Published on March 1, 2023
In a recent blog post, I talked about the glycemic index and the glycemic load. I covered what they are and how they are calculated, and went over the glycemic index and glycemic load data of a few common foods.
In this blog post, I am going to expand on this discussion by addressing the question of whether the glycemic index of our diet matters for our health.
Some people, both in academia and on social media, seem to think that the glycemic index is useless. The points raised are often factually correct, but I still disagree with the conclusion. I do think there is value in the glycemic index. However, there are a few things we need to know about and consider to make it really useful. It’s my goal for this post to remove any confusion in this area, and make sure you fully understand how you can use glycemic index data of foods to your benefit.
The Glycemic Index and Health Outcomes
The first thing we are going to do is take a look at what the scientific literature can tell us about whether the glycemic index matters for health. We’ll do that by looking at both observational and interventional studies for a few health outcomes: body weight/weight loss, type 2 diabetes, cardiovascular disease, and cancer.
Body Weight and Weight Loss
The cumulative evidence of dozens of observational studies does not suggest that there is an association between the dietary glycemic index and body mass index.
Fortunately, when it comes to body weight, we have better evidence in the form of well over 50 intervention studies in which investigators randomized participants to eating either a low- or a high-glycemic-index diet, usually for a few months, or up to a year.
If we consider all of these studies, as we should, they also suggest that there is no difference in body weight whether participants eat a low- or high-glycemic-index diet.
However, it’s important to note that many of these studies were not very successful in changing the glycemic index of the participant’s diets, and as a result, the glycemic index tended to be quite similar in the low-glycemic index arm and the high-glycemic-index arm of many of these studies. That’s unfortunate and really means that these studies cannot tell us whether someone would lose weight if they lowered the glycemic index of their diet. These studies just tell us that many people have a hard time changing their diet in meaningful ways (or that the studies were poorly done).
If we only look at studies that succeeded in changing the participant’s diets enough that there was a difference in the glycemic index between the intervention groups of at least 20 units, then we see that those randomized to a low-glycemic index diet lost 1.8 kg, or 4 pounds, more than those randomized to a high-glycemic-index diet. Not huge, but there is a bit of a signal there suggesting that lowering the glycemic index may help people lose weight as long as the change in the glycemic index is meaningful enough. In other words, lowering the glycemic index from 75 to 72 won’t make much of a difference. Lowing it from 75 to 55 quite likely would.
A word of caution here though. It is usually problematic to base our conclusion on just a portion of all completed and published studies, because that opens the door to picking only those studies that are consistent with whatever we want the result to be, and then justifying why we picked only those studies. The fact that weight loss was consistently seen in 28 studies that succeeded in manipulating the dietary glycemic index in a meaningful way is reassuring and may lead us to the conclusion that lowering the glycemic index of our diet substantially most likely would modestly lead to weight loss. However, the fact that the dietary glycemic index barely differed between participants randomized to low- versus high-glycemic-index diet arms in many studies should not be ignored. This observation suggests that many people are unable or unwilling to reduce the glycemic index of their diet, which is an important finding in its own right.
Now, how do we reconcile this with the fact that observational studies do not see a consistent association between the glycemic index and the body mass index? One potential explanation is that in most observational studies, the difference in the glycemic index between the groups with the highest vs. lowest glycemic index is probably too small because it’s usually less than 10 units. As in the well-known Nurses’ Health Study, for example (see table below). So in that way, many of the published observational studies have the same flaw as many intervention studies: they compare people eating diets that are not sufficiently different in their glycemic index.
Taken together, it seems clear that just lowering the glycemic index of one’s diet will not have a large impact on body weight all by itself. However, reducing the glycemic index meaningfully, by 20 units or more, is likely to have some modest effect on body weight. This finding suggests that low-glycemic index diets may be more satiating, meaning that people may need to eat fewer calories on low-glycemic index diets to feel comfortably full.
Type 2 Diabetes Mellitus
Given that the glycemic index is based on the degree to which a food raises blood sugar levels, it would make a lot of sense that a low glycemic index diet would be preferable when it comes to type 2 diabetes: either lowering the risk of developing diabetes, or at least reducing elevated blood sugar levels in those already suffering from diabetes.
