What is BMI?

The body mass index or BMI is a ratio of mass to height. It is calculated as kg per square meter or pounds per square inch of height. Numeric BMI values correspond to weight categories including underweight, normal, overweight and obese.

BMI is used as an indicator of the relative healthiness of a person. The Centers for Disease Control (CDC) and the World Health Organization (WHO) recognize that people who are overweight or underweight are at higher risk for certain health conditions. BMI also enables health professionals to discuss bodyweight objectively with their patients.

Understanding your BMI result

Underweight:

Being underweight could be a sign you're not eating enough or you may be ill. If you're underweight, a GP can help.

Risks associated with being underweight

Being underweight has its own associated risks, listed below:

• Malnutrition, vitamin deficiencies, anemia (lowered ability to carry blood vessels)
• Osteoporosis, a disease that causes bone weakness, increasing the risk of breaking a bone
• A decrease in immune function
• Growth and development issues, particularly in children and teenagers
• Possible reproductive issues for women due to hormonal imbalances that can disrupt the menstrual cycle. Underweight women also have a higher chance of miscarriage in the first trimester
• Potential complications as a result of surgery
• Generally, an increased risk of mortality compared to those with a healthy BMI

In some cases, being underweight can be a sign of some underlying condition or disease such as anorexia nervosa, which has its own risks. Consult your doctor if you think you or someone you know is underweight, particularly if the reason for being underweight does not seem obvious.

Normal weight:

Keep up the good work! For tips on maintaining a healthy weight, check out the food and diet and fitness sections.

Overweight:

The best way to lose weight if you're overweight is through a combination of diet and exercise. The BMI calculator will give you a personal calorie allowance to help you achieve a healthy weight safely.

Risks associated with being overweight:

Being overweight increases the risk of a number of serious diseases and health conditions. Below is a list of said risks, according to the Centers for Disease Control and Prevention (CDC):

• High blood pressure
• Higher levels of LDL cholesterol, which is widely considered "bad cholesterol," lower levels of HDL cholesterol, considered to be good cholesterol in moderation, and high levels of triglycerides
• Type II diabetes
• Coronary heart disease
• Stroke
• Gallbladder disease
• Osteoarthritis, a type of joint disease caused by breakdown of joint cartilage
• Sleep apnea and breathing problems
• Certain cancers (endometrial, breast, colon, kidney, gallbladder, liver)
• Low quality of life
• Mental illnesses such as clinical depression, anxiety, and others
• Body pains and difficulty with certain physical functions
• Generally, an increased risk of mortality compared to those with a healthy BMI

As can be seen from the list above, there are numerous negative, in some cases fatal, outcomes that may result from being overweight. Generally, a person should try to maintain a BMI below 25 kg/m2, but ideally should consult their doctor to determine whether or not they need to make any changes to their lifestyle in order to be healthier.

Limitations of the BMI

Your BMI can tell you if you're carrying too much weight, but it cannot tell if you're carrying too much fat.

The BMI cannot tell the difference between excess fat, muscle or bone.

The adult BMI does not take into account age, gender or muscle mass.

This means:

• very muscular adults and athletes may be classed "overweight" or "obese" even though their body fat is low
• adults who lose muscle as they get older may fall into the "healthy weight" range even though they may be carrying excess fat

Pregnancy will also affect a woman's BMI result. Your BMI will go up as your weight increases. You should use your pre-pregnancy weight when calculating your BMI. Apart from these limitations, the BMI is a relatively straightforward and convenient way of assessing someone's weight.