In female cynomolgus macaques, the amount of body fat is linked to different changes in gut bacteria depending on whether the animals eat a Western or Mediterranean diet.

We analyzed the available evidence and found that in female cynomolgus macaques, body fat appears to be connected to changes in gut bacteria, but those changes depend on the type of diet the animals eat [1]. Specifically, when these animals consume a Western-style diet — typically high in refined sugars, saturated fats, and processed foods — the relationship between their body fat and gut bacteria looks different than when they eat a Mediterranean-style diet, which is richer in vegetables, fruits, whole grains, nuts, and healthy fats [1]. What we’ve found so far suggests that diet doesn’t just influence gut bacteria on its own — it also changes how body fat interacts with those bacteria. This means two animals with the same amount of body fat might have very different gut bacterial patterns if one eats a Western diet and the other eats a Mediterranean diet [1]. The evidence we’ve reviewed does not say which diet is better, or whether these changes lead to health improvements or problems — only that the connection between fat and gut bacteria shifts based on what’s being eaten [1]. There is no evidence in our current review that contradicts this pattern. However, this finding comes from a single assertion based on observations in one type of primate, and we don’t yet know how this applies to humans or whether the changes in gut bacteria have any meaningful effect on health outcomes. In everyday terms: what you eat may change how your body’s fat and gut bacteria talk to each other — even if your weight stays the same.