Over the last few years, a growing body of evidence suggests that gut microbial communities play a fundamental role in many aspects of human health and diseases. Considering the influence of such a microbial community on human health and its multiple mechanisms of action as the production of bioactive compounds, pathogens protection, energy homeostasis, nutrients metabolism, and regulation of immunity, establishing the influences of different nutritional approach is of pivotal importance. The very low carbohydrate ketogenic diet is a very popular dietary approach used for different aims: from weight loss to neurological diseases. This article explores the relationship between the ketogenic diet (KD) and gut health, focusing on the potential benefits of probiotics in mitigating some of the diet's adverse effects.
Understanding the Gut Microbiota
The human gut microbiota, that means the types of organisms that are present in an environmental habitat, consisting of trillions of microbial cells and thousands of bacterial species [1]. It encompasses ~10−13 microorganisms belonging to the three domains of life Bacteria, Archaea, and Eukarya and it is involved in several and different functions [2,3]. Microbiome is the collection of the genes and their functions and, due to the new genetic and bioinformatics technologies, the study of the gut microbiome has been radically transformed. Several studies suggest that a mammalian host establishes their core microbiota at birth [7]; the colonization of the gastrointestinal tract by microorganisms, begins within a few hours of birth and concludes around three to four years of age. The composition of the microbiome is influenced by many factors [11] and the stability of the microbiome, reached between two to five years of age, is overlooked by Bacteroidetes, the largest phylum of gram-negative bacteria associated with both beneficial and detrimental effects on health [12,13].
The Role of Gut Microbiota
The microorganisms living in our gut influence the host through the production of bioactive metabolites, which are able to regulate many biological pathways involved in immunity and energy production. Indigested substances, named “microbiota accessible carbohydrates” (MACs), are represented by the walls of plant cell, cellulose, hemicelluloses and pectin and resistant starch; these polymers undergo microbial degradation and subsequent fermentation [3]. The prominent end-products of fermentation in the colon are short chain fatty acids (SCFAs) such as butyrate (C4H7O2-) produced especially by Firmicutes, propionate (C3H502-) by Bacteroidetes and acetate (C2H402) by anaerobes; they represent the greatest source of energy for intestinal absorptive cells. Moreover, specific bacteria have their own ability to produce many neuroendocrine hormones and neuroactive compounds involved in key aspect of neurotransmission, thus, microbial endocrinology interconnects the science of microbiology with neurobiology.
Factors Influencing Gut Microbiota
The variability among people and the adaptability of gut microbiota to substantial changes have permitted the manipulation of various external factors, restoring both the biological functions and richness of microbiota [31]. It has been demonstrated the high fat-high sugar Western diet negatively impacts gut health [36] and a high fat diet is closely related to inflammation [37], however, several studies [38,39,40] suggested the necessity to consider the structure and the function of different fatty acids.
The Ketogenic Diet: An Overview
The very low carbohydrate ketogenic diet (VLCKD) is a dietary protocol that has been used since the 1920 as a treatment for refractory epilepsy [48] and it is currently getting popularity as a potential therapy for obesity and related metabolic disorders [49]. Originally, VLCKD had been used as a treatment for epileptic patients that failed to respond to anticonvulsant medication [55]. Currently it has become popular for its benefits extended to neurodegenerative diseases, metabolic diseases and obesity [45]. These new patterns have been demonstrated as a successful tool able to reduce seizure symptoms, as well as they reveal a similar outcome, with lower side effects, while compared to the traditional regimes of VLCKD [60,61,62]. Due to the MAD and LGIT people may choose in a more flexible way the meal they want to consume, they do not have to calculate the specific ketogenic ratio but they may focus on ensuring sufficient and appropriate fats, both in quantity and quality.
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Ketogenesis and Ketone Bodies
The very low carbohydrate ketogenic diet (VLCKD) share several pathways that have been found during fasting state [64]. Ketone bodies, 3 hydroxybutyrate (3HB), acetate and acetoacetate (AcAc) are produced in the liver through the process of ketogenesis. Ketogenesis takes place especially in the mitochondria of liver cells where fatty acids reach the mitochondria via carnitine palmitoyltransferase and then breaks down into their metabolites, generating acetyl CoA.
