The current American diet lacks proficient dietary fiber intake. Despite current recommendations for fiber intake of 25g per day for woman and 38g per day for men, the average American only consumes ~15g of fiber per day (1,2,3,4). There are various types of fiber that can be consumed from several different sources; however, fiber can be divided into two categories: soluble and insoluble (5). Being that fiber is considered a prebiotic as well, this means that it is actually food to help feed the beneficial gut bacteria. . Food sources high in fiber are also rich in many essential vitamins, minerals, phytonutrients, and antioxidants. Many health benefits are attributed to fiber intake including reducing the risk of cardiovascular disease, diabetes, gastrointestinal disorders, obesity, and metabolic syndrome (6, 7, 8).


General fiber recommendations are based on average calorie consumption whereby 14g of fiber for every 1000 calories consumed is considered adequate (Institute of Medicine). The upper limit of fiber is set at ~50g per day; however, this can change based on personal tolerance (9). Fiber is considered a non-digestible carbohydrate that is resistant to the enzymes in the gut; consequently, depending on the source, fiber has almost zero calories.


Within the classification of fiber there are generally two specific types, which are soluble and insoluble. Soluble fiber is closely related to heart health because soluble fiber passes through the digestive tract attracting cholesterol. In an individual with high cholesterol, this is beneficial as soluble fiber may assist in lowering elevated blood cholesterol. Furthermore, due to the fact that fiber slows digestion, soluble fiber may aid in preventing spikes in blood glucose and insulin, two characteristics of type I and type II diabetics. In addition, soluble fiber absorbs water as it passes through the digestive tract. This absorption of water will bulk stool preventing diarrhea and constipation. In regards to insoluble fiber, this source may assist in weight loss because it may increase satiety. Insoluble fiber may be beneficial to digestive health because it passes through the digestive tract into the colon where it can feed the gut microbiome (10,11).


Among fiber, there is a specific group that has been shown to give benefit to exert benefits on the gut microbiome. This classification of fiber is a prebiotic known as oligosaccharides, which are chains of carbohydrate molecules 3-10 units in length. With consumption of fiber, the largest change in the gut microbiome seems to come from two specific classifications of bacteria in the gut: lactobacilli and bifidobacteria. Although both are abundant within the gastrointestinal tract, bifidobacteria reside mainly in the large intestine and they feed directly off of fermented oligosaccharides (8). In turn, you can look at this like a growing animal. Feed it and it can flourish or if you don’t, it is more prone to sickness, disease, and other complications. Similarly, the feeding bifidobacteria can allow it to increase in number thereby increasing defense mechanisms against pathogens (viral, bacterial, and fungal). Therefore, the dominant bacterial groups of the human gut microbiota may be classified into those that are either beneficial with health promoting effects; harmful or pathogenic; or those that demonstrate both or no effects.



Not only does fiber provide various health benefits in regards to the gastrointestinal tract, but foods high in fiber tend to be higher in vitamins, minerals, phytonutrients, and antioxidants. This is because high fiber foods include whole grains, fruits, vegetables, nuts, seeds, and legumes. Whole grains are superior compared to their refined counterparts because they contain the bran, germ, and endosperm. The bran is the outer portion containing fiber and B-vitamins; the germ is the inner portion containing high amounts of protein, minerals, and dietary fats; the endosperm is the portion between the bran and germ, which primarily contains the starchy carbohydrates of the plant (Whole Grains Council). When a grain is refined, it loses the bran and germ. This causes the grain to lack fiber, essential fatty acids, vitamins, and minerals (8 and 12).

With that being said – in regards to a ketogenic diet – foods such as whole grains, fruits, certain vegetables, and legumes will likely be omitted from the dietary plan due to their high carbohydrate concentration. While obtaining fiber from vegetables, seeds, and legumes will be crucial for maintenance of gastrointestinal health, these foods do contain low amounts of carbohydrates; therefore, eating them in abundance may not be optimal for maintaining ketosis. Supplemental fiber sources such as glucomannan, inulin, soluble corn fiber, fructooligosaccharides, and galactooligosaccharides may be beneficial to include in the diet – in small amounts – to increase fiber consumption.

