A BMI below 18.5 is classified as underweight by the World Health Organization. That’s the threshold where health risks start to climb, but the danger increases significantly the further below that number you fall. A BMI under 17.0 indicates moderate to severe thinness, and below 16.0 is considered an extreme limit associated with a markedly increased risk of serious illness, severe physical decline, and death.
How Low BMI Categories Break Down
BMI, or body mass index, is calculated from your height and weight. It’s an imperfect screening tool that doesn’t account for muscle mass, bone density, or body composition, but it remains a widely used starting point for identifying potential health concerns. The WHO uses three key cutoffs for low BMI in adults:
- Below 18.5: Underweight. This is where measurable health risks begin to appear.
- Below 17.0: Moderate to severe thinness. Nutritional deficiencies and physical symptoms are more likely at this level.
- Below 16.0: Extreme thinness. This range carries a sharp increase in risk for organ damage, inability to perform basic physical tasks, and death.
For context, a 5’6″ adult would hit a BMI of 18.5 at about 115 pounds, 17.0 at roughly 105 pounds, and 16.0 at around 99 pounds.
What Happens to Your Body at a Low BMI
Being underweight isn’t just about a number on a scale. When your body doesn’t have enough stored energy and nutrients to work with, it starts cutting corners in ways that affect nearly every system.
Bone and muscle loss are among the earliest consequences. Your body breaks down muscle tissue for energy when it doesn’t have enough fuel from food, and bones lose density without adequate nutrition to maintain them. Over time, this leads to osteoporosis, making fractures far more likely. Your immune system also weakens, leaving you more vulnerable to infections and slower to recover from illness. Anemia, a condition where your blood can’t carry enough oxygen, is another common complication.
Low pulse and blood pressure frequently accompany underweight status. While low blood pressure sounds harmless, it can cause dizziness, fainting, and fatigue that interfere with daily life. In severe cases, the heart muscle itself weakens, since it’s a muscle like any other and can atrophy when the body is chronically underfueled.
Effects on Fertility and Pregnancy
For women, a BMI below 18.5 is linked to a higher risk of anovulatory infertility, meaning the body stops releasing eggs. This happens because fat tissue produces a hormone called leptin, which signals to the brain that the body has enough energy reserves to support reproduction. When body fat drops too low, leptin levels fall, and the brain essentially shuts down the reproductive hormone chain. Periods may become irregular or stop entirely.
This pattern is especially common among athletes and people who combine intense exercise with restricted eating, a condition sometimes called relative energy deficiency in sport. The menstrual disruption isn’t just a fertility issue. It also accelerates bone loss, since the same hormones that regulate your cycle help maintain bone density.
Men aren’t exempt. Low body weight can reduce testosterone levels and sperm production through the same energy-sensing pathways. For women who do conceive while underweight, the risks include pregnancy complications and delivering infants with low birth weight, which carries its own set of health consequences for the baby.
Why the Threshold Shifts With Age
The 18.5 cutoff applies to adults under 65, but the picture changes for older adults. A large study tracking elderly men and women found that those with a BMI between 25 and 29.9, a range normally labeled “overweight,” actually had the lowest mortality rates. Being underweight in older age is particularly dangerous because it compounds the natural age-related loss of muscle and respiratory strength. It also weakens immune function in a population already more susceptible to infections and acute illness.
This doesn’t mean older adults should aim for obesity, but it does mean that a BMI of 19 or 20, perfectly healthy for a 30-year-old, may represent a risk factor for someone in their 70s. Many geriatricians consider a BMI below 22 or 23 worth monitoring in older patients.
How It Works for Children and Teens
Static BMI numbers don’t apply to anyone under 20. Because children’s body composition changes dramatically as they grow, the CDC uses age- and sex-specific growth charts instead. A child or teen is classified as underweight if their BMI falls below the 5th percentile for their age group. That means fewer than 5 out of 100 children of the same age and sex would have a BMI that low. Growth delays are a particular concern in this age range, since chronic undernutrition during childhood can affect height, brain development, and puberty timing.
What a Low BMI Actually Tells You
BMI is a rough filter. It can flag a potential problem, but it can’t tell you why your weight is low or whether it’s harming you. Someone with a BMI of 17.5 who has always been naturally lean, eats well, and has normal blood work is in a very different situation from someone at the same BMI who has been losing weight unintentionally or restricting food intake.
The causes of underweight range widely: high metabolism, chronic digestive conditions that impair nutrient absorption, thyroid disorders, depression, eating disorders, medications that suppress appetite, or simply not eating enough to match a high activity level. The underlying cause matters as much as the number itself, because treatment looks completely different depending on what’s driving it.
If your BMI is below 18.5 and you’re experiencing fatigue, frequent illness, hair loss, feeling cold all the time, or missed periods, those symptoms suggest your body is already being affected. The lower your BMI drops below that threshold, especially approaching 16.0, the more urgent the situation becomes. At that level, the WHO considers the risk of serious harm high enough to treat it as a medical emergency.