Achieving a healthier weight isn't always about losing weight. Some people would benefit from gaining it. If that's you, you will know it isn't as straight forward as it might sound. But with planning and perseverance it can be done.

What is 'underweight'?

If you think you might be underweight, check using the Body Mass Index calculator. Many things affect our health. But research has shown that people within a certain range of body size - the healthy weight range - tend to live the longest, and enjoy the best health. If you are underweight, you will be below that range, which means your health could be at risk in some way.

What is wrong with being underweight?

A number of people are fit and well when 'underweight' and simply have a slender constitution. But it could mean your bones are not as strong as they could be, and for women, periods might be irregular. It also means you have less 'reserves' if you fall ill for some reason.

If recent and unintentional weight loss has made you underweight, and you always feel tired, then do see your doctor in case you have an underlying health problem. If you consciously restrict how much you eat, and/or feel anxious about the thought of gaining weight, then you could have an eating disorder. Again, please do talk to your doctor.

Why do some people stay so slender?

Be aware

If you consciously restrict how much you eat, and/or feel anxious about the thought of gaining weight, then please do talk to your doctor.

Healthy people who stay so slender do so because they are in energy balance. In other words, they are (usually quite unconsciously) managing to eat the right amount of food to meet their calorie needs. Research shows that they don't have very high metabolic rate, nor do they magically 'waste' calories. It's a combination of their inherited body shape, their natural level of appetite, what they eat and the activity (for example they may rarely sit down!) they do.

How common are eating disorders?

Eating disorders include a range of different conditions where people have an abnormal attitude towards food, altered appetite control and unhealthy eating habits to the extent that it affects their health and ability to function normally. The commonest disordered eating is obesity, which affects more than 1 in 10 of the population. But we are all so familiar with obesity that we rarely see it as particularly abnormal.

Bulimia nervosa, or binge eating and purging, is also common - 8% of women will develop it at some point in their life. This is more than double the number of cases of anorexia, which affects about 3%. Men or boys are less often affected than girls or women, although eating disorders are fast becoming common among them too.