Gaining weight after dieting might also be due to changes in your gut microbiome. A 2016 study in Nature put obese mice on a diet so they would have the same weight and metabolic factors as control mice. Even though the mice lost weight, their gut bacteria composition remained the same. When the formerly obese mice returned to their former diet, they gained weight more quickly than the control mice. The researchers then implanted this mix of gut bacteria into the control mice, and they gained weight more quickly. Even though the study was performed on mice, the study suggests that the gut microbiome remembers the previous weight and might contribute to gaining weight back more quickly.
Not all diets affect your gut microbiota in the same way, and some can be beneficial to your health, according to a 2021 article in the Journal of Personalized Medicine. People who follow a low-calorie Mediterranean diet can increase bacteria that produce short-chain fatty acids and break down carbohydrates. However, this diet decreased Lactobacillus, which is often found in many over-the-counter probiotic supplements and helps maintain balance in your gut. A low-calorie keto diet exhibited similar effects on gut bacteria, but each diet saw changes in specific strains of gut bacteria.