Among Beaumont’s many observations, he noticed that St Martin’s mood affected his stomach. When the man became angry or irritable – and it’s hard to imagine not getting irascible when a surgeon is dangling food through the hole in your side – his rate of digestion changed. That was the first clear sign that the brain affects the gut. Almost two centuries later, this maxim seems all too familiar. We lose our appetite when our mood changes, and our mood changes when we feel hungry. Psychiatric problems and digestive problems often go hand in hand. Biologists speak of a “gut–brain axis” – a
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