Underweight adults

Being underweight can damage your health. Weighing too little can contribute to a weakened immune system, fragile bones and feeling tired.

If you're concerned that you're underweight, check your body mass index (BMI) using our Healthy weight calculator.

If your BMI is below the healthy range, this suggests that your weight may be too low.

If you're underweight, your GP or practice nurse can give you help and advice. There might be an underlying medical cause for your low weight, such as an overactive thyroid.

Or you may simply be underweight because your diet isn't providing you with enough energy (calories). This can happen for a number of reasons. Stress or other emotional problems can sometimes cause a change in eating patterns that is hard to recognise.

If diet is the cause of your low weight, changing to a healthy, balanced diet that provides the right amount of calories for your age, height and how active you are can help you achieve a healthy weight. Registered dietitian Azmina Govindji has provided the following guidelines.

Why are you underweight?

If our healthy weight calculator has told you that you may be underweight, think about why this might be:  

  • Have you been unwell?
  • Have you been skipping breakfast or lunch and just eating snacks on the go?
  • Have you lost your appetite, perhaps because you’re worried or stressed?
  • Have you been trying to lose weight? Are you more focused on being "thin" or looking a certain way than on being a healthy weight?
  • Does not eating make you feel as though you are more in control? If so, you need to talk to someone about your weight and eating habits. 

Talk to someone about your weight

There may be emotional issues that are stopping you from eating a healthy diet.  

If you feel anxious or worried when you think about food, or feel you may be using control over food to help you cope with stress or low self-esteem, you may have an eating disorder.

If you think you may have an eating disorder, you should tell your GP because help is available. Find out about eating disorders.

Why being underweight is bad for you

Being underweight can be bad for your health now and in the future, for the following reasons:  

  • If you are underweight, it's likely that you are not consuming a healthy, balanced diet, which can lead to you lacking nutrients that your body needs to grow and work properly. Calcium, for example, is important for the maintenance of strong and healthy bones. Being underweight increases the risk of osteoporosis in later life. If you're not consuming enough iron, you may develop anaemia, which may leave you feeling drained and tired.
  • Your immune system is not 100% when you’re underweight, making you more likely to catch a cold, the flu or other infections.
  • Women who have lost a lot of weight can find that their periods stop. This increases the risk of fertility problems.

A healthy diet for a healthy weight

If you're underweight, aim to gain weight gradually until you reach a weight that is healthy for your age and height. 

It’s crucial that you gain weight the right way, and not by eating chocolate, cakes, sugary drinks and other high-calorie foods full of saturated fat and sugar. These foods can increase your body fat instead of your lean body mass and can lead to an increased risk of developing high levels of cholesterol in your blood, or tooth decay.

Instead, aim for three meals and three snacks a day, and base your diet on healthy eating principles. That means:

  • Base your meals on starchy carbohydrates, such as bread, pasta, rice or potatoes. Choose wholegrain varieties wherever you can, or eat potatoes with their skins on for more fibre.
  • Eat at least five portions of a variety of fruit and vegetables a day. Find out how you can get your 5 A DAY.
  • Choose some lean protein, which can be meat, fish, eggs, beans, pulses and other non-dairy sources.
  • Include some milk and dairy foods, such as cheese and yoghurt. These are great sources of protein and calcium.
  • Have only a small amount of foods high in fat and/or sugar, or eat them less often.
  • Cut down on saturated fat, found in processed meats, pies, cakes and biscuits.
  • Cut down on sugary foods and drinks such as chocolate, cakes and biscuits, and sugary soft drinks.

Learn more about the different food groups and how they form part of a healthy diet

If you don't eat meat, find out how to have a healthy vegetarian diet.

Healthy high-energy food ideas

If you’re trying to gain weight, eat foods that are not only healthy, but also high in energy. Try the following:

  • For breakfast, porridge made with whole (full-fat) milk with chopped fruit or raisins sprinkled on top, or eggs on toast.
  • Fruit smoothies or milkshakes for a great snack (make them at home and take them to work or college). But remember that fruit juice can be sugary, so try to limit this to no more than 150ml a day. 
  • For a healthier lunch, try a jacket potato with baked beans or tuna on top, which contains energy-giving starchy carbohydrate and protein.
  • Peanut butter on toast for a high-energy snack.
  • Yoghurts and milky puddings, such as rice puddings.
  • Unsalted nuts.

