How is Obesity diagnosed?

Body Mass Index

BMI, or body mass index, is a simple and widely used method for estimating body fat.

In epidemiology BMI alone is used as an indicator of prevalence and incidence.

BMI was developed by the Belgian statistician and anthropometrist Adolphe Quetelet. It is calculated by dividing the subject's weight by the square of his/her height, typically expressed either in metric or US "Customary" units:

Metric: BMI = kg / m2
Where kg is the subject's weight in kilograms and m is the subject's height in metres.

US/Customary: BMI = lb * 703 / in2
Where lb is the subject's weight in pounds and in is the subject's height in inches.

The current definitions commonly in use establish the following values, agreed in 1997 and published in 2000:

A BMI less than 18.5 is underweight
A BMI of 18.5–24.9 is normal weight
A BMI of 25.0–29.9 is overweight
A BMI of 30.0–39.9 is obese
A BMI of 40.0 or higher is severely (or morbidly) obese
A BMI of 35.0 or higher in the presence of at least one other significant comorbidity is also classified by some bodies as morbid obesity.

In a clinical setting, physicians take into account race, ethnicity, lean mass (muscularity), age, sex, and other factors which can affect the interpretation of BMI. BMI overestimates body fat in persons who are very muscular, and it can underestimate body fat in persons who have lost body mass (e.g. many elderly).

Mild obesity as defined by BMI alone is not a cardiac risk factor, and hence BMI cannot be used as a sole clinical and epidemiological predictor of cardiovascular health.

Waist circumference

BMI does not take into account differing ratios of adipose to lean tissue; nor does it distinguish between differing forms of adiposity, some of which may correlate more closely with cardiovascular risk. Increasing understanding of the biology of different forms of adipose tissue has shown that visceral fat or central obesity (male-type or apple-type obesity) has a much stronger correlation, particularly with cardiovascular disease, than the BMI alone.

The absolute waist circumference (>102 cm in men and >88 cm in women) or waist-hip ratio (>0.9 for men and >0.85 for women) are both used as measures of central obesity.

Body fat measurement

An alternative way to determine obesity is to assess percent body fat. Doctors and scientists generally agree that men with more than 25% body fat and women with more than 30% body fat are obese. However, it is difficult to measure body fat precisely. The most accepted method has been to weigh a person underwater, but underwater weighing is a procedure limited to laboratories with special equipment. Two simpler methods for measuring body fat are the skinfold test, in which a pinch of skin is precisely measured to determine the thickness of the subcutaneous fat layer; or bioelectrical impedance analysis, usually only carried out at specialist clinics. Their routine use is discouraged.

Other measurements of body fat include computed tomography (CT/CAT scan), magnetic resonance imaging (MRI/NMR), and dual energy X-ray absorptiometry (DXA).

Risk factors

The presence of risk factors and diseases associated with obesity are also used to establish a clinical diagnosis. Coronary heart disease, type 2 diabetes, and sleep apnea are possible life-threatening risk factors that would indicate clinical treatment of obesity.

Smoking, hypertension, age and family history are other risk factors that may indicate treatment.