Can you improve your gut bacteria by spending more time in nature?
Whether spending more time in nature improves your gut bacteria is a question the available studies cannot answer; if you want to positively influence your microbiome, diet and body weight have a far better documented effect.
None of the available studies directly examines whether spending time in nature improves your gut bacteria. Based on these sources, that question simply cannot be answered, and it would be incorrect to suggest otherwise.
What we do know is that environment, diet and early exposure to micro-organisms are associated with large differences in the composition of your gut microbiome. However, the exact contribution of each of those factors remains largely unclear. 'Environment' is also a broad concept and encompasses much more than just contact with nature.
Things that have a far better documented effect on the gut microbiome include breastfeeding in early childhood, diet and body weight. Obesity, for example, is associated with lower bacterial diversity in the gut. Ethnic and racial background turned out, in one large study, to be more strongly associated with microbiome composition than many other measured factors.
It is also worth bearing in mind that 'more diversity' is not automatically better. Some gut bacteria can be harmful: certain E. coli variants produce a substance that damages DNA and has been found in colorectal cancer tumours in humans. And the gut microbiome can convert certain carcinogenic substances into more active, more harmful forms.
None of the included studies directly examines the effect of contact with nature on the gut microbiome. The remaining claims concern other determinants of the microbiome (diet, breastfeeding, obesity, ethnic background) and microbiome-disease relationships.