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SAN DIEGO -- Gut bacteria changed for the worse over time in black men who were prediabetic, researchers reported here.

Changes in the composition, diversity, and abundance of microbiota were already apparent for those with impaired glucose tolerance and prediabetes, said , at the University of Illinois College of Medicine, who presented the study at a press conference at the Endocrine Society's annual meeting prior to its formal presentation.

"Changes in the gut microbiota occur in the early stage of diabetes development," said Barengolts. "The gut bacteria signature -- the composition and abundance -- could be a useful tool in assessing a person's risk for developing obesity and diabetes."

Changes in intestinal bacteria were measured at 12 months and compared at baseline. The 116 participants were African-American veterans who were all in a vitamin D intervention funded by the Department of Veterans Affairs. They ranged from age 45 to 75, with an average age of 60.

They were put into one of four groups: those with a stable glucose tolerance, those with stable impaired fasting glucose or stable impaired glucose tolerance, those with worsened glucose tolerance, and those with improved glucose tolerance.

There were significant differences in bacterial composition between the first and second groups (P=0.03) at the phylum level. Bacteroidetes was higher and Firmicutes was lower with worse glycemic control in the second group. The Bacteroidetes/Firmicutes ratio was 1.9 versus 0.9 in group 1 and 2 (P=0.01) and 1.9 versus 1.1 in group 1 and 3 (P=0.04).

Proteobacteria decreased over the period in groups 2 and 4 compared with group 1 (P=0.04 for both).

At the family and genus levels, in group 2 versus group 1 there was less Prevotella, and a higher Bacteroides/Prevotella ratio in the second group at 5.6 to 2.7 (P=0.05). There was also less Enterobacteriaceae (P=0.03), and more Ruminococcae (P=0.01) and Veillonellaceae (P=0.02).

"We speculate that lower abundance of Prevotella may be associated with worsening glycemia, and, conversely, higher abundance of Akkermansia might be associated with improving glycemia, thus corroborating suggestions from previous studies," the researchers said.

Previous research has shown a relationship between diabetes and gut bacteria. And, said Barengolts, there are two studies underway, one in Italy and one in China, looking at bacteria transplant as a treatment for diabetes.

"While changes in microbiota have been described in obesity and diabetes, little is known about microbiota composition in various dysglycemic states," said the researchers. "[But] there is emerging evidence that intestinal microbiota is a contributor to the metabolic/glycemic phenotype."

Diet, especially the consumption of probiotics and prebiotics, may lead to healthier bacteria profiles, Barengolts said.

She also noted that it isn't clear from the study in which direction, if any, the causal arrows run. "There is interaction," she said. "But what is the egg and what is the chicken? We are not able to determine."

The study was fairly small, and the results may not be generalizable. But Barengolts said that previous studies have shown similar results for those with diabetes, and that findings are consistent across different populations. "Our genome and our metabolism is predominately similar," she said.

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