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What does it mean to be ‘underweight’?
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Being underweight can be defined as having a low body weight or a body mass index (BMI) less than 18.5kg/m2. BMI is a measure of a person’s weight in relation to their height, the other relevant categories are as follows:
- 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 what would be an ideal weight for you.
Discover even more of our expert health guides including our guide on how much you should weigh or check out our 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, including genetics and they 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, due to poor appetite or early satiety
- 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 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.
If you’ve been critically ill you may find your appetite is affected, this may be due to reduced appetite signals from the brain or because your stomach empties more slowly, making you feel fuller for longer and less inclined to eat.
Some people who are underweight may not be getting the right amounts 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.
In addition, women of reproductive age, who are underweight, may find that their periods are irregular or missing and may experience fertility issues.
10 ways to gain weight according to a dietitian
There are simple ways to increase the amount of energy and protein in your diet, choosing the right foods and fortifying or enriching meals so that every mouthful makes a difference is important. Following these suggestions may help prevent further weight loss and help you gain weight in a healthy and sustainable way:
1. A full plate of food can be overwhelming – instead eat ‘little and often’ throughout the day, aim for six smaller meals and snacks eaten more frequently, such as every 2-3 hours.
2. Have small, nourishing snacks between meals, like oatcakes, cheese and crackers, bread sticks with guacamole, yogurt, egg custard, rice pudding, unsalted nuts, dried fruit like raisins, dates, dried apricots 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 a fortified plant-based milk), smoothies, malted hot drinks and nourishing soups.
4. Try not to drink just before your meals because this may impact your appetite and make you feel too full to eat, likewise avoid fizzy drinks as they may make you feel full.
5. Make every mouthful count by boosting the calorie and protein content of your meals without increasing the overall quantity – add gravy or sauces, spreads, nut butters, chopped or blended fruit, cream, oils, cheese, dried milk powder (this can be added to enrich milk and used with cereals or in sauces), condensed milk and honey.
6. Boost the protein content of your meals with lean meat, fish, eggs or tofu, beans such as soya, pulses, cheese and nuts; these will help to maintain your muscle mass which is important for health.
7. Buy good quality frozen meals, arrange a meal delivery service or batch cook for times when you don’t feel able to cook.
8. Eat in a relaxed environment with minimal distractions to help stimulate your appetite and allow you to concentrate on your meal.
9. You may not feel like being active but a short walk or some gentle weight bearing, such as chair yoga, as well as strength training with light weights, may help to preserve and maintain lean muscle mass and in turn help improve your appetite.
10. Visual cues may help stimulate appetite messages in the brain – thumb through a cookbook or food magazine before your meals and, if at all possible, try to eat together with friends and family.
Be aware that if you have a health condition that limits the fat and sugar in your diet, such as 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 your 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 take 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 missed periods, 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 for further examination and to advise on suitable textured and 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 the services and health professionals that will be in a position to support you.
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This article was reviewed on 27 January 2025 by Kerry Torrens.
Tai Ibitoye is a registered dietitian and a doctoral researcher in food and nutritional sciences. Tai has experience working in different sectors such as in the NHS, public health, non-government organisations and academia.
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