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Food and diet - Underweight adults

NHS Choices Feature

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

ack of energy.

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 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 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 eating healthily, or have you been skipping breakfast or lunch and just eating snacks on the go?
  • Have you lost your appetite, perhaps because you're stressed or worried?
  • 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?
  • Are you not eating because it gives you a feeling of control or power?

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, you are more likely to be lacking vital 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 (fragile bone disease) later in life.
  • If you're not consuming enough iron, you may develop anaemia (a lack of red blood cells), which leaves 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.
  • For women, you may have interrupted periods and find it difficult to become pregnant. Women who are underweight can find that their periods stop. This increases the risk of problems with fertility.

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, help is available. Find out more about eating disorders.

A healthy diet for a healthy weight

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

It's crucial that you gain weight the right way, and not by eating chocolate, cakes and other high-calorie junk foods full of saturated fat and sugar, or with fizzy drinks. These foods can increase your body fat instead of your lean body mass.

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

  • Make meals with starchy carbohydrates, such as wholemeal pasta, brown rice, potatoes or lentils, as a base.
  • Eat five portions of fruit and vegetables a day. For ideas, see 5 A DAY.
  • Choose lean protein from meat, fish, beans and pulses.
  • Get three portions of calcium a day. One portion is a glass (190ml or 1/3 pint) of milk, a yoghurt or a small matchbox-size piece of cheese.
  • 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 sugar-rich 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 more about a  healthy vegetarian diet.

Healthy high-energy food ideas

If you're trying to gain weight, eat foods that are not only healthy but are also packed with energy. Try the following:

  • for breakfast, porridge 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)
  • for a healthy lunch, a jacket potato with baked beans or tuna on top, which contains energy-giving carbohydrate and protein
  • peanut butter on toast for a high-energy snack
  • yoghurts and milky puddings, such as rice puddings
  • nuts, which are high in 'good' unsaturated fats (choose unsalted varieties)

Underweight older people

Eating less and unintentional weight loss are common in older people. Surveys of admissions to hospital and to care homes regularly find that there is a much higher proportion of underweight older people than overweight older people, according to the British Geriatrics Society.  

Find out more about being over 60 and underweight.

Medical Review: November 08, 2012

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