【Background】
Many dietary guidelines and treatments for diabetes recommend consuming whole grains because a diet rich in whole grains contributes to a lower incidence of type 2 diabetes, heart disease, and colorectal cancer. However, the definition of "whole grain" includes highly processed and reconstituted grains. Finely milled whole grains can be added to highly processed foods and may increase energy intake and weight gain. Some reports finely milled grains can cause a spike in postprandial blood glucose levels. Therefore, this study examined the effects of consuming whole grains with different degrees of processing on glycemic control in diabetic patients.
【Methods】
The subjects were 31 type 2 diabetic patients (63 years old, BMI 32.4, HbA1c 7.5, all median). Glycemic variability was assessed during two weeks of consumption of minimally processed whole grains (oats, brown rice, and bread made from coarsely ground wheat and crushed wheat grains; "low-processed whole-grain diet") and two weeks of consumption of highly processed whole grains (instant oats, brown rice pasta, and bread made from finely ground whole wheat grains; "high-processed whole-grain diet"). A randomized crossover study was conducted to measure blood glucose variability using a continuous glucose monitor (CGM) during a 2-week washout period. The washout period was 2 weeks.
The subjects were instructed to determine their daily cereal intake from their food diaries for 4 days before the intervention and replace the equivalent amount with the test meal. No other dietary or lifestyle instructions were given. The nutritional composition of the test diets and the particle size breakdown of the whole grains included are shown in the table. The major nutrients and fiber content of both test diets were consistent. They met the American Association of Cereal Chemists (AACC) standards for whole grains.
The area under the blood glucose elevation curve (iAUC) was calculated using the CGM data up to 3 hours after each of the three meals. Data from 28 individuals who carried out the two intervention trials were used in the analysis.
【Results】
In both intervention periods, the intake of carbohydrates, starch, and fiber increased instead of decreasing fat intake compared to the pre-intervention period. There was no difference in energy intake, major nutrients, and fiber intake between the two intervention periods. There was no difference in energy intake, major nutrients and dietary fiber intake between the two intervention periods. 5.5 servings of the test meal per day were consumed during both intervention periods, with the highest intake at breakfast, 2.3 servings for the low-processed whole-grain meal, and 2.2 servings for the high-processed whole grain meal.
Post-breakfast iAUC was 9% lower during the low-processed whole-grain meal period than during the high-processed whole-grain meal period and 6% lower on average for all three meals. Bodyweight decreased on the low-processed whole-grain diet but increased on the high-processed whole-grain diet, with a difference of 0.81 kg.
【Discussion and conclusion】
The low-processed whole-grain diet improved blood glucose levels compared to the period of the high-processed whole-grain diet. The only difference between the two diets was in the structure and particle size of the whole grains. There was no difference in energy intake during the intervention period, but there was a difference in the change in body weight.
The difference in whole-grain particle size due to the degree of processing may have changed the digestibility of starch. The less processed whole grains passed through the small intestine and became short-chain fatty acids by fermentation in the large intestine and were absorbed without affecting blood glucose levels. On the other hand, the particles of highly processed whole grains are smaller, making it easier for water and digestive enzymes to access the starch, resulting in higher sugar absorption. Moreover, flouring of whole grains may affect the interaction of starch and protein, and alter digestibility.
Guidelines for the management of diabetes recommend whole-grain foods regardless of grain structure or particle size. However, the health benefits of whole grains may be diminished with increased processing.
【Research institution】
University of Otago (New Zealand), Karolinska Institutet (Sweden), Liddett Institute (New Zealand), Lincoln University (New Zealand)
Whole-Grain Processing and Glycemic Control in Type 2 Diabetes: A Randomized Crossover Trial
Diabetes Care dc200263, 2020
