Young adults born via Cesarean section are more likely to be obese than those delivered vaginally, suggesting C-sections could be feeding the obesity epidemic, researchers have found.
But the theory is controversial. One expert cautioned that scientists are still a long way from pinning the expanding waistlines on higher rates of C-sections.
In the new study, Brazilian researchers found that among more than 2,000 23- to 25-year-olds, 15 percent of those delivered via C-section were obese compared to 10 percent of those born naturally.
The team looked at a number of other factors that could potentially explain the connection, like heavier birth weight, or income and education levels (more-educated mothers had a higher C-section rate).
But even after accounting for these factors, C-section remained linked to a 58-percent increase in the risk of adulthood obesity, according Dr. Helena Goldani and colleagues.
The findings do not prove cause-and-effect, Goldani, of Universidade Federal do Rio Grande do Sul in Porto Alegre, told Reuters Health in an email. And there could still be other explanations for the finding, she said.
Still, it's possible that C-sections could directly affect the risk of becoming obese later in life, the researchers speculate.
That's because infants born via C-section are not exposed to the beneficial bacteria in the birth canal, and so they might take longer to accumulate Bifidobacteria and other microbes that could influence their metabolism.
Similarly, obese adults tend to have fewer of those friendly bacteria in their digestive tract than normal-weight people do.
A researcher not involved in the study said that while the theory is "interesting," this study offers no proof that C-sections are to blame.
"I think all they have shown is an association. They have not shown any mechanism or cause-and-effect," said Dr. Xavier Pi-Sunyer, director of the New York Obesity Research Center at St. Luke's Hospital.
A major weakness, he told Reuters Health, is that the study had no information on mothers' weight.
Obese women are more likely than thinner women to need a C-section. Likewise, they are more likely to have overweight or obese children.
"That is a huge gap in the data," Pi-Sunyer said.
He noted, though, that other researchers are looking into the question of why obese people tend to have a different composition of bacteria in the intestines than thinner people.
One theory is that the variation in intestinal bacteria comes first and contributes to obesity by causing people to burn fewer calories and store more as fat.
Whether that is the case remains uncertain, however, and the role of C-sections is even less clear.
"This is an interesting finding," Pi-Sunyer said. "But it raises more questions than it answers, and it requires a lot more research."
He said that future studies will have to account for more "confounding" factors, like mothers' weight and pregnancy-related diabetes, to show whether the link between C-sections and obesity is real.
Goldani agreed that further studies in other countries are needed -- including studies that measure newborns' intestinal bacteria, then follow them over time to chart their weight changes.
Brazil has long had one of the highest C-section rates in the world. Of the 2,057 adults in this study, who were born in the late 1970s, 32 percent were born via C-section. It's now estimated that C-sections are done in about 44 percent of Brazilian births, many of which are thought to be medically unnecessary.
The C-section rate is also high in the U.S., where it accounts for about 33 percent of births.
"An underlying message in this (report) seems to be that there should be fewer C-sections performed," Pi-Sunyer said.
That may well be, he added, but not because it will do anything to solve the obesity problem.