What is Body Mass Index (BMI)?

what is bmi imageBody Mass Index (BMI) is a calculation of an individual’s thickness or thinness which takes both height and weight into account.  The aim is to quantify the amount of tissue mass (muscle, fat, and bone) in an individual and then categorize that person as underweight, normal weight, overweight, or obese based on that value. (1)

The standard weight status categories associated with BMI ranges for adults are shown in the following table. (2)

BMIWeight Status
Below 18.5Underweight
18.5 – 24.9Normal or Healthy Weight
25.0 – 29.9Overweight
30.0 and AboveObese

Formula for calculating BMI

BMI for adults is determined by a simple formula in which an individual’s weight or mass is divided by the squared height. The formula is universally expressed in kilograms and meters and therefore must be converted if the weight and height is given in pounds and inches.  This conversion is done by dividing the mass by the height then multiplying by 703. BMI for children and teens ages 2 to 20 is interpreted differently, even though it is calculated using the same formula; it is determined using a BMI table that compares their weight and height along with growth charts. (3)

What is BMI used for?

The American scientist who coined the term Body Mass Index, Ancel Keys, wrote in the Journal of Chronic Diseases in 1972 that BMI should be applied to population studies and not to individual evaluations. (4) In epidemiological studies, BMI is the favored measure of excess weight to estimate relative risk of disease. (5)

Currently, the Center for Disease Control maintains that a high BMI can be an indicator of high body fatness and that it can be used to screen for weight categories, but it is not diagnostic of the body fatness or health of an individual. (6) In order to determine if a person’s high BMI is a health risk, a healthcare provider would have to perform further assessments. (7)

What does BMI indicate?

The higher an individual’s BMI, the higher their risk for heart disease, type 2 diabetes, gallstones, osteoarthritis, sleep apnea, and certain cancers.(8) Essentially, BMI itself is not a measure of health or physiological state such as resting blood pressure that indicates the presence or absence of disease; it is simply a measure of your size.(9) Measuring waist circumference is a more precise assessment that screens for possible health risks that come with overweight and obesity; if most of an individual’s fat is around the waist rather than the hips – they are at a higher risk for heart disease and type 2 diabetes.(10)

What impact does BMI have on health and other life facets?

A study published by the Journal of the American Medical Association (JAMA) in 2005 showed that overweight people had a death rate similar to normal weight people as defined by BMI, while underweight and obese people had a higher death rate. (11) Obesity, which is quantified by a BMI of 30 or above, is a serious health risk and quality of life issue which can lead to a range of conditions, including: (12 )

  • Diabetes
  • Arthritis
  • Liver disease
  • Several types of cancer
  • High blood pressure
  • High cholesterol
  • Sleep apnea

Quantifying one of these risk indicators in terms of percentage of population – adults with a BMI of 30.0 or above – the prevalence of high blood pressure is 38.4 percent for men and 32.2 percent for women, respectively, compared with 18.2 percent for men and 16.5 percent for women with BMI less than 25. (13) In economic terms, obesity and underweight conditions such as anorexia are an extra burden in health care and disability costs. BMI per se has no impact on health but does indicate whether individuals are more likely to be healthy or unhealthy.

Correlation between high BMI and overweight

A BMI from 25.0 to 29.9 is the overweight range, while 30.0 to 35.0 is the obese range. But the measure has limits as an indicator of health risks; some people have larger bones and more muscle mass.  For two such groups – athletes and body builders – BMI can be inaccurate. (14) (15)  Some studies, one of which was published in the Journal of the American College of Cardiology indicate that people actually tend to live longer if their BMI is higher than average. (16)  A study conducted by the International Journal of Obesity found that nearly half of those considered overweight by BMI had a healthy “cardiometabolic profile,” including normal blood pressure, cholesterol, and blood sugar. (17) This finding reconfirms the JAMA study, pointing toward the conclusion that BMI as a single measure of an individual’s cardiovascular health is highly insufficient.

Works Cited:

  1. https://en.wikipedia.org/wiki/Body_mass_index – cite_note-7
  2. https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html
  3. http://www.nhlbi.nih.gov/health/educational/lose_wt/bmitools.htm
  4. https://en.wikipedia.org/wiki/Body_mass_index – cite_note-7
  5. http://www.nhlbi.nih.gov/files/docs/guidelines/ob_gdlns.pdf
  6. https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html
  7. https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html
  8. http://www.nhlbi.nih.gov/health/educational/lose_wt/bmitools.htm
  9. http://www.health.harvard.edu/blog/how-useful-is-the-body-mass-index-bmi-201603309339
  10. http://www.nhlbi.nih.gov/health/educational/lose_wt/bmitools.htm
  11. http://www.ncbi.nlm.nih.gov/pubmed/15840860
  12. http://www.health.harvard.edu/blog/how-useful-is-the-body-mass-index-bmi-201603309339
  13. http://www.nhlbi.nih.gov/files/docs/guidelines/ob_gdlns.pdf
  14. https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html
  15. http://www.health.harvard.edu/blog/how-useful-is-the-body-mass-index-bmi-201603309339
  16. http://www.health.harvard.edu/blog/how-useful-is-the-body-mass-index-bmi-201603309339
  17. http://www.health.harvard.edu/blog/how-useful-is-the-body-mass-index-bmi-201603309339

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