A barrage of public information on the importance of microbes to human health has been spurred by human microbiome research. Although this research has primarily identified associations, and not causal relationships, between specific microbes or microbial patterns and health or disease, it is already clear that (1) microbes are important at an early age, perhaps even prenatal, to healthy human development; (2) disruption of colonization processes, especially through antibiotics, can have negative outcomes; and (3) diet impacts microbial colonization (Maukonen and Saarela, 2015).
One response by the public to this information is a growing interest in microbe-containing foods (fermented foods and probiotic foods) and their role in a healthy diet. Observational studies have suggested that some fermented foods, primarily fermented dairy products, are associated with health benefits, such as less weight gain (Mozaffarian, et al. 2011; Wang et al. 2013), reduced risk of cardiovascular disease (Sonestedt et al. 2011), reduced risk of stroke (Dalmeijer et al. 2013) and decrease in diabetes risk (Sluijs et al. 2012). But conclusions from observational studies are limited, primarily because causality cannot be determined from these studies. Therefore, the role of live microbes in such health benefits remains to be robustly demonstrated.
However, humans evolved consuming high levels of microbes, through their consumption of raw foods and their microbe-rich environments. Modern foods are comparatively lacking in live microbes, due to different pasteurization and processing steps during production.
To understand if adding microbes back to our diet is beneficial, knowing what level of live microbes people are consuming would be useful baseline information. To address this, Lang et al. 2014 conducted a study evaluating the microbial content of three different isocaloric model diets: a diet focused on convenience foods; the USDA recommended diet, emphasizing fruits and vegetables, lean meat, dairy, and whole grains; and a vegan diet, excluding all animal products.
The researchers found that the USDA recommended diet delivered the most microbes, at about 1 billion per day. The other diets contributed significantly fewer microbes, in the millions. In microbial terms, these are very low levels. The USDA diet contained yogurt, due to the recommendation for dairy products. This is the likely factor for the higher levels of microbes consumed in the USDA diet.
Adding fermented foods—that have not been processed to kill or remove live microbes—to your diet is one way to increase consumption of live microbes. Fermented foods include kimchi, raw sauerkraut, fermented vegetables and cultured dairy products. Dairy products such as yogurt, kefir, buttermilk and aged cheeses contain high numbers of live microbes. Many yogurts on the U.S. market contain added probiotic bacteria (such as Lactobacillus and Bifidobacterium species) in addition to the yogurt starter cultures. Kefir typically contains several different types of lactic acid bacteria and yeast. Aged cheeses can also contain a diverse range of live microbes. If you are interested in learning more about microbial diversity in fermented foods, check out a recent study that applied modern gene-based techniques to assess the microbiome assessment of fermented foods (Mayo et al. 2014).
Many fermented foods on the mainstream grocery store shelves do not contain live microbes at the point of sale. Baking sourdough bread, filtering wines and beers, pasteurization of sauerkraut and pickles are all processing steps that kill or remove live bacteria. As such, it may be necessary to look around to find markets that sell microbe-rich fermented foods. Or, you can make them yourself.
We may not know exactly what benefits we obtain from consuming fermented foods containing live microbes, but they may be nutritious additions to a healthy diet, are safe to consume, may provide a fun activity for the home kitchen and just may contribute to our immune, digestive and metabolic health.