“Healthy” whole grains: What the evidence really shows

You hear about them all the time: those allegedly nutritious whole grains you should be eating every day if your goals include being slim and healthy — and whose goals don’t include that?

Do whole grains, however, really live up to their reputation as a superfood? Let’s take a closer look at the scientific evidence behind the claims made about their benefits. Then you can decide whether you need a daily dose of grains in your diet or not.

Disclaimer: The effects of whole grains on human health is controversial, with inconclusive evidence for long-term health effects.1 While whole grains appear to be better than refined grains for many health outcomes, there is almost a complete lack of research on whether whole grains are better than no grains.

This guide is our attempt at summarizing what is known. It is written for adults who are concerned about whole grain intake and health.

Discuss any lifestyle changes with your doctor. Full disclaimer

First, what are whole grains?

Technically, whole grains are the seeds of cereal grasses. In their natural “whole” state, grains have a hard, inedible husk that covers three edible parts:

  • Bran: fiber
  • Germ: contains some B vitamins, minerals, fat, and protein
  • Endosperm: major portion of the grain; mainly starch with a small amount of protein, vitamins and minerals

Nutritionally speaking, whole grains have had their outer inedible husks removed but retain all three edible parts of the seed. By contrast, refined grains like white flour (including unbleached wheat flour) and white rice have their bran and germ removed during milling, leaving only the endosperm.

Most whole grains have some processing. For instance, whole wheat is ground or crushed to create whole-wheat flour; old-fashioned oats are steamed and rolled in order to make them more palatable and easier to digest.

Wild grains appear to have been eaten by hunter-gatherers in certain regions during the paleolithic era, including the areas known today as Southern Italy and Africa.2 During the Agricultural Revolution, larger quantities of wheat, barley, rice and other grains were grown and became a part of the human diet in many other areas.

Since then, people around the world have consumed a variety of grains based on cultural preferences and availability. Among the dozens of types of whole grains that exist, some of the most well-known and widely consumed include:

  • Barley
  • Brown rice
  • Bulgur
  • Corn
  • Oats
  • Rye
  • Whole wheat
  • Wild rice

According to the Whole Grains Council, the most commonly consumed whole grains in the US are whole wheat, oats, and brown rice.

Buyer beware: over the past several decades, the term whole grains has become a buzzword among the health conscious. Knowing this, manufacturers often include bold, eye-catching messages like “Contains 14 grams of whole grains” on boxes of cereal, whole-wheat pasta, granola bars and similar products, which often contain high levels of added sugar or refined grains, too, as well as being very high in carbohydrates.

In fact, a 2013 review of more than 500 grain-based products found that those displaying a “whole grains” stamp contained more sugar — and were more expensive — than similar products without the stamp.3

This does not necessarily make processed “whole-grain” products a healthy choice, despite their labels’ claims.

Nutrition in whole grains

Are whole grains really a nutrient-packed energy source? While some types contain a bit more protein and micronutrients than others, their nutrition profiles aren’t very impressive overall, aside from being very high in manganese (like many other foods).

For instance, a bowl of steel-cut oats — often suggested as an ideal meal to start your day — provides about 10 grams of protein (although lacking in some of the essential amino acids found in complete animal protein), 8 grams of fiber, and small amounts of thiamin, iron, magnesium, selenium and zinc. However, it also contains 46 to 48 grams of net carbs, even when prepared without milk, fruit, sweeteners or other additives.

How about using two slices of whole wheat bread to build your sandwich at lunch? This would provide about 7 grams of (incomplete) protein, half of your daily selenium needs, and small amounts of thiamin, niacin, and magnesium, which would come with 40 grams of net carbs and a comparably low 6 grams of fiber.

Brown rice’s nutritional profile is similar to that of whole wheat bread and oats, although lower in protein and fiber.

Additionally, trendy grains like quinoa and farro are often lauded for being higher in protein and easier to digest than other grains, but they too are very high in net carbs, relatively low in fiber, and lacking substantial amounts of key nutrients.

Most whole grains contain a substance called polyphenols, which are compounds found in plants that might potentially help protect cells from damage and reduce inflammation in the body. However, higher-quality studies are needed to confirm this.4 Indeed, we just don’t know enough about polyphenols to make them a priority for inclusion in our diets.

And that leads us to the quality of the existing research on whole grains and health.

