So, when obese communities colonised lean guts, they found that every morsel of food was already being devoured and every niche had been filled. By contrast, when the lean communities entered obese guts, they found a glut of uneaten fibre – and flourished. Their success only evaporated when Ridaura fed the mice with fatty, low-fibre chow, designed to represent the worst extremes of the Western diet. Without fibre, the lean communities couldn’t establish themselves or stop the mice from putting on weight. They could only infiltrate the guts of mice that ate healthily.
