More than 6 million people with type 2 diabetes live in Germany – and the trend is rising. Long-term complications can reduce the quality of life and lead to premature death. Eating behavior, physical activity and other lifestyle factors are considered to be a key to influencing the onset and development of the metabolic disorder. In particular, a diet with lots of insoluble fiber – primarily from whole grains – has been hypothesized to be protective. This is the conclusion of a number of large epidemiological observational studies. But people who eat a lot of whole grains often also lead a healthier lifestyle overall. So it was previously unclear whether the positive effects actually are due to the insoluble dietary fiber.
Tracking the effect of insoluble dietary fibers
Kabisch and his team wanted to understand exactly whether and how insoluble fiber can protect against type 2 diabetes. To this end, they conducted a randomized study, the Optimal Fiber Trial for Diabetes Prevention, or OptiFiT study for short. "The results indicate that insoluble fiber actually has an impact on blood glucose levels and possibly also on other metabolic parameters," said Dr. Stefan Kabisch, head of the study and postdoc in the Clinical Nutrition / DZD research group at the German Institute of Human Nutrition Research Potsdam-Rehbrücke (DIfE). Through previous analyses of the OptiFiT study, the researchers found that, among other benefits, insoluble fiber has a positive effect on the long-term blood glucose value (HbA1C). “The data from the current studies now show that there are subgroups of patients who particularly benefit from fiber. For example, test subjects with an additionally increased fasting glucose level improved their glucose tolerance, and obese test subjects improved their inflammatory values if they were in the fiber group,“ said Kabisch.
Design of the OptiFiT study
Between March 2010 and October 2012, a total of 180 subjects with a precursor of type 2 diabetes participated in the study. The participants received identical nutritional advice and were additionally divided into two groups. The first group received insoluble oat-based fiber in the form of drinking powder twice daily for two years. The second group received only a placebo, i.e. a dietary fiber-free supplement. In order to assess whether an improvement in the metabolic state was really apparent, the research team carried out oral glucose tolerance tests. Because the intervention was double-blinded, neither the test subjects nor the researchers knew who received which supplement. “From a purely methodological point of view, this is a very high-quality study that can reveal fairly precisely whether it was really the insoluble fiber that had the positive effect," said Kabisch, who led the study.
Special benefit for patients with elevated fasting glucose
The increased benefit of insoluble fiber for people with elevated fasting glucose levels may indicate that patients with fatty liver in particular may benefit from the treatment. "Prediabetics with elevated fasting glucose often also have fatty liver. Patients without fatty liver may not benefit as much from a high-fiber diet," Kabisch explained. But since not all patients in the OptiFiT study underwent a fatty liver test, the only interpretation left was via this indirect route. Prediabetics with elevated fasting glucose also tend to be more overweight than those with normal fasting glucose levels. However, according to the new evaluation of the OptiFiT study, overweight does not explain the particular positive response of prediabetics with elevated fasting glucose. "The additional benefit in terms of inflammatory processes in obese patients is an independent advantage. The new analyses therefore provide an important impetus in the direction of individualized nutrition therapy," said Kabisch.
With biomarkers for individualized nutritional therapy
In the next step, the researchers would like to use the data from the OptiFiT study for analyses of certain biomarkers that are related to fatty liver and the observed metabolic improvement. The aim is to be able to predict who will respond to which food components and how. In addition, follow-up studies are planned with new volunteer test subjects to confirm the current results.
Kabisch, S., Meyer, N. M. T., Honsek, C., Gerbracht, C., Dambeck, U., Kemper, M., Osterhoﬀ, M. A., Birkenfeld, A. L., Arafat, A.M., Weickert, M. O., Pfeiﬀer, A. F. H.: Obesity Does Not Modulate the Glycometabolic Beneﬁt of Insoluble Cereal Fibre in Subjects with Prediabetes—A Stratiﬁed Post Hoc Analysis of the Optimal Fibre Trial (OptiFiT). Nutrients 11, E2726 (2019) Open Access
Honsek, C., Kabisch, S., Kemper, M., Gerbracht, C., Arafat, A. M., Birkenfeld, A. L., Dambeck, U., Osterhoff, M. A., Weickert, M. O., Pfeiffer, A. F. H.: Fibre supplementation for the prevention of type 2 diabetes and improvement of glucose metabolism: the randomised controlled Optimal Fibre Trial (OptiFiT). Diabetologia 61, 1295-1305 (2018) Open Access
Kabisch, S., Meyer, N. M. T., Honsek, C., Gerbracht, C., Dambeck, U., Kemper, M., Osterhoff, M. A., Birkenfeld, A. L., Arafat, A. M., Hjorth, M. F., Weickert, M. O., Pfeiffer, A. F. H.: Fasting glucose state determines metabolic response to supplementation with insoluble cereal fibre: a secondary analysis of the Optimal Fibre Trial (OptiFiT). Nutrients 11: e2385 (2019) Open Access
Dietary fibers form a group of many different long-chain carbohydrates that the body’s intestinal enzymes cannot digest. A distinction is made between fiber that is soluble or insoluble in water. Soluble fiber is mainly found in fruits and vegetables, insoluble fiber mainly in cereals and legumes.