A key differences between the way our ancestors ate and our modern diet is how much fermentable fiber we consume. Fermentable fiber is found in processed or acellular carbohydrates. We all know that vegetables and fruits are important for health. But which ones and why? The new term MAC, or microbiota accessible carbohydrate is now being used. It describes how fiber functions in the colon where the microbiome is housed. There is growing evidence of foods that feed the good bacteria.  For example grain based fibers as in those found in bran are not fermentable. They don’t have the same beneficial impacts.

Why Fermentable Fiber is Unique

Fermentable fiber selectively stimulates a limited number of favorable species of bacteria. Specifically it increases lactobacillus and bifidobacterium. Short-chain fatty acids like butyrate will increase as well. These are crucial in promoting cellular growth and supporting the absorption of minerals. Fermentable fibre increases the acidity of the colon making it less hospitable to pathogens like parasites and fungus. It improves gut barrier function and host immunity. Remember we are the host to these bacteria and when they thrive so do we.

Where to find these Microbiota Accessible Carbohydrates

Our ancestors ate a wide variety of wild plants that were high in these kinds of fibre. The difference with the plants we eat are they are cultivated for higher density of energy and less fibre. We also typically eat about 20 plants whereas they ate over a hundred. MAC’s with high amounts of fermentable fibres can be found in fruits, vegetables, legumes, starchy plants, nuts and seeds.

A Comparison Of Healthy Diets

Archaeological evidence of preserved coprolite deposits from the northern Chihuahua desert show what a typical male adult consumed. The dietary intake of inulin, a prebiotic also known as FOS, was about 135 grams per day. Jeff Leach from the American Gut Project lived with the Hadza. These modern-day hunter-gatherer eat an average of a hundred grams per day of fiber per day. Contrast this with the average daily fibre intake of a North American which is 10 to 15 grams per day.

Jeff Leach has conducted an informal study of modern people who identified with different types of diets. Those in this study have a bias towards eating better than the average modern person yet their fiber intake is still much lower than our hunter gatherer ancestors.

Omnivore 19g
Paleo 25g
Pescatarian 28g
Vegetarian 33g
Vegan 43g.

Keep in mind that those in the study were not assessed for nutrient status or any other markers related to health. What this study indicates is the importance of consuming more vegetables. We know that the liver functions better with a diet higher in vegetables. We now know the microbes thrive on the fermentable fibers and the prebiotics that are found in a variety of vegetables. For any diet where health or healing is the goal, this is crucial.
Recently a relatively healthy and informed patient of mine asked about his intake of veggies. Like many paleo types, he focuses on consuming enough protein, healthy fats and the right supplements. His lab work came back low in lactobacillus and bifidobacterium bacteria which is not surprising.

Reducing processed or acellular carbohydrates is important for a number of reasons. Including cellular carbohydrates like potatoes, legumes and white rice that are typically not considered paleo is beneficial. When cooked and cooled these are resistant starches. Resistant starches are fermentable fibres that are particularly beneficial to the colon. As their name indicates they resist being digested in the small intestine and instead feed the bacteria in the colon. Legumes have the added benefit of stabilizing blood sugar if they are tolerated.

What About Prebiotics?

Prebiotics are another type of fermentable fibre. They cause the good bacteria to multiply so there is an increase over time. The numbers will stay high despite stopping a supplement as long as enough fermentable fiber is in the diet. And we do need to supplement prebiotics as our foods no longer contain adequate amounts.

Prebiotic fibers decrease gut permeability and the toxic overload caused by leaky gut. Increasing your consumption also improves insulin sensitivity, inflammation and liver health. Everyone knows about probiotics but prebiotics are in fact more important. Probiotics don’t impact the gut flora in a sustained way. They are useful for immune response or repopulating specific species. They do not increase number of beneficial bacteria and once you stop there is no increase. This is because many species of probiotics are transient residents of the digestive tract. They don’t colonize. Probiotics do stimulate the immune system and certain strains are better in certain circumstances. On that note some strains can be detrimental in some disease states as well. For example, Saccharomyces boulardii can be good at treating yeast overgrowth or diarrhea caused by antibiotic use.  It should not be used in Crohn’s disease during a flare.

Research Pitfalls

A lack of fiber may explain why studies of high-fat diets show negative results. In these studies, they typically feed rats or humans a high-fat diet, but they do it in the context of a diet that is low in fibre. While this mimics people eating an industrialized diet, it ignores our evolution and the role that fibre plays in mitigating the harmful effect of these fat-fed microbes.