Comments and deriving a body fat percentage formula
This article uses the fairly standard definition of obesity, using body fat percentages of 25% for men, and 30% for women. It found the commonly seen curved relationship of body fat percentage versus Body Mass Index, as shown in the first chart below.
I showed this chart because it has data from some super obese people. Most other studies don’t have subjects this heavy, which makes this study quite valuable.
There are some limitations of this study. Most importantly, they used impedance to estimate body fat percentage, but impedance measurements can be inaccurate in subjects in the super obesity range, so there is some doubt about whether the body fat measurements are "true". So, the curves (above) may not be as flat on top, as they appear to be.
This article found that a linear relationship between BMI and body fat could be achieved by plotting body fat divided by height squared, versus BMI. This is an interesting observation, shown below.
The article contains arithmetic formulas for this linear relationship.
The article examined the Obesity thresholds using BMI, compared to body fat percentages as the gold standard. They reported their results in weird way, making it impossible to determine their sensitivity and specificity. But they did mention a similar study by Hortobagyi1, which had these results for a BMI threshold of 30 kg/m2:
Since I don’t know the ages of the Hortobagyi study, its not easy to interpret the data. The men’s threshold looks good, but the women’s threshold looks too high. If the threshold were lower than 30 in women, it would have higher sensitivity. But keep in mind that this is based on using an arbitrary body fat percentage standard of 30% for women. More likely, the 30% number should be raised.
- Hortobagyi T, Israel RG, O’Brien KF. Sensitivity and specificity of the Quetelet index to assess obesity in men and women. Eur J Clin Nutr 1994: 48:369.