Your own gut might be undermining your New Year's resolution to lose weight.
Bacteria in the human digestive tract, sometimes referred to as "microbiomes," "microbiota" or "gut bacteria," are an increasingly popular subject among scientists. It has been linked to everything from food allergies, to neurodegenerative disease, to obesity and weight loss.
Now, research suggests microbiota can even hamper efforts to lose weight in the early days of a new diet.
A team of researchers from several institutions in the United States and Italy took faecal samples from two groups of people who had been eating two different diets for an extended period of time. One group had been on a standard American diet, while the other stuck to a restrictive diet that emphasises optimising nutrients per calorie. These are often called Calorie Restriction with Optimal Nutrition (CRON) diets.
The scientists deposited gut bacteria from these faecal samples into the digestive tracts of specially bred germ-free mice. Being germ-free, the mice were able to act as blank bacterial canvases for the human microbiota, taking on the same bacterial communities. Some of the mice ended up with bacteria from people long on the standard American diet; others got the bacteria from the calorie restriction dieters.
The mice were then placed on a plant-based, low-calorie diet that resembled the one the CRON dieters had already been on. The mice with the bacteria from the CRON eaters responded better to the diet.
"In our study, Americans consuming unrestricted diets maintained less diverse faecal microbiota than those of individuals adhering to plant-rich diet with restricted caloric intake," the researchers wrote in their study.
This suggests that sticking with a veggie-heavy diet for a longer period of time may give dieters a leg up over those who attempt to achieve rapid results.
The findings were published this week in Cell Host & Microbe, a peer-reviewed research journal.
Other evidence from previous studies has shown that practitioners of Western-style diets tend to have less bacterial diversity than those on traditional diets, but that lack of diversity may be remedied in part by changing their eating habits.
In the most recent study, the research team took another step that revealed a further insight: They placed the mice from both groups together. Once in the same living space, the mice began exchanging microbes, and the guts of the SAD mice began to resemble the calorie-restrictive mice.
The way they do this is rather disgusting - mice have a habit of eating faeces, which helps them obtain certain nutrients, such as vitamin B12. But the result does suggest that long term diet changes, taking certain probiotics or other treatments could influence the composition of gut bacteria.
Evidence has shown that people tend to quit on their New Year's resolutions, and have a hard time sticking to diets. There are, of course, likely countless reasons why these plans fail. But this research suggests part of the problem may be biological.
Even among bacteria, old habits die hard.