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. According to the Eatwell Guide, that means:

  • Eat at least five portions of a variety of fruit and vegetables every day.
  • Base meals on potatoes, bread, rice, pasta or other starchy carbohydrates. Choose wholegrain where possible.
  • Have some dairy or dairy alternatives (such as soya drinks and yoghurts). Choose lower-fat and lower-sugar options.
  • Eat some beans, pulses, fish, eggs, meat and other protein. Aim for two portions of fish every week – one of which should be oily, such as salmon or mackerel.
  • Choose unsaturated oils and spreads and eat in small amounts.
  • Drink plenty of fluids – the government recommends 6-8 cups/glasses a day.

If you're having foods and drinks that are high in fat, salt and sugar, have these less often and in small amounts.

Try to choose a variety of different foods from the five main food groups. Most people in the UK eat and drink too many calories, too much fat, sugar and salt, and not enough fruit, vegetables, oily fish or fibre.

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: 08/11/2017


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