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.
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.