Weight Loss

BMI An Imperfect Way to Measure Body Fat

The American Medical Association (AMA) Council on Science and Public Health report recently shared a new policy that clarified how BMI can be used as a measure in medicine. The new guidelines were revealed at the annual AMA House of Delegates meeting. This comes after many years of BMI (body mass index) being used as the standard for determining an individual’s risk of developing a variety of health issues, when related to the ratio of their height and weight.

The BMI is an attempt to quantify the amount of tissue mass (muscle, fat, and bone) in an individual, and then categorize that person as underweight, normal weight, overweight, obese, and morbidly obese based on that value.

The BMI can also be represented by the chart below, which displays BMI as a function of mass and height (this time, in pounds and inches) using contour lines or colors for different BMI categories. Note that:

  • The ideal BMI reading for a healthy weight is 18.5 to 24.
  • Being overweight means that you have a BMI score of between 25 and 29.9.
  • A score of 30 and above means that you are obese.
  • A score of 40 and above is considered morbidly, or seriously obese.

All of these categories help predict potential health issues.


For years, people have argued that BMI was an incomplete measure of a person’s health. Several factors can skew the results of the proportion between a person’s height and their weight, including gender, race/ethnic groups, age, or body composition. The AMA release noted that BMI is “significantly correlated with the amount of fat mass in the general population but loses predictability when applied to the individual.” Going further, the AMA now suggests that BMI be used in conjunction with other valid measures of risk including visceral fat, body adiposity index, body composition, relative fat mass, waist circumference, and genetic/ metabolic factors.

Visceral fat refers to fat behind your abdominal muscles that can’t be seen. It surrounds your stomach, liver, intestines and other organs. A variety of factors impact your level of visceral fat, including genetics (which determines how your body stores visceral fat), diet, exercise, stress levels, sleep quality, and alcohol ingestion.

Body adiposity index is a ratio between an individual’s hip circumference and their height. The index does factor gender and age. You can explore those metrics and calculate your BAI here: Body Adiposity Index (BAI) Calculator.

Body composition refers to the physical shape of your body. Gender, race/ethnicity, and age, all contribute to your body composition.”[BMI] also does not tell us the distribution of body fat in a person,” Eva Tseng, MD, MPH, an Assistant Professor of Medicine at Johns Hopkins, “We know that people with more central or abdominal adiposity [obesity] have a higher risk of premature cardiovascular disease, diabetes, and death compared to people with a similar BMI but less abdominal adiposity.” To determine body composition (levels of muscle and fat), doctors may use skin calipers, underwater weighing, a DEXA scan (low level xray), or bioelectric impedence (electrical currents).

Relative fat mass was recently developed by Cedars-Sinai scientists (and their research published in the journal Scientific Reportso) and has been found to be a more exact predictor of body fat percentage than BMI. To determine your RFM, use the following formulas:

  • Men: 64 – (20 x height/waist circumference) = RFM
  • Women: 76 – (20 x height/waist circumference) = RFM

Waist circumference can indicate risk of hypertension, can help identify those at ‘early health risk’ of central obesity-related illnesses, and can be an indicator of potential heart disease and type 2 diabetes. Waist circumference is strongly associated with all-cause and cardiovascular mortality. According to the AMA, waist circumference of a healthy waistline is:

  • 40 inches or less for men
  • 35 inches or less for women

The AMA’s updated guidelines on how to use BMI in conjunction with other tools take into account that every individual’s body is different. From their shape, fitness, genetics, race/ethnicity, gender, age, and more- no single index can categorize the entire story of our health.










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