Whole-grain research: weak evidence overall

Studies about whole grains seem to receive more than their fair share of media coverage these days. One such study to make recent international news headlines was a series of systematic reviews and meta-analyses exploring the health effects of different types of carbohydrate, which was published in The Lancet early in 2019.

After analyzing 185 observational studies and 58 clinical trials, researchers concluded that eating more whole grains and fiber might be an effective strategy for preventing obesity, heart disease, diabetes, cancer, and reducing risk of early death.5 Headlines also suggested that people would live longer if they ate more healthy grains and that the findings “were a blow to fashionable low-carb diets.”

We wrote about that study when it came out, noting that it was not comparing diets with whole grains to low-carb diets, but rather to diets full of highly-refined grains. (And we addressed the erroneous myth that low-carb diets are low-fiber diets. They are not!)

Diet Doctor: A low-carb diet does not mean a low-fiber diet

Importantly, the claims made for whole grains’ beneficial health effects are largely based on epidemiological or observational studies. This kind of evidence has a hard time proving cause and effect relationships. Especially in nutrition, it can rarely be relied upon to draw any firm conclusions due to bias, research methods and other factors, as Stanford professor John Ioannidis has repeatedly pointed out:

Video from Swiss Re “Food for Thought”: The role of bias in nutritional research

American Council on Science and Health: John Ioannidis aims his bazooka at nutrition science

Epidemiological studies rely on self-reporting and the notoriously inaccurate food-frequency questionnaire, in which people are asked to recall how many times they ate certain foods within a specific period of time — an extremely difficult task for most people.6

Additionally, since the studies are observational, they cannot claim that eating more whole grains actually causes people to have greater weight loss, health improvements or decreased risk of disease or death; they can just say that they observed a correlation. Remember the oft-said phrase “correlation is not causation”? Such studies cannot prove anything. Learn more about observational vs. experimental studies:

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Guide to observational vs. experimental studies

Guide In this guide, we discuss the differences between observational and experimental studies, the advantages and disadvantages of each.

What’s more, the observational associations between eating whole grains and staying slim and healthy are statistically weak. When an observational study’s hazard ratios are less than 2, any evidence tying a behavior to an outcome is marginal at best. In observational studies looking at whole grains and health, hazard ratios have consistently been well below 2, meaning that any relationship between the two may be due to confounding variables or healthy user bias rather than a true cause and effect.

An association between frequent whole-grain consumption and good health is a perfect example of the “healthy user bias.” We don’t know if the reason people who eat whole grains tend to be healthier than those who don’t is solely because of this one aspect of their diet, or because they tend to practice many other health-conscious behaviors like exercising regularly, drinking alcohol in moderation, avoiding sugar-sweetened beverages and cooking fresh food at home on a regular basis.

But whole grains just seem so healthy, you may be thinking. And it’s certainly understandable to feel that way, given what we’ve all been hearing from various health organizations over the past several decades. However, let’s examine the experimental (higher-quality) randomized control trial (RCT) evidence to see how well it supports the claim that whole grains can improve your health.

RCTs are designed to compare an intervention (such as consuming more whole grains) with a control (consuming refined grains or a standard diet). A very important question to ask in these whole grain RCT studies is what were whole grains in the diet compared to? Did they compare eating whole grains to eating refined grains or a standard diet? Or did they compare eating whole grains to eating low-carb vegetables? It makes a difference.

Virtually all the RCTs for whole grains have compared them to refined grains or a standard diet. Let’s take a closer look.

Whole grains and weight loss

Many nutrition authorities, such as on this website, keep sharing this message: Eating plenty of whole grains can help you achieve and maintain a healthy body weight. But how strong is the (RCT) evidence linking high whole-grain intake and weight loss?

A 2013 systematic review of RCTs — considered the strongest type of evidence, as it includes multiple RCT studies rather than relying on just one — found no differences in weight loss but slightly higher fat loss (less than 0.5% difference) in groups who consumed diets high in whole grains compared to groups who consumed diets high in refined grains.7 A newer systematic review published in 2019 found no evidence of whole-grain consumption having any effect on weight loss.8

I other words, overall the effect of whole grains on weight loss seems to be small or even non-existent. However, in some studies positive effects have been noticed.

