Between Paleo, Primal, ketogenic, vegetarian, vegan, Mediterranean, and the dizzying array of other dietary approaches, it’s rare for proponents of these disparate ways of eating to agree on much. Cutting back on sugar and refined grains is one area where there’s general consensus, and consuming a fiber-rich diet is another. Yes, even ketogenic diets, with their very small allowance for dietary carbohydrate, can be high in fiber. However, depending on the state of one’s health and digestive function, some fiber sources may be more appropriate than others.  

Leafy greens are a great way for people to add fiber to their diet with a lower likelihood for the flatulence and bloating that often accompany digestion of beans and grains. For those who need to control their carbohydrate intake, leafy greens provide fiber without the substantial glycemic impact of whole grains. Plus, unlike refined grain products that often need to be fortified in order to provide clinically relevant amounts of micronutrients, green vegetables are naturally high in several nutrients such as folate, vitamin K, magnesium, potassium, calcium and iron. (Some may wish to supplement with extra vitamin C during meals that contain greens in order to increase the absorption of non-heme iron.)

The chart below shows the percent daily value (DV) for 100-gram servings of select green vegetables:

Some of the benefits of fiber are attributable to short-chain fatty acids (SCFAs), such as butyrate, acetate, and propionate, which are the byproducts of bacterial fermentation. Colonic bacteria ferment fiber into these SCFAs, each of which appears to have unique effects. Propionate can be converted into glucose via intestinal gluconeogenesis or diffuse into the portal vein, where it can serve as a substrate for hepatic gluconeogenesis. Acetate can enter the systemic circulation (it’s the most abundant SCFA in circulation) and also crosses the blood-brain barrier.

Butyrate seems to have more far-reaching and beneficial effects than acetate and propionate. Butyrate is a fuel substrate for cells lining the colon, and is also a histone deacetylase (HDAC) inhibitor. HDACs are a family of enzymes that regulate specific genes via epigenetic mechanisms. Inhibition of HDACs may have positive effects on gut health and gut-associated immune function. Reduced histone acetylation is a feature of several neurodegenerative disorders, making HDAC inhibition an attractive target for dietary components and pharmaceutical drugs that might be helpful in these conditions. Evidence suggests Parkinson’s disease may have a strong underpinning in intestinal dysfunction, and since butyrate and other SCFAs help support intestinal barrier integrity by influencing tight junctions, perhaps there’s a role for higher fiber diets in patients with Parkinson’s. 

Additionally, although most of the butyrate produced by fermentation serves to nourish colonic cells, some of it enters systemic circulation and may be beneficial to the central nervous system in ways that are not yet completely understood. Butyrate also appears to have immune-modulating effects, which may hint at why many patients report improvement in autoimmune and gastrointestinal conditions after adopting Paleo or Primal style diets, which are typically high in fermentable vegetable fiber. Butyrate increases production of antimicrobial compounds in intestinal macrophages in vivo and downregulates the inflammatory response. It also drives differentiation of monocytes to macrophages via HDAC inhibition.

However, for all the benefits fiber has, we would be remiss if we failed to mention that for some patients, higher fiber intakes may actually worsen constipation and other gastrointestinal problems, so the amount of fiber that’s optimal may need to be determined on an individual basis—not unlike carbohydrate intake. While some individuals thrive on higher carb diets, many do not experience improvement in obesity, metabolic syndrome and associated comorbidities until they decrease carbohydrate intake to a level that helps them maintain healthy levels of blood glucose and insulin. The same may be true for fiber: each of us has our own unique “sweet spot” such that more may be better for some people, and less may be indicated for others.