Fiber in a Keto Diet
What is fiber? Fiber, a form of carbohydrate, is non-digestible in humans. It is prevalent in many vegetables and fruits.
You may have heard that fiber helps relieve symptoms of constipation and other digestive tract issues. That’s true for most people. [Erdogan 2016] [Chmielewska 2011] [Badiali 1995]
But unless you’re suffering with symptoms of constipation, this is the likely the least important reason to include fiber in your diet. Here are some reasons that we think deserve more consideration.
Dietary fiber reduces the body’s glycemic response to meals
Because fiber is non-digestible, foods high in fiber have no negative impact on blood glucose provided they are not otherwise high in digestible carbs. [Hervik 2019] [Thompson 2017 – USA] In fact, research indicates that high-fiber foods actually have the effect of lowering the glycemic impact of a meal in non-diabetics and both type 1 and type 2 diabetics [Thompson 2017– USA (Review of 12 controlled trials)] [Basu 2019 – USA (Epi CACTI Cohort)] [Potter 1981] [Silva 2013 – Systematic Review of 11 Randomized Controlled Trials].
Higher mean blood glucose concentrations in both fasting and postprandial states are associated with impaired insulin production and with weight gain. [Abdul-Ghani 2008] The objective of a ketogenic diet is to reduce insulin secretion and accordingly keep blood glucose levels at bay. [Gershuni 2018]
Accordingly, the impact of fiber in lowering insulin secretion and blood glucose concentration is an important element of an effective ketogenic diet plan that should help Keto Dieters maintain a state of nutritional ketosis.
Fiber-rich foods are the best sources of many essential micronutrients
Avocados, spinach, broccoli, broccoli rabe, cauliflower, cabbage, Brussels’ sprouts, and greens are all high in fiber but also high in both potassium and magnesium. Only 2% of Americans get a sufficient amount of potassium in their diet and less than half of Americans meet the minimum requirement for magnesium.
It would be difficult to obtain sufficient amounts of these micronutrients if high-fiber foods, such as those mentioned above, were excluded from your diet. The carbohydrate content of these super-foods is primarily fiber which should not be included when computing net carbs on a Keto Diet. Thus they are not only a permissible part, but also an essential part of a healthy Keto Diet.
Fiber-rich foods might reduce the risk of many diseases
A diet higher in fiber has been shown to reduce the risk of earlier death, cancer, diabetes, and heart disease. [Yang 2015 (All-cause mortality)] [McRae 2018 CRC (Colorectal cancer – 19 cohort studies)] [Aune 2011 (Colorectal cancer – 25 cohort studies)] [McRae 2018 Diabetes ] [ Liu 2002 (CVD)]
These studies are epidemiological studies which lack sufficient strength to formulate a dietary recommendation. It should be noted, however, that the relative risk of the noted associations ranged from around 25% to 75% and there was a consensus among the studies. That’s pretty convincing, even for epidemiology. And there is now some cutting-edge research in the form of both laboratory studies and human trials indicating that fiber, which is the primary source of energy for the human gut biome, may play a crucial role in deterring the onset of many diseases, including those mentioned above.
Fiber is food for the Gut Biome
The microbiome, sometimes referred to as the “microbiota”, plays a crucial role in mineral and micronutrient absorption, enzyme synthesis, endogenous production of vitamins and amino acids, and short-chain fatty acids (SCFAs). The fermentation byproducts of acetate, propionate, and butyrate are important for gut health and provide fuel for epithelial cells, strengthen the epithelial barrier, and provide protection against pathogens.
There is a growing body of laboratory and clinical research showing that lack of homeostasis in the microbiome, called dysbiosis, might play an important part in the pathological foundation of many major illnesses, including cancer, diabetes, and other autoimmune diseases such as Hashimoto’s and rheumatoid arthritis. [Desai 2016] [Kovatcheva-Datchary 2019]
What does this have to do with fiber, you ask? Although dietary fiber isn’t digested by humans for energy, it is the primary fuel for trillions of bacteria residing in the gut biome. There is now compelling scientific evidence that a diet deficient in fiber can lead to the aforementioned dysbiosis that is related to so many major diseases. [Desai 2016] [Birchenough 2019]
In a recent review published in 2020, researchers at Oslo Metropolitan University in Norway, analyzed 6 randomized controlled human trials investigating the effect of dietary fiber on gut microbiota and metabolic regulation. The researchers concluded, “Taken together, the papers on which the current review is based, suggest that dietary fiber has the potential to change the gut microbiota and alter metabolic regulation.”
There is even research now indicating that dysbiosis could be a factor in metabolic dysfunction that can lead to weight-gain or interfere with weight-loss. [Menni 2017]
New research on impact of high-fiber cruciferous vegetables
There is also a growing body of evidence showing the benefits of consuming foods containing phytochemicals that can prevent chronic diseases, including cancer. Sulphur-containing compounds, called isothiocyanates (ITCs), that are found in abundant quantities in high-fiber cruciferous vegetables, are among this group of promising phytochemicals. Isothiocyanate, and especially sulforaphane, both derivatives of the compound glucosinolate, occurring naturally in these green leafy plants, are biologically active compounds capable of mediating the anticancer effect of cruciferous vegetables.
Recent studies have demonstrated that eating cruciferous vegetables is associated with reduced risk of developing various forms of cancers through metabolic activation and detoxification, mediation of inflammation, angiogenesis, metastasis, and regulation of activity in the human epigenome. [Mitsiogianni 2019] [Soundararajan 2018] [Arumumgam 2018] [Priya 2013] [Clarke 2008]
British researchers Mitsiogianni et al stated, in their 2019 publication, “In the context of human malignant melanoma, a number of studies suggest that ITCs can cause cell cycle growth arrest and also induce apoptosis in human malignant melanoma cells.”
Fortunately, all of these cruciferous vegetables, although high in fiber, are low in digestible carbohydrates and therefore do not have a negative impact on blood glucose concentration. They are, therefore, an important part of a properly constructed Keto Diet.
Various factors related to preparation and cooking can affect the bioavailability of these important phytochemicals. Learn more about these super-healthy foods and how to prepare them to insure you get the most out their nutritious potential.
Fruit, Legumes and Grains for Keto Dieters?
Not all high-fiber vegetables are a good addition to a Keto Diet. Grains and legumes, although moderately high in fiber are also high in digestible carbs that can inhibit dietary ketosis. Most fruits are also too high in glucose, sucrose or fructose to be legitimate candidates for a Keto Diet. Strawberries, raspberries, and blackberries can possibly be consumed as part of meal provided the net carb content of the meal is otherwise sufficiently low.
Use our Carb-o-Rater to see the net carb content for all popular vegetables and fruits. Keep your net carbs at any single meal below 8 to 10 grams and keep your daily cumulative net carbs below 20 to 25 grams to keep your body producing those ketones.
The takeaway
The scientific literature overwhelmingly shows that there are many health benefits to including high-fiber cruciferous vegetables in a well-designed diet, including a Keto Diet. Because they improve glycemic control, they also help keep Keto Dieters in that much-valued state of ketosis.
