Obesity and its prime bedfellows (diabetes and heart disease) are epidemic.

Obviously a calorie is not always just a calorie. In recent decades, science has been moving fast to discover links to microbes. Enough evidence now shows that the microbiota is different in obese and normal weight people though the details are hazy.

Alert the media! Prebiotics or indigestible complex carbohydrates are potent actors in this drama connecting microbiota and health. For example, complex carbohydrates that increase Bifidobacteria and Lactobacilli, without increasing the deleterious Bacteroides, most likely promote cardio and immune health in obese subjects.

But first a snippet of a review

• The human gut is dominated by the phyla Bacteroidetes and Firmicutes (representing more than 90% of the total bacterial population).
• Exact composition varies depending on genetics, method of birth delivery, mode of infant feeding, and antibiotic exposure. 
• Adult microbiota is generally established by age three but diet and other factors can change it.
• Fermentation converts carbohydrate residues (prebiotics) into short-chain fatty acids (SCFAs); acetate, propionate and butyrate provide extra calories and enhance gut function and appear to reduce risk of many metabolic diseases including obesity.

An excellent review from the Netherlands The Role of Supplemental Complex Dietary Carbohydrates and Gut Microbiota in Promoting Cardiometabolic and Immunological Health in Obesity: Lessons from Healthy Non-Obese Individuals appeared in Frontiers in Nutrition in July 2017.

The paper detailed the prebiotic impact in healthy subjects (or skip ahead for quick summary from authors.)

• Increase in Bacteroides abundance. Interestingly, soluble corn fiber, abundant in subsidized Western processed foods decreased Bacteroides. However, a causal role of Bacteroidetes in obesity has not been proven.
• Decrease of Clostridium abundance has been linked to decreased severity of Type 2 diabetes and obesity however studies reporting the opposite also exist.
• Increase in Lactobacilli abundance yet levels have been inversely associated with obesity.
• Increase in Bifidobacterium upon supplementation; positive effects of Bifidobacterium counts on the host’s health have been well established.
• Increase in Actinobacteria. A few reports linked low abundance to unhealthy controls.
• Desulfovibrio abundance has been reported to be increased in Type 2 diabetes and in patients suffering from ulcerative colitis.

“Based on the data discussed above, it appears that dietary carbohydrate supplementation in healthy individuals seems to be associated with increases in the abundance of Bacteroides, Lactobacillus, Bifidobacterium, and Atopobium, whereas lower levels of abundance were generally observed in Clostridium and Desulfovibrio.”