Body Mass Index Explained

Body Mass Index Explained


What is BMI?

Body mass index (BMI) is a metric or measure that is used to provide a snapshot of an individual's health in one number. It’s purpose is to describe the relationship between an person’s height and weight. In clinical terms, it’s used to describe an individual's adiposity or body fat percentage. Its usefulness stems from the fact that it better estimates an individual's body fat than just body weight alone. For example, a patient who is 5’0 and weighs 150 lbs would be considered overweight, while a person who is 6’0 tall weighing the same would not. It’s not a perfect metric and has limitations; but it’s pervasiveness in the world of medicine and nutrition and ease of use are reasons to understand it.

Calculating BMI

BMI was developed using the metric system, and that equation is rather simple. It’s mass (kg) / (height (meters))^2. In the SI system, which is what is used in the United States, it’s mass (lb) / (height (in))^2 x 703. There are dozens of calculators available online.

NIH Body Mass Index Calculator

Classification of BMI

BMI is classified according to the table below. Numbers to remember are that the normal range is 18.5 - 24.9, overweight is 25.0 - 29.9, obese is 30.0 to 39.9 and morbidly obese is > 40.0. Over 30, the WHO uses three classes obesity.

Health Risks of a high BMI

The higher your BMI, the higher your risk for certain diseases such as heart disease, high blood pressure, type 2 diabetes, gallstones, breathing problems, and certain cancers. These health risks increase even more as the severity of an individual's obesity increases. Despite the limitations of BMI, these risk factors are well established in the literature.

Research has shown that in individuals with a BMI over 25 (classified as overweight) and a BMI over 30 (classified as obese) the risks for the following conditions also increases:

  • Death from any cause (all cause mortality)
  • Coronary heart disease (heart attack)
  • Type 2 diabetes
  • Cancers (endometrial, breast, and colon)
  • Hypertension (high blood pressure)
  • Dyslipidemia (high cholesterol, triglycerides)
  • Stroke (cerebrovascular accident)
  • Liver and Gallbladder disease
  • Sleep apnea and respiratory problems
  • Osteoarthritis (a degeneration of cartilage and its underlying bone within a joint)
  • Gynecological problems (abnormal menses, infertility)

Healths risk of a low BMI

Individuals with a BMI under 20, especially under 18.5, are at increased risk of medical problems as well. Unfortunately, because the underweight folks are fewer and farther between, they don’t get the same attention as the 1.3 billion overweight and obese adults. That said, being underweight has increased risk of:

  • Death from any cause (all cause mortality)
  • Malnutrition
  • Vitamin or mineral deficiency
  • Complications of vitamin or mineral deficiency such as osteoporosis,
  • Immunosuppression
  • Respiratory disease
  • Gynecological problems (abnormal menses, infertility)

Limitations of BMI

The level of adiposity for a given BMI value varies by race. For example, a white male with a BMI of 25 has 21% body fat, an Asian male has 23% and an African-American male 20%. This variability is true at all levels of BMI and for both males and females of all 3 races. This suggests that the mean BMI associated with the development of an adverse metabolic profile varies by race. The current WHO guidelines listed above apply to whites, hispanics and blacks. For Asians, the current cutoffs underestimate the amount of risk.

BMI is also considered to be an inaccurate measurement of adiposity in individuals with lean body mass (i.e., athletes, weight lifters, etc). Individuals with more muscle mass tend to have a higher BMI despite body fat percentages in the lower-normal range. For example, an individual who is 6’3” and weighs 235 lbs with about 10% body fat. The calculated BMI is 29.4, which puts that individual in the overweight range and very close to being classified as obese. Conversely, it may underestimate body fat percentage in older individuals or those who have lost muscle mass.

These limitations have lead researchers and industry experts to suggest that BMI is not an accurate assessment of adiposity. Despite it’s inaccuracy, it is simple to calculate and widely used. In the future, researchers may develop a metric that is a better assessment of adiposity. .

Other anthropometric measures

Despite the widespread use of BMI, other body measurements may be better predictors of adiposity and risk. These include neck circumference, waist circumference, waist-to-height ratio and waist-to-hip ratio. There is evidence to support the clinical utility and accuracy of these measurements but they are not as widely used in clinical medicine.

What is your risk of death based on your BMI?

What is your risk of death based on your BMI?

Muscle Mass & Death From Heart Disease

Muscle Mass & Death From Heart Disease