Our compliance-based analysis supports previous cohort studies, which reported a beneficial association of higher intake of insoluble oat hulls fibre on diabetes risk [6, 7, 11]. In particular, we can confirm our recent publication on the same data set, which compared IGT subjects based on their allocation to a blinded oat hulls fibre supplement . In the present methodological approach, fibre intake irrespective of group allocation was focussed on, but showed similar results. As in our previous papers, we demonstrate that the improvement in glycaemia is accompanied by an amelioration of fasting and postprandial insulin resistance, also reflected by increased hepatic insulin clearance, and a reduced level of liver fat markers. In addition to that our study highlights that a daily dosage of at least 14 g of total insoluble fibre is necessary in order to achieve a metabolic benefit, confirming current recommendations.
Our publication indicates that achieving a certain level of insoluble fibre intake is more important than just increasing the amount even on top of sufficient baseline levels. Somehow, the beneficial effect seems to undergo a saturation plateau, which contradicts epidemiological data on a steady decline of diabetes with higher levels of fibre intake [6, 11].
We demonstrate that the level of fibre intake after one year of lifestyle intervention and fibre supplementation is mainly independent of the baseline dietary state of the subjects. A strong health belief can affect the adherence to a certain treatment which is perceived as particularly effective by the patients . In our study, most subjects failed to increase their fibre intake by means of regular diet, which was shown in earlier intervention trials . The majority of our patients surpassed the desired level of insoluble fibres by supplementation only.
Previous diabetes preventions trials such as the Da Qing Study, the Indian Diabetes Prevention Programme, the Diabetes Prevention Programme and the Diabetes Prevention Study aimed for an intake of 15 g of total fibre per 1000 kcal [1,2,3,4]. Our data show that this target is valid, even though most of the glycaemic benefit seems to be achieved at a lower dose, and sole dietary advice is an insufficient measure to reach an acceptable level.
In confirmation of our earlier analyses of OptiFiT, we show that the glycaemic improvement is driven by reduced insulin resistance [8, 9, 13]. The exact mechanism leading to better insulin sensitivity is not well-understood. Based on an earlier RCT on proteins and insoluble fibre in participants with Metabolic Syndrome (“ProFiMet”), the role of the intestinal absorption of branched-chain-amino acids—potent activators of the mTOR pathway—was discussed  In OptiFiT, faecal samples were not collected. In ProFiMet, also, the involvement of bile acids was investigated . Up to now, we did not assess bile acid levels in OptiFiT. However, we noted that the tertile with the highest fibre intake and best metabolic outcome did also show a stronger improvement in fatty liver index. Even though the ProFiMet study—with healthier subjects—did not report an effect on liver fat, this finding should be followed up in novel trials. The putative role of changes in the gut microbiome cannot be answered by our trial. Our fibre supplement is mainly unfermentable; therefore, we do not expect a mechanistic contribution of short-chain fatty acids. Even apart from that, insoluble fibre could alter the bacterial balance, but without faecal samples, this assumption remains speculative.
Weight loss does not seem to explain the metabolic differences of the three tertiles, as they do not differ in their change of body weight or body composition.
Once again, this analysis, too, demonstrates an anti-inflammatory effect of insoluble oat hulls fibre, which was shown in our recent stratified approach based on obesity . Cohort studies have indicated that high intake of fibre—in particular insoluble fibre and fibre of cereal origin—is associated with lower risk for certain inflammatory disorders . In the context of obesity, metabolic syndrome and prediabetes, insoluble fibre might act by specifically reducing inflammatory processes in visceral adipose tissue.
Increased fibre intake in the context of regular nutrition can be accommodated with changes in the overall dietary pattern and weight loss . We did not see relevant or significant differences between the highest and lowest tertile with respect to changes in macronutrient composition, energy intake or weight loss. Besides nutrition, increased physical activity can lead to metabolic benefits . However, none of the tertiles achieved a significant increase in daily steps or energy expenditure by walking over the first 12 months of intervention.
We like to address some limitations of our work. Selection of completers with full dietary data might contain a selection bias, even though we did not find baseline differences between completers with and without usable food protocols or completers and non-completers. Our cohort of 119 subjects is of moderate size, allowing the conducted comparisons. As a common feature in nutritional RCTs, male subjects are underrepresented.
Our study was conducted with a well-defined insoluble fibre supplement from natural plant origin. However, the metabolic impact may depend on the type of cereal and its processing, leading to differences in particle size, specific composition and food matrix. Replication studies with different supplements and fortified food products are required.
By using 4-day food records and drug accounting of supplement tins, we are able to evaluate dietary compliance on the basis of regular diet and additional fibre intake. There are no biomarkers for intake of insoluble fibre, which limits our measures to provide a fully objective alternative indicator of adherence to recommendations and drinking powder. Even food sampling techniques with subsequent chemical analysis would need to be considered under the limitation of underestimated fibre contents [36, 37].
We could rule out a confounding effect of physical activity by using pedometers.
We experienced a one-year drop-out rate of 24% and additional 9% of subjects without reliable food records. These rates are within the range of those from other studies with lifestyle intervention or dietary supplements [13, 23, 38, 39]. Only few drop-outs due to incident T2DM occurred before the one-year visit in OptiFiT. Conducting our analysis for the full study period of two years would have meant to reduce the power to only 94 subjects, to accept a systematic bias by diabetes-related drop-outs, a lower compliance to lifestyle advice, supplementation and quality standards for our recurring food records. We therefore decide against an additional analysis of the full study period.
In summary, we demonstrate the effects of an intake of sufficiently high levels of insoluble fibre on the glycometabolic outcome in subjects with IGT. In average, intake of more than 14 g of insoluble fibre should lead to a reduction in capillary 2-h glucose levels, but an amount of 25 g should be aimed for in order to achieve the maximum benefit. Glycaemic improvements are accompanied by reduced insulin resistance, inflammation and potentially liver fat accumulation. Due to the supplementation design of the study, we can pinpoint the effect of healthy cereal products more convincingly towards the isolated component of insoluble fibre, irrespective of food matrix and additionally beneficial compounds. Both whole grain and fortified foods seem to be recommendable. Further targeted studies are required to assure replicability of our findings.