The findings may be important, because measuring populations of these bacteria and other microscopic organisms in the intestines could help spot people at risk for obesity and diabetes, or the bacteria might offer targets for treating these conditions, the researchers said.
The study was presented Sunday at the annual joint meeting of the International Society of Endocrinology and the Endocrine Society in Chicago.
"Further studies should be carried out to elucidate if the gut microbial changes are a cause or effect of metabolic diseases," study lead investigator Dr. Yalcin Basaran, an endocrinologist from Gulhane Military Medical Academy School of Medicine in Ankara, Turkey, said in a meeting news release.
"Manipulation of intestinal bacteria could offer a new approach to manage obesity and type 2 diabetes," Basaran added.
The new study included 27 severely obese adults, 26 adults newly diagnosed with diabetes, and a "control group" of 28 healthy adults. All of the participants were aged 18 to 65 and none had taken antibiotics within the past three months (antibiotics can alter intestinal bacteria).
The obese and diabetic adults had considerably lower levels of the most common types of gut bacteria than those in the control group, the investigators found. These levels were about 4 percent to 12.5 percent lower in the obese patients, and 10 percent to 11.5 percent lower in the diabetes patients.
Basaran's team also found that levels of the most common gut bacterial species, Firmicutes, were affected by body mass index (a measure of body fat based on height and weight) and by hemoglobin A1c, a common measure of blood sugar control over the past three months.
Waist circumference and A1c also affected levels of another gut bacterial species called Bifidobacteria, and weight and fasting blood sugar level affected levels of a third gut bacterial species called Clostridium leptum, the results showed.
The findings support other recent studies that have found a link between certain species of gut bacteria and obesity and diabetes, Basaran noted.
He believes that microbes in the gut "may be used as an important marker to determine the risk of these metabolic diseases -- obesity and diabetes -- or it may become a therapeutic target to treat them."
According to background information in the news release, people have at least 160 different species of bacteria and other microbes in their digestive system, with total microbial populations estimated to range from 10 trillion to 100 trillion. Some experts believe that gut microbes might trigger low-grade inflammation, which in turn may affect body weight and sugar metabolism.
Experts note that findings presented at medical meetings are typically considered preliminary until published in a peer-reviewed journal.
The U.S. National Institutes of Health has more about the bacterial makeup of the body.