‘Tis the season to embark on the latest diet regime promising to effortlessly shift those extra pounds. A few years ago, proponents were extolling the benefits of the Paleo Diet and the Belly Fat Cure; more recently it was raspberry ketones and the 5:2 intermittent fast.
But according to a group of Israeli researchers, that perfect one-size-fits-all diet does not exist—largely because, as they’ve just discovered, there is no universal response to any given food. In fact, they maintain, no one can even say with certainty which foods are categorically slimming. Even if a group of us were to eat the same meal, the way in which it is metabolized would differ markedly from one individual to another.
Blood sugar monitoring
Researchers Eran Segal, of the Department of Computer Science, and his colleague Eran Elinav, of the Department of Immunology, at the Weizmann Institute of Science in Israel came to this astounding conclusion after continuously monitoring the blood sugar levels of hundreds of people and comparing those levels with the food they were eating.1
The ostensible reason for the study was to identify those foods and dietary factors that underlie our blood sugar responses after meals to help curtail the current worldwide epidemics of obesity and diabetes.
All 800 of the study participants were kitted out with small monitors that religiously recorded and measured their blood sugar levels, and each participant was asked to keep a record of everything they ate in food diaries, using apps on their mobiles, plus how much they slept and exercised every day.
Besides studying the blood sugar responses to nearly 47,000 meals in total, the researchers also factored in total food intakes, and examined body measurements and stool samples.
The study was also unique in that it analyzed the microbiome, or gut microbes, of each of the participants.
No single response
Although blood sugar levels were to some degree related to age and body mass index (BMI), the unexpected result was that there was no uniform response to individual foods, not even to sugar. While many of the participants’ blood sugar levels shot up after consuming a meal laden with glucose, others didn’t respond to sugar so much as to white bread.
In fact, many of the participants had strikingly different responses to the same foods: one volunteer’s blood sugar zoomed up after eating a banana, but not after a biscuit with the same number of calories, while another volunteer had the opposite response. And everyone had highly individualized responses to an identical plate of food they ate for breakfast.
Also among the unexpected findings was that many so-called ‘healthy’ foods can actually be responsible for weight gain. For instance, one middle-aged woman, who was obese and suffering from prediabetes, and found most diets useless in controlling her weight and blood sugar, discovered that eating tomatoes on a regular basis was causing an immediate spike in her blood sugar levels.
Although it’s well known which foods contain healthy nutrients, these results put paid to the idea that certain foods are automatically good or bad for your waistline. Responses to blood sugar were also dependent on a number of lifestyle factors, such as whether the participants had exercised or slept before eating, and also on the state of their gut bacteria.
The ideal diet
From the details gathered by the Israeli team about blood sugar, plus individualized information about lifestyle, medical background and specific makeup of the gut microbiome, they were able to create algorithms to predict each of their participants’ responses to different foods and so design both their best and worst diets.
Both diets contained the same calories, but differed only in how the individual’s blood sugar responded to them; the good diets kept a person’s blood sugar steady and within healthy levels, while the bad diets caused large spikes in glucose levels after a just single week of following the regime.
Another interesting finding was that the good diets also caused major beneficial changes in the composition of an individual’s gut microbes, which suggests that the microbiome may both influence and be influenced by the host’s blood sugar levels.
But, as each of the good diets were very highly individualized, the researchers say they cannot generalize as to what constitutes the ideal diet for the population at large.
“The huge differences that we found in the rise of blood sugar levels among different people who consumed identical meals highlights why personalized eating choices are more likely to help people stay healthy than universal dietary advice,” says Professor Segal.
Their study also suggests that the glycaemic index (GI) and calorie rating of any food isn’t absolute, but depends on individual factors, including your particular food intolerances and the state of your digestion (see box, page 20).
“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 opposite responses to one another.”
This study also adds to the growing wealth of data that the state of our gut bacteria not only controls our digestion, but may also be linked to obesity, diabetes and even impaired glucose tolerance.
The Israeli researchers discovered that the presence of specific microbes correlated with how much blood sugar levels rose after eating. Once the scientists discovered the ideal foods that could lower any given person’s blood sugar levels, they also saw consistent and positive alterations of the gut microbial population.
This now adds to the scientific literature on biochemical individuality. We know from the work of innovators like cancer specialist William Kelley and his protégé William Wolcott, who coined the term ‘metabolic typing’, that people differ vastly in their rate of cellular oxidation—the pace at which the body converts food into energy—and that individual foods can have opposite effects in people of different metabolic types, depending on whether they ‘burn’ or ‘store’ food.
And now we know that the same food also has highly individual effects on blood sugar.
One man’s meat is another person’s chocolate cake.
The glycaemicindex (GI)
The GI measures the increase in blood sugar after eating a carbohydrate food in relation to the effect of pure sugar (which scores 100). Sweetcorn has a score of 55, which means that it raises blood glucose by 55 per cent, or just over half as much as pure sugar does. In general, carbs below 55 are considered low GI, scores of 55–70 are mid-GI and those over 70 are high GI.
In the past, it was widely believed that simple sugars dramatically raise blood glucose levels, while starches like potatoes and bread are digested more slowly. But the results of numerous studies show this is not so. Among the biggest surprises are potatoes, reported to have an average GI of 84, making them one of the higher GI foods around. Initially, nutritionists analyzed food by the amount of energy (calories) they provided. This was rather crudely worked out in a laboratory by burning a food to measure the amount of heat energy it produced.
But caloric measures are useless for diabetics, who need to know the glucose value of foods to control their blood glucose intakes and avoid the need for insulin injections. And the glucose values of foods can’t be measured in the lab, but must be tested in living individuals.
This led to a painstaking series of tests in the 1980s, whereby every foodstuff was analyzed for its potential to produce glucose in the bloodstream. Human guinea pigs were fasted to create a baseline to measure against, then given a single food to eat. Regular blood samples were taken over a four-hour period to chart changes in blood glucose levels.
One surprise was finding that all carbs caused a glucose peak roughly 30 minutes after ingestion. Previously, it had been thought that only simple carbs (like sugar and honey) were fast-acting, while complex ones (like potatoes and cereals) were slow-acting. There were also dramatic differences in levels of blood glucose and, as the technical term for blood glucose is ‘glycaemia’, these differences were measured in terms of their glycaemic index: the higher the glucose peak, the greater the amount of blood glucose produced by that food.
Not surprisingly, virtually all foods have a GI lower than pure glucose. Also, as expected, the most refined carbohydrates cause the highest blood glucose spikes. But there were some unexpected findings too. Cooking was found to have major effects on the GI. Carrots, for example, produce three times the amount of blood glucose when cooked than when eaten raw. In fact, any cooking or processing raises the GI significantly.
‘Bad’ carbohydrates produce a sharp rise in blood glucose, causing hyperglycaemia, an excess of glucose in the bloodstream. These carbs include all forms of white sugar (in processed snacks, sweets, biscuits and cakes), foods made from white flour such as bread and some pastas, and white rice, as well as corn and beer.
‘Good’ carbohydrates cause only a slight increase in blood glucose. These include whole grains, brown rice, pulses like lentils and dried beans, most fruits, and most vegetables high in fibre, such as leeks, cabbage, broccoli, cauliflower, salad greens and green beans.
But all of these foods were tested in only a handful of people. Thanks to the Israeli research, we now know that the GI, like metabolic typing, depends on many individual factors, not least of which is the state of the microscopic population of microorganisms to which we all play host.