Junk food kills the gut bacteria that can help keep people thin, while coffee and some beers increase them, a new book by a leading academic claims.
Microbes that live in people's bodies make up 90pc of living cells and when disrupted could be a major cause of obesity, according to Tim Spector, a professor of genetic epidemiology at King's College, London.
In a new book called The Diet Myth, he researched the links between microbes, food and health in an attempt to gain an insight into the burgeoning obesity crisis.
Prof Spector's research also found sweeteners in diet fizzy drinks had adverse affects on metabolism, leading to weight gain, while fasting diets such as the 5:2 diet could benefit microbes and metabolism.
But he found some elements were down to genetics, with a third of people naturally possessing microbes which prevented them getting fat, while genes even determined some people's preference for salads, broccoli or garlic.
A statement from the publishers Weidenfeld and Nicolson, said: "Compared to our recent ancestors who lived outside cities, with rich and varied diets and without antibiotics, we have only a fraction of the diversity of species of microbes living in our guts.
Prof Spector also said: "Feeding junk food to my son's poor gut microbes - a massacre in the name of science."
In its report, entitled The Rising Cost of a Healthy Diet, the ODI analysed data from five countries, including the UK and USA.