Author: Geetha Gopalsamy
Gopalsamy, Geetha, 2017 The potential of resistant starch to function as a novel infant prebiotic and enhance Zn bioavailability., Flinders University, School of Medicine
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Prior to the current scientific and community interest in the gut microbiota, there was an awareness that adult consumption of resistant starch, a component of dietary starch which is undigested in the small intestine and available for fermentation in the colon, could confer health benefits. More recently it has emerged that the manipulation of the developing gut microbiota during early childhood can have effects on the health of the host that persist into adulthood. These key concepts have converged to form the genesis of this doctoral thesis. Utilising in vitro methodology, this thesis examines the capacity of infant faecal inocula to firstly ferment a form of resistant starch, high amylose maize, and then to determine if any such fermentation is associated with microbial changes that might predict a health benefit to the host. If the prebiotic properties of resistant starch are available earlier in the human lifespan, when both the microbiota and the immune system have a greater degree of plasticity, then this might confer novel, durable benefits upon human health. The study presented in Chapters 3 and 4 of this thesis, is unique in many ways: it has the highest number of participants compared to any similar infant in vitro fermentation study, it retains a measure of individual variation in fermentation capacity by not pooling participants’ faecal inocula and finally, it collects pre-weaning and weaning samples from the same infants in order to demonstrate how the introduction of solids might affect the prebiotic potential of resistant starch. The major aim of Chapter 3 was to determine if the faecal inocula of young infants firstly have the capacity to ferment resistant starch, as determined by an increase in the production of short chain fatty acids and a reduction in pH following 24 hours of in vitro incubation. While there was evidence of some fermentation capacity by pre-weaning faecal inocula, the relative increase in SCFA production following incubation of the weaning faecal inocula with the resistant starch suggests that RS intake might be only of clinical significance if provided to infants who have already commenced solid food. In contrast, currently available prebiotics such as fructo-oligosaccharides and galacto-oligosaccharides are administered even to pre-weaning infants. Nevertheless, these results provide the first evidence that resistant starch, in the form of high amylose maize, can be fermented by the faecal inocula of infants who have only recently commenced solids.
Another aim of Chapter 3 was to examine whether there was any difference in the capacity of weaning faecal inocula to ferment a modified starch in comparison to its unmodified counterpart. Sections of the community have expressed concerns regarding the use of modified starches in infant products. This is despite an absence of supporting evidence and the existence of strict government restrictions regarding the degree of chemical modification and the percentage of modified starch that can be added to infant products. The results showed that there was no significant difference in the capacity of the weaning faecal inocula to ferment HAMS in comparison to the commercially available mHAMS. The major aim of Chapter 4 was to use molecular methods, including next generation sequencing (NGS), to characterise the changes to the microbiota following incubation of the infant faecal inocula with HAMS and a mHAMS. In contrast to the pre-weaning infants, incubation of the weaning faecal inocula with the resistant starches led to dramatic changes in the composition of the microbiota. For example, following incubation with either of the resistant starches, the abundance of Bifidobacteria was higher at 24 hours than at 0 hours, suggesting that both substrates were able to selectively stimulate the growth of this beneficial microbe from human infant microbiota. NGS analysis also demonstrated that in weaning infants, the abundance of Bacteroides significantly increased during fermentation of both HAMS and mHAMS. In weaning infants, fermentation of the resistant starches also increased diversity. Thus in infants who have only recently commenced solids, fermentation of HAMS and mHAMS may indeed have a prebiotic effect. Human trials will be needed to confirm any health-promoting effects. The fermentation of carbohydrates also creates an acidic environment in the large intestine which can facilitate the absorption of divalent cations such as calcium and magnesium. In infants, deficiency in zinc, another divalent cation, is of major significance in the arena of global child health. Thus, in combination with the in vitro work already mentioned, this thesis also considers whether the fermentation of high amylose maize might promote the absorption of zinc in a young animal model. To explore such conjecture, a review of the capacity of the large intestine to participate in the absorption of zinc was required. This is presented in Chapter 5 of this thesis. Based on this review, it was discovered that conditions that might reveal a latent capacity for colonic zinc absorption had not been adequately tested. This includes examination of the effect of preceding zinc deficiency on the capacity of the colon to participate in the absorption of zinc. Thus, in Chapter 6, the main aim was to use a weanling rodent model to examine if preceding zinc deficiency affects the capacity of resistant starch to influence zinc bioavailability in a growing mammal. The study revealed that although HAMS increased zinc status, a finding that correlates with previously published studies, there was no apparent effect of preceding zinc status on the capacity of the RS to promote the absorption of zinc. However, in light of other observations also made in Chapter 6, it is apparent that manipulation of host dietary zinc has implications for microbial fermentation activity, and by inference, the composition of the gut microbial community. The effects of dietary zinc on the activity of the microbiota as opposed to the effect on host parameters, have not been well described and the findings of this rodent study bring to light the need for further research into this topic.
Keywords: resistant starch, prebiotic, high amylose maize starch, Zinc, infant prebiotic,
Subject: Medicine thesis
Thesis type: Doctor of Philosophy
Completed: 2017
School: School of Medicine
Supervisor: Prof Graeme Young