High fiber intake, dietary protein, and prevention of type 2 diabetes

KEYWORDS:

• Cereal fiber
• colonic fermentation
• dietary fiber
• glycaemic index
• insulin resistance
• short chain fatty acids
• type 2 diabetes

A high fiber diet for the prevention of type 2 diabetes (T2DM) is recommended in most guidelines [1,2]. However, it is still commonly assumed that protective effects of dietary fiber consumption are related mainly to low energy density and subsequent weight loss, or to viscous properties of soluble, fermentable types of fiber [3]. This is surprising, considering (i) no relevant differences in types of fibers-related to effects on satiety and minor weight loss [3,4]; and (ii) emerging evidence that it is mainly insoluble, typically non-fermentable fibers from cereals and whole grains and not soluble, fermentable fibers that are consistently associated with some 30% reduced risk of developing T2DM risk in large prospective cohort studies [3,5]. These observations are challenging a convincing role of commonly proposed protective mechanisms related to fiber intake. These include fiber-induced reductions of postprandial glucose excursions and as such the glycemic index of carbohydrate-rich foods; and colonic fermentation of complex carbohydrates that are not digestible in the upper gut, with subsequent increased production of short chain fatty acids (SCFA) in the colon [5]. Notably, cereal fibers are insoluble in water and, except from some resistant starch products, are at best moderately fermentable in the colon [3]. Further, cereal fibers (in US cohorts: Mainly bran products from wheat and corn) are typically non-viscous [3]. Therefore, these types of fibers are neither expected to relevantly increase colonic SCFA production [6,7], nor do they influence the glycemic index of foods [3]. In contrast, soluble viscous fibers (which are more abundant in fruits, berries, certain vegetables, the germ fraction from oats and barley; and fiber supplements such as psyllium and guar) typically are strongly fermented and improve the glycaemic index, but do not appear to reduce diabetes risk [3,5,8].

Although numerous studies have shown that both low glycemic index diets and increased availability of SCFA can indeed have beneficial metabolic effects [5,9], it is important to note that most related interventions were done in animal studies [3]. Further, even in rodent studies long-term intake of the soluble, highly fermentable fiber guar, as opposed to short-term exposure, resulted in weight gain, increased body fat mass, and insulin resistance, whereas both short- and long-term intake of non-fermentable, insoluble cereal fibers (cellulose) improved metabolic health [3,10,11]. It appears that the short-term beneficial effects of fermentable-fiber induced higher SFCA production could be outweighed by SCFA contribution to total energy intake in the long-term [10,11], an effect that has been shown to be relevant also in humans [12]. More generally, a very high fiber intake without the use of fiber supplements may be achieved only by relevantly increasing the intake of carbohydrates, which contribute to energy intake.

Until recently, convincing mechanistic explanations of how insoluble, non-fermentable types of cereal fibers may influence T2DM risk were not offered. However, possible mechanisms have been identified more recently and include cereal–fiber mediated interference with the digestion and/or absorption of dietary protein, thereby modulating the blood amino acid metabolic signature [13] and preventing the activation of the mammalian target of rapamycin (mTOR)/S6-kinase 1 (S6K1) signaling cascade; thus improving insulin resistance [7]. These findings could also contribute to explaining why high cereal fiber versus high dietary protein intake in overweight or obese, sedentary people is associated with diabetes risk in opposite directions [3,14]. Notably, adverse effects of high protein intake on insulin sensitivity do not appear to apply in active people who relevantly stimulate also mTOR regulated increases in lean mass [3], which may compensate for protein-intake induced worsening of insulin resistance and is in line with the observation that in active athletes even very high protein intake does not appear to increase diabetes risk [3]. In addition, insoluble cereal fibers have been shown to alter the bile acid metabolic signature [15] and also may mechanically bind bile acids, thereby reducing cholesterol in plasma and restricting the absorption of dietary fats, which is beneficial for T2DM [8]

The effectiveness of preventing T2DM by moderate alterations in lifestyle has been proven in two previous trials [referenced in [16]] and large epidemiological studies have identified insoluble cereal fibers as one of the most effective nutrient components for diabetes prevention [17]. However, relevantly increased fiber intake was not usually achieved. We have therefore recently designed the randomized controlled ‘Optimal Fiber Trial’ (OptiFit) where long-term high insoluble cereal fiber intake was facilitated using supplements, for twice daily consumption over 2 years. The fiber supplements were purified fiber extracts derived from oat hulls, which contained 70% cellulose, 25% hemicellulose, and 3–5% lignin, to be dissolved in 300 mL water. Two out of three of the participants in the high cereal fiber group did reach their target for fiber intake [16] and increased cereal fiber intake indeed tended to reduce the incidence of T2DM over 2 years; and showed significant improvements in HbA1c and blood glucose concentrations over 1 year in women, despite otherwise identical lifestyle interventions in the placebo group [16]. Further, weight loss was almost non-existent at the end of the study and not different between the high-fiber and placebo groups, supporting the hypothesis that the observed beneficial metabolic effects of high insoluble cereal fiber intake were not linked to weight change [16]. Future targeted studies are needed to further investigate whether diverse types of fibers show similar effects on metabolic outcomes. These may include the investigation of possible gender-related effects and potential roles of fiber solubility and fermentability, although accurate assessment of fiber intake in observational studies remains to be a challenge. Fiber intake may also help preventing diabetes by changing the gut microbiome [8], although the insoluble cereal fiber extracts that were used in our studies did not relevantly influence dominant groups of the gut microbiota [6]. Importantly, OptiFit also showed that long-term high cereal fiber intake can indeed be achieved in a sizable percentage of participants by the use of fiber extract supplements [16]. Insoluble cereal fibers are widely available by-products of flour production, generally show no gastrointestinal effects due to their non-fermentability [6,7], and could safely be added to various foods. By using cereal fiber supplements, substantially improving cereal fiber intake in the general population could be achievable, with potentially relevant beneficial metabolic effects at a population level.

Declaration of interest

The author has no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

The manuscript was not funded.