It is perhaps not surprising that whole grains have occasionally been shown to outperform refined grains in experimental weight-loss studies. Some RCT studies have found that among normal weight and overweight adults, those who consumed whole grains for periods ranging from four to 16 weeks experienced greater increases in resting metabolic rate and greater decreases in belly fat, insulin resistance, inflammation and body weight compared to those who consumed refined grains.9

In one study, people who ate whole rye products had greater fat loss compared to those who consumed refined grains, whereas consuming whole wheat didn’t seem to provide any fat-loss benefits over refined wheat.10

Bottom line: Adding whole grains to the diet does not appear to have any significant beneficial effect on body weight, on average. However, replacing refined grains with whole grains may have modest benefits.

Whole grains and diabetes

Can eating whole grains on a regular basis help prevent diabetes and keep blood sugar from rising too high after meals?

Results from experimental trials looking at blood sugar response to whole grains have been mixed. A 2017 systematic review of RCTs found that whole grains seem to raise blood sugar less than refined grains do, at least in healthy people.11

However, an even more recent review of RCTs showed that non-diabetic individuals had almost identical blood sugar responses after eating whole or refined wheat or rye. The blood sugar increase, however, was much higher after eating white rice compared to whole-grain rice.12

Still, even though white rice raised people’s blood-sugar levels more quickly, whole-grain (brown) rice was also found to raise blood sugar to some extent in these studies, with differences likely related to individual tolerance, different methods of measuring, and possibly other factors as well.

At this time, there are only a few RCTs comparing blood sugar responses to whole vs. refined grains in obese people and those with diabetes. Overall, they have shown that replacing processed with whole grains modestly improves blood sugar and insulin regulation.13

So is this a good strategy for diabetes reversal or even adequate glycemic control? Probably not. For instance, in one of these studies, when diabetic adults ate 50 to 100 grams of whole-grain oats on a daily basis, their HbA1c and fasting blood sugar levels were still well above target range after one month, as well as when researchers followed up with them one year later.14

Wholegrain food still life

Whole grains and heart disease

Whole grains are often referred to as “heart-healthy” foods. So what is the evidence supporting the cardio-protective effects of whole grains?

Experimental studies often show improvements in certain heart disease risk factors when whole grains are substituted for refined grains. Two meta-analyses of RCTs found minor reductions in LDL cholesterol and triglycerides in groups who consumed whole grains compared to groups who consumed refined grains, with oats appearing to have the most cholesterol-lowering power.15

However, reducing isolated risk factors does not necessarily translate into improved health, especially if one marker improves while others worsen (such as low-density lipoprotein improving but insulin resistance worsening.)

Moreover, in 2017, authors who performed a systematic review of nine RCTs concluded that there isn’t enough evidence to support claims that whole grains lower CVD risk. They made that conclusion due to the lack of high-quality controlled research, including small sample sizes and a high risk of bias (including funding from pro-cereal organizations) found in some of the trials they assessed.16

This highlights the importance of understanding how low-quality research has biased the support for the “heart-healthy” claim for whole grains. When scrutinized with a higher level of scientific integrity, the data doesn’t seem to hold up.

Whole grains and cancer risk

Whole grains are often promoted by various groups, such as cancer agencies, as a food that helps fight cancer, especially colorectal cancer. This is based on some observational studies with weak correlations suggesting that people who eat the most whole grains may be at lower risk for certain cancers, while others show no association at all.17

Experimental research, however, that tests the effect of consuming whole grains on cancer risk is entirely lacking. Remember, because observational studies show associations and not causation, they’re considered a very low quality of evidence. There currently are no good RCTs about whole grains and cancer risk.

So based on the lack of robust evidence and the conflicting results of lower-quality observational trials, at this point it doesn’t sound as though there’s much convincing evidence that whole grains are protective against cancer.

Whole grains and other attributed health benefits

Whole grains have also been linked to a few other health improvements:

  • Reduced inflammation: Chronic system-wide inflammation is believed to be at the root of many chronic diseases, including heart disease.18 Two meta-analyses of RCTs found that consuming whole grains helped reduce the inflammatory markers C-reactive protein (CRP) and interleukin-6 (IL-6) but did not change other markers of inflammation — again, compared to eating refined grains.19
  • Better gut health: Bacteria that reside in your colon produce short-chain fatty acids as a byproduct of digesting fiber. Results from RCTs suggest that consuming whole grains seems to boost production of these short-chain fatty acids — which nourish the gut and may improve insulin sensitivity — that is, again, compared with consuming refined grains.20

While these studies may sound convincing, keep in mind that they are not comparing whole grains to a grain-free, low-carb diet, but rather to the consumption of highly refined grains.