A lot of observational studies have looked into the question. A recent meta-analysis, meaning a study of studies, combining the data from 20 individual observational studies, showed that the risk of developing type 2 diabetes is 19% higher in people eating a high-glycemic index diet compared to a low-glycemic index diet. Specifically, what was done here is that for each study, the entire population was divvied up into quartiles or quintiles based on their glycemic index. Investigators can then compare the risk across these quartiles or quintiles, as illustrated in the figure below.
Let’s look more closely at one of these studies to really understand how these types of studies are done. The well-known Nurses’ Health Study was one of the studies that looked into the relationship between the dietary glycemic index and the risk of type 2 diabetes. In a first step, the investigators formed quintiles based on the glycemic index. There was an equal number of participants in each quintile, about 15,000 women, so the ~15,000 whose diet had the lowest glycemic index were grouped in quintile 1, and their median glycemic index was 48.8. Median means that within this group, 50% had a lower glycemic index and 50% had a higher glycemic index. The ~15,000 women with the next lowest glycemic index were grouped in quintile 2, and so on.
The investigators then followed these groups for about 20 years and looked at who developed type 2 diabetes. For this purpose, the investigators decided that quintile 1, that is, those with the lowest dietary glycemic index, would be considered the reference group, and they set the risk of developing type 2 diabetes for this group to 1 (or 100%). They then calculated that risk for each of the other quintiles, relative to that of quintile 1. From this, we can see that in analyses adjusted for age only (model 1), the risk was 12% higher in quintile 2, 11% higher in quintile 3, 26% higher in quintile 4, and 42% higher in quintile 5, all compared to quintile 1.
Now, why did they adjust this analysis for age? For the simple reason that the risk of type 2 diabetes goes up quite a bit as we get older, and accounting statistically for a major risk factor like that can make it easier and more reliable to detect other risk factors. The numbers in parentheses are the 95% confidence intervals. In short, what these mean is that there is always some uncertainty associated with these estimates of increased risk in each of the quintiles. For example, we cannot be 100% certain that the risk of type 2 diabetes would always be exactly 42% higher in quintile 5 compared to quintile 1. However, the data tell us that if we had a chance to run this study another 100 times in this population, the risk would be higher by between 32% and 53% in quintile 5 in 95 out of 100 studies. This can make us pretty confident that the observed association is based on a real association between the dietary glycemic index and type 2 diabetes risk, and not just the result of some random variation (careful here though: real association does not mean causation).
Now, one problem with observational studies is that people in quintile 5 may be quite different in other ways from those in quintile 1. To illustrate this, I have picked just two examples.
As we can see in the table below, in this Nurses’ Health Study cohort, those who ate a diet with a higher glycemic index also had lower physical activity, and they also drank way less coffee.
Let’s consider this issue of differences in physical activity. Whenever we are conducting an observational study, we are interested in whether an exposure, in this case, the dietary glycemic index, is associated with a health outcome, in this case the risk of developing type 2 diabetes. We can only detect an association, but what we are trying to get insights on is whether there may be a cause-and-effect relationship. Like, does eating a diet with a higher glycemic index cause the risk of developing type 2 diabetes to be increased?
One big obstacle in these investigations is the risk of what scientists call confounding. Confounding means confusion of effects. In this example, we found that in the Nurses’ Health Study, there was a consistent association between the dietary glycemic index and the risk of type 2 diabetes. However, we also saw that the dietary glycemic index was clearly associated with lower physical activity and lower coffee intake. Lower physical activity is almost certainly associated with a higher risk of type 2 diabetes, so we must ask the question of whether this relationship between the dietary glycemic index and type 2 diabetes was confounded by physical activity, meaning those who followed a higher glycemic index diet only had an increased risk of type 2 diabetes because they exercised less, and not because of the higher glycemic index of their diet.
To address this potential issue, the investigators routinely adjust for these potential confounding variables by including them in a statistical model. Model 4 in the table below is what we call the full adjusted model that includes numerous additional variables that may affect type 2 diabetes risk or that may confound the relationship between the glycemic index and type 2 diabetes risk. Both physical activity and coffee consumption were variables in that model, along with many others such as the body mass index, family history of diabetes, and many dietary variables. We see that the estimates have changed slightly, but not all that much. This is reassuring because it means that the glycemic index was associated with type 2 diabetes risk, independent of the lower physical activity and lower coffee intake of people eating high glycemic index diets.
