What you’re getting yourself into:
- 2700 words, 8-12 minute read time
- Most of the major differences in performance and metabolism between sexes can be explained by size and body composition, not biological sex itself.
- Of the true sex differences, the most important ones have to do with differences in sex hormones and fiber types.
- Additionally, females’ fat and muscle tissue is better equipped than males’ for handling both carbs and fat.
- All of these differences make females better metabolically suited for… just about everything related to health and performance except for short, intense bursts of activity that rely on glycolytic capacity.
- If you prefer pictures to words, the highlights of this article are presented in an infographic at the bottom.
So, just for starters, how much of a difference IS there between males and females? Or at least, how large are the physiological differences in major parameters that relate to strength and performance?
Not very large at all.
For starters, males and females are very metabolically similar, at least when looking at metabolic rate. About 90% of daily energy expenditure can be explained by fat-free mass, fat mass, and activity level. Females *do* tend to have slower metabolisms than males, but the difference is primarily a function of muscle mass and body size, not biological sex.
In terms of muscle mass differences, females tend to have about 2/3 the muscle mass males do, with a larger difference in upper body muscle mass (about 1/2) than lower body muscle mass (about 3/4). And although males tend to be stronger than females, that difference is explained *almost* entirely (97%) by muscle mass differences. That means if a male and female have the same size muscles, they should have roughly the same strength.
On the aerobic side of things, males tend to be slightly faster than females with equivalent levels of training. However, the difference is almost entirely explained by body composition differences (males tend to be leaner), hematocrit differences (higher levels of testosterone lead to slightly higher red blood cell counts), and differences in heart size.
So, just to get this out of the way early, the VAST majority of the differences between males and females that are relevant to performance aren’t necessarily sex differences, but rather can be primarily explained by differences in body composition. A female and a male with similar training and similar amounts of muscle and fat will perform similarly. The point of this article is to delve into those differences that DO exist that aren’t explained entirely or almost entirely by size and body composition, and talk about the difference they can make in training and diet.Most differences in performance & metabolism between sexes can be explained by size & body comp Click To Tweet
To discuss metabolic differences, the main source for this article is this recent (absolutely fantastic) review article.
The article starts out with an interesting quandary. Females tend to have about 2/3 the muscle mass and 2x the fat of males, but tend to have substantially better metabolic health. On the surface, you’d expect someone with more muscle and less fat to be more metabolically healthy. However, the numbers tell a different story. In males, depending on the study, rates of elevated fasting blood glucose are 50-100% higher, whole body blood glucose clearance is ~15% slower, and the rate of glucose uptake in muscle is ~30-50% slower.
So the obvious question: Why the difference?
Short answer: Females are more metabolically equipped for just about everything.
Longish answer: Keep reading.
The Role of Estrogen
When discussing sex differences in just about any realm, the first place most people think to look is sex hormones, and for good reason. The majority of the difference is muscle mass is attributable to males’ higher testosterone levels, and a lot of the difference in metabolic characteristics can be explained by females’ higher estrogen levels.
Your muscles have estrogen receptors, and, in fact, there’s good reason to believe that estrogen plays a major role in the beneficial adaptations that occur with aerobic training. When compared to sedentary males, endurance-trained males have 3-5x as many estrogen receptors in the muscles (suggesting they become more sensitive to the effects of estrogen), and it’s been found that, at least in mice, estrogen receptors on mitochondria increase the rate of glucose uptake into the muscle when activated.
Now, I’m sure male athletes reading this are starting to get a little uneasy. The last thing you’d want is estrogen affecting your muscles, right? Isn’t this just another reason to avoid cardio forever? Wrong. Males who are born with abnormalities in the estrogen system (faulty aromatase enzymes or mutated estrogen receptors) are more prone to insulin resistance and diabetes. As long as your estrogen levels are normal, the only major thing that happens due to increased muscle sensitivity to estrogen is improved glucose uptake into your muscle and improved metabolic health.
Another major reason to believe that estrogen is a major player in females’ superior metabolic health is that sex differences in insulin sensitivity don’t arise until puberty (at which time, it decreases in males and increases in females per kg of lean body mass). Furthermore, females’ insulin sensitivity declines again after menopause, but is often improved when they go on estrogen replacement therapy.
However, as with most things, too much can be just as bad as too little. Some studies have shown that females using oral contraceptives have about 40% lower insulin sensitivity than females not on the pill, when matched for BMI, body composition, and physical activity.
(Note: HRT and hormonal contraceptives don’t follow those trends in all cases, and the literature isn’t unanimous. As always, don’t base medical decisions on blog posts. Ask your doctor about your options and the potential risks and benefits.)Women are more metabolically equipped for just about everything. Click To Tweet
So the major takeaway: Estrogen is a good thing for metabolic health, within the normal physiological range. It’s a major reason females are more metabolically healthy than males (and increased sensitivity to estrogen is one reason metabolic health improves in males with endurance training). When it’s too low (like after menopause), when something in the estrogen system is out of whack (like nonfunctional aromatase or estrogen receptors), or when it’s too high, metabolic health suffers.
