Our diet is determined by culture, economic status, geography, and perhaps, most importantly, our taste buds. What we choose to include in our diets often dictates our short- and long-term health, for better or for worse. With the rising demand for accurate nutrition labels and conspicuous caloric counts, it’s hard not to feel pressured to aim for a healthy and balanced diet. However, given an average of 3 meals and 1,500-2,000 calories per day, there really isn’t much room to spare if we want to be eating “healthy”. But what actually constitutes a healthy diet? And how exactly do the foods and nutrients we consume contribute to our health? These questions are not always easy to answer, even for nutritionists or scientists. But with an evolving public mindset focused on healthy living, uncovering the effects of diet on our bodies continues to be a hot topic in research with the hopes of one day discovering the proper balance of foods and nutrients optimal for a healthy lifestyle.  

Living in such a health-conscious society, the use of specific diets designed to improve health is becoming more and more common among the general public. Go to any nutrition-based website or blog and you’re bound to find an article claiming to offer the best diet to cut those last 10 pounds off your stomach. Paleolithic, Mediterranean, ketogenic, and vegetarian diets are just some of the many dietary regimens typically suggested by resident bloggers and nutritionists alike. These diets focus on favouring or excluding certain nutrients, such as a decrease in carbohydrates and increase in protein with a Paleolithic diet, or the removal of animal proteins for vegetarians. The health-promoting benefits of diets are usually attributed to changes in the multitude of macro- and micronutrients associated with a shift in food consumption.

One of the greatest downstream beneficiaries of the foods we consume are the bacteria that inhabit our gut, collectively known as the gut microbiome. This complex network of organisms has recently become a popular research topic due to the ability of gut microbes to contribute to and affect disease states both positively and negatively. Gut bacteria can improve our health through a variety of symbiotic functions, including the formation of a niche that prevents colonization by harmful bacteria and the metabolization of nutrients that our bodies are not naturally equipped to process, such as complex non-digestible fibers. As such, these microbes can play a significant role in determining our state of health, warranting at least some caution when it comes to the consumption of food that may alter the gut microbiota in a negative manner.

Remarkably, scientists have known that the gut microbiome could be affected by diet since 1910, where it was observed that transitioning from a protein diet to one based on carbohydrates changed the activity and morphology of intestinal flora seen under a microscope. These days, the gut microbiome is known to be profoundly impacted by one’s diet due to the highly modifiable nature of the system. Changes in bacterial composition occur quickly and typically come in response to the different levels of nutrients present in various diets. For example, consuming foods rich in carbohydrates filled with non-digestible monosaccharide linkages – also known as microbiota-accessible carbohydrates, or MACs – increases the diversity and number of helpful bacteria in the gut, such as Bifidobacterium and Lactobacillus. This can be achieved through the use of prebiotics, which are foods containing an abundance of MACs and other compounds aimed at boosting the growth of these beneficial bacteria in the gut. In contrast, high-fat diets have been shown to increase levels of harmful bacteria, such as those from Proteobacteria, while decreasing bacterial diversity in mice. Saturated fats in particular are the usual suspects of these deleterious effects, whereas unsaturated fats may be similar to MACs, promoting positive bacteria growth and diversity. Interestingly, microbial diversity in the gut is often negatively correlated with chronic inflammatory diseases, making it a useful marker of a healthy microbiome.

Along with diversity, the dietary metabolites processed by bacteria can also dictate the health of the gut. Beneficial bacteria produce short chain fatty acids (SCFAs) from foods consumed by the host. SCFAs, such as butyrate and acetate, are derived from the breakdown of MACs and certain proteins. These metabolites play critical roles in maintaining intestinal barrier integrity and regulating inflammation, and act as a recoverable energy source from non-digestible nutrients. Therefore, foods that produce high amounts of SCFAs, including prebiotics and specific plant or dairy proteins, are excellent inclusions in any diet.

Perhaps the most important role that SCFAs have on the overall health of the host is their effect on the immune system. First and foremost, SCFAs act as suppressors of inflammation in several disease models. SCFAs mediate this effect through their ability to act as signalling molecules in anti-inflammatory pathways and as up-regulators of gene expression via histone deacetylase inhibition. The downstream effects of these pathways include the growth of anti-inflammatory T regulatory helper cells and a decrease in inflammatory molecules and T cell responses. This prevents chronic inflammation in the gut, which is dangerous due to its potential to lead to autoimmune disorders and cancer.

With all this in mind, it is clear that there likely is no combination of foods and nutrients that can produce the perfect body or prevent all diseases. However, many diets do offer a broad range of beneficial effects on health despite the inherent limitations that come with specific dietary restrictions. For example, diets focused on greatly reducing carbohydrates, such as Paleolithic and ketogenic diets, are effective for losing weight, but potentially lose some of the benefits of SCFAs that are abundant in MACs. In contrast, vegetarian or vegan diets are rich in SCFAs and have been found to promote improved health outcomes compared to meat-based diets but require supplementation of micronutrients like vitamin B12 and zinc, which are important for bodily function.

Taking a spin on the norm of excluding certain “unhealthy” foods, the Mediterranean diet instead promotes a balance of food groups and inclusion of a variety of different nutrients. This diet includes many fruits, veggies, and unsaturated fats while limiting the amount of red meat consumed, producing a phenotype that increases bacterial diversity with high levels of SCFA production. Widely considered as one of the healthier long-term diets, the Mediterranean diet passes both the eye and microbe test in terms of health outcomes, perhaps teaching us a lesson that diversity and balance in food groups are what is important when deciding on what to eat.

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