There are many medical reasons for being underweight. Before trying to put on weight, you need to know that it is safe to do so. The best way is to make sure you are supervised by someone appropriately qualified.

Check if you’re a healthy weight for your height using the BMI healthy weight calculator.

If you’re underweight, your GP can check for any health conditions that may have caused you to lose weight, such as:
an overactive thyroid gland (hyperthyroidism)
type 1 or type 2 diabetes
coeliac disease, a condition that prevents your body from absorbing food properly
eating disorders, such as anorexia nervous
body dysmorphic disorder