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鈥檛 know until you try. Some people will have an unhealthy reaction to the cheap stuff, with surging blood sugar levels. But others won鈥檛, and instead have a sharp rise in blood sugar after the sourdough. Some will surge on both, others barely at all.
This article is part of a series on nutrition that delves into some of the hottest trends of the moment. Read more here.
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?
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Increases in blood glucose and lipids are quite normal after eating, but if they go too high too quickly 鈥� called spiking 鈥� 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 ones high in sugar and fat. Such thinking also underpinned the development of the glycaemic index (GI), an influential measure of how quickly the body converts a foodstuff into glucose, which then enters the bloodstream. High GI foods include and sugary fruit. For low GI, think wholegrains and leafy vegetables. The higher the GI, the unhealthier the food. Or so the story went.
Over the past few years, nutrition researchers have thoroughly debunked that assumption. Differences in genetics, circadian rhythms and the make-up of our gut microbiomes translate into . As a result, we now know that there is no such thing as a healthy diet that works for everybody.
That doesn鈥檛 mean there is no such thing as a healthy diet at all, however 鈥� it is just that it needs to be individualised. That is the goal of precision nutrition, also known as personalised nutrition.
Research on how to do this has been going on for some time. In 2018, a team led by Tim Spector at King鈥檚 College London launched the (PREDICT) to measure people鈥檚 metabolic responses to food, with the ultimate goal of designing individualised healthy diets.
The shock discovery is that our body's response to food is highly idiosyncratic
In the first phase, the researchers recruited more than 1000 people, took various biometric measurements including genomes and microbiomes, fed them identical meals and then measured their glucose and fat responses 鈥� which . From this, it was possible to roughly predict other individuals鈥� responses to food based on their biometrics alone, which suggested that diets could be personalised without having to actually measure blood glucose and fats.
PREDICT is still ongoing, with around 25,000 subscribers to the Zoe nutrition app now taking part in the . However, the results are in from a smaller, related experiment that gives a flavour of what we might expect. Based on their responses to food, 177 for 18 weeks via the Zoe app, and followed up to see any health impacts. Roughly the same number received generic dietary advice. The participants given personalised advice had and also lost weight and shrank their waist circumference, although there were no differences to other measures including insulin, glucose and cholesterol levels.
鈥淚mportantly, we also showed that those who were most adherent to their personalised nutrition programme saw the greatest improvements,鈥� says Spector, who is a co-founder of Zoe.
The proof of the pudding will come from larger trials like PREDICT. The US National Institutes of Health also has a with a goal to 鈥渇undamentally transform nutrition science鈥�. Last year, it started recruiting 10,000 people to take part in an . There are no results as yet.
Still, even if they are positive, cooking up this research into widespread dietary advice , not least due to the . Likewise, a from the UK鈥檚 Food Standards Agency concluded that the benefits of personalised nutrition 鈥渟eem somewhat marginal when compared to what is already understood about a healthy diet鈥�.
This article is part of a special series investigating key questions about nutrition.
Article amended on 8 November 2024
We have removed a quote that we misattributed to Regan Bailey
