From LA Times:
A report published in the new journal Science Translational Medicine has made an interesting discovery concerning the relationship between sugar intake and the balance of intestinal flora. Researchers have discovered that a diet high in sugar and fat substantially alters the bacterial composition in the gut, making it difficult to maintain a healthy weight.From NIH 2014 "Obesity as a Consequence of Gut Bacteria and Diet Interactions":
At the experimental level, analysis of fecal samples revealed a potential link between obesity and alterations in the gut flora (drop in Bacteroidetes and increase in Firmicutes), the specific gut microbiome being associated with the obese phenotype. The sparse data in humans supports the fact that fat storage is favoured by the presence of the gut microbiota, through a multifaceted mechanism.
It results from a long-term disbalance between energy intake and expenditure, that is, increased consumption of more energy-dense, nutrient-poor foods containing high levels of sugar and saturated fats in combination with reduced physical activity. Obesity is now characterized by a cluster of important chronic metabolic disorders, including insulin resistance, type 2 diabetes, fatty liver disease, atherosclerosis, hypertension, and hypercholesterolemia, and by a low grade of systemic inflammation , being the cause of exacerbation of all the above and leading to increased morbidity and mortality.There's a lot of good studies referenced there, you might want to read the NIH study.