The result of the meta-analysis mentioned earlier is a summary of 20 such individual studies, and these – taken together – show that the risk of those in the highest category (quintile or quartile) is 19% higher than that of those in the lowest glycemic index category.
What about intervention studies? Unfortunately, no randomized controlled trial has been conducted that compared diets differing in the glycemic index and that had incident type 2 diabetes as an endpoint. And we will probably never have such a study, because it’s unlikely that we’d find thousands of participants who are willing to be randomized to eating high or low-glycemic index diets for many years or even decades.
However, we do know for sure that high-glycemic index diets lead to greater blood glucose concentrations after meals than low-glycemic index diets (after all, that’s what the glycemic index of foods is based on). And higher blood glucose concentrations will usually also trigger higher blood insulin concentrations. Chronically higher glucose and insulin concentrations, in turn, are quite likely to decrease the body’s sensitivity to insulin, which means people would be expected to become slightly more insulin resistant on high- compared to low-glycemic index diets.
This is mostly consistent with data from randomized controlled trials, which cumulatively suggest that insulin resistance is likely higher when people eat high- as compared to low-glycemic index diets. This is not consistently seen in all intervention studies, however, and we will discuss an important factor later that may explain these discrepant findings in some studies.
Another major risk factor for type 2 diabetes is excess body weight. As we have discussed above, the available data also suggest that body weight is slightly higher on high-glycemic index diets.
Taken together, observational studies show an association between the consumption of high-glycemic index diets and an increased risk of type 2 diabetes. This is consistent with data from randomized controlled trials suggesting that body weight and insulin resistance are also elevated on high-glycemic index diets. While the evidence is not conclusive, the consistency between observational and interventional studies is reassuring.
Lastly, and maybe not surprisingly, low-glycemic index diets also reduce the degree of hyperglycemia in patients with manifest type 2 diabetes.
Cardiovascular Disease
For coronary heart disease, a meta-analysis of 10 observational studies showed a 14% increased risk in those consuming a high-glycemic index diet compared to those consuming a low-glycemic index diet. Again, this is comparing the highest vs. the lowest quintile or quartile.
There are also several randomized controlled trials that have investigated the effect of high- vs. low-glycemic index diets on various risk factors for cardiovascular disease. Taken together, these suggest a modestly higher blood pressure on diets with a high glycemic index, but no consistent impact on other risk factors such as the fasting serum lipid profile or biomarkers of inflammation. As discussed earlier, high-glycemic index diets also slightly increase body weight and the risk of type 2 diabetes, both of which are risk factors for cardiovascular disease.
For stroke, the data suggest no statistically significant difference between high and low glycemic index diets. In the figure below, the 95% confidence interval includes the value zero, so if we ran these studies again and again, we could not be at least 95% confident that the risk of stroke would be higher in the group consuming high-glycemic-index diets at least 95% of the time.
Taken together, the available data suggest that the risk of coronary heart disease is slightly elevated on diets with a high glycemic index, possibly related to slightly higher blood pressure, higher body weight, and a higher risk of hyperglycemia and type 2 diabetes.
Cancer
Cancer is a particularly interesting endpoint when considering the glycemic index of the diet. That is because most cancer cells feed predominantly on glucose, and a hypothesis in the scientific community is that higher blood glucose concentrations would provide a better growth environment for tumors.
Cumulatively, meta-analyses of observational studies found an increased risk with high-glycemic index diets for breast cancer, colorectal cancer, and bladder cancer, but no significant association between the dietary glycemic index and other types of cancer.
Summary
In summary, the available data do suggest that eating a high-glycemic index, when compared to a low-glycemic index, diet slightly increases the risk of type 2 diabetes, coronary heart disease, and bladder, breast, and colorectal cancer.
I am saying ‘suggest’ because all of these findings are based on observational studies, and so we can never be certain that the observed associations are reflective of a cause-effect relationship.
According to intervention trials, high-glycemic index diets likely also slightly increase body weight, insulin resistance, and blood pressure, all of which are risk factors for type 2 diabetes, coronary heart disease, and even certain types of cancer (including some types of breast cancer as well as colorectal cancer). These findings from randomized controlled trials slightly raise our confidence that the associations seen in observational studies may be reflective of a cause-effect relationship.
