What does it mean to be ‘underweight’?

Being underweight can be defined as having a low body weight or a body mass index (BMI) less than 18.5kg/m2. Body mass index is a measure of a person’s weight in relation to their height. There are different categories associated with BMI ranges:

  • A BMI between 18.5 to 24.9kg/m2 is considered to be a ‘healthy weight range’.
  • A BMI between 25 to 29.9 kg/m2 is considered as overweight.
  • A BMI over 30kg/m2 is considered obese.

It is worth mentioning that BMI is not always synonymous with health. There are some people who may be classified as underweight according to their BMI but are healthy. Conversely, having a BMI classed as overweight or obese does not mean you do not have a healthy weight, as it does not take into consideration muscle mass and bone density. BMI is just one way to classify your weight and it is best to speak to your doctor or a registered dietitian about your ideal weight.

Discover even more of our expert health guides or check out our catalogue of delicious, healthy recipes to inspire your next meal.

What can cause someone to be underweight and what are the potential health consequences?

There are various factors that can cause someone to be underweight. Some people may be underweight due to genetics and may not have any existing health issues. However, there are also people who are underweight or unintentionally lose weight due to other reasons such as:

  • Inadequate food intake
  • Being unable to meet the body’s increased nutritional demands when acutely or chronically ill
  • Issues with the glands that secrete hormones – such as hyperthyroidism (overactive thyroid), Addison’s disease or undiagnosed diabetes
  • A problem with the gut, such as a Crohn's disease, ulcerative colitis or coeliac disease
  • Loss of appetite and motivation to cook or eat
  • Difficulty with chewing and swallowing (dysphagia)
  • Side effects of some medications and treatments
  • Eating disorders, which include, but are not limited to, anorexia nervosa, avoidant/restrictive food intake disorder (ARFID) and bulimia nervosa

Some people who are underweight may not be getting the right amount of calories, protein and other nutrients from their diet and may be at risk of malnutrition. Malnutrition can lead to health issues such as increased risk of illness and infection, delayed wound healing, low mood, reduced energy levels, increased risk of fractures, reduced muscle strength and reduced independence.

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In addition, some women who are underweight may find that their periods are irregular or missing and may experience fertility issues.

7 ways to gain weight according to a dietitian

There are simple ways to increase the amount of energy and protein in the diet that may help prevent further weight loss and gain weight in a healthy way:

  1. Eat ‘little and often’ throughout the day – try to have smaller meals more frequently
  2. Have small, nourishing snacks between meals, like biscuits, cheese and crackers, vegetable sticks with a creamy or avocado-based dip, yogurt, unsalted nuts, dips, samosas, dough balls and plantain crisps*
  3. Introduce nourishing drinks into your diet. Try milky hot beverages, yogurt-based drinks like lassi, milkshakes (either made with full fat dairy milk or plant-based milk), smoothies, malt drinks and soups*
  4. Try to not drink just before meals to avoid feeling too full to eat
  5. Make every mouthful count by boosting the calorie and protein content of your meals without increasing the size – add extra gravy, spreads, nut butters, chopped or blended fruit, cream, oils, cheese, dried milk powder, sugar or honey*
  6. Eat in a well-ventilated and relaxed environment to stimulate your appetite and help you concentrate on your meal
  7. Buy frozen meals or batch cook for times you don't feel able to cook

*If you have a health condition that limits the fat and sugar in your diet, e.g diabetes or high cholesterol, you should discuss with your doctor and registered dietitian whether it is suitable to include these in your diet.

When to see a doctor

If you have unintentionally lost a significant amount of weight within the last 3-6 months and noticed signs like clothes becoming baggy, extreme tiredness, change of mood and reduced physical performance, then it is best to see a doctor. They can make a full examination and identify any issues that may be leading to weight loss. A doctor can also refer you to a registered dietitian for tailored dietary advice. If you have an existing medical condition, or are experiencing any other symptoms, like missing periods or muscle weakness or pain, you must seek advice from your doctor.

If you're unable to eat due to difficulties swallowing or chewing, you must see a doctor so that they can refer you to a dietitian and speech and language therapist to examine further and advise on suitable textured modified foods.

If you think you may have an eating disorder, talk to someone you trust. Consider speaking to a doctor – help is available and they will be able to direct you to services and health professionals that will support you.

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This article was published on 1 February 2021.

Tai Ibitoye is a registered dietitian and a doctoral researcher in food & nutritional sciences. Tai has experience working in different sectors such as in the NHS, public health, non-government organisations and academia.


All health content on bbcgoodfood.com is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other health care professional. If you have any concerns about your general health, you should contact your local health care provider. See our website terms and conditions for more information.

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