Abstract
Background:
Type 2 diabetes affects around 8.5% of adults globally and if untreated, hyperglycaemia can lead to nerve damage, blindness, kidney failure and increase the risk of cardiovascular disease. Therefore, for people with diabetes, reducing postprandial glycaemia is a priority to optimise health and reduce metabolic complications. Consumption of wholegrains such as oats, rice and wheat are associated with reduced risk of diabetes, cancer and heart disease and a lower glycaemic response compared to consumption of refined wholegrains. However, commercially available wholegrain foods vary from minimally to highly processed, and it is unclear whether processing of the grains (degree of intactness) influences the apparent health benefits. Short chain fatty acids (SCFA) are produced by microbial fermentation in the colon following wholegrain consumption and absorbed into the bloodstream with positive effects on appetite, body composition and insulin sensitivity. However, the effects of wholegrain particle size on SCFA production are currently unknown. Analysis of volatile metabolites and SCFA in exhaled breath provides a novel approach in understanding how foods are digested and metabolised in real time. As a non-invasive method, this may assist in understanding how differences in the degree of processing affects digestion and metabolism of wholegrains and SCFA production.
Design:
People with normal glucose tolerance (NGT; n=11 to 12) and type 2 diabetes (T2DM; n=11 to 12) participated in two randomised crossover studies examining glycaemic response and volatile metabolites in end tidal breath over 3-6 hours using proton transfer reaction mass spectrometry (PTR-MS) following consumption of three test foods. Each participant consumed all three test foods providing 50 g of available carbohydrate, on three separate occasions. These included: a glucose drink and two types of wholegrain breads differing only in wheat grain particle size one made with finely milled wholegrain wheat, the other with a mixture of intact, kibbled and finely milled wheat.
The aim of Study 1 was to measure the change in 17 volatile metabolites in exhaled breath and blood glucose over 3 hours following food consumption in two groups of participants, those with NGT and T2DM conducted with NGT and T2DM groups to measure the change in 17 volatile metabolites in exhaled breath and blood glucose over 3 hours following food consumption. The aim of Study 2 was to measure 17 volatiles metabolites and blood SCFA in three test foods over six hours (as similar to that described in Study 1). In addition, included matched NGT and T2DM participants for age, sex and BMI and was conducted in a more stable testing environment. Thus, improving upon limitations identified from the first study. Breaths collected in Tedlar bags were analysed with gas chromatography mass spectrometry (GC-MS) for identification of compounds in breath. Blood and breath SCFA were measured over 6 hours to examine the response to each food, which was hypothesised to rise between 4-6 hours following consumption of the bread products, but not for the glucose drink.
Outcomes:
Following consumption of the three test foods, breath VOCs followed three main trends; namely increasing over time, decreasing over time or a peaked response (initial increase then subsequent decrease back to baseline). Participants with diabetes had higher breath VOC concentrations at baseline and larger changes in VOC concentrations post-consumption.
The results of the first study (as measured by AUC) showed several breath VOCs (2-butanol, propanol, acetate and butyrate) were influenced by the digestibility of the carbohydrate, related to the intactness of the grain structure. In the T2DM group, the AUC of 2-butanol, propanol/acetate, butyrate and propionate (m/z 57, 61, 71 and 75, respectively) was the largest following consumption of the glucose drink and lowest following the bread with the largest particle size (more intact grain structure) (p=<0.05). However, in the NGT group, the lowest VOC response of propanol/acetate, propionate and butyrate (m/z 61, 75 and 89) was observed following consumption of the bread with the smallest particle size (most refined) (p<0.05).
In the second study, incremental area under the curve (iAUC) was used to measure relative change in breath VOCs following consumption of each test food. In the T2DM group, the glucose drink led to the largest iAUC of 2-butanol, propanol, acetate, butyrate and propionate (m/z 57, 61, 71 and 75, respectively) compared to either of the bread products (p<0.05). In the NGT group, no differences in response of breath VOCs were observed for any food comparisons (p>0.05). This was possibly linked to tighter glycaemic control for normoglycaemic participants. No increase of SCFA in blood or breath was detected between baseline and 5 h for any test food in either the NGT or T2DM group. This was likely due to delayed gastric emptying and slow transit time for the bread formulations investigated.
Conclusions:
PTR-MS distinguished different patterns in the volatile metabolites in exhaled breath post-consumption between test foods and between subjects with and without type 2 diabetes, demonstrating significant differences in metabolism following consumption of wholegrain foods of differing intactness. This research extends the current published literature by showing that wholegrain foods with a larger particle size led to a reduced response in four exhaled metabolites during digestion and metabolism in participants with T2DM (m/z 57, 61, 71 and 75). Findings from Study 2 were unable to determine the potential for breath gas analysis for the determination of blood SCFA, possibly due to insufficient testing time post-consumption, which will need to be addressed in future research. This thesis has demonstrated the potential of breath gas analysis to facilitate greater understanding of metabolism in response to food consumption and that in future it could lead to a non-invasive tool for physicians and dietitians to monitor dietary intake in patients.