As one would expect, high-glycemic index diets also lead to higher blood sugar levels in people who already have diabetes, so this is one case where it’s clear that low-glycemic index diets are preferable to high-glycemic index diets.
Why Foods Differ in Their Glycemic Index, and How The Glycemic Index Could Be Improved
What all of this suggests is that people eating a high glycemic index diet have a higher risk of these diseases because they have larger increases in blood sugar levels after every meal.
Right?
Well, the whole concept of the glycemic index is built on exactly this hypothesis: that eating high glycemic index foods gives us more substantial blood sugar spikes, and that more substantial blood sugar spikes increase the risk of chronic disease.
Well, as plausible as that sounds, the currently available data do only provide limited support for this idea.
Why do I say that?
Let’s imagine what the diet of someone who eats mostly high glycemic index foods looks like. They may eat a lot of white rice, cornflakes, white flour bread, beer and soda, and sweets.
Now compare that to the diet of someone who eats a lot of low-glycemic index foods. Let’s say sourdough rye bread, oatmeal made from steel cut oats, lots of beans and lentils, boiled sweet potatoes and starchy vegetables such as parsnips and carrots, and maybe some dairy foods. So they are eating the same amount of total carbs, but they just focus on lower glycemic options.
What we can see is that these foods differ in a lot of ways, and the glycemic index is just one of these. In general, low-glycemic index foods are less likely to be refined products, and so they are typically more nutrient-dense, meaning they have more vitamins and minerals per calorie. They also tend to be less energy dense, meaning they have fewer calories per g. They tend to be richer in fiber, and some low-glycemic index foods such as beans and lentils also contain protein.
So even though in the observational studies participants were grouped into quintiles or quartiles based on the glycemic index of their diet, we need to be clear that the diets of those in the fifth quintile differed in a lot of ways from those in the first quintile. In observational studies, we can try to adjust for some of these differences, but that’s not always successful and we need to be clear that we cannot totally adjust for all of the factors that differ between low- and high-glycemic-index foods, because we would then adjust away all of the differences between these groups, right?
The same is true in randomized controlled trials. When investigators are creating the high- and low-glycemic index diets, they have some high- and low-glycemic options of almost identical foods, such as oatmeal from instant oats vs. oatmeal from steel-cut oats. But, that’s an exception, and in all studies, people end up eating totally different diets in the high- vs. low-glycemic index arm.
Therefore, we need to be clear that what the research suggests is that the risk of type 2 diabetes, coronary heart disease, and certain types of cancer is higher in people eating these types of foods with a high glycemic index. We cannot be 100% certain that it’s really their higher glycemic index that makes them unhealthy. To me, it’s very likely a combination of different factors. Practically, that’s not relevant, because whatever the reason, the data tell us that we probably benefit from limiting our consumption of high-glyemic index foods.
I actually suggest that the glycemic index may be more important than these studies suggest, for a couple of reasons.
First, take a look at our example of a high- vs. low-glycemic index diet. The average glycemic index of these high-glycemic index foods here is 83, while it’s 43 on the low-glycemic index diet. That’s a difference of 40 units, meaning that if someone really took the concept of the glycemic index to heart, one could fairly easily lower the glycemic index of the diet by 20, 25, or more units.
Contrast that to the published observational studies. In most studies, the difference in the glycemic index between those in the fifth quintile and those in the first quintile is less than 10, meaning that their diets didn’t differ all that much. That is because in most populations, people eat a mixture of foods, and while there may be some individuals who eat extreme diets, most people’s diets don’t differ that much from that of their neighbors. That suggests that if we made a serious attempt to minimize high glycemic index foods, the long-term benefits could quite plausibly be greater than what the existing studies suggest.
And then there is another issue: take a look at these foods in the rightmost column in the figure below. On average, these foods also have a very low glycemic index of 43. Let’s say you were eating a lot of baked potatoes with a glycemic index of 88, and you wanted to lower that. You could eat more beans, say, kidney beans, with a glycemic index of 40. Or instead, you could do even better and just gulp down some agave syrup, with a glycemic index of 19. Or maybe eat a muffin with a glycemic index of 48.