Potential Downsides of the Keto Diet
For many people, the keto diet is a drastic change from their usual way of eating as it means eating 70-80% of your calories as fat. In addition, carbohydrates are limited to less than 50 grams per day. It’s not uncommon to experience digestive issues, especially diarrhea and constipation in the initial stages of this diet, resulting from the sudden change in the way you eat. High-fat foods can be harder to digest and move more slowly through your GI tract. This can cause nausea, vomiting, stomach cramps, and diarrhea. These potential side effects of the keto diet can result in unhealthy changes to your gut microbiome - the trillions of friendly bacteria and other microorganisms that live in your digestive tract. The keto diet might cause changes in your microbiome because the microbes in your gut also have to eat. Studies show increasing dietary fat from 20% to 40% (a typical Western diet) leads to unfavorable microbiome changes such as less microbial diversity.⁶ That may affect the health of your digestive tract and immune system in particular. A reduction in microbial diversity contributes to dysbiosis. Dysbiosis means the mix of microbes in your gut becomes unbalanced with more unfriendly microorganisms and fewer beneficial ones. A keto diet is closer to 75% fat, and it too reduces microbiome diversity quickly.⁷ There is very little information on the long term effects of keto on the microbiome, but some of the research that has been done is surprising.
Ketogenic Diet and Gut Microbiome: The Interplay
Only few experimental studies sought the relationship between VLCKD and gut microbiome [47,50,52,53,67,68,69,70] investigating how VLCKD impacts composition and characteristics of intestinal microorganisms. The effects of VLCKD on gut microbiome have been explored in mice and humans with mixed results. Recently, [53] it has been explored the role of VLCKD on gut microbiota related to the anti-seizure effect on mice. In this study, they found that mice, within four days of being on a diet, had significant changes in gut bacterial taxonomy. Moreover, they observed a decrease in subset of ketogenic γ-glutatamylated (GG) amino acids (i.e., γ-glutamyl-leucine) both in the gut and blood. This fact, in turn, had the effect of increasing the ratio of GABA to glutamate in the brain of mice. The ketogenic diet, composed by short fatty acids SFAs, monounsaturated fatty acids MUFAs and polyunsaturated fatty acids PUFAs, has been provided for 16 weeks by Ma and colleagues [51] and it reveals that mice had a variety of neurovascular improvement strictly linked to a lower risk of developing Alzheimer’s disease. These beneficial effects may be connected with the changing on gut microbiota composition, more specifically with the growing quota of beneficial bacteria, Akkermansia Muciniphila and Lactobacillus, which have the ability of generating short chain fatty acids SCFAs. Interestingly, they found a reduction in pro-inflammatory microbes such as Desulfovibrio and Turicibacter. A 2016 study [67] investigated whether or not a VLCKD could ensure benefits in the gut microbiome in murine model of autism. The authors administrated a VLCKD for several days (10-14) observing changes in gut microbiome; they concluded that the VLCKD had an “anti-microbial” effect by decreasing the overall richness of microorganisms both in cecal and fecal matter, and as well as improved the ratio of Firmicutes to Bacteroides species. A lowered firmicutes: bacteroides ratio is common in ASD and the VLCKD, by improving this ratio, was able to enhance ASD behavioral symptoms. A study in children by Xie and colleagues [68], investigated the connection between microbiome and refractory epilepsy in 14 epileptic and 30 healthy infants. Patients with epilepsy demonstrated an imbalance of gut microbiota before starting the VLCKD. The authors found a higher amount of pathogenic proteobacteria (Escherichia, Salmonella and Vibrio), which significantly decreased after VLCKD treatment and an increase of Bacterioidetes both in healthy subjects and in patients.
Impact on Gut Microbiome Composition
One study shows that ketosis might have the opposite effect of a Western high fat diet. Specifically, animal studies show that staying in ketosis can reduce inflammation in the gut. Experts do believe the type of fats you eat on the keto diet play a role in how your microbiome changes. Eating more saturated fats from foods such as red meat or bacon causes unfavorable changes. Artificial sweeteners may also impact your microbiome. Since sugar is off limits on keto, many people rely on artificial sweeteners to satisfy their sweet tooth.Some studies show that using large amounts of these sweeteners contributes to microbiome changes and disruptions in healthy bacteria.
Addressing Gut-Related Side Effects
There are some common gut related effects on the digestive system when the body enters ketosis, namely constipation and nausea. Nausea can occur because of the switch to consuming such a high quantity of high fat foods. In addition, a 2020 study looked at the effects of high dairy consumption and low dairy consumption on the gut microbiome. Fruits and vegetables are the most common dietary sources of fibre, vital for regular bowel movements. When you suddenly stop eating or drastically reduce fruits and vegetables because of their carb content, your body is suddenly missing out fibre. This can cause bowel movements to slow down or stop altogether, resulting in constipation. Constipation is a perfectly normal side effect of the keto diet but that does not mean you need to suffer. There are some less common digestive effects of the keto diet that you want to be aware of. If you experience any significant gut changes, we would suggest easing off a strict keto diet for a while until your body adjusts.
