Do Whole Grains Cause Diabetes? - A Review of Evidence

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This article will seek to weigh in on the debate over whether carbohydrates are “bad for you.” In particular, whether eating carbohydrates increases the risk for type 2 diabetes. This is to respond to a common conclusion many have that carbohydrates increase the risk for type 2 diabetes because sugar is associated with diabetes. Contrary to this notion, there is research to suggest that the type of carbohydrate determines whether one may be at an increased or decreased risk for type 2 diabetes.

Whole grains appear to lower the risk of diabetes, whereas refined starches may be associated with an increase in one’s risk. It is the opinion of the author that one must be careful not to make sweeping generalizations, but rather distinguish between refined grains and whole grains, when it comes to the conversation surrounding type 2 diabetes.

All Grains Are Not the Same

If most people were asked the question: “What causes diabetes?” the vast majority of people will simply say: “Sugar,” and if questioned further, likely the individual will have the impression that “sugar” also means “carbs.” The practical implication of this way of thinking is a growing number of people are under the impression that all carbohydrates are “bad.” However, it needs to be emphasized that there are different types of carbohydrates. More specifically, carbohydrate sourced from grains can be divided loosely into two categories: 1) Whole grains and 2) Refined grains. Whole grains are defined by the American Association of Cereal Chemists in the following way: “Whole grains shall consist of the intact, ground, cracked or flaked caryopsis, whose principal anatomical components—the starchy endosperm, germ, and bran—are present in the same relative proportions as they exist in the intact caryopsis.”^[1]

In contrast to this, refined grains are whole grains that have been stripped of one or more of their key parts (bran, germ, and endosperm). For example, white flour is devoid of the bran and germ, leaving only its endosperm. The result? Refined grains have much less fibre, minerals, and vitamins. Though refined grain often tastes better and are easier to chew, nutritionally, they are far inferior to whole grains. Both whole grains and refined grains are ubiquitous throughout society. Brown rice, quinoa, and raw oats are examples of whole grains. On the other hand, muffins, white rice, doughnuts, and Wonder bread are examples of refined grains. As we will see, this distinction is important, because the risk for type 2 diabetes differs based on the type of grain.

Not All Starches Increase the Risk for Type 2 Diabetes

In a 2015 research paper, Dr. Walter Willett of Harvard and his associates state: “starch was associated with a higher risk” of type 2 diabetes.^[2] This was the finding from a prospective cohort study of over 70,000 women from the Nurses’ Health study (1984–2008). They found that those in the highest quintile of starch consumption had a 23% more elevated relative risk for type 2 diabetes compared to those consuming the lowest level of starches.^[2] Just based on the above, one may quickly jump to the conclusion that we should avoid all potatoes, bread, rice, and pastas. However, this would be erroneous, because the study also found that carbohydrate intake was not associated with a risk for type 2 diabetes, even for those who were consuming the highest amounts of total carbohydrates.^[2]

Fibre Made the Difference

This may seem like a confusing contradiction to some; however, it makes a lot of sense when considering the role of fibre. The study found that in those who consumed starches in which the amount of starch was far greater than the corresponding amount of fibre (high starch:fibre ratio), the risk for type 2 diabetes was increased up to 12–39%.^[2] Similarly, those consuming a diet that was high in carbohydrate but low in fibre content (high carbohydrate:fibre ratio) elevated their risk by about 9%. Why is this important? Because high carbohydrate-to-fibre and high starch-to-fibre ratios are indicators of refined grain intake. The authors state: “In addition, all 4 ratios of carbohydrates- or starch-to-total or -cereal fiber were positively associated with risk of T2D, and the strongest association was between starch-to–cereal fiber and risk of T2D, which reflects a diet rich in highly refined and processed grains.”^[2] So, the rise in the risk of type 2 diabetes was not a result of carbohydrates as a whole, but to the type of carbohydrates people were consuming. Not all starches and not all carbohydrates are equal. Refined grains and starches likely increase the risk of diabetes, whereas whole grains do not.

Whole Grains May Reduce the Risk of Type 2 Diabetes

Now, some may be quick to point out that the above study did not directly assess whole-grain intake, and thus argue that saying “whole grains are not associated with an elevated risk of type 2 diabetes” is sort of jumping the gun. This is true in a sense, since Dr. Willett’s paper did not directly report on whole-grain intake. However, their paper bore out that the fibre content of the grains is an important factor. Low fibre appears to elevate risk, whereas high fibre does not. Whole grains are high in fibre; refined grains have almost no fibre. Thus, it is not a stretch to suggest that whole grains do not elevate the risk of type 2 diabetes. This is even more so when one considers evidence from other research studies. There are other papers which argue whole grains do not increase diabetes risk, but in fact may actually decrease it. One group of researchers published a meta-analysis in 2013 to assess this very thing.^[3] They got sixteen studies and statistically analyzed those studies to see if a combined association between grain intake and type 2 diabetes emerged or not. What they found was that whole-grain consumption indeed did result in a significant reduction in the risk of type 2 diabetes. In fact, they found that for every three servings per day of whole grains, type 2 diabetes risk was lowered by about 32%.^[3] Refined grains did not have any effect on the risk of type 2 diabetes. This is not the only report that has found this association; other researchers have drawn the same conclusion as well. Qing et al conclude in their paper that 48–80 g/d (3–5 servings per day) of whole grains could be associated with an approximately 26% reduction in type 2 diabetes risk.^[4] In 2015, researchers found that an increase of 20 g/d of whole grains was associated with a 12% relative reduction in the risk of type 2 diabetes.^[5] The authors conclude, based on their model, that a population consuming 45 g/d of whole-grain ingredients would decrease its risk of type 2 diabetes by 20% versus those consuming 7.5 g/d of whole-grain ingredients.^[5]

But Isn’t Bread Particularly Bad?

