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Weight loss and body mass index (BMI)

Working out a person's body mass index or BMI is one way the NHS uses during a health check to see if they are a healthy weight for their height.

BMI is calculated by dividing an adult's weight (in kilograms) by his or her height (in metres, squared) or  (kg/m2).

BMI can also be calculated by multiplying weight (in pounds) by 705, then dividing by height (in inches) twice.

A person with a BMI of 18.5 to 24.9 is considered to be at a healthy weight. A person with a BMI of 25-29.9 is considered to be overweight. 

A BMI over 30 is considered obese. A BMI of 40 or above indicates that a person is morbidly obese. This can increase a person's risk of death from any cause by 50% to 150%.

BMI is quite a blunt tool, and doesn’t take into account waist circumference or muscle mass.

For children, BMI calculations take into account their age and sex. Using child growth charts a BMI centile reading is worked out to check whether they are inside a healthy range.

Medical causes of obesity

Usually obesity is the result of overeating, but in fewer than one out of every 100 cases, excess weight gain is a symptom of another disease.

Medical causes of obesity can include:

Hypothyroidism. This is a condition where the thyroid gland, located in the neck, produces too little thyroid hormone. Thyroid hormones regulate our metabolism. So too little hormone slows the metabolism and often causes weight gain. If your doctor suspects thyroid disease as a cause of your obesity, he or she may perform blood tests to check your hormone levels.

Cushing's syndrome. This describes a hormonal disorder caused by prolonged high levels of the steroid hormone cortisol, leading to a build-up of fat in characteristic sites such as the face, upper back, and abdomen.

Depression. Some people with depression overeat, which can lead to obesity.

There are also certain inherited conditions and other diseases of the brain that can cause excess weight gain.

Certain medications, notably steroids, some antidepressants, high blood pressure drugs, and seizure medications can also cause increased body weight.

A doctor can determine if any of these conditions are responsible for your obesity.

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WebMD Medical Reference

Medically Reviewed by Dr Rob Hicks on December 03, 2013

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