Low BMI: causes, risks, and what to do

If you have a BMI below 18.5, you're in the underweight range on the BMI scale.¹ A reading in this range is a flag worth investigating alongside the wider clinical picture.

A naturally lean build, a fast metabolism, an undiagnosed thyroid condition, or a nutritional gap can all produce the same number. Working out which is causing your individual score is a job for a GP.

What is a "low BMI"?

The standard BMI scale used across UK clinical practice has four bands: underweight, healthy weight, overweight, and obese.² A score below 18.5 sits in the underweight band.¹

A reading in this range doesn't automatically signal a problem. Some adults are naturally lean, and athletes carrying a lot of muscle can also return readings that don't reflect their health.³ The number can also be the first visible sign that your body isn't getting enough fuel, or that something else is affecting your weight.

Which of those applies to you isn't something a calculator can tell you.

Potential causes of a low BMI

Plenty of things can pull a BMI below 18.5, and we can broadly put these into four categories.

A faster metabolism or higher activity level

Energy needs vary between people. A higher resting metabolic rate, regular intense training, or a physically demanding job can all make weight harder to maintain, particularly where energy intake doesn't keep pace. The pattern is more often seen in younger adults, athletes, and anyone who's recently increased their training load.

An underlying medical condition

Several conditions can cause unintended weight loss or make it difficult to gain weight, including:

• An overactive thyroid (hyperthyroidism), which speeds up your metabolism.

• Coeliac disease, which prevents proper absorption of nutrients from food.

• Type 1 diabetes, particularly when poorly controlled.

• Inflammatory bowel disease, including Crohn's and ulcerative colitis.

• Persistent infection or chronic illness.

If your BMI has dropped recently, or sits well below the healthy range without an obvious explanation, a GP can run blood tests to check for these conditions.¹

A nutritional gap

A diet that doesn't deliver enough energy to meet your daily needs is a frequent cause of low BMI.¹ Sometimes the issue is eating too little overall. Sometimes it's eating regularly but missing key macronutrients, particularly protein and healthy fats. A registered dietitian can help you identify what's missing and build a plan that adds calories in without compromising your overall health.

Mental health and disordered eating

Stress, anxiety, depression and disordered eating patterns can all affect appetite and body weight, often in ways that go unnoticed until the number drops.⁴

Eating disorders are more common than most people realise and they affect people of every age, gender and background. They're also treatable, and treated earlier they're treated more easily.⁴ If any of this sounds familiar, Beat and your GP are both good places to start, and neither will expect you to have it worked out first.The health risks of being underweight

Weakened immune function

Where a low BMI reflects undernutrition, immune function can be affected, because immune cells depend on a steady supply of nutrients including zinc, iron, vitamin A and protein.⁵ That can mean more frequent infections and slower recovery from them.

This applies to undernutrition rather than to a low reading on its own. An adult who is naturally lean and eating well isn't in the same position as one whose low BMI reflects a nutritional gap.

Bone health

Low body weight is a recognised risk factor for low bone density and osteoporosis.⁶ It's partly mechanical, since less weight on the skeleton means less stimulus for bone building, and partly nutritional, since calcium, vitamin D and protein are all needed to maintain bone mass. A BMI below 18.5 is associated with a higher fracture risk, particularly in older adults and post-menopausal women.⁶

Fatigue and low energy

If your energy intake doesn't meet your body's demand, energy levels can drop. Persistent tiredness, poor concentration, irritability and difficulty recovering from exercise are all commonly reported.¹

Hair and skin

Hair thinning, hair loss, dry skin and brittle nails can appear early when nutrition isn't keeping up with demand.¹ These changes are usually reversible once the underlying cause is addressed.

Fertility

Very low body weight is associated with disruption to reproductive health in women. The driver is energy availability rather than body fat alone, which is why athletes who are eating enough on paper but training heavily can be affected too.¹ Periods may become irregular or stop entirely, a state called functional hypothalamic amenorrhoea.

In men, undernutrition can affect testosterone levels and sperm production.⁷

Key facts: understanding low BMI

• A BMI below 18.5 falls into the underweight category on the standard NHS scale.^1,2

• Low body weight is a recognised risk factor for low bone density and osteoporosis.⁶

• Unintentional weight loss should always be discussed with a healthcare professional.¹

• Where a low BMI reflects undernutrition, immune function can be affected.⁵

• A low BMI, particularly where it is unexplained or accompanied by symptoms or unintentional weight loss, may warrant further clinical assessment.

When to see a GP

If your BMI sits at 18.5 or below, or you've experienced weight loss you didn't intend, book an appointment with your GP. This is the single most important step on this page.

A GP can run blood tests to rule out underlying conditions, including thyroid problems, coeliac disease, and diabetes. They can also refer you to a registered dietitian for a structured plan tailored to your medical history.

Symptoms worth flagging at the appointment include:

• Persistent tiredness or low energy.

• Feeling cold often, even at normal indoor temperatures.

• Loss of appetite or changes in your relationship with food.

• Frequent illness or slow recovery from infections.

• Hair thinning, hair loss, or skin changes.

• Unintentional weight loss that puts your BMI in the underweight category.

• For women: irregular periods or periods that have stopped.

Numan doesn't offer weight-gain treatments, and we'd advise against searching for one online. The right pathway for a low BMI is a clinical assessment, ideally in person, with a healthcare professional who can see your full medical picture.

Next steps: monitoring your health

If you'd like to confirm your BMI, our BMI calculator walks you through the inputs and returns your reading and the relevant clinical band.

Some adults sit in the underweight band while carrying healthy levels of muscle and normal metabolic markers. Our BMI limitations guide explains where the calculation can mislead, and what else clinicians look at alongside it.

To see where the long-term target sits, our BMI chart lays out the 18.5 to 24.9 band and why it remains the recommended range for most adults.

The numan take

Low BMI gets less airtime than high BMI, but the clinical risks are real. Weakened immunity, lower bone density, reduced fertility, and persistent fatigue are all documented consequences of being underweight for longer periods of time.

None of these conditions are self-diagnosable. A GP appointment, with blood tests and a full medical history check, is the best way to find out what's behind your reading and what to do about it.

References

^{1 NHS. Advice for underweight adults [Internet]. London: NHS; 2024. Available from: https://www.nhs.uk/live-well/healthy-weight/managing-your-weight/advice-for-underweight-adults/}

^{2 NHS. BMI healthy weight calculator [Internet]. London: NHS; 2024. Available from: https://www.nhs.uk/health-assessment-tools/calculate-your-body-mass-index/}

^{3 Wu Y, Li D, Vermund SH. Advantages and limitations of the body mass index (BMI) to assess adult obesity. Int J Environ Res Public Health. 2024;21(6):757.}

^{4 Treasure J, Duarte TA, Schmidt U. Eating disorders. Lancet. 2020;395(10227):899–911.}

^{5 Bourke CD, Berkley JA, Prendergast AJ. Immune dysfunction as a cause and consequence of malnutrition. Trends Immunol. 2016;37(6):386–398.}

^{6 De Laet C, Kanis JA, Odén A, Johanson H, Johnell O, Delmas P, et al. Body mass index as a predictor of fracture risk: a meta-analysis. Osteoporos Int. 2005;16(11):1330–1338.}

^{7 Shukla S, Shrivastava D. Nutritional deficiencies and subfertility: A comprehensive review of current evidence. Cureus. 2024;16(8):e66477.}