Quoted by
BBC News: "What does 'underweight' mean?
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.
Our body mass index calculator can help you find out if you're underweight.
We've also got a calculator to work out your hip to waist ratio.
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 means their bones aren't as strong as they could be and they have fewer 'reserves' if they fall ill. It can also affect a woman's fertility.
If you've experienced recent and unintentional weight loss and you're always tired, you should see your doctor in case there's 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.
Talk to your doctor or contact Beat.
Why do some people stay slender?
People who stay slender do so because they're in 'energy balance'. In other words, they (often unconsciously) eat the right amount of food to meet their calorie needs.
Research has shown they don't have high metabolic rates and don't magically 'waste' calories. It's a combination of their inherited body shape, their natural level of appetite, what they eat and their activity levels.
How do I gain weight?
You need to take in more calories than you burn. Combining this with toning exercises helps, too.
Set yourself a realistic target and aim to gain weight gradually, at around 1kg (2.2lb) a month. For that you'll need an extra 300kcal to 350kcal a day. This is best done as part of a balanced diet.
Keep a food diary for a couple of weeks to learn more about your eating habits and to identify where you can add calories.
One way is to make meals a little larger or more calorific. Have an extra slice of toast at breakfast, drink grape juice rather than orange juice, make coffee with milk or serve an extra spoon of potato, rice or pasta.
Although exercise burns calories, it's important to include some form of activity into your plan to keep your bones strong and your muscles toned.
Aim for 30 minutes' physical activity five days a week and combine this with some resistance training. Take care not to be too active. If you're always on your feet, make time to relax each day. "