I guess most people can readily see that muffins or pure agave syrup would not be a healthier choice than kidney beans, and – I’d argue – also not healthier than baked potatoes. The problem here is probably the biggest limitation of the glycemic index, and it has to do with why some foods have a lower glycemic index than others.
You see, foods with a high glycemic index, without exception, are rich in easily digestible starch, which is just a chain of glucose molecules. Usually, these high-glycemic index foods are also low in fiber. Eating these foods, such as white rice, white flour bread, cornflakes, potatoes, or instant oatmeal, the blood glucose level rises quickly because the starch in these foods can be easily and quickly digested, and glucose can enter the bloodstream very soon after the meal, and all at once.
Among the foods with a lower glycemic index, there are some that are also rich in starch, but they also contain a lot of fiber and sometimes other nutrients, such as protein. They commonly also require more chewing. Foods in this category include legumes such as beans and lentils, many whole grains, and most starchy vegetables. Because of the greater need for chewing and their content of fiber and/or protein, the starch is digested much more slowly, and glucose enters the bloodstream more gradually. From a nutritional perspective, this is desirable in many ways: with these foods, the body has more time to deal with the incoming glucose, and the foods – because they are usually more intact and less processed – provide other nutritional benefits such as fiber, micronutrients, and phytochemicals. All of these may add to the health benefits of these low-glycemic index foods.
The second factor that lowers the glycemic index of foods is something entirely different though. Remember that when we calculate the glycemic index of a food, we measure changes in the blood GLUCOSE concentration after eating that food. Therefore, foods with a high glycemic index tend to be those that are rich in glucose, mostly in the form of starch, because starch is just a chain of glucose molecules.
Foods that contain carbohydrates other than glucose tend to have a lower glycemic index simply because there is less glucose that can enter the bloodstream after these foods have been eaten. An example for this is the main carbohydrate in dairy foods, lactose, which is only 50% glucose and 50% galactose, another simple sugar. The second example is fructose. Fructose can be found as a simple sugar in fruit and fruit juices, but also in many concentrated sugars and syrups. These include table sugar, which consists of 50% glucose and 50% fructose, high-fructose corn syrup, agave syrup, maple syrup, and honey.
The calculation of the glycemic index entails that people eat 50g of available carbohydrates. In the case of rice, cornflakes, and potatoes, that’s almost 50g of glucose in the form of starch. In the case of agave syrup, which usually consists of about 90% fructose and 10% glucose, it’s only about 5 g in the form of glucose and 45 g in the form of fructose. So obviously blood GLUCOSE levels are going to rise less substantially after eating agave syrup, and that’s why agave syrup has such a low glycemic index (19). If we were to look at blood FRUCTOSE levels, though, it would be a different matter, but we don’t consider this when we are calculating the glycemic index.
Now, you may ask, are blood FRUCTOSE levels relevant as well? I think so, even though the liver filters out much of the fructose we absorb and as a result fructose levels in circulating blood tend to be much lower than circulating glucose levels (usually under 10 mg/dL, ever after a meal rich in fructose). But the fructose then accumulates in the liver, and the liver has to deal with it, so that’s probably not a good thing if we are eating a lot of fructose. Different conversation for another time.
For now, let’s just say that I guess most people would agree that aside from its low glycemic index, agave syrup is clearly not a health food if it’s eaten in large quantities. Mostly, agave syrup, just like table sugar, maple syrup and even honey, is a dense source of calories that doesn’t add much nutritional value.
Now, the fact that the dietary glycemic index could be lowered by eating more agave syrup or maple syrup has led many people, both in academia and on social media, to the conclusion that the glycemic index is useless. I disagree. I’ll repeat what I wrote in the last blog post: it’s just important not to use the glycemic index by itself, as the only measure of the overall healthfulness of a food.
What I would suggest instead is that we don’t use their low glycemic index as an excuse to eat a lot of added sugars and syrups. In other words, let’s minimize high-glycemic index foods and instead eat more whole foods with a low glycemic index that are not fructose bombs.
How about fruit and dairy then? Both have a low glycemic index partly because they contain carbohydrates other than glucose. Fructose in the case of fruit, galactose in the case of dairy. In my mind, a good guiding criterion here is whether the food has nutritional value. I think whole fruit and whole dairy foods are clearly nutritious foods in many ways. I have myself published many papers on dairy, and one review article on fruit, and – looking at the cumulative evidence – whole fruit and berries, milk, yogurt, and cheese are not associated with negative health outcomes (different story, however, for products with added sugar such as puddings or ice cream).