Fiber has many health benefits due to its impact on the gastrointestinal tract (8). Dietary fiber has shown promising benefits in lowering risk of cardiovascular disease through various mechanisms including improving blood lipids profiles, lowering blood pressure, and improving insulin sensitivity (13). Fiber has been shown in several studies to reduce incidence of myocardial infarction, decrease total cholesterol and increase HDL (good) cholesterol, and reduce high blood pressure (hypertension) (14, 15, 16).

In regards to diabetes, fiber has shown promising benefits in lowering the magnitude of type II diabetes. It is speculated that this occurs through multiple mechanisms such as lowering the glycemic response to food by slowing carbohydrate absorption; improving weight management, which has an indirect effect on maintenance of type II diabetes; lastly, high fiber foods are higher in vitamins and minerals that can help with the maintenance of diabetes (8). One study found that water insoluble fibers (wheat bran and maize amylose) as well as water soluble fibers (carboxymethyl cellulose, guar gum, and xanthan gum) reduced post-consumption glycemic response (17).

Supplemental vitamins and minerals – specifically vitamin C, vitamin E, zinc, and magnesium – have demonstrated increases in HDL cholesterol, a marker of cardiovascular health; and Apolipoprotein A1, a marker of lipid metabolism (18). In addition, there is research linking decreased antioxidant and phytonutrients consumption to a reduction in reactive oxygen species (ROS) (19) Reactive oxygen species are molecules that are produced during normal metabolism of lipids, carbohydrates, protein, and DNA that result in the production of a radical that may cause cell damage resulting in various metabolic disorders (20). Foods higher in fiber tend to have more antioxidants and phytonutrients to combat ROS such as vitamin E, vitamin C, curcumin, flavonoids, lycopene, etc. (19).

There has been extensive research on the effects of fiber and gastrointestinal health. Among these benefits includes increased regularity, short chain fatty acid production, and the previously stated prebiotic and probiotic benefits. Fiber has been shown to increase stool weight due to its bulking properties traveling through the gastrointestinal tract and absorption of water (8). However, different fiber sources have been shown to cause adverse side effects when consumed in high doses. For example, inulin and fructooligosaccharides have been shown to cause gastrointestinal issues and diarrhea in relatively small doses (~5-10g) (21, 22). Furthermore, fibers such as isomaltooligosaccharides were first established as a fiber source. However, recent research from The Applied Science and Performance Institute has shown that isomaltooligosaccharides have a relatively high glycemic index and are almost fully digested – not fitting characteristics of fiber (21). Furthermore, soluble maize fiber and polydextrose has shown minimal side-effects on the gastrointestinal tract (23). In addition, polydextrose and resistant starch have been shown to be tolerable in amounts up to 50g without gastrointestinal symptoms (24).

Metabolic syndrome encompasses diseases such as obesity, diabetes, and cardiovascular disease. As stated previously, fiber has been shown to increase beneficial gut bacteria, which has been associated with body weight management (25). Furthermore, consuming higher fiber foods has a very strong correlation with lower overall body weight (26). An increase in daily fiber intake has been shown to decrease body weight by approximately 2.2 pounds over 12-weeks with no other alterations in diet. This was compared to a group that gained approximately 1 pound over the same 12-week intervention where the subjects did not consume additional fiber. Although speculation, this may be due to the increase in satiety that the fiber caused as well as the suppression of ghrelin and increase in peptide YY, which stimulate and decrease appetite, respectively (27).


Fiber is a very beneficial nutrient that is essential to health through several different mechanisms. This includes increasing the flora in the gut microbiome; providing various micronutrients, antioxidants, and phytonutrients; as well as preventing and reducing symptoms of a wide variety of metabolic disorders/diseases. In regards to a ketogenic diet, due to carbohydrate restriction, fiber sources may be lacking. Therefore, try and allow most of your carbohydrate intake to be primarily from dietary fiber. Be wary of the “high fiber” bars out on the market that contain added fibers like ”isomaltoolgiosaccharides” that may actually get digested and cause a glucose and insulin response. Instead, eating green leafy vegetables is a great way to make sure you are still getting fiber into your diet, even with little carbohydrate intake.


Keto Conclusions

  • Fiber is lacking in the American diet.
  • The ketogenic diet requires adequate fiber intake .
  • Fiber can be divided into two categories: Soluble and Insoluble.
  • Certain fibers can alter the gut microbiome.
  • Foods that are high in fiber also contain many other beneficial micronutrients.


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