Over the next two decades, surgeons in the United States refined the procedure. They cut the small intestine near each end, then rejoined it to circumvent all but about 40 centimetres. Known as a jejunoileal bypass, it caused remarkable weight loss but also an array of unpleasant side effects, including bloating, diarrhoea, anal burning and dehydration. Bacterial populations in the bypassed intestine continually rose and the liver became inflamed. “Everybody realized that five years after you have this, you lose your liver,” says David Cummings, a endocrinologist at the University of Washington in Seattle.