Abstract
OBJECTIVES: Determine the effect on glycemic response of equi-carbohydrate partial exchange of fruit for cereal, identify mechanisms of its effectiveness, and establish the metabolic safety of long-term fruit consumption in individuals at risk of type 2 diabetes. METHODS: Randomised repeated measures human intervention studies were conducted with healthy participants ingesting 40 g available carbohydrate meals of wheaten starch biscuit (WB), WB plus kiwifruit (WB + KF), and WB plus the same sugars in the same quantity as consumed in the kiwifruit (WB + KFS). Postprandial glycemic responses were measured. Associated in vitro studies aimed to identify mechanisms of glycemic response reduction by fruit. Physicochemical and rheological properties of fruit digested under simulated gastro-intestinal conditions, and the impact of the fruit remnants on physical and enzymatic processes of digestion were determined. In a follow-up parallel group intervention study, prediabetic (HBA1c > 4%) participants consumed 2 kiwifruit per day for 12 weeks, to establish the metabolic safety of long term kiwifruit consumption through clinical biomarkers. RESULTS: Partial substitution by kiwifruit (WB + KF) significantly (P < 0.05) and substantially (−35%) reduced glycemic response peak height compared with unsubstituted WB. Also, substitution by kiwifruit (WB + KF) caused significantly (P < 0.001) lower (−20%) peak glycaemic response than substitution with the kiwifruit sugars alone (WB + KFS), indicating the glycemia reducing effect of the fruit was due to more than fructose substitution of digestible starch. The mechanistic studies showed several gut-level factors by which kiwifruit could reduce glycemic impact; high gastric acidification and buffering capacity, swelling and dispersion of the fruit cell walls (dietary fiber) leading to rheological changes retarding digestion, glucose diffusion and intraluminal mixing, all involved in transport of digestion products to the gut wall for absorption. The long term study could not detect adverse metabolic effects. CONCLUSIONS: Including fruit in the diet in a carbohydrate exchange format may suppress glycemic response through a number of mechanisms and without metabolic risk. FUNDING SOURCES: New Zealand National Science Challenge.