The Role of Diet on the Gut Microbiome, Mood and Happiness
  "After the diet change, we observed significant changes in measures of anxiety, well-being and happiness, and without changes in gut microbiome diversity. We found strong correlations between greater consumption of fat and protein to lower anxiety and depression, while consuming higher percentages of carbohydrates was associated with increased stress, anxiety, and depression." 

I think most of us would agree that makes sense? Eat like crap, feel like crap?  "We also found strong negative correlations between total calories and total fiber intake with gut microbiome diversity without correlations to measures of mental health, mood or happiness.... inversely correlated with gut microbiome diversity."

I kept reading this second paragraph trying to understand the "without correlations" part. And also the  negative correlations part. And then the full-text has this, which helped I think:   "Furthermore, total calories and fiber had a negative correlation with gut microbiome diversity, and anxiety and depression decrease as the gut diversity increases."