BMI & Obesity

The body mass index (BMI) is a ratio of height to weight. Along with waist circumference, it is a key measure used by the National Heart Lung and Blood Institute (NHLBI) and the National Institute of Diabetes and Digestive and Kidney Disease (NIDDK) to determine obesity and overweight statistics. Based on BMI, the NHLBI and NIH define overweight and obesity in relationship to disease risk and overall mortality associated with excess body weight.

BMI Calculation

BMI can be calculated by dividing weight in pounds by height in inches squared, then multiplying by 703. There are also many online BMI calculators. BMI charts and graphs are also commonly found in doctors' offices and health websites.

Healthy BMI

A BMI of 18.5 to 24.9 is considered healthy. Individuals in this category are at the lowest risk of diseases associated with excess body weight, such as type 2 diabetes, heart disease and some cancers. The NHLBI recommends that individuals with healthy BMIs be reassessed every 2 years.

Underweight Classification

A BMI under 18.5 is considered underweight, and places individuals at risk for osteoporosis, poor immunity and protein-energy malnutrition. While some individuals are naturally thin, being underweight may also be an indicator of an underlying condition causing unintentional weight loss such as cancer. Individuals with BMIs under 18.5 should undergo regular nutritional screening to evaluate nutrient intake and the cause of their low body weight.

Overweight and Obese Classifications

Individuals with BMIs 25 to 29.9 are classified as overweight, while individuals above or equal to 30 are classified in the obese category. Individuals with a BMI above 25 are at risk for cardiovascular and other diseases, while a BMI above 30 increases overall risk of death. The obese category is further broken into obese I, II and III. Obesity class I is defined as 30 to 34.9, class II as 35 to 39.9 and class III as 35 to 39.9. The final classification, labeled as extreme obesity, is a BMI of 40 and above. As BMI increases, health risks associated with excess body weight increase as well. The NHLBI recommends weight loss for individuals with a BMI above 30. For BMIs above 25, weight loss is also recommended if two or more of the following risk factors are present: high blood pressure, high LDL cholesterol, low HDL cholesterol, high triglycerides, high blood sugar, family history of premature heart disease, physical inactivity and/or cigarette smoking.

Misconceptions

Body builders and highly muscular individuals are at risk for misclassification due to high amounts of muscle mass. Muscle weighs more than fat, which causes individuals with a larger amount of muscle mass to have a higher body weight and thus a higher BMI. Elderly individuals can be inaccurately measured for the exact opposite reason. The elderly have lower muscle mass and increased fat, making their weight lower with a possibly underestimated BMI. It is also important to remember that BMI will be affected if height and weight are not measured correctly.