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The Role of Probiotics in Mitigating Keto's Effects
Probiotics are live bacteria and yeasts that provide health benefits when you take them at recommended doses. The bacteria and yeasts in probiotics are the same or similar to those found in a healthy microbiome. Cultured and fermented foods such as yogurt, sauerkraut, and kimchi also provide live bacteria, although not in the same consistent and high amounts as probiotics. Probiotics work best when paired with prebiotics; these are the indigestible compounds found in many plant foods. Prebiotics are a source of food, or fertiliser, for your probiotics. Also, some probiotic manufacturers pair prebiotics with probiotics.
Benefits of Probiotics on Keto
Probiotics promote a healthy gut microbiome linked to digestive health and metabolic balance, and taking probiotics may be especially beneficial for individuals on the ketogenic diet. Probiotics are helpful for many people, those on a high-fat diet may particularly benefit from increasing gut microbe diversity with probiotic supplements. What you eat can have a significant impact on the composition of your microbiome, and increasing dietary fats naturally promotes the growth of certain beneficial bacterial strains that thrive on lipids and fatty acids, such as Firmicutes phylum. Probiotics help maintain a healthy balance of microbes, which is crucial for digestive health and strong immune defenses, and taking probiotics supplements may keep common keto challenges such as keto flu at bay.
Strong Immune System Function
Evidence published in the Journal of Microbiology, Immunology, and Infection found that probiotics play a crucial role in regulating the production and activity of immune cells. A healthy microbiome enhances your immune system’s ability to prevent infections, detect and eliminate potential abnormal and carcinogenic cells, and lower the risk of autoimmune diseases. These findings have been confirmed by a study published by the British Medical Journal that found that Lactobacillus GG supplementation lowers the frequency and severity of upper respiratory tract infections by almost 20 percent.
Support Metabolic Health
Probiotics are closely linked to various metabolic benefits, making them an excellent addition to the ketogenic lifestyle. Beneficial gut bacteria ferment certain foods, which generates large amounts of short-chain fatty acids (SCFAs). SCFAs stimulate the secretion of glucagon-like peptide-1 (GLP-1), an essential hormone needed for balanced insulin control and glucose metabolism. Research published in Diabetology and Metabolic Syndrome found that Lactobacillus, Bifidobacterium, and Clostridium species can improve insulin resistance, a critical marker of metabolic health.
Weight Loss
A study published in Nutrients suggests that beneficial bacteria affect the production and regulation of various hormones needed to regulate appetite, satiety, and energy metabolism, which plays a crucial role in weight management. The authors of the same study found that certain strains of Lactobacillus, such as Lactobacillus acidophilus, Lactobacillus plantarum, and Lactobacillus bulgaricus, showed significant weight-loss effects, especially if taken in combination with Bifidobacterium species. The relationship between a diverse intestinal microflora and balanced appetite-regulating hormone production may also explain the benefits of probiotics for sugar cravings linked to weight loss and weight maintenance.
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Enhanced Fat Digestion
Friendly gut microbes recycle bile salts needed for the breakdown and absorption of fats and fat-soluble vitamins, which is particularly beneficial if you are doing keto. When gut microbes interact with bile released during digestive processes, bile salts undergo conjugation, a chemical process that increases their solubility and allows their reabsorption into the enterohepatic circulation. “More than 90 percent of bile salts are recycled by the gut microbiome,” explains Dr. Berg. “Probiotics can play a significant role in promoting proper fat digestion and nutrient absorption.” Probiotics are also widely available as dietary supplements and a safe and highly effective way to increase your gut bacteria diversity.
Incorporating Probiotics into a Keto Diet
While a good probiotic with prebiotics should be a starting point to protect and enhance your digestive health while on the keto diet, don’t overlook other diet strategies. One of the best ways to combat constipation and support healthy gut microbes is to increase your fibre consumption. Load up on as many low-carb vegetables and fruits as you can each day while staying within your carbohydrate goals. When planning meals and snacks, focus on nuts, seeds, and oily fish such as salmon or sardines. Finally, try to limit your use of artificial sweeteners.
