Let's face it — every diet out there boasts of its efficacy. Yet if every claim is true, why do so many still struggle even when they stick faithfully to their chosen regime?
A new Israeli study suggests that even if everyone ate the same meal, the individual effect would differ from one person to another. The findings, published Nov. 19 in the journal Cell, hint at the power of personalized nutrition plans in helping people discover which foods might help or harm their health goals.
Because high blood sugar levels are closely associated with health problems such as diabetes and obesity, a standard called the Glycemic Index (GI) was developed some years ago to rank foods based on how they affect blood sugar. The Glycemic Index ranks carbohydrates based on their glycemic response — that is, the rate at which they are converted to glucose within the body. The GI is used by doctors and nutritionists to develop healthy diets, and measuring it with a continuous glucose monitor is reasonably easy.
The GI system, however, only averages how small groups of people respond to various foods. The new study led by Eran Segal and Eran Elinav of the Weizmann Institute of Science, tracked the blood sugar levels of 800 people over a week. The team collected their data via health questionnaires, body measurements, blood tests, glucose monitoring, stool samples and an app that recorded lifestyle and food intake. In addition, the volunteers received a few standardized/identical meals for breakfast. In all, the results from 46,898 meals were measured.
The researchers found that the GI of any given food is not a set value, but in fact depends on an individual's own metabolism. Age and body mass index (BMI) were already known factors in the level of blood glucose after meals. However, the new data also revealed that different people show significantly different responses to the same food, even though their individual responses did not change from one day to another.
"Most dietary recommendations that one can think of are based on one of these grading systems," says Segal. "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." Elinav adds: "[This] 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.”
One of the problems with nutrition studies, the researchers say, is getting participants to accurately record their dietary intake. Useful results rely on the truthfulness of the data — but whether it's mere forgetfulness or something more dishonest, compliance can be a problem. In this study, the participants were asked to eat a standardized breakfast such as bread or a glucose-rich dish each morning and enter everything into a mobile app food diary. In return, the researchers provided an analysis of the participants' responses to food. This proved to be a strong motivator and the information in the diaries closely matched the biometric data obtained from glucose monitors.
The personalized feedback proved invaluable for some. A middle-aged woman with obesity and pre-diabetes, who had tried and failed to see results with a range of diets over her life, learned that certain "healthy" eating habits may actually have been adding to her problems. It was found by the team that her blood sugar levels spiked after eating tomatoes, which she ate multiple times over the course of the week of the study. "For this person," Elinav says, "an individualized tailored diet…may have included other ingredients that many of us would not consider healthy, but are in fact healthy for her."
The team wanted to find out why such differences exist between people. There is growing evidence to suggest that gut bacteria are linked to obesity, glucose intolerance, and diabetes so the researchers analyzed stool samples collected from each participant. It was found that specific microbes correlate directly with how much blood sugar levels rise after eating. By studying the results of 26 additional study participants, the researchers were able to reduce GI levels and alter the makeup of the microbes in the gut.
"It's common knowledge among dieticians and doctors that their patients respond very differently to assigned diets," says Segal. "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," says 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."
The researchers hope to craft their findings to reduce the number of "inputs" needed and so simplify the means by which people can be provided with personalized nutritional reports. "In contrast to our current practices," Elinav says, tailoring diets to the individual may allow us to utilize nutrition as means of controlling elevated blood sugar levels and its associated medical conditions."