Norra MacReady

October 27, 2010

October 27, 2010 (San Diego, California) — Changes in gut microorganisms may play a role in calorie absorption in food and could influence whether someone gains or loses weight, investigators said here at Obesity 2010: The Obesity Society 28th Annual Scientific Meeting.

In a small study of lean and obese men fed high- and low-calorie diets, the ratio of 2 major types of intestinal flora differed according to the diet's energy content, lead author Rainer Jumpertz, MD, explained.

Dr. Jumpertz, a visiting fellow at the National Institutes of Health in Phoenix, Arizona, and colleagues studied 12 lean and 9 obese men who were fed 3400-calorie and 2400-calorie diets in a random crossover design.

For 3 days, the men ate meals calculated to maintain their weight "to keep them metabolically stable," Dr. Jumpertz told Medscape Medical News. Then each man was randomly assigned to either the higher- or lower-calorie regimen for 3 days. This was followed by another 3 days on the maintenance diet, followed by the second calorie-controlled diet, and concluded with another 3 days on the maintenance diet. The experimental diets were matched for micronutrients such as vitamins and minerals, so they differed only in calorie content.

In each diet condition, stool samples from the men were subjected to bomb calorimetry, and calorie absorption was determined by calculating the percentage of stool calories relative to calories ingested. Microbiota species were identified by sequencing bacterial rRNA genes from the fecal bacterial DNA. The 2 major phyla present were Firmicutes and Bacteroidetes.

Calorie absorption did not differ significantly among lean and obese volunteers on either experimental diet, although the lean volunteers absorbed significantly more calories on the 3400-calorie than the 2400-calorie regimen (P = .04), and overall there was considerable variation between individuals.

"We were very surprised to find rapid changes in the gut microbiota upon overfeeding individuals relative to their body size, meaning that the Firmicutes and the Bacteroidetes, which comprise about 97% of the phylae in the gut microbiota, changed rapidly after only 3 days. That was a new finding that hasn't been seen before," Dr. Jumpertz said. Higher calorie absorption was associated with an increase in the presence of Firmicutes, but when calorie absorption dipped, Bacteroidetes predominated (P = .04).

"What we can conclude from our results is that the more we overfeed individuals, the more the Firmicutes are represented: the Firmicutes go up and the Bacteroidetes go down with increasing nutrient load, and vice versa." He noted that these findings support observations by other investigators that Bacteroidetes outnumber Firmicutes in humans after people stay on a low-calorie diet for 1 year.

"This study in man exemplifies the complexities of the relationships between the microbiota, the host, and nutrient intake," said Eamonn Quigley, MD, professor of medicine and physiology at the University College Cork Medical School, in Cork, Ireland. "The story of this relationship is far more complex than initially suggested by animal work, and while this study by no means discounts a role for the microbiota in nutrient absorption and the genesis of obesity, it also illustrates how the microbiota itself may be manipulated by changes in nutrient intake." Dr. Quigley was not involved in this research.

These findings suggest that individual differences in calorie or nutrient absorption may be a result of variations in gut microflora, Dr. Jumpertz said. "I think the most important clinical finding of this study is definitely the association between changes in gut microbiota with nutrient absorption itself. This kind of implicates that a calorie might not really be a calorie for everybody."

Dr. Jumpertz and Dr. Quigley have disclosed no relevant financial relationships.

Obesity 2010: The Obesity Society 28th Annual Scientific Meeting: Poster 201-P. Presented October 12, 2010.

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