Difference in Fat
Though females tend to have more fat, there are differences in where that fat is stored, and also the characteristics of that fat.
For starters, males tend to have more visceral fat (fat stored around the organs in the abdominal cavity), and females tend to have more peripheral subcutaneous fat (fat stored between the muscles and the skin). This gives rise to the “apple” and “pear” shaped, or android and gynoid fat distribution patterns.
A major reason that visceral fat is particularly nasty is that it’s more sensitive to catecholamines (adrenaline and noradrenaline), meaning more of it gets broken down and released into the blood stream. Subcutaneous fat goes directly into general circulation, but visceral fat is sent first to the liver. Your liver and your pancreas are the major organs that regulate blood glucose, and the increase in fatty acids sent to your liver from visceral fat can decrease your liver’s insulin sensitivity, which can throw off glucose homeostasis.
Since females tend to have less visceral fat, they’re less prone to fatty acid-induced hepatic (liver) insulin insensitivity.
Visceral fat is also more active in producing inflammatory cytokines as well. Inflammation (and how it’s affected by and interacts with fat tissue) is a big topic, so for the purposes of this article, just be aware that that’s also not a good thing, and we’ll leave it there.
So the fat distribution pattern in females is a more beneficial one, and the fat itself also helps females metabolically.
Fat produces two hormones that positively impact metabolic health: leptin and adiponectin.
Leptin helps suppress appetite and improve insulin sensitivity. Interestingly, although females have up to 4x higher leptin levels, they have greater central leptin sensitivity than males, largely due to the effects of estrogen. However, its effects seem to be mainly central (i.e. altering hunger), at least in humans. Resting leptin levels don’t seem to affect metabolic rate in humans the same way they do in animal models.
Adiponectin is associated with better insulin sensitivity. Depending on what study you look at, females have somewhere between 34% (obese females vs. obese males) and 127% (lean young females vs. matched males) higher adiponectin levels. Adiponectin works by activating AMPK (the AMPK pathway is implicated in many of the positive effects of aerobic training), increasing glucose uptake and fat oxidation in muscle. However, females have fewer adiponectin receptors than males, and a strong correlation between adiponectin levels, AMPK activation, and glucose uptake is only seen in males.
Taken as a whole, though females DO have higher levels of leptin and adiponectin, they probably only play a minor role in the metabolic differences between males and females.
One last little tidbit before we move on from fat differences: Fat tissue absorbs glucose from the blood at roughly 40% of the rate of muscle tissue, meaning that although muscle is a more important factor for glucose disposal, fat tissue does play a non-negligible role. When you culture male and female fat cells in a petri dish, the rate of glucose uptake is higher for female fat cells than male fat cells, which could (potentially, though you shouldn’t put too much faith in in vitro research) also play a role in females’ superior glucose handling.
The most important muscular difference is that females tend to have a greater proportion of Type 1 fibers (roughly 27-35% greater Type 1 fiber area relative to total fiber area) and greater capillary density.
Those are two major factors. More Type 1 fibers and greater capillary density mean better tissue perfusion (ability to get more blood to the muscle to provide oxygen and clear metabolites) and greater capacity for glucose and fatty acid oxidation (because Type 1 fibers are the ones with more mitochondria and aerobic enzymes). Insulin resistance and type 2 diabetes are negatively correlated with Type 1 fiber percentage and capillary density in both lean and obese people.
(As an aside, that’s a major reason why black people – particularly of West African descent – tend to do exceptionally well in power-dependent sports like football and basketball, but also suffer from higher rates of diabetes and heart disease. On average, they have a higher proportion of Type II muscle fibers, which are awesome for explosive sport performance, but not so great for metabolic health.)
So females have a greater proportion of Type 1 fibers and the assistance of higher estrogen levels, which largely explains how their muscles handle glucose better. However, it doesn’t end there. Female muscles also handle fat better, even when comparing female Type 1 fibers to male Type 1 fibers.
Females have roughly 40% higher plasma fatty acid concentrations than males, and they’re able to put those fatty acids to good use. FAT/CD36 is the most important protein for bringing fatty acids into muscles and transporting them to the mitochondria. FAT/CD36 concentrations increase in both sexes as a result of aerobic training, but they’re higher in females regardless of training status.
This is a great thing for cardiac risk factors. After you eat, triglycerides and VLDL (very low density lipoprotein, which primarily functions as a transport vessel for fat) levels increase. They return to baseline faster in females because their muscles can absorb more fat, and do so quicker.
