One of the most important areas of human nutrition research at the moment is focused on the gut microbiome. This term describes the population of bacteria which live in the human intestine, which comprises more than 1000 different species and tens of trillions of cells. It is estimated that around a third of the make-up of the microbiome is common to all humans, but the remaining two thirds varies in terms of species and the extent of gut colonisation and that, with the number of different permutations of colonists that are possible, is essentially a specific pattern that is unique to each individual. As these bacteria express more than 3 million different genes, their activities within the human gut have a large number of effects on human physiology.

A review article by Milani and colleagues, published in the Journal of Human Nutrition and Dietetics, describes the effects of the gut microbiota on human health. These are highly varied and depend upon the nature of the species involved. Whilst some bacteria are pathogens (eg E. coli, B. fragilis_ and are linked to adverse health outcomes (irritable bowel syndrome, necrotizing enterocolitis, colorectal cancer) others (eg Lactobacilli, Bifidobacteria) are now known to have basic activities that promote health and are a fundamental part of human physiology and metabolism:

• Digestion and adsorption
• Vitamin synthesis
• Immunomodulation
• Inhibition of growth of pathogens

In addition to these functions the gut microbiome is now understood to influence aspects of metabolic function, appetite and influence risk of obesity and diabetes. The nature of the gut microbiome is shaped by a number of different factors. These include genetics, exposure to microbes in the environment and the diet. A lot of the features of the microbiome are set in early life. Babies are born with a sterile gut and initially acquire microbes during delivery (vaginal compared to caesarean deliveries differ in the microbes that babies acquire), breast or bottle feeding, infections and exposure to pets. As we age the microbiome will respond to further infections, rounds of antibiotic treatment and the nature of the food we consume. As set out by Milani and colleagues, there are differences between the microbiomes of meat-eaters and vegetarians and bacterial species respond to dietary intakes of fibre and possibly fats.

If you would like to know more about the role of bacteria in human health, then the review article is short, easily digested and free to access at the Journal of Human Nutrition and Dietetics website.