And then there is another limitation of the glycemic index that I haven’t touched on. The glycemic index of foods or diets doesn’t consider in which combination foods are eaten. We’ll discuss in the near future how we can eat high-glycemic index foods such as baked potato or white rice in a way that minimizes their impact on blood sugar levels.
I said earlier that I think the glycemic index may be more important for our health than is reflected in the current literature. What if we compared high-glycemic index diets that are so typical of what many people eat these days to low-glycemic index diets that are also low in fructose from added sugars and syrups? My best guess is that the differences in health outcomes would be much more substantial than what we are seeing in the published literature now.
How I use this Information
As you know, I cannot make any dietary recommendations here, so I’ll just share how I use this information for my family. I personally see the glycemic index as one factor to inform my diet, even though it’s not perfect, and it certainly should not be considered by itself. There are lots of other aspects that make up a healthy diet.
The first conclusion I draw from these data is that it’s almost certainly a good idea to minimize the consumption of high-glycemic index foods, with a glycemic index of 60 or more, by themselves. With regard to blood sugar levels, eating these foods by themselves is similar to eating pure sugar.
Second, I do eat high-glycemic index foods such as rice or potatoes pretty regularly, but never by themselves, which we would call ‘naked’ carbs. I eat them only in complex meals with other foods that have a lot of fiber, protein, and fat, all of which minimizes the overall blood sugar response.
And third, I minimize the consumption of fructose-rich foods such as table sugar, agave syrup, maple syrup, honey, and the like. Basically everything we would use to sweeten other foods. And that is even though these foods have a low glycemic index. An exception here are, in my opinion, fruit and berries. These have a lower glycemic index partly because they contain fructose, and some people may argue fruit consumption should also be limited. I disagree with that. Together with two colleagues, I have authored a review paper on this very issue, and – if we look at the cumulative evidence – we could find no evidence that eating a lot of fruit is associated with poor health outcomes in observational studies or randomized controlled trials.
Summary
In summary, we have seen that eating a diet with a high glycemic index is associated with an increased risk of type 2 diabetes, coronary heart disease, and certain types of cancer, specifically breast, colorectal, and bladder cancer. There are also data from intervention trials showing that a high- when compared to a low-glycemic index diet leads to higher blood sugar levels after a meal in healthy people and also those with prediabetes or diabetes, and a high-glycemic-index diet also raises body weight, insulin resistance, and blood pressure compared to a low-glycemic index diet.
We have also discussed that these effects are seen even though existing studies are a bit limited in some ways. For one, the range of glycemic indices in different groups that are being compared is fairly narrow, suggesting that people who make a more substantive change in their glycemic index could reasonably expect greater benefits. Existing studies usually also don’t take into consideration that fructose in added sugars has a low glycemic index, meaning that low-glycemic index diets in both observational and interventional studies could have that low glycemic index partly because they contain a lot of fructose. In my opinion, a low-glycemic index diet that is also low in fructose from added sugars or syrups is likely a better choice than one that includes a lot of fructose.
If you haven’t already, you can download a poster with the glycemic index and glycemic load data of many common foods here.
As always, please don’t hesitate to leave a comment below if you have any questions.
References
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3 Responses
Could you discuss the risk of using/eating saturated fats from dairy products? The literature you refer to, – the articles I read the abstracts of – seems to indicate no problems vis-à-vis CVD. But some of the authors cited/referred to suggest that plant based fats would be more healthy. I am sorry that I cannot formulate the problem more succinctly. Of course I have a personal interest in the answer as my wife likes cheese and I liked whipped cream. So rather than being guided by my own bias, I would appreciate it if you would share your knowledge of the field on this issue. I am impressed by your nuanced approach, with abundant references to the literature.
Thank you for the suggestion, Ton. I have myself done a randomized controlled trial on the impact of low-fat and full-fat dairy foods on cardiometabolic disease risk. Will cover this in detail at some point for sure.
Cheers
Mario
Thank you for your reply.
That is something to look forward to!
I sure do. Ton