Specialty diets and prebiotics

By Susan Hewlings, Ph.D., R.D., Director of Scientific Affairs Nutrasource/GRAS Associates

While going on a specific diet or fasting can have many advantages for weight control, food intolerances and overall health, it can also impact your microbiome in both positive and negative ways. Therefore, any time you change your diet or follow a specific diet it is important to do everything you can to maintain or improve gut health. One way to do so is to continue your probiotic and prebiotic intake as best you can within the constraints of the diet. When following certain practices such as intermittent fasting, supplements may be the ideal option since they are less likely to add calories, which is particularly important when practicing calorie restriction.

Calorie restriction, commonly known as intermittent fasting has become a popular dietary intervention. It appears to have gained popularity because many people find it easier just to fast periodically rather than restricting calories daily. Research suggests the associated health benefits are similar and that the main thing is to restrict the calories. Some of these health benefits may be mediated by the gut microbiome. Reduced nutrient intake to the gut microbiome may beneficially disrupt the composition and function of what would typically be a “fed” gut microbiota.1 Therefore, it is thought that fasting may provide a period of “gut rest,” that improves microbiota diversity, gut barrier function, immune and inflammatory response, and production of postbiotic metabolites, such as short chain fatty acids (SCFAs).2 Supplementing prebiotics is said to enhance and support the benefits associated with calorie restriction via their regulation of short chain fatty acids and bile acids.3 In addition, intake of a prebiotic such as inulin may help to benefit appetite management which would help one stick to the calorie restriction.1

Another explanation for the benefit of calorie restriction is thought to be mitigated by the obvious change in macronutrient intake. This is most notable in a keto-diet. The term “keto” or “keto friendly” has become a popular household word and often appears on food labels despite lack of a standard definition. This is in response to the popularity of keto diets for weight loss. While the exact definition of such a diet can vary based on calories and macronutrient composition, the basic goal is lower carbohydrate intake to induce a metabolic state called ketosis.4 Though there are accepted variations, a classic keto diet includes a ratio of fat to carbohydrate plus protein grams of 3:1 or 4:1, or 90% of the energy comes from fat and only 10% from carbohydrate and protein combined mixture.5 This macronutrient ratio induces a state where ketones such as acetoacetate, 3-B-hydroxybutyrate and acetone increase in the blood because of increased fatty acid mobilization. To achieve this state one must lower carbohydrate intake on average below 100g/day. While these diets have been used medically for decades they cycle in and out of popularity in the fad diet media. There is evidence to support the effectiveness of these diets for weight loss and other cardiovascular risk factors.6 However, the long-term health effects are highly debated. Among the many potential negative health effects, the effect of a keto diet on gastrointestinal health is highly questioned, especially considering that diet can rapidly and significantly impact the gastrointestinal microbiome and have negative effects on intestinal health.7  For example,  it has been observed that low carbohydrates intake  decreases substrates available for Bifidobacteria and Lactobacillus an increase in Bacteroides spp.8 These changes do seem to resolve when the host returns to consuming a more balanced diet and therefore appears to be transient. However, for medical purposes such diets may need to be consumed long term. The challenge in defining what negative or positive changes occur is that different results may occur depending on the baseline health of the host. A host seeking a keto diet for resolution of irritable bowel syndrome (IBS) may be starting the diet with dysbiosis and altered microbial composition secondary to the IBS and thus see an improvement because of resolution of IBS and the associated inflammation. While others starting the diet to lose weight but otherwise healthy may be starting the diet with a more balanced microbiome and thus may experience different alterations.9 Another factor to consider is the length of the dietary intervention. For example, one pilot study of patients with Glucose Transporter 1 Deficiency Syndrome (GLUT1 DS) provided fecal samples at baseline and after 3 months of following a keto diet. At 3 months there were no differences in Firmicutes and Bacteroidetes but a significant increase in Desulfovibrio spp. a sulfate-reducing bacteria with potential detrimental effect on gut health. The authors suggested that prebiotic supplementation for patients consuming such a diet may be warranted to offset long term implications of dysbiosis.10 Interestingly, a study was conducted to evaluate the changes in gut microbiota produced by a low calories keto diet and symbiotic supplementation. They reported that while the synbiotics didn’t significantly affect the microbiota changes experienced because of the diet, they did see an increase in short-chain fatty acid producing bacteria and anti-inflammatory bacteria such as Odoribacter and Lachnospira. Additionally, the administration of Bifidobacterium animalis subsp. lactis prebiotics fiber during the diet was significantly associated with the percentage of weight loss and change in glucose, C-reactive protein and lipopolysaccharide-binding protein.11  This is in agreement with other studies which have shown that probiotics combined with prebiotics i.e. synbiotics facilitate weight loss during interventions of various macronutrient distributions.12  This supports the potential benefits of providing supplemental synbiotics or prebiotics when it is necessary to follow a keto diet long term.



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