Phenolics from wholegrain oat products as modifiers of starch digestion and intestinal glucose transport

Abstract

Plant phenolics have been reported to modify glycemic response of carbohydrate rich foods. However, this effect has not been documented for phenolic rich wholegrain oat foods. In the present study the influence of oat phenolics on starch digestibility and intestinal glucose transport were investigated in vitro. Phenolic extracts were prepared from 3 oat cultivars (HiProtein, Dancer, and GMI 423) and 1 commercial oat (CO1). Cinnamic acid derivatives were confirmed by LC‐MS to be the primary phenolic species with avenanthramides (AVEs, 31.6–70.7% of free phenolics) and ferulic acid (52.2–89.3% of bound phenolics) as the major constituents of free and bound fractions, respectively. α‐Amylase activity was enhanced by presence of oat phenolics at 20 μM (96.7–117.8%, P<0.05), while modestly inhibited at 500 μM (83.0–95.4%). α‐Glucosidase activity was significantly inhibited (P<0.05) by free (57.4–82.4%) and bound (49.6–61.3%) oat phenolics, albeit with high IC50 values (499.7–938.9 μM). Oat variety had minimal impact modulation of starch digestibility. All free and bound oat phenolic extracts (0–100uM) attenuated transepithelial transport of D‐glucose‐1,2,3,4,5,6,6‐d7 (d7‐glu) over 60 min in dose‐dependent fashion. While effects were modest at 10 and 50 uM, treatment with 100uM free phenolic extracts from the 4 oat flours reduced d7‐glu transport by 35–64% relative to phenolics‐free glucose control. Similarly 100uM of bound oat phenolics demonstrated similar effects with an 45–70% reduction in d7glu transport relative to control.To determine the extent to which these effects may be present during digestion of wholegrain oat foods, oats from each cultivar were formulated into wet cooked porridges (WP) and CO1 was additionally used in manufacture of a model puffed cereal (PC) and snack bar (SB). Foods were subjected to a coupled in vitro digestion/Caco‐2 intestinal cell model where bioaccessibility of phenolics was assessed simultaneously to starch digestion and ultimate d7‐glu intestinal transport. Results suggested that phenolic bioaccessibility was derived primarily from free phenolics and was modestly affected by oat variety (HiPro 13.6%> CO1 12.6% > Dancer 11.5%> GMI 423 10.8%). Type of processing had a more significant impact to bioaccessibility of phenolics from CO1 oats as SB had significantly higher bioaccessibility than CO1 porridges, especially for AVE C (46.0% vs. 0.2%). Glucose release through in vitro digestion did not differ between oat products. However, total bioaccessible phenolic content from single servings of oat foods ranged from 2.6–69.5μM suggesting levels from foods were below the effective doses observed for enzyme inhibition. However, 60 min d7‐glu transport by Caco‐2 monolayers was significantly reduced from digesta of GMI 423 WP and CO1‐PC by 34% and 20% relative to control (P<0.05). These results suggest that oat phenolics might contribute to a reduction of glycemic response from whole grain oat products but these effects are likely tied to oat phenolic content and bioaccessibility from consumer foods.Support or Funding InformationGeneral Mills Inc and Purdue University Ingestive Behavior Research Center