Abstract
Background
Postprandial hyperglycaemia is a risk factor for diabetes and its complications, and it is influenced by the rate of carbohydrate digestion and absorption. Human salivary amylase (HSA) initiates starch digestion in the mouth and plays an important role in starch digestion as it may continue starch digestion in the stomach during gradual gastric acidification. Previous research has shown that Hayward kiwifruit lowers the glycaemic response to co- ingested cereal foods. However, the underlying mechanisms have not been identified; many factors, including acidity combined with the buffering capacity of fruit organic acids, and the action of the kiwifruit protease, actinidin, may play a part, and interact with one another during digestion.
Aim
To investigate effects of individual factors (fruit buffering capacity, acidity, pepsin and actinidin) and their interactions on HSA activity during 30 minutes of simulated gastric digestion and so to elucidate their role in the glycaemic response-lowering capacity of kiwifruit co- ingested with cereal foods.
Methods
A semi-dynamic gastric model with automated titration was developed to mimic gastric digestion. This project had 6 stages: Stage 1: Determining the buffering capacity of various fruits. Stage 2: Development of a CaCl2 fortified potassium-citrate buffer to approximate kiwifruit organic acid (referred as Ca-KF buffer), and testing pH effects on HSA activity during gastric acidification to pH 2.0. Stage 3: determining effect of Hayward kiwifruit, SunGold kiwifruit and Granny Smith apple on HSA activity during acidification. Stage 4: Testing effect of pepsin on HSA activity during acidification. Stage 5: Determine interaction of pepsin and actinidin from Hayward kiwifruit on HSA activity during acidification. Stage 6: Measure effect of pepsin with the three fruits in stage 3 on HSA activity in a Weet-BixTM meal. HSA activity was measured as sugar released in 15 and 30 minutes using a colorimetric assay. Protein analysis by SDS-PAGE and a kinetic assay were used to examine effects of pepsin-actinidin interaction on HSA survival in stage 5.
Results
Buffering capacity differed between fruits. HSA activity decreased as pH dropped from 7.0 to 2.0 with little decrease in activity above pH 4.0. The Ca-KF buffer significantly inhibited HSA activity at a pH range of 3.0 to 6.0 (p<0.01) compared to CaCl2 fortified saline solution. Pepsin inhibited HSA activity at pH 4.0 and below. Hayward kiwifruit significantly inhibited HSA activity at pH 4.0 and below (p<0.01) with the strongest inhibition at pH 4.0. A significant but small inhibition was contributed by SunGold kiwifruit (p<0.01) as well as Granny Smith apple (p<0.05) at pH 3.0 and below. HSA activity was inhibited more by pepsin alone than by pepsin and actinidin combined at pH 3.0 and below. Actinidin inhibited HSA activity more than pepsin alone at pH 4.0.
Conclusion
The low pH and strong buffering capacity of kiwifruit lead to immediate inhibition of HSA activity in the stomach as well as promoting early activation of pepsin, which also inactivates HSA. Actinidin in Hayward kiwifruit actively degraded HSA at its optimal pH around 4.0 but pepsin degraded both actinidin and HSA when pH fell below 3.0. Overall, Hayward kiwifruit had a potential in retarding HSA in the stomach and in turn might lower glycaemic response to co-ingested starch.