What people should eat varies so widely between individuals that a universal healthy diet is unrealistic. Nutrition science is moving toward tailored eating plans known as precision nutrition.

For about ten years geneticist Tim Spector (King’s College London) ate the same lunch daily — a tuna-and-sweetcorn sandwich on brown bread and a banana — thinking it was healthy. When he began measuring his metabolic responses, he found it produced large post-meal surges in blood sugar and fats, both linked to higher risks of diabetes, heart disease and obesity. Yet that same meal can be fine — even beneficial — for other people.

Recent research by Spector and others shows people’s metabolic reactions to identical foods are often highly individual. That variability suggests there is no single diet that will be optimal for everyone. Instead, researchers are developing strategies to tailor diets to individuals, a movement commonly called precision nutrition. This shift may help explain why decades of universal dietary guidelines have not ended the global rise in obesity and diabetes.

The idea of diet shaping health goes back centuries, and systematic dietary guidance began in the late 19th century with recommendations for variety, moderation and limiting excess fats, sugar and starch. Those broad principles remain useful, but the assumption that one standard “healthy diet” exists is now being challenged.

Surprising individual responses

A pivotal finding came from work at the Weizmann Institute in Israel, where researchers investigating artificial sweeteners measured volunteers’ blood-glucose responses. They expected similar responses across people, but instead found dramatic differences: some people had large glucose spikes, others none, and many lay in between. That variability contradicted assumptions behind the glycaemic index (GI), which treats foods as having a predictable average effect on blood sugar. The glycaemic index (GI) is a ranking of carbohydrate-containing foods according to how quickly they raise blood glucose. It’s useful because high-GI foods tend to cause rapid post-meal glucose spikes which is linked to higher risk of diabetes and weight gain.

Follow-up experiments comparing foods such as industrial white bread and artisan wholegrain sourdough showed that, for some individuals, white bread produced smaller glucose rises than wholegrain bread — the opposite of expectation. The team realised large studies that averaged small groups masked highly personalised responses.

Building on this, Elinav and Segal’s team studied hundreds of people, collecting detailed data — age, sex, lifestyle, BMI, waist-to-hip ratio, microbiome profiles from stool samples, and continuous glucose monitoring while participants logged meals, sleep and activity. From more than 52,000 meals they recorded, responses to identical foods remained strikingly individual.

Using machine learning, the researchers found that personal biometric data, particularly gut microbiome composition, was a strong predictor of a person’s glucose response to a meal. This indicated it might be possible to design low-GI diets tailored to individuals. In small trials with people at risk of diabetes, personalised diets improved glucose control; some “good” diets included foods typically seen as unhealthy for others — beer, chocolate or ice cream could be acceptable in moderation for certain people, while other foods (for example, tomatoes for some) could be unsuitable.

What drives the differences?

One notable discovery is that genetics appears to play a smaller role than expected. Even identical twins often show different responses to the same meal, implying that factors such as the gut microbiome*, circadian rhythms*, current diet* and lifestyle* are more influential. In Spector’s data, genetics accounted for only a minority of the variation in glucose responses; much more was explained by non-genetic factors.

*as part of our week 1 self-assessment, hopefully you’ve discovered how you’re tracking on a few of these and later in the Summer challenge we will dive into FIBRE and it’s importance for the gut microbiome.

Practical implications and limits

These findings suggest a future where a few tests — blood, stool or urine combined with simple measurements — could generate tailored dietary advice. Commercial products and apps aiming to provide personalised nutrition guidance (for example, Zoe and projects linked to the researchers above) have emerged. Early results are promising, but experts caution that sleep, activity and meal timing also affect outcomes, making real-world implementation complex. Additionally, personalised plans face the same human barriers as standard advice: people often struggle to follow dietary recommendations.

Researchers and clinicians emphasise that moving toward personalised nutrition does not overturn general healthy-eating principles. A diverse diet rich in fibre, fruits, vegetables, nuts and legumes, appropriate quality fats, and limited processed foods remains broadly advisable. Precision nutrition aims to refine and personalise within that healthy framework, not replace it with permission to over-consume indulgent foods.

In Summary

The broad strokes of good nutrition still matter, but which specific foods are best can differ markedly between individuals. Precision nutrition seeks to map those differences so dietary advice can be more effective and better matched to each person’s biology and lifestyle. If someone wanted to learn how they are impacted by different foods, then they can track their personal glycaemic response by wearing a continuous glucose monitor (CGM) while carefully logging what they eat, when they sleep and how they exercise — the CGM records glucose continuously so they can see post-meal spikes. Several consumer apps and services (including research-linked programmes such as Zoe and other personalised-nutrition apps) pair CGM data with food logs — and sometimes microbiome tests — to identify which foods trigger rises in glucose and to suggest tailored swaps or meal plans.

The original basis of this article has come from a NEW SCIENTIST publication by author Grace Wade who focuses on public health, nutrition and biotechnology. She holds a dual degree in journalism and science in human cultures, with a concentration in environment, science and society, from Northwestern University’s Medill School of Journalism in Illinois.