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Comparing whole grains to no-grains

To recap, the RCTs to date around whole grains demonstrate that, yes, eating a less-processed food is better than eating a highly-processed one.

What would the outcomes be if studies compared consuming three servings of whole grains to three servings of non-starchy vegetables per day on a low-carb diet? It could be that under such an RCT healthy whole grains might lose a bit of their halo — but we don’t know that yet because such a trial has never been done, at least not one that we can find in the research literature.

One RCT study, however, suggests that the no-grain approach might be superior. Although not a head-to-head trial pitting whole grains vs. no grains, back in 2007 researchers showed that a grain-free, very-low-carb ketogenic diet was much more effective than a low-glycemic-index (low-GI) diet, which included whole grains, for weight loss and decreasing blood sugar in overweight adults with type 2 diabetes.21

While this result probably comes as no surprise to those of us following a low-carb or keto lifestyle, in order to strengthen the evidence, we need more RCTs comparing the health effects of low-carb grain-free diets to whole grain diets in both healthy people and those with diabetes.

Whole grains don’t provide unique nutritional benefits

While it’s true that whole grains contain some vitamins, minerals, fiber, and polyphenols, these nutrients can be found in many other foods, often in greater concentrations:

  • Fiber: Looking for fiber? Eat cruciferous vegetables, flaxseed, chia seeds, nuts, avocados, broccoli, blackberries and raspberries — all of which provide more fiber than whole grains and far fewer digestible carbohydrates.
  • Magnesium: Although most whole grains are considered a good source of magnesium, mackerel, cooked greens, almonds, hemp seeds, pumpkin seeds and sunflower seeds contain equivalent or greater amounts per serving.
  • Thiamin: Seafood has the edge over whole grains for thiamin, and pork is higher in this B vitamin than any other food.
  • Niacin: Chicken, turkey, red meat (especially liver), fatty fish and avocado are better sources of niacin than whole grains.
  • Polyphenols: The jury is still out when it comes to the effects of plant phenols on health. However, you certainly don’t need to consume whole grains to get them. Vegetables, berries, olive oil, tea, spices, coffee, and dark chocolate contain an array of potentially beneficial polyphenols.22

Tolerance for whole grains varies among individuals

While whole grains may get more credit for health benefits than they deserve, some people can eat them regularly without experiencing any issues — and so they would have no pressing need to avoid them. Others, however, cannot.

Individuals with celiac disease need to avoid gluten-containing whole grains. Gluten — a protein found in wheat, barley rye and the trendy wheat derivatives farro, spelt and triticale — can cause damage to the digestive tract and other symptoms when consumed in even small amounts by those with celiac disease.

People with celiac disease or non-celiac gluten sensitivity must eat a gluten-free diet in order not to experience symptoms.23 These individuals may be able to tolerate other types of whole grains like brown rice or quinoa. However, they might benefit from avoiding all grains, as even types that don’t naturally contain gluten are sometimes cross-contaminated with those that do during processing and packaging.

There are also people for whom whole grains might cause other problems, such as IBS and skin problems.24 In addition, although not widely accepted among everyone in the scientific community, some researchers suggest that removing grains from the diet may be beneficial for people with mental health issues.25

While there isn’t much published research about adverse reactions to whole grains, many people report that they feel better and notice health benefits, such as improved digestive function when they minimize or avoid grains of all types.

Whole grains can increase blood sugar more than commonly believed

Whole grains rank surprisingly high on the glycemic index (GI), the scale that measures how much a specific food raises blood sugar.26

The amount of processing grains undergo will influence their GI. Yet even minimally processed steel-cut oats have a moderate GI of 55, and quick-cooking oatmeal has a GI over 70.27 By contrast, cabbage and spinach have very low GIs of 15 and 6, respectively, and meat, fish, cheese, and fats are zero-GI foods.

As discussed earlier, whole grains have been shown to raise people’s blood sugar levels less than refined grains do in most experimental studies. But what is the blood sugar response to an entirely grain-free diet?

