Achieving a healthier weight isn't always about losing pounds; some people would benefit from gaining some. Often, that isn't as simple as it might sound, but with the right plan it can be done.

What's 'underweight'?

Many things affect our health, but research has shown that people whose body weight is within a certain range tend to live the longest and enjoy the best health. Those who are underweight are below this range, which means their health could be at risk.

To find out if you're underweight, check a body mass index calculator.

What's wrong with being underweight?

A number of underweight people are fit and well; they simply have a slender constitution. However, for many people being underweight may mean their bones aren't as strong as they could be and a woman's periods may be irregular, which can affect fertility. It also means they have fewer 'reserves' if they fall ill.

If recent and unintentional weight loss has made you underweight, and you always feel tired, 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, you may have an eating disorder. Again, please talk to your doctor.

Why do some people stay so slender?

Healthy people who stay slender do so because they're in 'energy balance'; in other words, they (usually quite unconsciously) eat the right amount of food to meet their calorie needs. Research shows they don't have a very high metabolic rate and they don't magically 'waste' calories either. It's a combination of their inherited body shape, their natural level of appetite, what they eat and their activity levels (for example, they may rarely sit down).

For tips on gaining weight healthily, see our Weigth Gain Tips.

Article by Lyndel Costain, from http://www.bbc.co.uk/health/healthy_living/your_weight/whatis_gain.shtml