Abstract
Wholegrain foods vary in the extent of their grain processing, but most use re- constituted wholegrain flour. Grain particle size can affect starch digestibility and therefore the glycaemic response. This thesis primarily examines whether grain particle size in wholegrain foods affects the postprandial glycaemic response, using a range of wholegrain foods produced under different food processing conditions.
People with normal glucose tolerance (n=15 to 20) and type 2 diabetes (n=15 to 20) participated in a series of randomised crossover studies in which acute postprandial capillary blood glucose response was examined following ingestion of three wholegrain wheat products (wheat porridges, breads, and crackers). Each product provided 50 grams of available carbohydrate, and the products within each food category were matched for energy and macronutrient composition but differed in grain particle size. Wheat porridge products differed in both grain particle size (flour vs kibbled wheat) and starch gelatinisation (ungelatinised and gelatinised). The composition of the breads from least to most intact was: 100% roller-milled wheat flour, 100% stoneground wheat flour, 50% kibbled wheat + 50% roller-milled wheat flour, and 30% intact wheat + 30% kibbled wheat + 40% roller-milled wheat flour. The grain particle size of the crackers matched those of the least intact and most intact breads. The postprandial appetite response to each test food was assessed via a questionnaire using Visual Analogue Scales (VAS) given every fifteen minutes for the first hour and every half hour thereafter. The palatability of each food was assessed via a VAS questionnaire.
Among people with type 2 diabetes, wholegrain foods comprising larger compared with smaller grain particle size generated a significantly lower blood glucose iAUC across all product categories. The mean difference in blood glucose iAUC between wheat porridges made from kibbled grain versus fine flour was: 187 mmol/min×L (95% CI: 54, 320), p=0.006 for gelatinised porridge and 237 mmol/min×L (95% CI: 143, 331), p=<0.001 for ungelatinised porridge. The bread including 30% intact kernels and 30% kibbled wheat generated a blood glucose iAUC significantly lower than all other breads with smaller grain particle size, and the stoneground flour bread generated a blood glucose iAUC significantly lower than the roller-milled flour bread (mean difference 137 mmol/min×L (95% CI: 21, 254), p=0.020). The cracker with 30% intact kernels and 30% kibbled wheat generated a significantly lower blood glucose iAUC than the 100% flour cracker (mean difference 159 mmol/min×L (95%CI: 40, 278)), p=0.009).
Among people with normal glucose tolerance, wholegrain crackers that included intact and kibbled grains generated a blood glucose iAUC that was 36% lower than crackers made from fine flour (95% CI: 15%, 52%), p=0.002. For wheat porridges, the effect of grain particle size on blood glucose iAUC was larger for gelatinised wheat porridges (mean difference 27 mmol/min×L (95% CI: -3, 57) than for ungelatinised wheat porridges (mean difference 0 mmol/min×L (95% CI: -22, 23), p=0.981). There was no evidence of differences in blood glucose iAUC among the breads.
Grain particle size did not significantly affect postprandial sensations of appetite for any of the test foods. For most food processing conditions, the inclusion of intact or kibbled grains in wholegrain foods did not reduce palatability scores. Older participants with type 2 diabetes rated the palatability of the test meals quite differently to the younger normoglycaemic participants. Among older participants the bread including intact and kibbled grain was considered more palatable than the other breads. However, the inclusion of intact grains reduced the palatability scores for crackers among younger normoglycaemic people and for ungelatinised wheat porridge among older people with type 2 diabetes.
In a separate randomised crossover study including people with known risk factors for type 2 diabetes (age 55 – 75, BMI ≥28 kg/m2, and physically inactive), the postprandial capillary blood glucose and venous insulin responses to three wholegrain breads were tested: (1) fine roller-milled flour, (2) fine stoneground flour, and (3) coarse stoneground flour. Each bread provided 50 grams available carbohydrate and was matched for energy and macronutrients. Flour particle size did not significantly affect either blood glucose or insulin iAUC.
The findings in people with type 2 diabetes confirm and extend previous evidence demonstrating the extent to which wholegrain foods including cracked and intact grains or coarsely milled flour generate lower postprandial responses compared with extensively milled wholegrains. This applies to breads, low moisture products such as crackers, and wheat porridges. The studies involving wheat porridges demonstrate the potential importance of considering other aspects of food processing, such as gelatinisation, in addition to particle size. The extent to which the product is gelatinised also has a striking effect on glycaemic response. Amongst normoglycaemic individuals, the effect of increasing particle size on lower glycaemic response was only apparent in crackers, as well as the effect of gelatinisation on higher glycaemic response in the preparation of wheat porridge. Given the high prevalence of prediabetes and the increasing prevalence of type 2 diabetes, these findings support more nuanced nutrition recommendations relating to the consumption of wholegrain foods than currently provided. In addition to advising people to choose wholegrain carbohydrate foods, advice should also include guidance for consuming wholegrain products with more intact grain structures and less gelatinised starch content.