Underweight older people

Eating less and unintentional weight loss are common in older people, but getting older doesn't mean that losing weight is inevitable. Find out what to do if you are over 60 and underweight.

Page last reviewed: 08/11/2014

Next review due: 08/11/2016


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The 9 comments posted are personal views. Any information they give has not been checked and may not be accurate.

Tigi0 said on 07 December 2014

I agree with HJMarchant. While it would be ideal for an underweight person to gain weight with a healthy diet, healthy food is not always nice to eat (imagine having to be on a diet of wholemeal grains, porridge, dried fruits, beans, and unsalted nuts). Sometimes one has to be a bit pragmatic and allow for some unhealthy food, because the problem often is a lack of appetite. By focusing solely on healthy food it is going to be very difficult to grain weight.

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adwoaE said on 12 July 2012

I'm 16 years old and I have a good appetite, I eat very well about 4-5 times a day and in big portions but I do not gain any weight. I have fast metabolism which is affecting my weight gain, I am not very active in sports and I am desperate to gain weight because I am quite thin for my age and compared to my friends my age. I currently weight 50kg (7stones) and desire to be 55-60kg. I have been 50kg for a year and a half now. What can I do to slow down my metabolism and gain the desired weight?

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rubynolegs said on 26 June 2012

I was 11 stone 10 lb, but now 6st 8lb. I have been ill, gastric tract and bowel probs. I paid for a Colonoscopy for speed May 2010 and had 3 Adenomas and 7 Polyps removed. Have GERD/Hiatus Hernia/Gastritis/H.Pylori etc and never feel hungry. When I make a cup of tea I take something out of the fridge to eat (as i never feel hungry or get a message from the brain telling me to eat it is easy to forget, so this is a new strategy).
It is a struggle to eat more than 2 cocktail sausages, 2 cookies, and a Yoghurt with a cup of tea for a main meal.
I asked for some help but GP will not referr me. April 2012 had bloody stools and took to GP. 10 days later I was told one of the Doctors destroyed it.
I am going Private to see what is going on inside but I wish I kew how to get constructive help from GP. It isnt as easy as it sounds when you read the words "Do tell your GP'''
Any tips would be warmly accepted, but I just always feel full up.....not an easy one...Thank you

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HJMarchant said on 05 May 2012

From my own experience, I know that people with eating disorders can be desperate to stick to the "healthy eating rules", and thus deprive themselves of valuable nutrients, and of the chance to return to normal patterns of eating. Please think about who is likely to have been sent to your site - often people with eating problems who have made themselves underweight through eliminating foods classed as "unhealthy". It is normal to have things like cakes, bacon sandwiches and so on, obviously not at every meal, but as part of a good overall diet. And women in particular need some fat in their body makeup, not just lean tissue.
Please at least consider issuing a disclaimer along the lines of "this is general advice which may not apply to you". Girls are statistically more likely than boys to suffer from eating disorders which leave them underweight - please consider mentioning the fact that the normal female body does contain a proportion of fat and is not just composed of lean muscle.

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kensingtonabida said on 05 February 2012

I came across this post when trying to search for tips on how to put on weight healthily. I've always been naturally slim but through stress have dropped from a size 6 to 4, which shows considering I'm 5ft8in. I've taken the tips on here and have started to eat porridge in the mornings with blueberries on top and almonds as a snack. I've been eating more fruits and vegetables to get all the nutrients that I'm lacking in. Hoping that given time, I'll be back to a healthy size.

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mum0f3 said on 22 September 2011

What do i do?

My 14 year daughter is 5'2 and weighs 38kg - worked out bmi to be about 16. She is confident, happy, doing very well at school, has lots of friends, does not have a poor body image etc etc but isn't much of an eater. I've spoken to her about possible future health issues eg fertility, osteoperosis etc

Is there any help out there for 'poor eaters'?

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devluva940 said on 23 July 2011

aswell as bein 2 underweight im also 2 thin so i dont want 2 gain weight but still luk thin so wat can i do?

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A Sutton said on 28 March 2011

Hi Julia
If you click on the grey 'T' icon just below the image in the video window above, the transcript of the video will come up in a new window and you can print it from there.

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Julia Z said on 30 June 2010

I find the video very clear and useful BUT please can you also put this on screen in writing. This way I can print it out for clients and give them a written copy.

You could also suggest ideas for vegetarians (who eat dairy and eggs but not meat) who may need ot gain weight.
Thank you

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