For many years, microbes, e.g. bacteria, fungi, viruses and other microorganisms, have solely been associated with diseases and unhygienic conditions. In response, mankind has spared little effort to eradicate these organisms, be it in everyday life through ubiquitously available disinfectants or in a medical setting, e.g. through antibiotics or antifungal medication. While there are places where a germ-free environment is vital, e.g. an operation room, one needs to differentiate between spaces in which microorganisms physiologically should not exist (e.g. in our bloodstream) or spaces in which they should flourish (e.g. the colon).
Bearing this in mind, in recent years, researchers have found that the microorganisms living on and in our body (i.e. any surface connected to the outside world, e.g. skin, nasal and oral cavities, our gastrointestinal tract et cetera), our microbiota, are, in fact, not harmful to us. On the contrary, they may contribute to our health and well-being and protect us against actual pathogens (by forming a so-called “colonization resistance" [1]). In addition, research has revealed the complexity and diversity of our gut microbiota which may be in part responsible for the development of certain diseases including obesity and type 2 diabetes [2].
Each individual’s gut microbiota
Each gut microbiota is as individual as one’s fingerprint [3]. It is partially heritable and stabilizes in the first 3-5 years of one’s life, with the mode of delivery (vaginally versus C-section) [4], type of infant feeding (breast-fed versus formula), prematurity and early administration of antibiotics all playing a role. Ideally, in regards to our microbiota, a baby is born vaginally which provides an initial contact with vaginal lactobacilli, is being fed with breastmilk [5] which serves as a natural probiotic including hundreds of other nutritious and protective substances and avoids antibiotic-intake or hospitalization.
As adults, our (gut) microbiota remains susceptible to influences from the outside world. For the health of our gut, it is important to create a living environment that is optimal for the species associated with health benefits. One way to do this is by maintaining a diet which helps certain phyla of bacteria in our large intestine to strive.
A diet for us and our microbes
We have a biomass of microbes [6] in our colon of up to 1.5 kg consisting of approx. 160 species of microbes, 90% of which can be assigned to either the phyla of bacteroidetes or firmicutes [7]. Current research indicates that the composition of these two phyla plays a crucial role in the way food is being metabolized and is potentially associated with an individual’s (lean or obese) phenotype thus, in case of the latter, may lead to type 2 diabetes.
While each person’s diet, lifestyle, genetics and social status play a role in the development of obesity, gut microbes of lean and obese people differ greatly. In one study [8], feces of identical twins discordant for obesity were transplanted into germ-free mice. While on the same diet, mice that had received microbes from the obese twin grew to be more obese and developed more body fat than the mice that had received microbes from the lean twin. This may indicate a role for microbiota in regulating body weight. Generally, the feces in obese subjects have shown less diversity in microbiota compared with the microbiota of lean subjects.
Although little is certain regarding the ideal ratio of microorganisms in our large intestine, the by-products of their metabolic pathways have been studied carefully. So-called short-chain fatty acids (SCFA) [9], namely butyrate, propionate and acetate, are associated with certain health benefits, e.g. improved insulin sensitivity, anti-inflammatory potential and inhibition of cholesterol synthesis.
While humans lack the ability to digest dietary fiber, our microbiota metabolizes these undigested food remnants and supplies us with important nutrients, synthesizes vitamins and amino acids, SFCAs and acts as a protective barrier. Consuming sufficient amounts of dietary fiber is therefore essential in order to provide nutrition to billions of microbes in our colon. As a side note, a diet high in (saturated and trans-) fat and sugar has shown to alter microbes and reduces their diversity.
Conclusion
Although it is unclear whether a healthy gut microbiota is the cause or effect of a healthy body, chances are that with a healthy diet, you may not only benefit your health but the health of your microbiota as well. The Dietary Guidelines for Americans [10] currently recommend a daily intake of at least 25 g/day for women and 38 g/day for men which may help with weight reduction, reduce postprandial glucose response and beneficially influence blood lipids. Changes in gut microbiota may occur as early as 24 hours [11] after dietary change and if maintained for a certain period, may stabilize. Future studies will tell how to best target and strengthen the microbes within us. Although we are just starting to understand the impact of our microbiota on our health, keeping in mind that we need to feed them while feeding ourselves will possibly go a long way.