If being too fat is a health risk, you might think being too thin is good for you. But contrary to that old adage, "you can never be too rich or too thin," being overly skinny carries its own health risks.

Underweight is defined as having a body mass index (BMI) below 18.5. For example, someone who is 5'5" and weighs 110 pounds would have a BMI of 18.3, which is considered underweight.

While complaints of being "too skinny" may spark envy from those who tip the scales in the other direction, weighing too little can increase your risk of osteoporosis and fertility problems, weaken your immune system, and cause other health problems.

As well, if you aren't happy with your body - because you want more curves or muscle - being too skinny can make you feel self-conscious and reduce your self-esteem.

There are a number of reasons why someone may be underweight. Some people are just naturally thinner than others, because they tend to burn more calories or eat less. Other people may lose weight as a result of certain medications or an underlying medical condition. Sudden weight loss without trying can also be an indication of a health problem, so talk to your doctor.

For some people, being too thin is a self-induced condition, known as anorexia nervosa. Anorexia is a condition where sufferers may diet to the point of near-starvation or exercise excessively all in the name of weight control. Though anorexia is a psychological illness, it can have serious physical complications, including heart and lung problems, osteoporosis, and, in some cases, death.

Many people with anorexia do not see their weight as a problem and may go to lengths to hide their habits. If you believe someone you know is suffering from anorexia, encourage them to seek help for their condition.

Written and reviewed by the MediResource Clinical Team