Underweight BMI
A BMI under 18.5 falls in the WHO's underweight category. Like every BMI category, it is a population-level screening threshold, not an individual diagnosis — but a sustained low reading is worth understanding rather than ignoring.
Worked example
At 165 cm, the healthy-weight range (BMI 18.5–24.9) is 50.4–67.8 kg. Below 50.4 kg at this height falls into the underweight category.
Associated health risks
- Weakened immune response — inadequate energy reserves can impair the body's ability to fight infection.
- Nutrient deficiencies — insufficient intake often means insufficient vitamins and minerals alongside insufficient calories.
- Bone density loss — particularly relevant for older adults, where a low BMI is linked to higher fracture risk.
- Fertility and hormonal effects — very low body fat can disrupt menstrual cycles and fertility.
These are associations observed at a population level, not a certainty for any individual — the same caveat that applies to every BMI category.
When to see a healthcare provider
Unintentional weight loss, a BMI that has recently dropped into this range, or an underweight reading paired with fatigue, hair loss, irregular periods, or frequent illness are all reasons to talk to a healthcare provider. They can check for underlying causes — from thyroid issues to digestive conditions — that a BMI number alone cannot reveal.
Common questions
- What BMI counts as underweight?
- The WHO adult threshold for underweight is a BMI below 18.5. This applies the same threshold to men and women; children and teens are assessed differently, against age- and sex-specific growth charts.
- What are the health risks of being underweight?
- Being underweight is associated with a weakened immune system, nutrient deficiencies, fertility issues, and — particularly in older adults — a higher risk of osteoporosis and frailty. As with a high BMI, these are population-level associations, not certainties for any individual.
- Is a low BMI always a health concern?
- Not necessarily. Some people are naturally lean with no underlying issue, and a single low reading can also reflect recent illness, high activity levels, or genetics. A sustained low BMI, unintentional weight loss, or accompanying symptoms are what warrant medical attention.
- What should I do if my BMI is under 18.5?
- Talk to a healthcare provider, especially if the low reading is new, unintentional, or paired with fatigue, hair loss, irregular periods, or frequent illness. A provider can check for underlying causes that a BMI number alone cannot reveal.