Your basal metabolic rate (BMR) tells you how much energy your body burns to just “keep the lights on” – it’s the energy used to power the basic functions of your vital organs, to accomplish sufficient protein and cell turnover to keep your tissues functioning properly, etc. If you didn’t leave your bed all day and didn’t move a muscle, your basal metabolic rate is the amount of energy you’d still burn in a day.
I don’t think it will come as a surprise to anyone that men tend to have higher BMRs than women. So, why is that?
If you’ve read the earlier articles in this series, you’ll know that BMR scales with fat-free mass: in general, people who have more fat-free mass tend to have higher BMRs. So, is that the only reason why men tend to have higher BMRs? Or does the relationship between fat-free mass and BMR differ between the sexes?
Ultimately, it appears that differences in fat-free mass can fully explain sex differences in basal metabolic rates.
This was first observed by Cunningham in 1980. He was re-analyzing the data of 223 subjects used to develop the Harris-Benedict BMR equations, and found that fat-free mass worked well as the sole predictor of BMR. In equations using factors like height and weight, he found that men have higher BMRs per unit of height or per unit of weight (in other words, if a man and a woman are both 170cm tall and weigh 70kg, you’d expect the man to have a higher BMR). But, male and female BMR could be predicted using the same equation when scaling to fat-free mass; it didn’t appear that the relationship between fat-free mass and BMR differed between the sexes (in other words, if a man and a woman both have 50kg of fat-free mass, you’d expect them to have the same BMR).
Subsequent research has supported this initial finding. For instance, a 1990 study by Mifflin and colleagues analyzed data collected from almost 500 subjects (247 women and 251 men). They found that the equations describing the relationship between fat-free mass and BMR were very similar in both sexes, so the “final” version of the FFM-based Mifflin-St Jeor equation didn’t differentiate between men and women.
A 2002 study by Heymsfield and colleagues, analyzing the data of 131 men and 158 women, had similar results:
Finally, for an even larger and even more recent example, a 2021 study by Pontzer and colleagues included an analysis of nearly 1,700 subjects in the doubly labeled water database, and it also found that the relationship between fat-free mass and BMR was the same in men and women.
I could bore you with dozens of additional examples, but I think you get the point: research consistently finds that fat-free mass is the best single predictor of BMR, and it finds that the relationship between fat-free mass and BMR is the same in both sexes. This has been a stable and reliable finding in large-sample research studies spanning the last four decades. That’s why our preferred equation for predicting BMR from fat-free mass (the 1991 version of the Cunningham equation) applies equally to both sexes.
Interestingly, that also means that women generally have a slightly higher energy expenditure per unit of fat-free mass than men do.
As we covered in a previous article, energy expenditure is associated with fat-free mass, but it’s directly determined by the specific tissues composing the body, and the individual metabolic rates of those tissues. Some lean tissues – like the heart, brain, kidneys, and liver – burn a ton of energy at rest. Others – like muscles, bones, and the lean component of adipose tissue – are still lean tissues, but they burn relatively few calories at rest (per unit of mass, at least).
As total lean mass increases, the composition of that lean mass changes. People with less total lean mass tend to have a higher percentage of high-metabolic-rate tissues than people with more total lean mass. That applies within each sex (i.e. smaller men tend to have a higher BMR per unit of fat-free mass than larger men, and smaller women tend to have a higher BMR per unit of fat-free mass than larger women), and it also applies between sexes.
To illustrate, men tend to have about 30-35% more total fat-free mass than women (the average man has around 60kg of fat-free mass, and the average woman has around 45kg of fat-free mass). But, men tend to have brains that are only about 10-15% larger, kidneys that are only 5-10% larger, and hearts that are only about 20% larger. So, since women have slightly more high-metabolic-rate tissue per unit of fat-free mass, they also tend to have a slightly higher BMR per unit of fat-free mass. On average, women burn around 30 Calories per kilogram of fat-free mass, while men burn around 27.5-28 Calories per kilogram of fat-free mass.
Of course, you may not know your body-fat percentage or how much fat-free mass you have. So, how does sex affect the relationship between total body weight and BMR?
Looking back at the study by Mifflin and colleagues cited above, the researchers found that men burned an average of 166 more Calories than women per unit of body weight. This is reflected by the equations they developed for predicting BMR from height, weight, age, and sex:
Mifflin-St Jeor equation for men:
BMR = 10 × weight (in kg) + 6.25 × height (in cm) – 5 × age + 5
Mifflin-St Jeor equation for women:
BMR = 10 × weight (in kg) + 6.25 × height (in cm) – 5 × age – 161
The equation is the same until the final term – after multiplying weight by 10, height by 6.25, and age by 5, you add 5 Calories for men, and subtract 161 Calories for women, for a difference of 166 Calories.
Other research has taken a similar approach to predicting BMR from height, weight, age, and sex. The difference of 166 calories reported by Mifflin is a pretty middle-of-the-road value. You can find values as low as 77 Calories, or as high as 241 Calories, but an average difference of about 150-200 Calories is pretty typical.
However, the gap is probably a bit larger at higher body weights, and smaller at lower body weights. In a study by Müller and colleagues, researchers found that the gap between men’s and women’s BMR (scaled to body mass) was larger for subjects with an overweight or obese BMI than for subjects with a BMI below 25.
This is corroborated by research by Henry and colleagues (the research used to develop the Oxford/Henry BMR equations discussed earlier in this series) on over 11,000 subjects. At very low body weights, men and women may have BMRs that differ by less than 100 Calories, but the gap grows as body weight increases. When comparing men and women at the same body mass, men burn about 10% more calories than women at 50kg of body mass, about 15% more at 85kg of body mass, and the gap gradually continues growing from there. This gap increases as body weight increases primarily because sex differences in body composition get larger at higher body weights.
So, let’s briefly recap to wrap up this article:
- Fat-free mass is the primary predictor of BMR, and the relationship between fat-free mass and BMR is the same in both sexes. In other words, a man and a woman with the same amount of fat-free mass would be expected to have the same BMR.
- Smaller people generally have higher BMRs per unit of fat-free mass, because people with less fat-free mass tend to have more high-metabolic-rate tissue per unit of fat-free mass. Because of this, women generally have higher BMRs per unit of fat-free mass than men do.
- Per unit of total body mass, men’s BMRs are generally about 150-200 Calories higher than women’s, but the gap is a bit smaller at lower body weights and larger at higher body weights.
Other articles in this series will continue exploring this topic, discussing how age impacts BMR, why athletes have higher BMRs (it’s not just a matter of having more muscle mass!), and how weight gain and weight loss affect your BMR. After that, we’ll explore how we can use all of this information to improve on the (current) best BMR prediction equations. You can also try our BMR calculator, which incorporates all of the information covered in this series, in order to estimate your BMR as accurately as possible.
