It's no secret that lots of us struggle with food. In many parts of the world, large segments of the population are developing health problems such as diabetes and obesity at alarming levels, suggesting that current dietary policies are ineffective at keeping people within their healthy weight range.

According to a new study, this situation isn't altogether surprising. Israeli researchers tracked the blood sugar levels of 800 participants over the course of a week and found that even if people all eat the same meal, how they end up metabolising it will differ from person to person. In other words, what might be healthy for you won't necessarily be equally healthy for somebody else.

The researchers say the problem lies with the manner in which most dietary systems measure results. Take the glycemic index (GI), for example, which ranks foods based on how they affect blood sugar. There's no doubting it's a useful starting point to gauge how foods can impact glucose levels, but it's based on studies that average out how groups of people respond to food – not particular individuals like you or me.

"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.

Segal and his fellow researchers say GI is not a set value for any given food, but rather depends on the individual eating that food. To test this, they monitored week-long glucose levels in an 800-person cohort, measuring individual responses to a whopping 46,898 meals.

Their findings, which are published in Cell, showed a high variability in the response to identical meals, which suggests that universal dietary recommendations may have limited utility in the real world.

"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 individual responses to foods produced some interesting results. In one case, the researchers found that a middle-aged woman with obesity and pre-diabetes who had long struggled with dieting experienced blood sugar level spikes after eating tomatoes – a food many would not think to categorise as 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."

The researchers hope their study will draw attention to the need for personalised nutrition reports in place of blanket 'healthy diet' advice.

"After seeing this data, I think about the possibility that maybe we're really conceptually wrong in our thinking about the obesity and diabetes epidemic," said Segal. "The intuition of people is that we know how to treat these conditions, and it's just that people are not listening and are eating out of control – but maybe people are actually compliant but in many cases we were giving them wrong advice."