Deep within our bowels, there's a flourishing, miniature ecosystem called the gut microbiome - a delicate balance of bacteria that has a big say on our health and digestive system. Now scientists in Belgium have identified 69 different factors - from chocolate to contraceptives - that influence the make-up of our gut microbiomes.

It's not just what we ingest either: sleep, habits, and lifestyle can all have an impact on our internal balance of bacteria as well. The researchers hope that the study will lead to a better understanding of these microbiota, how we keep them healthy and how we treat diseases in this part of the body.

As Jessica Hamzelou at New Scientist reports, some of the findings were expected: the amount of fruit and fibre we eat has long been known have an influence on the health of our gut. Other factors were less well established, such as whether people eat dark, milk, or white chocolate, and the types of hormonal contraceptives they use.

Anti-inflammatory drugs, antihistamines, and the level of alcohol intake were also flagged up in the study as changing the balance of microbiota deep within our bodies. In fact, the factor with the strongest influence turned out to be the shape and consistency of an individual's bowel movements, which is perhaps to be expected.

In the study, 1,100 volunteers were analysed, filling out detailed surveys on their diets, lifestyles and habits, as well as undergoing blood tests. "We've turned these people inside out," said study leader Jeroen Raes. "We compared all the microbiota we could get our hands on."

It's the largest study of its kind carried out so far, and it gives scientists a deeper insight into how our digestive system works. We still don't fully understand exactly what makes for a healthy gut microbiome, but we do know that an unhealthy one can lead to several diseases, including cancer. 

Our gut bacteria is also linked to mental health, and conditions such as obesity.

As the research progresses, further down the line, it's possible that your doctor could tell you to eat more milk chocolate to improve the health of your gut, for example.

However, all of these 69 factors only accounted for around 7 percent of the variations in the microbiome - the researchers think that genetics may play the most significant role overall. Raes says it's also possible that the bacteria themselves shape the patterns of their own ecosystem.

The study is part of a larger piece of research imaginatively titled the Flemish Gut Flora Project, and a total of 5,000 participants will be involved by the end. The biodiversity in our stomachs is so complex that the experts think some 40,000 samples will eventually be required to build up a full understanding of it.

"Analysing the 'average' gut flora is essential for developing gut bacteria-based diagnostics and drugs," says Raes. "You need to understand what's normal before you can understand and treat disease."

The team's work has been published in the journal Science.