A decade ago, spurred by the success of the Human Genome Project and the affordability of genetic sequencing, scientists began to explore the promise of “nutrigenomics”.

Could personalised nutrition, informed by knowledge of an individual’s DNA, help prevent and even treat diet-related diseases?

The results of early studies from Harvard, Stanford and elsewhere were compelling: Genetic differences seemed to predispose individuals to lose different amounts of weight on different types of diets. A multimillion-dollar industry soon sprang up, premised on marketing DNA-based diets.

But subsequent research has failed to show any statistically significant difference in weight loss between overweight people who “eat right for their genotype” and those who do not.

In fact, the effect of genes on obesity has been hard to tease out; various studies put the figure at anywhere from 35 per cent to 85 per cent.

Nutritionists have long observed that no one weight-loss strategy works for everyone, and that individuals show striking differences in their responses to different diets. What, then, explains the large variation in individual metabolism?

Last year, Tim Spector and Sarah Berry, epidemiologists at King’s College, London, and Dr Andrew Chan of Harvard Medical School began an ambitious new search for the answer. Their new study, called Predict, is the world’s largest and most comprehensive experiment to look at individual responses to food.

Their preliminary results, presented on Monday at the American Society for Nutrition’s annual conference, documented, for the first time, substantial and surprising variations in how well participants processed fats and carbohydrates, even among identical twins.

How efficiently a person metabolised one macronutrient was no predictor of how that person might respond to another.

“We are getting closer to being able to provide guidance for each person for what their ideal diet should be,” said Dr Eric Topol, a geneticist at the Scripps Research Translational Institute in La Jolla, California, who was not affiliated with the study.

“We’re not there yet, but the new study is another major milestone to get us there.”

For decades Professor Spector has been exploring the causes of individual variation in disease risk, including diet-related ailments.

In 1992, he set up TwinsUK, a research registry that now includes more than 13,000 identical and fraternal twins. Based on the twins, he concluded that genes contributed 70 per cent of an individual’s risk for obesity, on average.

Intrigued, he began a series of studies to tease out which factors influenced the remaining 30 per cent. In 2014, he began the British Gut Project, a crowdsourced effort to understand the diversity of gut microbes, their response to different dietary interventions and their effect on weight.

Among his registry of twins, he noticed, even identical pairs shared only about 50 per cent of their gut bacteria.

Professor Spector then started Predict to explore how variations in individual responses to fats and carbohydrates might contribute to obesity.

Eating foods that contain fats and carbohydrates causes glucose, insulin and triglyceride levels in the blood to rise and fall; spikes that are too high, too prolonged and too frequent are associated with inflammation, weight gain, heart disease and diabetes.

The study included 700 identical twins, 300 individual British volunteers and 100 subjects from the US, and gathered data on almost everything that can affect metabolism: Gut microbiota, sleep duration, exercise, body fat composition and more.

These initial results, however, analysed only the rise and fall of glucose, insulin and triglyceride levels in the blood after participants had eaten standardised meals.

The team concluded that genes play a limited role in how a person processes fats and carbohydrates. Among identical twins, only about half of the amount and duration of an individual’s post-meal blood glucose level could be attributed to genetic influence – and less than 30 per cent with regard to insulin and triglyceride response.

Predict tracked and analysed nutrition between identical twins. Photo: Getty

The more important factors in how our bodies metabolise food, it seems, are environmental: Sleep, stress, exercise and the diversity and population of our individual gut microbiome.

“That is really exciting for scientists and individuals,” Dr Berry said. “It has shown us how much is not genetic and therefore modifiable.”

She noted that the proportion of fats and carbohydrates in a meal explained less than 40 per cent of an individual’s response to that food.

That finding “reinforces the message that we should focus on whole lifestyle approaches rather than individual foods and nutrients,” she said.

A more detailed view of our metabolic differences has come only recently, with the advent of affordable machine learning, wearable sensors and genetic sequencing.

The result has been a surge of interest in the field.

In February, another large-scale, multi-year personalised nutrition study was started at the Swiss Federal Institute of Technology, in Lausanne.

“This research is fascinating and it’s important,” said Tim Caulfield, who researches health law and policy at the University of Alberta in Canada. Nonetheless, “if history tells us anything, it tells us that it’s unlikely that this is going to revolutionise nutrition.”

For one thing, he said, the basic parameters of a healthy diet are already well known: Plenty of whole grains, pulses, dark leafy greens and other vegetables, enough healthy oils and seafood, and very little red meat or refined carbohydrates. The problem is not that the guidelines are wrong or insufficiently personalised, Professor Caulfield said, but that people are not following them.

Want a proper look at your health? Experts suggest casting your eyes down. Photo: Getty

Even the focus on a person’s food choices or individual metabolism can distract from other significant contributors to the obesity epidemic, he said: “It is a fantastically complex issue that has to do with our built environment, with socioeconomics, with our food environment, with marketing, and with our activity levels – so many things.”

As a study, Predict is still in its early days; whatever individualised recommendations it might provide, there is no evidence yet that they can improve a person’s health any better than standard dietary guidelines can. Nonetheless, its scope and rigour are novel.

“It will require further validation, and doesn’t equate with preventing heart disease or cancer or other outcomes,” Dr Topol said. “But it’s still important if we’re ever going to get to the ‘food as medicine’ ideal.”

But for now, Professor Caulfield has some very low-tech advice for anyone in search of personalised nutrition: Look at the bathroom scale.

“That number is way more predictive of future health than most of the information you can get from these direct-to-consumer companies,” he said.

-New York Times