This post will look at the Body Mass Index bmi difference between men and women.

BMI or body mass index, is one of those acryonyms that became popular many decades ago. The BMI measurement is a way to decide if a person is too fat or carries too much weight for their height.

Back then, many decades ago, it was ‘better than nothing’ and it was pretty simple. Indeed the sum is 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.

 
Are men heavier than women? Normally, Yes. What about their Body Mass Index (BMI)?

Usually, BMI values of girls are slightly higher than boys (from age 7 to 16). As long as men and women are not overweight, BMI values of men are also slightly higher than for women( after age 18).

 

 
 

Overweightness is a different story. At around 18 years of age, young men and women’s Body Mass Index values become equal at the 85th percentile (overweight threshold) of 25 kg/m².

In addition, 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 World Health Organization (WHO) don’t think so. Furthermore, they both chose a single number threshold of 25 kg/m² as the overweight threshold for men and women, young and old. It appears that the reason for this is simplicity.

 

 
 

However, there is considerable published scientific evidence that suggests men and women should have different overweight thresholds.

Four kinds of evidence exist:

1. Body fat percentages
2. Median BMI differences
3. Statistical fairness
4. Visual appearance

Body Fat percentages

 
From a whole host of methods used to measure body fat we know that: women carry more fat than men. There is no dispute about this, it is normal.

Definitions of ‘overweight’ based on body fat percentages use a male threshold of 25% body fat. However, a the woman’s overweight threshold is 30%, 33% or 35% body fat.

 

 
 

Why do women have 3 different thresholds ( 30%, 33% or 35%) for body fat?

 
The different thresholds show 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. Furthermore, muscle weighs more than fat.

So, 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 be a rare coincidence for a man’s extra muscle to exactly balance a woman’s extra fat.

Studies have measured the average BMI of people with body fat percentages at the overweight threshold and these are 25% in men and 33% in women.

 

 
 

The Science and Studies behind bmi difference between men and women

 
One medical 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.

Furthermore, a third research study³ suggests the BMI gap should be 1.4 kg/m². There seems to be no agreement or 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. (This calculation uses 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 difference between men and women

 
There have been numerous large population surveys that measure Body mass index. The studies use 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 median BMI changes with age. This is 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 general belief that the American population weight was closer to ideal in the 1970’s than the 1990’s. Thus, the gap of 1.85 kg/m² between men and women’s BMI seems slightly more suitable.

Statistical fairness

 
One problem with the current CDC and WHO thresholds of overweight at 25 kg/m² is that it is unfair to men.

Hence, the thresholds label a larger percentage of the male population as ‘overweight’. If the male threshold was higher or the female threshold lower for ‘overweight’ 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 overweight thresholds with equality between men and women as an important statistical criteria.

The study suggests 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.

 

 
 

Visual Evidence

 
A study of the visual appearance of people⁷, judged by their resemblance to standard figures shapes, suggests an obesity threshold of a BMI of 31.5 for men and 29.9 for women. The gap between bmi thresholds of men and women is 1.6 kg/m².

Furthermore, at the overweight threshold (figures 6 & 5), there is a gap of 1.8 kg/m² between men and women.

Summary

 
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.

This is the gap between men and women’s Body Mass Index values that the halls.md v2 model uses for the ‘overweight’ thresholds.

 

 
 

References

1. 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
2. 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
3. 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
4. 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
5. 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
6. 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
7. 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