The Difference of Metabolism Between Men Vs. Women

"Scientists reveal a new difference between males and females."

Take it for face value, and it sounds like the making of a terrible joke.

And then it gets worse:

“According to a new study, this difference may play a key role in how females size up males as potential mates.”

Before you start to get wild ideas, I assure you that this study is not what you think it is.

The difference, in actuality, concerns energy expenditure.

Compared to females, males have much greater variability in the energy they burn at rest or on the move.

The Truth about “Females” and “Males”

In this review, the terms “females” and “males” will be used because they’re familiar to people — and make it easier for some to follow the study findings.

But for accuracy, it’s important to note: This research is talking about people assigned female at birth (AFAB) versus people assigned male at birth (AMAB).

Also, you may notice we’re not using “women” and “men” interchangeably with “females” and “males.”

That’s because, in science, sex and gender aren’t the same thing.

Here’s why:

“Females” and “males” refer to a person’s sex — the “biological differences between females and males.”

“Women” and “men” refer to a person’s gender, which includes “the continuum of complex psychosocial self-perceptions, attitudes, and expectations people have about members of both sexes.”

Even this is a simplified explanation.

The reason: Just because a person is assigned female at birth doesn’t mean they’re born with the “XX” genotype.

Likewise, just because they’re assigned male at birth doesn’t mean they’re born with the “XY” genotype.

There are several more possibilities.

What This Means and Why It Matters

Charles Darwin observed in 1871 (based on his book, The Descent of Man) that the males of a species typically have more variability than the females.

Males will show a much more extensive range of possibilities for their biology, physiology, and intellect.

To visualize how this works, imagine you lined up a representative sample of 100 men according to their height — shortest on the left, tallest on the right.

Then you did the same with 100 females, with each female in the line representing a percentile of the female population.

The range of heights from one end of the spectrum to the other would be much greater among males than females.

Height is just one trait in which males have more significant variability than females. Among the others:

• Physical characteristics, like body size, aerobic capacity, and some measures of strength and power
• Cognitive traits, like knowledge, creativity, and decision-making skills (especially for risk assessment)
• Behavioral characteristics, like time, spent sleeping, moving, and sitting

The most variable aspects among males are often the ones most valued by females in mate selection.

In terms of evolution, they were all proxies for access to resources.

For example:

You couldn’t grow tall or strong without eating.

However, a high level of physical activity suggests you are not worried about where your next meal comes from.

Same with creative expression. Someone close to starvation or at high risk of being eaten wouldn’t stop to look at the sky and be amazed.

That brings us to the questions this study asks:

• Is energy expenditure — the number of calories a person burns throughout the day — also more variable in males than females?
• If so, does that mean it factors into mate selection? Or, to phrase it most stupidly, do females dig a fast metabolism?

The Study Breakdown

The study’s authors tapped into a database of adults whose energy expenditure has been quantified with “doubly-labeled water.”

To see if males have more significant variability in metabolic rate than females, they looked at three measures:

• Basal energy expenditure (BEE): Basal energy expenditure measures the minimum number of calories you burn to keep your body alive.
• Activity energy expenditure (AEE): This is the number of calories you burn through all activity, from moving around throughout your day to involuntary fidgeting to intentional exercise.
• Total energy expenditure (TEE): This is the total number of calories you burn daily when all body processes and activities are considered.

The Study Findings

On all three metabolic rate measures, males have more significant variability than females.

One straightforward explanation is that energy expenditure follows from variability in many traits that influence your metabolism.

For example, we know that height and lean mass (muscle, bone, and everything else that isn’t fat) significantly affect how many calories you burn, both at rest and when you’re up and moving.

If those two traits are more variable in men, then males would also show more significant variability in energy expenditure.

But there are other reasons.

The range of metabolic rates observed in this study wasn’t dependent on those other factors.

Even if you compared males and females of equal height, lean mass, fat mass, and age, the males would still have much more variability in BEE, TEE, and AEE.

You can also look at behavioral traits, like how active or passive someone is.

For example, if males have high variability in physical activity, you’d expect TEE and AEE to be highly variable.

And they are.

But so is basal energy expenditure (a measure of the minimum number of calories you burn to keep your body alive).

By definition, BEE doesn’t include any behavioral traits because you aren’t doing any behaviors.

That brings us to a related question:

Why Do Females Have Less Trait Variability Than Males?

A central fact of human biology is the unequal cost of reproductive success.

Initiating a pregnancy costs the male next to nothing in terms of time and energy.

Given enough opportunities, one male could, theoretically, impregnate an almost infinite number of females.

However, completing a pregnancy might cost a female everything.

At a minimum, it’s nine months of gestation and physically traumatic childbirth, followed by a year or more of lactation.

Only females who survive childbirth can pass their X chromosome on to future generations, and even that’s dependent on their offspring living long enough to bring their children into the world.

So, from an evolutionary perspective, the traits that favor female reproductive success would get passed down from the survivors.

The qualities that don’t contribute to that success would disappear from the gene pool.

Males, meanwhile, don’t have any such restraints on their bloodlines.

All it takes is viable sperm.

Of course, some male traits will be more desirable to potential mates. For example, males who offer those traits will have many more opportunities to reproduce than males with less desirable qualities.

But, thanks to “genetic drift,” less desirable traits will get passed on to future generations.

What this means for males is that a much wider variety of traits can enter and remain in the gene pool as long as those traits include the ability to fertilize an egg.

In closing, studies like this make us smarter. They help us understand how we came to be what we are today.

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References:

1. Halsey LG, Careau V, Pontzer H, Ainslie PN, Andersen LF, Anderson LJ, et al. variability in energy expenditure is much greater in males than females. J Hum Evol. 2022 Oct;171:103229.
2. Tseng J. Sex, gender, and why the differences matter. AMA Journal of Ethics. 2008 Jul 1;10(7):427–8.
3. Stanford University. Female & Male, Gendered Innovations in Science, Health, & Medicine, Engineering, and Environment.