I have wondered about the role of fiber. I found it hard to get enough in my diet. As I have learned from this article, as long as I am in nutritional ketosis I should not worry.
There are a few dietary recommendations that have been so ingrained in our minds that we accept them without question. The need for dietary fiber and the proposed benefits of a high fiber diet seem to be two of these.
Adults are generally encouraged to consume at least 25 to 30 grams of fiber per day. A well-formulated ketogenic diet (WFKD), while necessarily restricted in its carbohydrate content, can include several classes of foods that provide fiber (e.g., vegetables, seeds, nuts), but on average may only provide about half this amount.
However, the nutritional ketosis resulting from a WFKD causes the liver to produce beta-hydroxybutyrate (BOHB), and BOHB has the potential to replace some of the functions of dietary fiber. Many of the benefits of fiber are attributable to its fermentation by bacteria, which produce short-chain fatty acids (SCFAs) in the colon, especially one called butyrate. SCFA has metabolic properties that are very similar to those of BOHB. Therefore, a well-formulated ketogenic diet may provide many of the benefits of fiber, without a high carbohydrate intake.
Snapshot: Two Pathways to Colonic Fuel and Health
Followers of a Western Diet depend upon fiber to produce butyrate. Butyrate is a preferred fuel to nourish the cells lining the large intestine (colon).
We now know that a ketogenic diet can induce the liver to produce anywhere between 75 to 150 grams of beta-hydroxybutyrate (BOHB) every day. Both butyrate and BOHB can be used by mitochondria as an efficient fuel, and both have anti-inflammatory and epigenetic effects.However, in this inflammation-modulating role, BOHB is more potent.
So, here’s our (Drs. Phinney and Volek) hypothesis: while high dietary fiber plus an optimum microbiome is critical to one’s health when eating a Western diet, this less important for the ketogenic diet. This is because adequate dietary carbohydrate restriction can induce the liver to produce many-fold more grams of BOHB than one could produce as butyrate via colonic fermentation. Thus, the keto-adapted individual might still be able to function without the need for as much fiber.
Strong biochemical evidence indicates that many of the benefits of fiber fermentation in the colon can be replaced by the normal physiological production of BOHB by the liver during a well-formulated ketogenic diet.
What is Fiber? Fiber is composed of various components of plants such as vegetables, grains, legumes, nuts, and fruits that are neither digested nor absorbed in the small intestine but are fermented by bacteria in the colon.
Fiber is often further separated into two categories:
Soluble (absorbs water, increases stool bulk, prone to fermentation)
Insoluble (promotes motility, less prone to fermentation)
Soluble fiber plays a supporting role in colon health, but only if one has an optimized microbiome that produces butyrate. Fiber primarily facilitates the production of SCFAs like butyrate, they are the real stars of the show.
Short-Chain Fatty Acids: Three SCFAs – acetate, propionate and butyrate – are produced in the intestinal lumen by bacterial fermentation of dietary fiber. Butyrate provides many of the health-promoting effects associated with colonic fermentation of fiber.
Butyrate acts as a histone deacetylase (HDAC) inhibitor. HDACs are enzymes that regulate specific genes and can help reduce unwanted cell growth and oxidative stress. BOHB is also an HDAC inhibitor. Both butyrate and BOHB bind to the same cell surface receptor in the colon, which means they can have similar tumor suppressing effects upon binding. This is likely one of the underlying mechanisms by which fiber intake is understood to contribute to a reduction in the incidence of colon cancer.
The Influence of Fiber on Components of Health: Some of the benefits that have been attributed to fiber including: improved gut health and motility, reduced risk of Cardiovascular disease (CVD), improved glycemia and insulin sensitivity, as well as help with weight control. Fiber contributes to these aspects through the action of butyrate.
While fiber is thought to increase gut motility and ease of bowel movements, (butyrate regulates fluid and mineral balance) the effect of minerals like magnesium and sodium, as well as hydration status, cannot be overstated. Stool water content significantly effects motility. This is especially true for those on a WFKD. Careful attention to magnesium, sodium and fluid intakes can improve constipation.
Cardiovascular Health. When we look more closely at the effect of fiber on coronary heart disease (CHD), we need to consider factors such as increased SCFA production and decreased caloric intake, as well as increased antioxidants, vitamins and minerals. These factors are known to influence inflammation, which is increasingly acknowledged to be a driver in the development of CHD. In fact, a sustainable diet that reduces inflammation may prove to be more beneficial for cardiovascular health than fiber.
Type 2 Diabetes and Weight Control. Most well-designed studies have not shown significant, sustained weight reduction that is attributable to fiber in individuals consuming high fiber real-foods.
Interestingly, individuals with Type 2 Diabetes (T2D) have fewer butyrate-producing bacteria in their gut, suggesting a potential protective role of butyrate in this disease. Based on current evidence, it is difficult to attribute a significant impact in improving T2D to fiber alone.
Gut Microbiome. It has been shown that diet can dramatically alter the content of the gut microbiota and the microbiome can affect your health in general.
The gut microbiota is also a potential factor in obesity and T2D as well as auto-immune diseases and other conditions. Therefore, as we continue to learn more about the role the microbiome plays in the modulation of health and disease, we will learn how the composition of the diet can influence these outcomes.
Dietary Fiber in the Context of a Ketogenic Diet. As we look at the proposed benefits of fiber, we can see that for those individuals who are on a WFKD, many of these positive effects are also achieved. We want to emphasize that butyrate appears to be the powerful actor behind fiber. Thus, in the context of a WFKD, the internal production of BOHB by the liver can reduce or eliminate the need for butyrate to achieve the desired health outcomes.
As a result, the need for a high dietary fiber intake may in fact be conditional and based upon the overall and consistent composition of the diet. If an individual is in nutritional ketosis, it may not be necessary to consume a high fiber diet (>25g per day) to achieve the health effects attributed to fiber.
Source: Stephen Phinney, MD, PhD Brooke Bailey, Ph.D Jeff Volek, PhD, RD on March 4, 2019
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