Many of those who associate grains with causing diabetes will often have little issue with grains like oats, but commonly take issue with bread. So, does eating bread increase the risk of type 2 diabetes? In the same study mentioned above, the researchers did not find that eating whole-grain bread was associated with an increased risk of type 2 diabetes. Instead, whole-grain bread consumption was associated with a 19% decreased risk of diabetes.^[3] Whole-grain breakfast cereal, brown rice, and wheat bran had a similar effect.

Brown Rice Is Superior to White Rice

There is evidence to suggest that brown rice is not a cause of diabetes, but rather may have a protective effect. In a study examining a large study group, consuming two or more servings of brown rice per week was associated with an 11% reduction in the risk of type 2 diabetes compared to eating brown rice less than once a month.^[6] Conversely, consuming five servings or more of white rice per week was associated with a 17% increased risk of type 2 diabetes versus eating white rice only once a month.^[6] Similarly, in a study already mentioned above, white rice was associated with an increased risk of type 2 diabetes.^[3] White rice is not a health food, and it is very unfortunate that approximately 70% of the rice consumed in the US is white rice.^[6] The science suggests that there is a link between white rice and diabetes risk. On the contrary, brown rice does not seem to pose a risk of type 2 diabetes.

Two Important Lessons Emerge

Reports like these mean it is irresponsible to make a sweeping generalization that carbohydrates “cause diabetes.” In fact, it would suggest to the contrary; that the effect of whole grains is in the opposite direction. Thus, these reports seem to suggest two things:

1. Not all grains are associated with increasing the risk for type 2 diabetes.
2. Whole grains may actually have a protective effect rather than a detrimental one.

Fibre Makes a Lot of Difference

So, why the difference between refined versus whole grains? What do whole grains have that refined grains do not? Well, the one major difference between these two is their fibre content. As mentioned above, high fibre intake is greatly beneficial, whereas low fibre intake may increase the risk of type 2 diabetes. Whole grains contain a lot of fibre, whereas refined grains do not. Why is fibre particularly important for type 2 diabetes? The answer is because type 2 diabetes is a condition in which there is a dysregulation of glucose and insulin: Blood sugars are high, as is insulin. Thus, foods that spike these two rapidly, or too much, will likely not be very helpful. Fibre has been shown to improve insulin sensitivity, and it also slows the speed at which the stomach empties its contents, meaning that the sugar from foods like brown rice is released into the intestine more gradually.^[3][7] The result will be a less rapid rise in sugar getting into the blood, and a corresponding slower rise in insulin.^[4] Researchers demonstrate that refined grains do just that. In one investigation, whole grains, like steel-cut and large-flake oats, was found to have a lower glycemic index compared to the lower-fibre, more refined quick or instant oats.^[8] The effect of foods like white rice is similar. With little nutrients and no fibre, white rice has a higher glycemic index, meaning it will result in a greater rise in blood glucose and a higher insulin response.^[6] On the other hand, brown rice is high in fibre and has the opposite effect.

But Don’t Grains Cause Glucose and Insulin Dysregulation?

Whole grains also have been shown to improve insulin sensitivity.^[3] Research suggests that whole grains actually favour a better control of glucose and insulin metabolism.^[5] These are supported by human studies. A study out of Korea suggests that the switch from eating white rice to brown rice is beneficial. They got subjects and divided them into a whole-grain group and a refined group. The whole-grain group ate brown rice, and the control group ate white rice. After several weeks, the researchers found that switching to the brown rice resulted in improvements of fasting glucose and insulin concentrations.^[9] It is not only rice that has a potentially beneficial effect on insulin. The all-too-often vilified cereal, bread, and pasta may have positive influences on postmeal insulin responses as well. In one study, researchers compared individuals with metabolic syndrome consuming either a whole-grain diet or a refined-grain diet. The whole grains included in the study were things like whole-wheat bread, whole-meal pasta, barley soup, oat biscuit, and whole-grain breakfast cereals. The refined grains diet consisted of foods such as wheat bread, rice, pizza, cornmeal porridge, and breakfast cereals like Rice Krispies.^[10] After 12 weeks, only the whole-grain group had improvements in the insulin response. More specifically, postmeal insulin change was reduced by 29%, and even the postmeal triglyceride levels were lower. No significant changes were found in the refined grains group in these parameters.^[10]

Conclusion

One can easily go on PubMed (a large, widely-used database for health research) and do a search for whole grains and diabetes, and they will find many papers that report that whole-grain intake is actually beneficially associated with type 2 diabetes. The notion that whole-grain intake will “cause diabetes” is wholly without a scientific foundation. Whole grains, likely because of their high fibre intake, may have beneficial effects that is lacking in refined grains. Not all grains are equal, and thus overly generalized recommendations to avoid carbohydrates because this can lead to diabetes are not warranted.