# How are BMI ranges established?

The ideal BMI is often quoted as being 18.5 to 24.9, and obesity is often defined as a BMI over 30.

Were these ranges constructed based off established science connecting these numbers to health, or are the ranges arbitrary? If they are based off peer reviewed research could you please provide as much detail as possible.

Generally, for determining normal range of any parameter, a reasonably large sample of apparently healthy population is tested for that parameter. The normal range is then taken to be 2.5th to 97.5th percentile value. Values above and below this range are categorized as abnormal (abnormally low or high).

For some parameters, e.g. eyesight, one side of range is better than normal and may be called super-normal and not abnormal.

Round, convenient values are often taken as limits of normal ranges and mild/moderate/severe categories, so that they can be easily remembered and applied in busy clinics. Some of the blood sugar, blood pressure and BMI cutoffs follow this principle.

For some parameters, prospective studies of outcome (life expectancy, morbidity and mortality) may show a particular range to be the best or optimal and such range may be used to determine normality. Yu Chen et al (http://www.bmj.com/content/347/bmj.f5446) found a U shaped association between BMI of Asians and cardiovascular deaths.

The risk is often continuous, but the values are categorized into groups so that odds ratio can be calculated. Odds ratio provides easy method to compare risk in different categories. Regarding BMI, the World Health Organization page (http://apps.who.int/bmi/index.jsp?introPage=intro_3.html) also mentions:

The health risks associated with increasing BMI are continuous and the interpretation of BMI gradings in relation to risk may differ for different populations.

There are many who strongly oppose conversion of continuous data to categorical: http://biostat.mc.vanderbilt.edu/wiki/Main/CatContinuous

• So only 5% of the healthy population are overweight? Sep 21, 2015 at 7:22
• Based on initial study used to define cut-offs, of apparently healthy people, upper 2.5% will be said to have abnormally high BMI, while lower 2.5% will be said to have abnormally low BMI. These values can change in later studies or studies using this criteria on another population. Moreover, those with higher than normal BMI are also subcategorized into overweight, obese and morbidly obese. Sep 21, 2015 at 13:00