Gastric bypass is only available for more extreme cases but there is now intense interest in developing drugs that mimic the actions of PYY.

BBC: The second brain in our stomachs

When we eat, it normally takes 20 minutes for food to get from the stomach to the ileum, causing the release of PYY and the message to the brain, "I'm full".

BBC: The second brain in our stomachs

His new shrunken stomach was then attached further down his small intestine, to a section known as the ileum which secretes a different gut hormone, PYY, which is responsible for making you feel full.

BBC: The second brain in our stomachs