Building off that, females have greater stores of intramuscular triglycerides than males. Now, these aren’t the nasty sort described in the last article, but rather the beneficial sort I briefly mentioned in the footnote.
Just a little aerobic physiology 101 – the greater the proportion of fat you can burn at any given exercise intensity, the better. It spares glycogen, reduces rate of perceived exertion (which is strongly related to glycogen levels), and pushes back how long it takes to “hit the wall.” Most importantly, there’s a strong relationship between how much fat is stored in the muscle (not independent fat cells interspersed with the muscle tissue as the last article mainly dealt with, but fat stores within the muscle fibers themselves) and how readily available it is to use during exercise.
What’s more, it’s not just that females have more intramuscular triglycerides than males, but those fatty acids are also more accessible. Males tend to have a few large lipid droplets, and fewer perilipins (proteins on the outside of the lipid droplets that break down the triglycerides and help transport them to the mitochondria). Females, on the other hand, have more numerous, smaller lipid droplets, and more perilipins. Smaller lipid droplets have a higher surface area to volume ratio, meaning they’re more accessible to perilipins and lipases to break down the stored fat to be oxidized in the mitochondria.
Going a bit further down that rabbit hole, females also have higher levels of the protein Stearoyl CoA desaturase 1, whose role is to (as the name implies) convert saturated fatty acids into unsaturated fatty acids. There is some data to suggest that muscle lipases have a higher affinity for less saturated fats.
So now to the important stuff: how all this actually affects training.
Regardless of training status, females use more fat at any given exercise intensity than males do, meaning that, all other things being equal, they’re more resistant to fatigue.
Conversely, males have a higher glycolytic capacity than females. That means that they can burn through more glucose in the absence of oxygen, which lends itself to better performance for short-intense bursts of effort, but which also means more lactate accumulation and longer recovery times after all-out efforts. This is related to both the higher percentage of Type II fibers, and also higher levels of glycolytic enzymes (glycogen phosphorylase, pyruvate kinase, phosphofructokinase, and lactate dehydrogenase in particular).
Differences in Substrate Use
There are some interesting differences in the proportion of fat and carbs males and females use at different times.
In the fasted state, males and females tend to burn about the same proportion of fat and carbs. However, after eating, females tend to preferentially store more fat and oxidize more glucose immediately, relative to males. When eating isocaloric, high-carb diets (increasing from 55% to 70% over the duration of the study), glycogen concentrations increased in males, but not in females because the additional carbohydrate was immediately used as fuel instead of stored.
In the fasted state, plasma triglyceride levels increase in both sexes, but after a 48-hour fast, muscle triglyceride storage increases in females, and liver triglyceride storage increases in males.
During training, as previously mentioned, females burn a greater amount of fat relative to glycogen at any exercise intensity. However, after training, that reverses. Females then tend to burn an increased proportion of carbs, whereas males burn an increased proportion of fat.
Just to reiterate, sex differences related to acute performance aren’t that huge, and are less a function of sex per se, and more a function of body composition. Furthermore, be aware that everything in this article is representative of trends, but may not hold true when comparing individual males and females, obviously.
Of the differences that do exist, the largest contributing factors are fiber type differences and sex hormone differences. And, in essence, they set females up to be more metabolically suited to just about everything. They clear VLDL and triglycerides better, have better insulin sensitivity, have a more favorable fat distribution, and burn a greater proportion of fat at any given exercise intensity, making them less fatigueable. The only place where males have the edge is in glycolytic capacity and explosive (but not maximal strength) performance (both related to Type II fiber proportion).
So what do we do with all that?
For starters, ladies, do not be afraid of carbs. Not only are they delicious and awesome, but you have better insulin sensitivity, and the more of them you eat, the more of them you burn.
Second, you do not have a harder time losing weight because you’re a female. Yes, you’ll probably have to eat fewer calories than a male who weighs the same amount you do, but the primary factors in determining your calorie needs are body size, body composition, and activity level, with sex playing little to no role. If you’re more jacked and/or more active than a guy who weighs the same as you, then you can eat more than him. If not, you can’t.
Finally, as far as training goes (though we’ll get more into training as this series progresses), odds are pretty good that you can do more work and benefit from more work than a guy can. Your muscles are inherently more glycogen-sparing and fatigue-resistant. You can probably do more reps with a given percentage of your 1rm before fatigue sets in, and do more total work (relative to 1rm) before you hit a wall due to higher proportion of Type 1 muscle fibers, greater proportion of fat being burned instead of glycogen, and lower glycolytic capacity.
So with that, I’ll put a bow on Part 1 of a (planned) 4-part series. This article was to set a basic groundwork with metabolic differences, Part 2 will cover structural differences and delve into training implications much more, Part 3 will mainly be about the menstrual cycle and contraceptives, and Part 4 will cover the female athlete triad.
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