A study on the effects of prebiotics on satiety and hunger came across my radar the other day. It had an interesting finding — that increased fiber had opposite satiety effects in men and women:
Over the remainder of the day, the high dose of scFOS reduced food intake in women, but increased food intake in men, suggesting a gender difference in the longer-term response.
I filed it away as interesting. Do women somehow extract more energy from fiber than men? Maybe.
But then, today, I came across a piece of info that I’m familiar with, but never really connected to anything: gender differences in colonic transit time. In short, it’s been found that women generally have a markedly longer colonic transit time. Average transit time for men: 33 hours. Average transit time for women? 47 hours.
That’s quite a difference. This study showed that when men and women are placed on an identical diet, women have lower stool weight and longer transit time. This study also confirmed longer transit time in women. And this study showed that the gender/transit time discrepancy was due to colonic transit time, and also that menstrual cycle was not the reason for the discrepancy (as was previously thought).
As we all know by now, the colon is where all the microbiota and fermentation action is. If we were to draw some kind of hypothesis from this, this one would be the most obvious: perhaps women derive more energy from fiber. The longer fiber remains in the colon, the longer the microbiota have to break it down, and thus produce more short-chain fatty acids. And if women do indeed derive more energy from fiber, then, just maybe, there’s a reason women might be built to do that? Do women need to support a larger/more diverse gut microbe community?
If that’s the case, we might observe something indicating increased fiber preference from females in the anthropological record. Do we?
We’ve talked about the Hadza quite a bit here — they are an African tribe and one of the last remaining true hunter-gatherer groups in the world. And they are the major subject of inquiry by the American Gut project. We’re going to learn a lot about the human microbiome through that work.
A lot of study has been done into the foraging and food preferences of the Hadza. Their diet is highly seasonal, and relies on a handful of staples whose availability changes season to season. We’ve discussed previously their strong reliance and preference for honey. Honey is the Hadza’s top-ranked food, and in some months out of the year, it accounts for up to 70% of their energy intake. The reason for honey being their top preference is quite clear: it is the most energy dense food in the world. As such, the Hadza’s second-ranked food is meat. Third is baobab fruit. Fourth is berries.
If we’re just talking about men, that is.
One of the most fascinating things about Hadza food preference research is the fact that women rank berries above meat. For women, berries are the second-ranked food. Not only that, but meat ranks fourth — after baobab fruit. That seems significant. Berries and baobab are not very energy dense — they are largely composed of dietary fiber. It seems that, after honey, men continue to prefer energy density, while women shift to a preference for fiber.
What’s more, the overall eating pattern of Hadza women shows that while men tend to consume infrequent but high energy foods, women eat far more frequently than men, and are consuming “low quality” (read: lower energy density, higher fiber) foods throughout the day. To me, that sounds like a diet geared toward deriving energy from a continuous, steady fermentation of fiber from colonic microbiota.
Hadza men, on the other hand, are largely tasked with the job of hunting game — leading to an obvious situation in which they kill, prepare, and consume meat (and are the exclusive consumers of organ meat) much more often. Does this point to an evolved gender dimorphism, in which women have a greater capacity for energy harvest from fiber, an enhanced gut microbiota, and a greater need for fiber?
And lastly, this all seems to comport with studies of gender food preferences in modern, Western populations: research has shown that men have a greater preference for meat, while women have a greater preference for fruit and vegetables. On their own, these studies might not exactly be perfect, given the role of culture and other factors, but the fact that they line up with what we find in a non-industrialized, hunter-gatherer population is relevant, I think.
So that’s all pretty fascinating. And adds quite a bit of fuel to this women/fiber hypothesis fire.
Now I’ll go ahead and heap another log. A pretty speculative one.
As you should know by now, a major area of interest for this blog is the role that the human microbiome might play in chronic disease. And there’s a pretty specific class of chronic disease that is particularly under the microscope. Given that we know what an important role microbiota play in our immune system — they are major regulators of immune activity, and about 70% of our immune system is considered to be in our gastrointestinal system — there’s no wonder that autoimmune diseases are at the top of the list when looking at the microbiome and human disease.
Now, let’s see. If women have a greater need for dietary, fermentable, prebiotic, gut bacteria-feeding fiber, would it perhaps stand to reason that they might suffer disproportionately from a modern Western diet devoid of that? Ummm:
Taken together, autoimmune diseases strike women three times more than men. Some diseases have an even higher incidence in women.
The fact that women have enhanced immune systems compared to men increases women’s resistance to many types of infection, but also makes them more susceptible to ADs.
All very interesting.