identify these issues so they’re addressed alongside any dietary changes.