The relationship between BMI and body fat is that it really depends on the population and the method of assessing body fat.
Regarding populations, if you compare BMI and body fat among people who are several standard deviations away from average height, the BMI and body fat relationship is very poor; BMI tends to underestimate body fat among taller people and overestimate it among shorter people.
Furthermore, some populations of people tend to have higher body fat percentages at a given BMI than other populations. As such, there is poor correlation between BMI and body fat percentage across different racial, ethnic, and cultural dimensions .
The relationship between BMI and body fat is affected by the method of assessment, with larger discrepancies being seen among more accurate assessments of body fat. The corollary is also true, with BMI being more correlated with body fat percentage when less accurate methods of assessment are used.
For example, BMI shares a relatively high correlation to body fat percentage when assessed by waist circumference (~r=0.78), but there was a high level of discrepancy observed when body fat was assessed by more precise measures.
BMI is a screening tool commonly used to assess a person's weight in relation to their height. The tool provides a general evaluation of a person's health risks relative to their weight. But, unlike body composition, it does not provide specifics about the distribution of a person's weight.
BMI is calculated by taking total body mass in kilograms (kg) and dividing it by height in meters (m) squared. The results are written as kg/m2.
The number is then put into a chart to find the category. The BMI categories include:7
- BMI less than 18.5 = Underweight
- BMI 18.5 to 24.9 = Normal
- BMI 25 to 29.9 = Overweight
- BMI 30 to 34.9 = Obese class I
- BMI 35 to 39.9 = Obese class II
- BMI more than 40 = Obese class III
Whereas BMI measures weight relative to height, body composition measures fat in proportion to lean body mass. For this reason, body composition offers a more accurate and comprehensive insight into a person's overall health.
Take, for example, an athlete with excessive muscle and very little fat. This individual will likely have a high BMI. In this instance, though, having a high BMI does not mean they have obesity or a greater risk of health conditions that are associated with obesity.
In order to get an accurate evaluation of how much fat, muscle, and all-around lean mass this individual has, an evaluation of their body composition is needed.