Brynn. You don't REALLY want this, it's your internal bacteria that want it.

If you reach for that tasty piece of chocolate, even though you are trying to lose weight, are you doing it out of your own volition? Or are you actually being controlled by the bacteria in your gut?

This is the question posed in BioEssays by Dr Carlo Maley of the University of California San Francisco. “Bacteria within the gut are manipulative,” said Marley. “There is a diversity of interests represented in the microbiome, some aligned with our own dietary goals and others not.”

If it sounds ridiculous that life forms too small to see are controlling our behavior, remember the bacteria within you outnumber your own cells at least 10 to one (some estimates say 100 to one)

UC San Francisco. Proposed mechanism by which gut bacteria control our behavior.

Our guts are filled with multiple bacterial species, with different preferred foods. While we benefit from their ability to break down nutrients we cannot, according to co-author Dr Athena Aktipis, of Arizona State University, this symbiotic relationship has its tensions. The healthiest diet for certain species of bacteria is not always to our benefit, or that of others in the internal ecosystem.

The question is whether these bacteria can influence what we eat to their benefit. Maley and Aktipis believe they can, by releasing molecules into our digestive system that are transmitted through the immune, endocrine and nervous systems to signal to the brain what to put in our mouths. Think of it as the bacteria's way of ordering fast food.

The authors suggest a test of their theory the microbes are running the show. In Japan, where seaweed is an important part of the diet, bacteria that specialize in digesting seaweed are common. If seaweed-eating bacteria are transferred into the gut of someone on a western diet will they start to get cravings? Of course, it could easily be the other way around - bacteria that specialize in seaweed become common in areas where it's a common dish.

Fortunately however, we are not simply at the whim of these small but numerous rulers, but can stage a revolt. "Our diets have a huge impact on microbial populations in the gut," Maley says. "It's a whole ecosystem, and it's evolving on the time scale of minutes." Within 24 hours of switching to a new diet changes to the species distribution inside us are measurable, as those that benefit from the new food intake multiply.

This explains why probiotics can improve not only our health but our mood. "Because microbiota are easily manipulatable by prebiotics, probiotics, antibiotics, fecal transplants, and dietary changes, altering our microbiota offers a tractable approach to otherwise intractable problems of obesity and unhealthy eating," the authors write.