Dried Fruit as a Means for Lowering the Glycemic Response to High Glycemic Index-carbohydrate Foods

BACKGROUND:

All studies assessing the glycemic index (GI) of traditional dried fruit show that they are low-to-moderate GI foods and that the insulin response is proportional to their GI. A recent study compared the glycemic response of two doses of raisins (28 and 69g) versus white bread showing that both doses of raisins significantly reduced post-prandial glucose and insulin levels compared with white bread. However, the effect of combining dried fruits with high-GI carbohydrate foods has never been addressed. The potential impact of combining nuts (i.e. pistachios) and high-GI carbohydrate foods has already analyzed with positive results. The investigators found that a dose of 56g of pistachios consumed alone had a minimal effect on post-prandial glycemia, but when taken with a high-carbohydrate meal attenuated the relative glycemic response. Although foods with high fibre content generally have a low-GI, other factors also contribute to a food's glycemic response. Factors thought to contribute to the glycemic response of dried fruits include the viscous texture when chewed; their whole food matrix; the presence of phenolic compounds and organic acids and the type of sugar present. In the case of dried fruit, about 50% fructose (low-GI) is present. Therefore, the consumption of dried fruit with high-carbohydrate foods may lead to glycemic control benefits by lowering the GI of a food. In addition to potentially lowering the GI of a food, dried fruits may also affect glycemic control by providing 'catalytic' doses of fructose. Fructose, through its metabolite fructose-1-P, has been shown to have "catalytic" effects on hepatic glucose metabolism by inducing glucokinase activity in hepatocytes. In specific, fructose-1-P displaces fructose-6-P from glucokinase's regulatory binding protein in the nucleus causing the release of glucokinase from its regulatory protein, allowing it to translocate to the cytosol, resulting in increased phosphorylation of glucose. Infusion studies in humans have shown that this mechanism relates to a ~30% decrease in hepatic glucose output under hyperglycemic conditions in participants with type 2 diabetes (T2D) and a ~3-fold increase in glycogen synthesis by C13-nuclear magnetic resonance (NMR) spectroscopy under euglycemic conditions in healthy people. Clinical translation of these findings has proven promising. Catalytic doses of fructose at 7.5g and 10g have been shown to decrease the postprandial glycemic responses to high GI meals (oral glucose, maltodextrins, or mashed potatoes) from ~15-30% in healthy participants and those with pre-diabetes or diabetes . These acute effects have been shown to be sustainable over the longer term as well. Systematic reviews and meta-analyses of controlled feeding trials have shown that small doses of fructose in exchange for other carbohydrates decreases HbA1c at a level which exceeds the clinically meaningful threshold of 0.3% proposed by the Federal Drug Administration (FDA) for the development of new oral anti-hyperglycemic agents. Therefore, the consumption of dried fruit with high-carbohydrate foods may lead to glycemic control benefits by acting as a vehicle for 'catalytic' doses of fructose.

OBJECTIVES:

To investigate the effect of using dried fruit to modify the glycemic response of high GI foods, the investigators propose the following 3 objectives:

1. To assess the GI of 4 common types of dried fruit (raisins, sultanas, dates, apricots) (GI effect)
2. To assess the ability of the 4 common types of dried fruit (raisins, sultanas, dates, apricots) to decrease the postprandial glycemic response to white bread by displacing half of the available carbohydrate (displacement effect)
3. To assess the ability of the 4 common types of dried fruit (raisins, sultanas, dates, apricots) to decrease the postprandial glycemic response to white bread by providing a 'catalytic' dose (7.5g) of fructose ('catalytic' fructose effect)