The paleo diet, which excludes grains, was found to be more effective than conventional dietary recommendations for lowering blood sugar and insulin levels in people at risk for metabolic syndrome, according to a 2015 meta-analysis of high-quality controlled studies.28

In fact, there are hundreds of online personal stories of people with diabetes who have achieved normal blood sugar control for the first time in years as a result of adopting an entirely grain-free low-carb or keto diet. Of course, this isn’t surprising, given that whole grains are so high in carbohydrates.

Moreover, even people without diabetes or prediabetes can experience blood sugar elevations after eating whole grains. Recently, Lily Nichols — a young, healthy non-diabetic dietitian — wore a continuous glucose monitor (CGM) for ten days in order to learn more about her own blood sugar patterns in response to various foods.

After consuming her typical lower-carb, real-food meals, she could see that her blood sugar increased only minimally. However, when she experimented with eating a small portion of old-fashioned oats as part of a low-fat meal, the CGM revealed that her blood sugar more than doubled within an hour.

Summary: so, should we eat whole grains or avoid them?

Whole grains appear to have a fairly clear edge over refined grains, with lower quality studies showing they can be an integral part of a beneficial diet. However, there’s a lack of evidence comparing eating a diet high in whole grains to eating a grain-free diet.

This means that for people who tolerate grains and enjoy eating them, minimally processed and whole grain types are likely best. But if you don’t feel a need to eat grains, you’re probably fine without them.

Keep in mind that whole grains don’t offer any unique nutritional benefits that can’t be found in other foods. And again, recommendations to consume them regularly to promote better health aren’t based on the highest quality scientific evidence.

What’s more, given their high carbohydrate counts, they don’t have any place in a low-carb lifestyle other than on rare occasions, if at all. Fortunately, Diet Doctor has plenty of tasty grain-free low-carb bread and porridge recipes available that can be enjoyed instead.

Do you need whole grains? In all likelihood, no. At the end of the day, why not experience the benefits of eating a wide variety of keto-friendly whole foods that are delicious, satisfying, and often nutritionally superior to whole grains?

/ Franziska Spritzler, RD

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  1. Cochrane Database of Systematic Reviews 2017: Whole grain cereals for the primary or secondary prevention of cardiovascular disease [strong evidence]

  2. Proceedings of the National Academy of Sciences of America 2015: Multistep food plant processing at Grotta Paglicci (Southern Italy) around 32,600 cal B.P. [historical data; ungraded]

    Science 2009: Mozambican grass seed consumption during the Middle Stone Age [historical data; ungraded]

  3. Public Health Nutrition 2013: Identifying whole-grain foods: a comparison of different approaches for selecting more healthful whole grain products [overview article; ungraded]

  4. Nutrients 2018: Whole grains and phenolic acids: a review on bioactivity, functionality, health benefits and bioavailability [overview article; ungraded]

  5. The Lancet 2019: Carbohydrate quality and human health: a series of systematic reviews and meta-analyses [Weak statistical correlation; very weak evidence]

  6. Cancer Epidemiology, Biomarkers & Prevention 2005: Is it time to abandon the food frequency questionnaire? [overview article; ungraded]

  7. American Journal of Clinical Nutrition 2013: Whole grain and body weight changes in apparently healthy adults: a systematic review and meta-analysis of randomized controlled studies [strong evidence]

  8. Advances in Nutrition 2019: Whole-grain consumption does not affect obesity measures: An updated systematic review and meta-analysis of randomized clinical trials [strong evidence]

  9. Resting metabolic rate

    American Journal of Clinical Nutrition 2017: Substituting whole grains for refined grains in a 6-wk randomized trial favorably affects energy-balance metrics in healthy men and postmenopausal women [moderate evidence]

    Belly fat

    Plant Foods for Human Nutrition 2018: Effects of whole grain wheat bread on visceral fat obesity in Japanese subjects: a randomized double-blind study [moderate evidence]

    Insulin resistance

    Metabolism 2018: A whole-grain diet reduces peripheral insulin resistance and improves glucose kinetics in obese adults: A randomized-controlled trial [moderate evidence]

    Inflammation and body weight

    Gut-British Medical Journal 2019: Whole grain-rich diet reduces body weight and systemic low-grade inflammation without inducing major changes of the gut microbiome: a randomised cross-over trial [moderate evidence]

  10. European Journal of Clinical Nutrition 2017: Wholegrain rye, but not wholegrain wheat, lowers body weight and fat mass compared with refined wheat: a 6-week randomized study [moderate evidence]

