Type 2 diabetes, which unlike Type 1 does not involve an inability to produce insulin but rather involves insulin production being overworked, is overwhelmingly related to obesity. That is why unlike Type 1 it is more of an adult disease. To prevent it requires more than using a type of food, it involves consuming less of all foods.
Whole grains are defined as consisting of all three main components of the grain kernel: endosperm, germ, and bran. To make their conclusion about whole grains - a popular dietary fad in Denmark - they used a prospective Danish cohort of more than 55,000 participants, who were between 50-65 years old when the study started. During the initiation of the cohort study in the early 1990s, healthy participants filled out questionnaires regarding their eating habits. Through these, the researchers established the participants’ total wholegrain intake per day, which of the most common cereals they got their wholegrain from, (wheat, rye, oats, in grams per day), and the total number, and different types, of wholegrain products (in grams per day) – rye bread, other wholegrain breads, oatmeal porridge and muesli. The cohort study was linked with data from Denmark’s national diabetes register, to investigate which participants developed type 2 diabetes during a 15 year period – which in total was over 7,000 people.
If you know anything about food frequency questionnaires, you understand the problem. Only the healthiest people recall their choices accurately, which skews the results.
Using their self-reported claims, the participants were divided into 4 different groups, based on how much wholegrain they reported eating. Those with the highest consumption ate at least 50 grams of wholegrain each day. This corresponds to a portion of oatmeal porridge, and one slice of rye bread, for example. The proportion who developed type 2 diabetes was lowest in the group which reported the highest wholegrain consumption, and increased for each group which had eaten less wholegrain. In the group with the highest wholegrain intake, the diabetes risk was 34 percent lower for men, and 22 percent lower for women, than in the group with the lowest wholegrain intake.
Based on that they confirm government claims that we should all switch out white flour for whole grains. They even go on to caution against low carb diets because, they write, "there hasn’t been a single study which has shown negative health effects.”
Citation: Cecilie Kyrø, Anne Tjønneland, Kim Overvad, Anja Olsen, Rikard Landberg, 'Higher Whole-Grain Intake Is Associated with Lower Risk of Type 2 Diabetes among Middle-Aged Men and Women: The Danish Diet, Cancer, and Health Cohort', The Journal of Nutrition, Volume 148, Issue 9, 1 September 2018, Pages 1434–1444, https://doi.org/10.1093/jn/nxy112