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Are you worried about being underweight? Or perhaps your friends or parents have mentioned it?
We all grow and develop at different rates. You may have friends who are taller, heavier and more muscular than you. Lots of boys don't reach their adult height and weight until they are over 18.
You can check whether you're a healthy weight by using our healthy weight calculator.
If you are underweight, your doctor, practice nurse or school nurse can give you help and advice.
There may be an underlying medical cause for your low weight that needs to be checked out. Gut problems like coeliac disease , for example, can make people lose weight.
Read about other medical problems that can cause unexplained weight loss .
Maybe you're having mental or emotional problems that have affected your eating habits. Depression](/condition/depression) and [anxiety , for example, can both make you lose weight.
Or perhaps you haven't been eating a healthy, balanced diet.
Whatever the reason, if you're concerned about your weight or your diet, the best thing to do is tell someone you trust, such as a parent, your school nurse or doctor. There's a lot that can be done to help.
Being underweight can leave you with low energy and affect your immune system. This means you could pick up colds and other infections more easily.
If your diet is poor, you may also be missing out on vitamins and minerals you need to grow and develop.
The good news is that, with a little help, you can gradually gain weight until you get to a weight that's healthy for your height and age.
It's important that you gain weight in a healthy way. Try not to go for chocolate, cakes, fizzy drinks and other foods high in fat or sugar.
Eating these types of foods too often is likely to increase your body fat, rather than building strong bones and muscles.
Ideally you should:
We all need some fat in our diet, but it's important to keep an eye on the amount and type of fat we're eating.
Try to cut down on the amount of saturated fat you eat – that's the fat found in foods such as sausages, salami, pies, hard cheese, cream, butter, cakes and biscuits.
Cut down on sugary foods, such as chocolate, sweets, cakes, biscuits and sugary soft drinks.
See more healthy eating tips for teenagers.
Strength training can also help to build strong muscles and bones. Find out how to increase your strength and flexibility.
Aim to eat three meals and three snacks a day to bump up your energy intake in a healthy way.
Make time for breakfast. Try porridge made with milk and sprinkle some chopped fruit or raisins on top. Have whole milk until you build your weight back up.
Or how about eggs on toast with some grilled tomatoes or mushrooms?
A jacket potato with baked beans or tuna on top makes a healthy lunch and contains both energy-rich carbohydrates and protein. Adding cheese will provide calcium. Or try pasta salad with chicken breast and tomatoes.
Have a healthy snack before bed. Lower-sugar cereal with milk is a good choice, or some toast. Go for wholemeal where possible.
See Change4Life for more healthy meal ideas.
Find out how many calories the average teenager needs.
Aim for three snacks a day to bump up your calorie intake. Try:
See Change4Life for more healthy snack ideas.
You should also make sure you get plenty of sleep. About 8 to 10 hours a day is ideal for teenagers.
Avoid smoking and alcohol.
If you feel anxious when you think about food, or you feel you may be using control over food to help you cope with stress , low self-esteem or a difficult time at home or school, then you may have an eating disorder.
If you feel you may have an eating disorder tell someone, ideally your parents, guardians or another adult you trust.
The eating disorders charity b-eat has a Youthline, where you can get confidential advice.
Important: Our website provides useful information but is not a substitute for medical advice. You should always seek the advice of your doctor when making decisions about your health.