Year

2016

Degree Name

Doctor of Philosophy

Department

School of Medicine

Abstract

Background: Whole grain intake is associated with reduced risk of chronic diseases, such as cardiovascular disease and Type 2 diabetes. Unlike more commonly recognised whole grains, sorghum is a so-called “ancient”, gluten-free whole grain food that is gaining attention from food manufacturers, consumers and researchers as a beneficial ingredient in novel product development. Sorghum is widely cultivated globally due to its adaptability to semi-arid/arid conditions and high temperatures, and has significant potential for sustainable grain production in harsh climates. It is a dietary staple in some communities of Asia, Africa and Central America, however in countries such as the United States and Australia, sorghum is predominantly used as an animal feed, with only small quantities utilised for the manufacture of human foods. If sorghum is to be accepted as a common food, attention to food formulations and processing methods that produce desirable sorghum-based cereal products, whilst preserving the grain’s beneficial nutritional and functional attributes, is required.

Sorghum whole grain has been shown to have lower starch and protein digestibility in vitro, resulting in reduced metabolisable energy and hence smaller weight gains in animals, and some cultivars are high in dietary fibre and rich in polyphenolic compounds. Based on their purported mechanisms, these attributes may contribute to positive effects on metabolic markers and body weight and therefore may more broadly influence chronic disease prevention in humans when they consume sorghum-based foods regularly. However, there is a paucity of human studies to date.

Research Hypothesis and Study Aims: This thesis proposes that sorghum grain consumption may assist in chronic disease prevention via weight management, especially in populations where food is ubiquitous and obesity-related chronic diseases are prevalent. Research examining the effects of sorghum intake on health outcomes, such as weight loss and disease biomarkers, requires clinical studies. These studies may also expose potential protective roles of the sorghum grain as part of a whole diet. The key hypothesis of this thesis is that sorghum is a viable alternative to more commonly consumed whole grain cereals in the human diet and may have positive benefits on factors associated with metabolic health including weight management. A review of the literature described sorghum grain structure and composition which relate to mechanisms by which sorghum grain components may influence weight management and chronic disease risk; and summarised the evidence on effects of sorghum consumption on health outcomes related to chronic disease prevention. This review provided insights into the knowledge gaps and study imperatives that informed the experimental components of the thesis.

In order to conduct human trials successfully, particularly with participants who do not typically consume traditional sorghum foods, suitable ready-to-eat sorghum grained cereal foods in the form of flaked breakfast cereal biscuits were formulated and tested prior to intervention trials commencing. A randomised, crossover, feeding study using the flaked sorghum biscuits was conducted to investigate mechanisms related to general metabolic markers and antioxidant status, as well as acute satiety. A randomised controlled trial (RCT) was then conducted with the aim to test the weight-loss effects of longer-term whole grain sorghum flaked biscuits consumption incorporated into an energy-restricted diet plan. This study also examined the effects on key biomarkers of metabolic health, inflammation and oxidative stress.

Methods: Gaps in the knowledge of effects of sorghum consumption on indicators of metabolic disease, including weight, were identified and elements of study designs were incorporated into the experimental components of the thesis. Whole grain flaked cereal biscuits were formulated on a pilot line by our industry partner (Sanitarium Health and Wellness) using a multistep process involving steaming, drying, rolling and baking. For the acute satiety study, forty subjects (20 males and 20 females) were tested on four occasions after a 12-hour fast. At baseline, they consumed 50 grams (or 3 biscuits) of one of four treatment meals: white, red or brown whole grain sorghum flaked biscuits or a whole grain flaked wheat biscuit control. Subjective satiety was measured at 8 time-points over four hours and food intake at the subsequent meal and for the rest of the day recorded. In a subset of 20 subjects, plasma glucose, insulin, and four appetite-related gut peptides (GIP, GLP-1, PYY and ghrelin) were also measured. In the subsequent RCT investigating chronic sorghum consumption, sixty subjects (14 males and 46 females) were randomised to either a sorghum (intervention) or wheat (control) group. Both groups received advice on an energy-restricted diet from an Accredited Practising Dietitian and were provided with 45 g of cereal products to include daily in their prescribed diets for 12 weeks. The primary outcome was weight loss. Secondary outcomes included: plasma glucose, HbA1c, insulin, total cholesterol, HDL-c, LDL-c, TAG, and various markers of inflammation and oxidative stress (IL1β, IL-6, IL-8, TNFα, hsCRP, HPX and TAC), measured at 0 and 12 weeks. Subjective satiety ratings were assessed at 0, 6 and 12 weeks.

Results: A vast array of nutritional and bioactive sorghum grain components, such as slowly digestible starches, polyphenols including anthocyanins in red sorghum, unsaturated fatty acids, and dietary fibre (including resistant starch), were identified in the literature, with potential actions in health protective metabolic processes. There was a paucity of human studies that are necessary to better understand effects of sorghum consumption (as a whole food), especially those related to health outcomes and indicators of disease. These findings justified the human studies described. During the acute meal test study, subjects reported significantly lower satiety ratings after consuming wheat compared to sorghum biscuits. Incremental AUC of postprandial GLP-1, GIP and in males, PYY, were significantly higher (p=0.018, p=0.031, p=0.036 respectively) for sorghum breakfasts compared to wheat. Energy intake at a subsequent meal did not differ between treatments, nor did glucose, ghrelin and TAC responses. The red sorghum biscuit showed the greatest alteration in appetite hormones, suggesting that the specific combination of its components (such as unique anthocyanin flavonoids) requires further investigation. Hence, these results informed the choice of a red sorghum-based biscuit to test in the subsequent 3 month RCT. This trial did not identify any significant differences in weight loss or any clinical variables between a sorghum cereal group and a wheat control in an energy-restricted diet. For both groups, the majority of clinical indices changed significantly over time (p

Conclusions: Research presented in this thesis provides human evidence for acute satiety effects from whole grain sorghum ingestion in the form of flaked cereal biscuits. However, it could not be concluded that whole grain sorghum assists specifically with weight management, or is superior to wheat (another whole grain product). Further clinical trials are necessary to establish an evidence base for weight loss effects from chronic sorghum consumption. In the longer term, this research would help to further evaluate the weight loss potential of sorghum consumption, especially red sorghum grown in abundance locally in Australia. Weight loss effects would be considered to assist in the prevention of chronic diseases, particularly those associated with obesity. Nevertheless, new knowledge was generated from these studies demonstrating the potential of sorghum consumption to enhance acute satiety. Sorghum is not only a gluten-free whole grain cereal, but also a promising ingredient in cereal foods targeting appetite control. Commercially, the results of the feeding trials may contribute to the development of the sorghum industry globally by adding to the evidence base on human health effects. This thesis also showcases the collaboration between food industry and science to progress sorghum, a relatively unknown human food in Australia, through the food product innovation pipeline with positive outcomes for all stakeholders including researchers, food manufacturers and also consumers.

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