  11. Nutrients 2017: Whole grain intake and glycaemic control in healthy subjects: a systematic review and meta-analysis of randomized controlled trials [strong evidence]

  12. American Journal of Clinical Nutrition 2018: The effects of whole-grain compared with refined wheat, rice, and rye on the postprandial blood glucose response: a systematic review and meta-analysis of randomized controlled trials [strong evidence]

  13. Nutrients 2016: Short- and long-term effects of wholegrain oat intake on weight management and glucolipid metabolism in overweight type-2 diabetics: a randomized control trial [moderate evidence]

    Metabolism 2018: A whole-grain diet reduces peripheral insulin resistance and improves glucose kinetics in obese adults: A randomized-controlled trial [moderate evidence]

    American Journal of Clinical Nutrition 2014: Effects of whole and refined grains in a weight-loss diet on markers of metabolic syndrome in individuals with increased waist circumference: a randomized controlled-feeding trial [moderate evidence]

  14. Nutrients 2016: Short- and long-term effects of wholegrain oat intake on weight management and glucolipid metabolism in overweight type-2 diabetics: a randomized control trial [moderate evidence]

  15. American Journal of Clinical Nutrition 2015: Whole-grain and blood lipid changes in apparently healthy adults: a systematic review and meta-analysis of randomized controlled studies [strong evidence]

    European Journal of Nutrition 2018: Comparative effects of different whole grains and brans on blood lipid: a network meta-analysis [strong evidence]

  16. Cochrane Database of Systematic Reviews 2017: Whole grain cereals for the primary or secondary prevention of cardiovascular disease [strong evidence]

  17. Nutrition Reviews 2016 : Consumption of whole grains and cereal fiber in relation to cancer risk: a systematic review of longitudinal studies [Weak statistical correlation; very weak evidence]

  18. European Cardiology Review 2017: Inflammation revisited: atherosclerosis in the post-CANTOS era [overview article; ungraded]

  19. Medicine (Baltimore) 2018: Whole grain diet reduces systemic inflammation [systematic review of randomized controlled trials; strong evidence]

    Journal of the American College of Nutrition 2018: Effects of whole-grain consumption on selected biomarkers of systematic inflammation: a systematic review and meta-analysis of randomized controlled trials [strong evidence]

  20. American Journal of Clinical Nutrition 2017: Substituting whole grains for refined grains in a 6-wk randomized trial has a modest effect on gut microbiota and immune and inflammatory markers of healthy adults [moderate evidence]

    Nutrition 2016: Effects of whole-grain cereal foods on plasma short chain fatty acid concentrations in individuals with the metabolic syndrome [randomized trial; moderate evidence]

    British Journal of Nutrition 2015: Increased gut hormones and insulin sensitivity index following a 3-d intervention with a barley kernel-based product: a randomised cross-over study in healthy middle-aged subjects [moderate evidence]

  21. Nutrition & Metabolism 2008: The effect of a low-carbohydrate, ketogenic diet versus a low-glycemic index diet on glycemic control in type 2 diabetes mellitus [randomized trial; moderate evidence]

  22. Frontiers in Nutrition 2018: The role of polyphenols in human health and food systems: a mini-review [overview article; ungraded]

  23. Journal of the American Medical Association 2017: Celiac disease and nonceliac gluten sensitivity: a review [overview article; ungraded]

  24. Gastroenterology 2014: A diet low in FODMAPs reduces symptoms of irritable bowel syndrome [randomized trial; moderate evidence]

    Skin Therapy Letter 2018: Skin and diet; an update on the role of dietary change as a treatment for skin disease [overview article; ungraded]

  25. Frontiers in Human Neuroscience 2016: Bread and other edible agents of mental disease [overview article; ungraded]

  26. American Journal of Clinical Nutrition 2008: Glycemic index, postprandial glycemia, and the shape of the curve in healthy subjects: analysis of a database of more than 1000 foods [Weak statistical correlation; very weak evidence]

  27. British Journal of Nutrition 2015: Systematic review of the effect of processing of whole-grain oat cereals on glycaemic response [systematic review of uncontrolled studies; weak evidence]

  28. American Journal of Clinical Nutrition 2015: Paleolithic nutrition for metabolic syndrome: systematic review and meta-analysis [strong evidence]