In the United States, we tend to get less than 20 grams of fiber a day, only about half the minimum recommended intake. Compare that with populations where many of our deadliest diseases were practically unknown, like rural China and rural Africa: They were eating huge amounts of whole plant foods and consuming up to a 100 grams or more of fiber a day, which is what we estimate our Paleolithic ancestors were getting, based on dietary analyses of modern day primitive hunter-gatherer tribes and by analyzing coprolites, or human fossilized feces. In other words, paleopoop.

“These most intimate of ancient human artifacts [were often] ignored or discarded during many previous [archaeological] excavations,” but careful study of materials painstakingly recovered from human paleofeces says a lot about what ancient human dietary practices were like, given their incredibly high content of fiber in undigested plant remains. It strongly suggests that for more than 99 percent of our existence as a distinct species, our gastrointestinal tract has been exposed to the selective pressures exerted by a fiber-filled diet of whole plant foods. So for millions of years before the first stone tools and evidence of butchering, our ancestors were eating plants. But what kind of plants?

One way you can tell if animals are natural folivores or frugivores is to map the area of absorptive mucosa in their gut versus their functional body size. Folivores are those meant to eat mostly foliage or leaves, while frugivores are better designed to eat fruit. (The faunivores, which is another name for carnivores, eat the fauna.)

If animals are charted this way, they fall along distinctive lines. So where do humans land? In my video Paleopoo: What Can We Learn from Fossilized Feces?, I show a fascinating chart that maps where humans fall on this spectrum. Based on our functional body size and absorptive area, while eating our greens is important, it appears the natural dietary status of the human species is primarily that of a fruit-eater.

Why does it matter how much fiber we used to eat? One theory for the rising levels of obesity in Western populations is that the body’s mechanisms for controlling appetite evolved to match how many plants we used to eat. Our ancestors ate so many plant foods we were getting about 100 grams of fiber a day. So for millions of years, food equaled fiber. No surprise then that one of the physiological mechanisms our body evolved to suppress our appetite involved this fiber.

Fiber is metabolized by our gut flora into short-chain fatty acids, which bind to and activate receptors on the surface of our cells that alter our metabolism. For example, activating receptors on fat cells increases the expression of the weight-reducing hormone leptin, and other hormones are affected as well. Until recently, food meant fiber and an increase in food intake meant an increase in fiber intake. This made our gut bacteria so happy they made lots of short-chain fatty acids, which activated the cell-surface receptors that released a bunch of hormones that made us lose our appetite and down-regulated hunger. So we ate less. But if we ate less, there was less fiber in our gut, which meant that less of those hormones were released, boosting our appetite and causing us to get hungry and want to eat. So fiber effectively regulates our appetite.

But what if all the sudden food doesn’t equal fiber, like on the standard American diet? Then we just keep getting these signals to eat, eat, and eat since there’s so little fiber. We’re always hungry. If we haven’t eaten our 100 grams of fiber for the day, our body may wonder if we’re starving.

Discovering this mechanism makes the food and pharmaceutical industries very excited. They figure they can now come up with new drugs in the fight against the current obesity onslaught.

Or we could just eat as nature intended.

Isn’t that a really fascinating mechanism? All along I was thinking of fiber more from just an energy density perspective (as in my video Eating More to Weigh Less), but the appetite-suppressing hormones are a whole new frontier. That underscores the urgency of the fact that 96 percent of Americans don’t even reach the recommended minimum intake of fiber, as I discuss in Do Vegetarians Get Enough Protein?.

Other paleo videos include:

For more on bowel function in the modern age, see:

My other videos on fiber include:

And to learn more about what your gut bacteria can do for you, check out my videos:

In health,

Michael Greger, M.D.