Scientists have developed a mathematical formula that can help identify which diets will best help people lose weight and reduce their risk of disease, based on the unique composition of their intestinal bacteria.

In recent years it's become increasingly evident that the bacteria inside our guts play a huge role in keeping us healthy and digesting our food, but this is the first time scientists have managed to map out in detail how bacteria interact during metabolism. The research could allow doctors to offer personalised diet advice and, in the future, might even see them treating diseases by supplementing or removing particular gut flora.

So how does our gut bacteria control weight? Even though most people think of food in terms of calories, the reality is that not everyone breaks down food in the same way - which is why some people can live on cheeseburgers and lose weight, and others will eat the same diet and quickly become obese. And an increasing body of research shows that our gut bacteria drives this difference.

The new study, led by Chalmers University of Technology in Sweden, took things one step further and mapped out exactly how each type of microorganism in the gut interacts with food, our bodies, and each other. This allowed them to create a mathematical formula that predicts how well someone will respond to a specific diet.

"This method allows us to begin identifying each individual bacteria type's metabolism and thus get a handle on the basic mechanisms in human metabolism," lead researcher Jens Nielsen said in a press release.

They tested their calculation out by assessing the gut bacteria of 45 obese or overweight volunteers. They found that 18 of them had low gut bacteria diversity (meaning they didn't have as many different types of microbes in their intestines) and 27 had high gut bacteria diversity. They then put all participants on a low calorie, high protein diet.

At the end of six weeks, everyone had lost weight, but those who had low diversity in their intestinal flora also showed a significant improvement in their metabolic health - which means they'd successfully lowered their risk of type 2 diabetes and heart disease. The same effect wasn't seen in those with high gut bacteria diversity.

While the results weren't exactly groundbreaking, what's significant is that their formula had accurately predicted them. This proof of concept now means that they can use those same calculations to help make personalised diet recommendations based on gut flora.

The most immediate benefit will be to those at high risk of type 2 diabetes and heart disease, but eventually anyone else who wants to lose weight or simply improve their metabolic health could also benefit from the research.

"In the long term we might be able to add intestinal bacteria for patients whose metabolism does not function properly," said co-researcher Karine Clement.

The team is already working on follow-up experiments to test this in real patients, and has now started their own spin-off company to develop products that might help do this… faecal transplants, anyone?

And the benefits don't end at weight loss - gut bacteria has also been linked to diseases such as depression and anxiety, as well as people's response to cancer treatments. If the team can find a reliable way to predict people's risk of these conditions based on their gut bacteria, doctors could then tailor their treatments to get the best results possible. 

The results have been published in Cell Metabolism.