BMI or body mass index, is one of those acryonyms that became popular many decades ago, and as a way to decide if a person is too fat, too much weight, for their height. Back then, many decades ago, it was “better than nothing”, and it was pretty simple, just weight over height squared, where weight is in kilograms, and height is in meters2. That is, the formula for BMI is kg/m2. Or, think of it as BMI = kilograms / ( meters * meters).
The BMI gap, the body mass index difference between men and women.
By Steven B. Halls, MD FRCPC
Are men heavier than women? ( Normally, Yes ). What about their Body Mass Index (BMI)?
Usually, BMI values of girls are slightly larger than boys (from age 7 to 16). As long as men and women are not overweight, BMI values of men are slightly larger than women ( after age 18).
"Overweightness" is a different story. At around 18 years of age, young men’s and women’s Body Mass Index values become equal at the 85th percentile (overweight threshold) of 25 kg/m2. And for middle-aged adults, overweight women tend to have higher Body Mass Index values than overweight men.
This is confusing, and if you think about it too much, you’ll go crazy because there is no single correct way to answer this question:
Should men and women have different overweight thresholds?
The CDC and WHO didn’t think so, and they chose a single number threshold of 25 kg/m2 as the overweight threshold for men and women, young and old. Simplicity.
But there is considerable published scientific evidence suggesting that men and women should have different overweight thresholds. Four kinds of evidence exist: Body fat percentages, median BMI differences, statistical fairness and visual appearance.
Body Fat percentages
From various methods used to measure body fat, we know that: women carry more body fat than men. There is no dispute about that. It is normal.
Definitions of "overweight" based on body fat percentages use a mens threshold at 25% body fat, but the women’s overweight threshold is at 30%, 33% or 35% body fat.
Why do women have 3 different thresholds ( 30%, 33% or 35%) for body fat? It just shows that there is no agreement among scientists, so different scientists choose different percentage numbers.
It is normal for women to carry more fat than men, and it is also normal for men to have more muscle than women. Muscle weighs more than fat.
Suppose a man and woman are the same height. Will they weigh the same and will their body mass index be the same?
Normally, No. It would have to be a rare coincidence for the man’s extra muscle to exactly balance the woman’s extra fat.
Studies have measured the average BMI of people with body fat percentages at the overweight threshold (25% in men, 33% in women).
One study suggests a suitable BMI "gap" of 2 kg/m2 between men’s and women’s BMI1. Another study2 suggests the BMI gap should be 5 kg/m2. Another study3 suggests the BMI gap should be 1.4 kg/m2. There is no single correct answer.
There are formulas that predict body fat percentage, based on Age, BMI and Gender. Using five such formulas3, and averaging the results, there is an average BMI gap of 2.3 kg/m2 between men and women. ( using ages 25 and 35 years, body fat 25% for men, 33% for women.)
In summary, body fat percentage evidence suggests that Men should have a BMI that is approximately 2.3 kg/m2 higher than Women.
Average BMI differences
There have been numerous large population surveys that measured Body mass index. Using the median BMI values, which are somewhat representative of "average" people, these studies always show that median men have higher BMI values than median women. But the "gap" between men and women‘s median BMI changes with age, because fatness generally increases with age, to a greater extent in women than men.
In America, the NHANES I study4 in 1971-74, and the NHANES III study5 in 1988-94, show that the "gap" between men and women has narrowed from 1.85 to 1.45 kg/m2 recently.
There is a generally held belief that the American population was closer to being ideal in the 1970s than the 1990s, so the gap of 1.85 kg/m2 between men and women’s BMI seems slightly more suitable.
One problem with the current CDC & WHO threshold of "overweight" at 25 kg/m2 is that it is unfair to men. It labels a larger percentage of the Male population as overweight. If the male threshold were higher, or the female threshold were lower, then the proportion of overweight men and women would be more equal.
A Danish study6 used a purely statistical method to define alternate BMI thresholds for overweightness, with equality between men and women as an important statistical criteria. Their study suggested a BMI of 27.2 kg/m2 for men and 25.0 kg/m2 for women, as overweight thresholds. The "gap" between these figures is 2.2 kg/m2.
A study of the visual appearance of people7, judged by their resemblance to standard "figures" shapes, suggested that the Obesity threshold was at a BMI of 31.5 for men and 29.9 for women, which is a gap of 1.6 kg/m2. And at the Overweight threshold (figures 6 & 5), there is a gap of 1.8 kg/m2.
Body fat percentage evidence suggests a "gap" of 2.3 kg/m2. Median BMI evidence suggests a gap of 1.85 kg/m2. Statistical fairness and Visual appearance also suggest gap values. The average of these values is 2.0 kg/m2. That is the "gap" between mens and womens Body Mass Index values that the halls.md v2 model uses in it’s overweight thresholds.
- Wellens RI, Roche AF, Khamis HJ et al. Relationships between the Body Mass Index and body composition. Obes Res, Jan 1996;4(1):35-44
- Wang J, Thornton JC, Burastero S et a. Comparisons for body mass index and body fat percent among Puerto Ricans, blacks, whites and Asians living in New York City area. Obes Res, Jul 1996;4(4):377-84
- Jackson AS, Stanforth PR, Gagnon J, et al. The effect of sex, age and race on estimating percentage body fat from body mass index: the Heritage Family Study. Int J Obes Relat Metab Disord, Jun 2002;26(6):789-96
- Must A, Dallal GE, Dietz WH. Reference data for obesity: 85th and 95th percentiles of body mass index (wt/ht2) and triceps skinfold thickness. Am J Clin Nutr, Apr 1991;53(4):839-46
- Kuczmarski RJ, Carroll MD, Flegal KM, Troiano RP. Varying body mass index cutoff points to describe overweight prevalence among U.S. adults: NHANES III ( 1988 to 1994). Obes Res, Nov 1997;5(6):542-8
- Price RA, Sorensen TI, Stunkard AJ. Component distributions of body mass index defining moderate and extreme overweight in Danish women and men. Am J Epidemiol, Jul 1989;130(1):193-201
- Bulik CM,Wade TD, Heath AC et al. Relating body mass index to figural stimuli: population-based normative data for Caucasians. Int J Obes Relat Metab Disord, Oct 2001;25(10):1517-24