How to find a quality probiotic supplement
To maximize the health benefits of probiotic supplementation, it’s crucial to choose a high-quality probiotic supplement that contains a variety of well-researched microbes, such as Lactobacillus and Bifidobacterium strains. It’s equally as important to opt for a high-potency supplement that contains at least 60 billion colony-forming units (CFU), a measure of how many viable live microbes are present in each serving. Taking probiotics with less than 60 billion CFU per serving may not deliver adequate amounts of beneficial microbes into the gut. In addition, it’s recommended to take probiotics packaged in a delayed-release capsule to ensure the survival of probiotics during their passage through the stomach acid. A high-quality liquid probiotic supplement is an ideal option for individuals who prefer not to take pills or capsules. While most probiotics supplements don’t contain carbohydrates, it’s essential to check the label for fillers or other added ingredients that may contain carbs.
Probiotic Foods that are Keto-Friendly
On a keto diet most types of fruit are not an option as they contain too much carbohydrate. Berries though are your friend. When it comes to vegetables, think above ground and green. Not only are green leafy veg packed full of vitamins and minerals, but they also tend to be lower in carbohydrate than root vegetables - perfect for the keto diet. Other probiotic foods that are keto friendly include fermented foods such as kimchi and sauerkraut, made from fermented cabbage.
Prebiotics
Prebiotics, the food the beneficial gut bacteria thrive on, are another healthy addition to a keto diet. Prebiotic fiber will help to keep your bowels regular as well - very important when on a keto diet! Also try to include high-fiber fermented foods such as kimchi and sauerkraut in your diet. The sugars in these foods have already been broken down in the fermentation process, so they shouldn’t affect your ketosis.
KD in Pediatric Age Groups
Since the beginning of the last century, the KD has been widely adopted as a treatment for epilepsy and is still a recommended therapeutic option for refractory epilepsy in pediatric age groups [17]. However, the specific mechanisms that lead to better seizure control with the KD are not yet fully understood, and several hypotheses have been proposed [18]. Various authors have suggested that the KD may affect neurotransmitter levels involved in seizure onset. The activity of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), for instance, might be enhanced in patients who undergo a KD, both through the activation of glutamic acid decarboxylase and by inhibiting transaminase activity. It has been supposed that the glycolytic adenosine triphosphate (ATP) production enhanced by the KD, and the reduced brain glucose utilization, induce ATP-sensitive potassium channels, increasing the epileptic threshold and then reducing seizures, as has been found in mouse models [19,20]. Therefore, the KD-induced elevation of blood ketone bodies and fatty acids could potentially modulate neuronal membrane excitability through the activation of two-pore domain potassium channels [21]. This mechanism represents a plausible anticonvulsant effect of the KD.
Obesity
During the past decades, obesity prevalence has increased among children, reaching almost 20-30% in several countries [31,32]. Such epidemiologic changes have led to the development of different diet programs, including KD. Despite the concern that a low-carbohydrate, high-fat, and high-protein diet might be unhealthy for obese children, several trial have proven its efficacy in obesity treatment [33,34,35]. In fact, different meta-analyses reported in the literature have shown the benefits in terms of weight loss of a low-carbohydrate diet compared to a low-fat one [36]. Nonetheless, reducing carbohydrate intake has also proven to lead to a greater reduction of triglycerides, cholesterol, and diastolic blood pressure levels, while increasing HDL [33]. Such benefits could have been achieved through different mechanisms promoted by KD, including an appetite reduction related to higher levels of glucagon-like peptide 1, cholecystokinin, and ghrelin (so-called “satiety” hormones), or perhaps caused by the suppression of the appetite promoted by ketones themselves [9,10].
Cancer
Despite its relatively rare incidence (400,000 children and adolescents from 0-19 years-old per year worldwide), cancer represents a leading cause of death and a major concern in pediatric age groups [39]. However, different efforts made by researchers worldwide have led to surprising results: the 10-year survival rate for leukemia in children increased from 27% to 81% in the past 30 years [40]. By reducing glucose levels, KDs limit the metabolism of cancer cells, which cannot instead process ketone bodies [41]. Such deprivation could also impair glucose-dependent signaling. Moreover, low glucose levels induced by KD lead to a suppression of the lactate/pyruvate cycle, which in turn blocks neovascularization, activation of epidermal growth factor induced by hypoxia, and angiogenesis [42]. Finally, increased levels of ketone bodies could inhibit NLRP3 inflammasome, therefore limiting inflammation, which plays a pivotal role in cancer pathogenesis [43].
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