Universal diets don’t work because “healthy food” varies from person to person, a new study has found.

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Sadly, only a fraction of all people truly eat healthily these days – we all have our own struggles with food, either because we eat too much, or we eat things that are bad for us, or we don’t have enough money to buy healthy food. But whatever the cause may be, dietary conditions like diabetes are spreading more and more, and by now, it’s clear that dietary policies are inefficient. A new study adds even more complexity to that problem.

Researchers recruited 800 volunteers and had them eat the same thing over a week, testing the blood sugar to see how different people end up metabolizing it. They found that even after eating exactly the same thing, the blood sugar was significantly different from some people to others. This translates to something else: what’s healthy for you may not be healthy for someone else.

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“Most dietary recommendations that one can think of are based on one of these grading systems; however, what people didn’t highlight, or maybe they didn’t fully appreciate, is that there are profound differences between individuals – in some cases, individuals have [the?] opposite response to one another, and this is really a big hole in the literature,” said Eran Segal of the Weizmann Institute of Science in Rehovot, Israel.

But it’s not just diets that can be misleading because of this – our entire dietary system may need some tweaking.

“Measuring such a large cohort without any prejudice really enlightened us on how inaccurate we all were about one of the most basic concepts of our existence, which is what we eat and how we integrate nutrition into our daily life”, said Eran Elinav, a co-author of the study. “In contrast to our current practices, tailoring diets to the individual may allow us to utilise nutrition as means of controlling elevated blood sugar levels and its associated medical conditions.”

Monitoring how people’s bodies reacted to regular food was quite surprising; in one case, a middle-aged woman with obesity and pre-diabetes who had long struggled with diets experienced a spike in her blood sugar levels after eating tomatoes: a food that’s really not considered unhealthy.

“For this person, an individualised tailored diet would not have included tomatoes but may have included other ingredients that many of us would not consider healthy, but are in fact healthy for her,” said Elinav. “Before this study was conducted, there is no way that anyone could have provided her with such personalised recommendations, which may substantially impact the progression of her pre-diabetes.”

Indeed, the way to go seems to be personalisation: finding out what works and what doesn’t for each person; however, this doesn’t mean that we shouldn’t believe that “healthy foods” aren’t healthy for us – there’s still a very good chance that they are, but it would definitely help to see how it works for us, personally.

Journal Reference: Cell, Zeevi and Korem et al.: “Personalized nutrition by prediction of glycemic responses”http://dx.doi.org/10.1016/j.cell.2015.11.001