Is personalised nutrition better than one-size-fits-all diet advice?

Consider two slices of bread, one from an artisanal sourdough boule, the other from a cheap, mass-produced white loaf. Which do you think is healthier?

The correct answer is that you don’t know until you try. Some people will have an unhealthy reaction to the cheap stuff, with surging blood sugar levels. But others won’t, and instead have a sharp rise in blood sugar after the sourdough. Some will surge on both, others barely at all.

The same is true for other foods and other nutrients, especially fats, which can also surge dangerously in the bloodstream after eating. How our metabolisms respond to food is highly idiosyncratic, a shock discovery that is upending decades of nutritional orthodoxy and promising to finally answer that surprisingly knotty question: what should we eat to stay healthy?

Increases in blood glucose and lipids are quite normal after eating, but if they go too high too quickly – called spiking – they can cause trouble. Frequent spikes in glucose and a type of fat called triglyceride are associated with the risk of developing diabetes, obesity and heart disease. For decades, nutrition researchers assumed that all humans responded to a given food in roughly the same way, with uniform increases in blood sugar and fats.

Glycaemic index

Under that assumption, dietary advice was simple and one-size-fits-all. Reduce consumption of the foods that cause spikes. Unsurprisingly, those were mostly…