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How reliable is the Glycemic index?Almost everyone who's ever started a diet in the past decade, will have heard about the glycemic index (GI) and have been advised to eat predominantly foods that have a lower glycemic response, that is, foods that will not increase blood sugar level so much.
A high blood sugar level is closely associated with health problems such as diabetes and obesity, and it's easy to measure using a continuous glucose monitor.
A standard developed decades ago, called the glycemic index (GI), is used to rank foods based on how they affect blood sugar level and is a factor used by doctors and nutritionists to develop healthy diets. However, this system was based on studies that average how small groups of people responded to various foods.
This method of the glycemic index (GI) was developed at the University of Sydney where the glycemic response to foods were compared with those of glucose after an overnight fast.
The emphasis on the glycemic index led to an oversimplification of food items as being 'good' or 'wrong' based on their GI-ratings. It can even falsely portray unhealthy foods as “safe.”
Frequently agave syrup and coconut sugar are being advertised as “healthy” because of their low glycemic indexes, yet they lack nutrients such as fiber and vitamins and are nothing but 'empty sugar'.
Individual glycemic responsesA bigger problem with the glycemic index is that glycemic responses to foods are highly individual.
While the University of Sidney did their research on just 10 persons, a study with no less than 800 healthy and pre-diabetic volunteers, whose blood glucose levels were monitored every five minutes for a prolonged period of time, a startling difference between individual response on specific foods were shown.
The new study, led by Eran Segal (from the dept of Computer Science and Applied Math) and Eran Elinav (from dept. of Immunology) of the Weizmann Institute of Science in Israel, found that the GI of any given food is not a set value, but depends on the individual.
The researchers were able to develop an algorithm that successfully predicts blood sugar responses to various foods at the individual level and they say the future of nutrition is diets that are optimized for the individual.
For all participants, they collected data through health questionnaires, body measurements, blood tests, glucose monitoring, stool samples, and a mobile-app used to report lifestyle and food intake. In addition, the volunteers received a few standardized/identical meals for their breakfasts.
As expected, age and body mass index (BMI) were found to be associated with blood glucose levels after meals. However, the data also revealed that different people show vastly different responses to the same food, even though their individual responses did not change from one day to another.
"There are profound differences between individuals--in some cases, individuals have opposite response to one another, and this is really a big hole in the literature," says Segal.
"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," says Elinav. "In contrast to our current practices, tailoring diets to the individual may allow us to utilize nutrition as means of controlling elevated blood sugar levels and its associated medical conditions."
Moving Toward Personalized NutritionCompliance can be the bane of nutrition studies. Their outcomes rely on participants, away from the laboratory, rigidly following a diet and honestly recording their food intake. In the Weizmann study, the participants (representing a cross-section of Israel's population and all unpaid) were asked to disrupt their weekly routine in two ways: They were to eat a standardized breakfast such as bread or glucose each morning and also enter all of their meals into a mobile app food diary. In return, the researchers would provide an analysis of the participants' personalized responses to food, which relied on strict adherence to the protocol. Elinav and Segal say this proved to be a strong motivator, and participant meal reporting closely matched the biometric data obtained from their glucose monitors.
The individualized feedback yielded many surprises. In one case, 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 her "healthy" eating habits may have actually been contributing to the problem. 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, an individualized 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," Elinav says. "Before this study was conducted, there is no way that anyone could have provided her with such personalized recommendations, which may substantially impact the progression of her pre-diabetes."
To understand why such vast differences exist between people, the researchers conducted microbiome analyses on stool samples collected from each study participant.
Growing evidence suggests gut bacteria are linked to obesity, glucose intolerance, and diabetes, and the study demonstrates that specific microbes indeed correlate with how much blood sugar rises post-meal. By conducting personalized dietary interventions among 26 additional study participants, the researchers were able to reduce post-meal blood sugar levels and alter gut microbiota. Interestingly, although the diets were personalized and thus greatly different across participants, several of the gut microbiota alterations were consistent across participants.
High intake of animal protein and fat, in particular, favors the growth of Bacteroides, whereas carbohydrate intake promotes growth of Prevotella species.
Several studies have demonstrated that high ecological diversity of gut microbes is associated with good health, which is presumably due to an increase in the diversity of bacterial functions.
"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."
"It's common knowledge among dieticians and doctors that their patients respond very differently to assigned diets," he adds. "We can see in the data that the same general recommendations are not always helping people, and my biggest hope is that we can move this boat and steer it in a different direction."
The researchers hope to translate what was learned in this basic research project so that it can be applied to a broader audience by reducing the number of data needed in order to provide people with personalized nutritional advice.
Take home messageWhile we, like many of us may have known already that the glycemic index had its flaws, because it takes food items out of context and makes some nutrition-devoid foods like coconut sugar appear healthier than they are, we always thought.. hey, it is still useful as long as you focus mostly on real foods anyhow and don't consume carbohydrates on their own, unless you are an athlete and need them for energy.
But to see that a food item can spike someone's blood sugar level and do nothing much for someone else, was quite the surprise.
It seems that along with focusing on real foods and trying to avoid empty calories as much as possible, we would do well to improve the health of our gut bacteria by giving them better food to thrive on.
Since fiber is their favourite food, aim for a daily intake of 30g of fiber. To get this from regular foods, this translates to half a kilogram of vegetables, fruits and legumes every day. Yes indeed this is twice as much as we are told to eat and even 4x as much as most Dutch eat!
Another way to increase the health of your gut is to consume fermented foods like Sauerkraut, kefir or yoghurt and add a few spoonfuls of coconut oil to your food to diminish the amount of unhealthy bacteria in your gut.
However, it may still be a good idea to supplement with a high-quality probiotic supplement, containing many different strains.