Carbohydrate consumption is among the most controversial topics in the field of nutrition. On the one side there are many who still cling to public dietary guidelines, which suggest that we should all get approximately 50% of our calories from carbohydrate, while on the other side there are those who seem to think that the less carbohydrate you consume, the better off you are. I have discussed the science on carbohydrate intake before, most notably in my long article on the topic for Bret Contreras and in my 4-part series on carbohydrate intake. In these posts I highlight several reasons why I think most people are best off with a macronutrient ratio where carbohydrate makes up ~20-40% of the daily energy consumed. This might be classified by many as a low-carbohydrate diet, but the fact is that this intake level could be considered the “norm” if we look at the evolutionary history of our species as a whole. I think it’s more accurate to classify modern, grain-based diets, which typically contain 45-60% carbohydrate, as high-carb diets.
In today’s article I want to further investigate the difference between carbohydrate intake in ancestral diets and modern diets. As we know, different types of carbohydrates can have very different effects in our body, something that is especially important to remember in the discussion of digestible vs. indigestible (to the human host) carbohydrates. Net carbohydrate intake refers to carbohydrates absorbed and digested in the intestinal tract. Since fiber is indigestible (to the human host), it doesn’t count towards the net carb intake.
In Darwinian medicine, a lot of the focus is on the evolution of the human body and what type of diet the human body is best adapted for. Up until the last couple of years, the trillions of microbial travelers that have co-evolved with us were largely ignored, a neglection that we’re now starting to realise has had a profound impact on public health. The human superorganism isn’t well adapted for a refined diet devoid of fermentable carbohydrates. Adequate fiber is important for a number of reasons, some of which are that fiber increases satiety, helps maintain a healthy gut microbiota, and puts the brakes on the insulin response that follows after a high-carb meal.
Modern diets contain too much starch, glucose, and fructose, and too little fermentable carbohydrates
Most of the carbohydrates (e.g., starch, fructose) in a Western Pattern Diet – which is high in refined grains, vegetable oils, and sugar – are digested by host enzymes and absorbed in the small intestine. Only a small percentage pass all the way down to the colon where it is fermented by gut bacteria and turned into short-chain fatty acids. In other words, the net carb intake is almost as high as the total carb intake.
If you’re a hunter-gatherer you simply can’t avoid getting a solid dose of fermentable fibers through your diet every day, unless you rely heavily on animal source food (e.g., Arctic dwellers) and/or honey as your main sources of energy (see why below). A recent paper by Boyd Eaton and Melvin Konner suggests that hunter-gatherer diets generally contain at least 70 grams of dietary fiber/day, which is many times higher than the average intake in most nations today (1). We’re literally starving our microbial inhabitants.
Several factors have contributed to the decline in dietary fiber intake since our days as Paleolithic foragers:
- Uncultivated fruits and vegetables are markedly more fibrous than domesticated versions (1). Domesticated types also contain more sugar, which brings up its own set of issues.
- An overarching theme is that hunter-gatherers processed a lot of the plant foods they ate inside their body, while we today refine and process foods prior to consumption.
- Grains contain less fermentable fiber compared to fruits and vegetables and are also less fibrous on a calorie-by-calorie basis (1).
- We now have easy access to a wide spectrum of calorie-dense, low-fiber food. Hunter-gatherers typically had to consume a greater quantity of food to meet their requirements.
- We have replaced fibrous plant foods with more palatable modern foods.
- We are less physically active today than any other time in human history. One of the implications of this decline in physical activity levels is that we need fewer calories to maintain energy homeostasis, and we thereby also consume less fiber.
Since wild plant foods such as tubers and fruits tend to be very fibrous, a high total carbohydrate intake for a hunter-gatherer doesn’t necessarily transfer over to a high net carbohydrate intake. In other words, we have to take into account that a big bulk of the carbs pass all the way down to the colon where it functions as substrate for the production of fatty acids, meaning that even though the initial macronutrient estimations might have suggested that the person is getting most of his daily calories from carbs, it is actually fat that ends up being the major source of energy. One extreme example of this fermentation process is seen in grazing cattle, which at first sight might seem to eat a high-carbohydrate diet, but actually consume a high-fat diet because complex polysaccharides undergo fermentation in the rumen.
In the modern world, we’ve altered the balance of things. For people eating a highly refined, Western diet, net carbohydrate intake is almost as high as total carbohydrate intake, something that helps explain why inflammation driven diseases are widespread. It’s also imortant to nota that it’s not just those people who are eating “crap” who take in too little fermentable fibers. Due to the reasons mentioned earlier, even those of us who eat a healthy diet can benefit from giving our fiber intake and net carbohydrate/total carbohydrate ratio some second thought.