I was reading through a muscle study the other day when I noticed something: it only included men.

Then I scanned through the study’s references, and nearly all of them included mostly men or only men.

Hmmmmmm … the hamster wheel in my brain started turning. Strong muscles are helpful for both men and women. Is there something amiss in the research?

This is not a new problem

First, an interesting little factoid:

In 1973, the first clinical trial on estrogen for heart disease prevention was conducted on 8,341 men … and zero women.^[1] It wasn’t until nearly 30 years later that a proper large trial was done on women. The results were surprising, and suggested that the widespread use of estrogen replacement therapy harmed countless women in earlier decades, due to an absence of quality women-specific evidence.^[2]

Did you know that the federal government established a mandate to include more women in trials, and has an office specifically advocating for it?^[3] Despite this, we encounter a shortage of women in nutrition trials all the time, and even more so, an astonishing absence of trials focusing on fit women.

Case in point: when we started teasing apart research for our Fitness Guide, we encountered a very interesting issue - research on women was split into two distinct areas:

Women’s health (some research)

There are hundreds of academic papers covering the complicated realm of nutrition and women’s issues, such as:

• The effect of menstrual cycles on food intake^[4]
• The potential of supplements to reduce PMS symptoms^[5]
• The effects of nutrition of conditions like endometriosis^[6] and uterine fibroids^[7]

Women’s fitness (little research)

On the other hand, there’s incredibly little research specifically done on fit women. You may counter with “who cares? It’s more important to do research on helping sick people than helping really fit people get fitter.”

And that’s a legitimate concern, especially when you factor in that women’s health issues are woefully underresearched from a nutrition aspect, being swamped by a much greater amount of pharmaceutical research.

But … this isn’t a zero sum game, and it doesn’t negate the fact that research on nutrition for fitness is far more expansive for men than for women.

Fit women lost out big time when it came to nutrition research

And they lost out, specifically, to fit men.

As nutrition research started to really ramp up in the 1980s, research labs received more and more money to study fit men, including from government sources and supplement companies. For example, you’ll find many men-only creatine trials, every single year from the 1980s^[8] to 1990s^[9].

But nearly 40 years later, fitness looks way different. Back then, men lifted weights and women (generally) did not.

That no longer holds true. Women are increasingly taking up strength training. Your typical CrossFit gym is chock full of women; women compete directly with men in American Ninja Warrior (side note … this show is awesome); and social media is littered with pictures of strong women with visible muscles.

So where are all the women-only trials, when it comes to nutrition and fitness? Why are there still tons of trials on trained men, like this 2018 trial^[10] testing the ketogenic diet’s muscle and fat effects … in men only? Or this one from 2017^[11], again on keto’s muscle and fat effects, and again … in men only?

There are legitimate reasons for the research scarcity

There isn’t actually a research cabal that refuses to study nutrition for fit women. At least, not one that I’m aware of (admittedly, cabals don’t like being detected).

Take keto as an example. Keto studies on fit people often do include women, although in fewer numbers than men, like in this 2018 study^[12] that included nine men and three women. Part of the issue is that studies are limited by who they can manage to recruit. Trials already take years to get results published: from hypothesis creation, to conducting the trial then collecting and analyzing the data, to writing the paper and waiting for journals, etc etc. So it may be that a ton of men signed up, and not very many women. Another issue is that trials aren’t all federally funded, especially when it comes to supplements. If a supplement company markets its mass-building supplement to men, then funding a supplement trial including only men works just fine for them.

That being said, I have noticed a slight uptick in trials conducted either only in fit women, or including more women than men. But nothing groundbreaking. For example, this tiny study^[13] on keto and body composition included four trained women and one trained man. Baby steps… I guess?

The vast majority of female-only studies are done on overweight or obese women, such as this^[14] keto study. From a public health standpoint, it makes sense to focus on this population. And excess bodyweight could have different physiological impacts in overweight women as opposed to overweight men. But the same thinking also applies to those at healthier bodyweights looking to get fitter: why should fit guys benefit from thousands of trials focusing on samples of fit men, while fit women only get a few dozen trials focusing on fit women?

Another issue: Menstrual cycles

Menstrual cycles are kind of a big deal, not just in real life, but in the research world as well.

If you run a four-week pilot study of a supplement that may impact body weight, suddenly you have extra considerations. How best should you account for women in different phases of the menstrual cycle? If the supplement ends up linked to digestive complaints, could it be interacting with menstrual cycle cramping or bloating? Could some body weight measurements be less telling due to fluid retention?

One issue is that no two women are exactly the same when it comes to menstrual cycle lengths and symptoms.^[15] So you can’t just say that everything will average out, because there is no average woman! Some women may have terrible cramping, others minimal. Some women have heavy periods, some have no periods. This becomes even more of an issue when trials have small sample sizes, so that one woman’s symptoms can skew results more than in a large trial.

But this doesn’t mean there shouldn’t be fitness trials in women. In fact, it means the opposite. Women are unique, and should be able to refer to trials conducted in women! Muscle trials done on trained, fit men shouldn’t simply be presumed to apply to trained, fit women.

What’s the takeaway for you?

If I had a magic wand (or ran a hedge fund), I’d fund the heck out more nutrition research, including trials specifically on fit women. We need to learn way more about things like intermittent fasting in women, carb intakes in women, evolving diets in women of different life stages, microbiome-diet interactions specifically in women, etc etc. But I’m just a nutrition researcher, with, sadly, no hedge fund.

Why is this so important? It’s essential to learn how diets and supplements impact women and men differently. Otherwise we cannot understand what's truly optimal for half the population.

For you, I’d recommend just learning that these issues exist. The more people are aware, the more likely these issues get addressed.

And when someone tries to convince you to take a supplement or follow their diet, adding that “yeah, there’s actually studies on this”, take it with many many grains of salt. Did they read the full text, and analyze the study for methodological quality? Does the study population include people like yourself (e.g. if you’re a fit woman in your 50s, did the study include people like you?)? Research is a helpful tool, but be wary when people waive the word “research” around like it automatically addresses all concerns.

Or just send them to Examine.com.