5 Evaluación de los efectos socio-económicos en la fase de desarrollo o explotación El efecto de la demanda turística

8.1 Summary

The aim of this project was to identify and assess possible gut health benefits of functional food ingredients or ingredient combinations of New Zealand origin. Ingredients were pre- selected by scientists from The New Zealand Institute for Crop & Food Research Ltd (now Plant & Food Research) for their known or suspected antimicrobial activity, primarily targeted against H. pylori, but also with known or suspected antioxidant or other activities conducive to health. The selection process for the ingredients was detailed in Chapter 1. A systematic approach of in vitro assays for antimicrobial activity (Chapter 3) and immunomodulatory activity (Appendix 3) was performed, in order to select the best ingredient(s) for further study and for incorporation into a concept functional food by members of the larger project of which this work forms a part.

Manuka honey was identified as an outstanding candidate, with BroccoSprouts® also selected for further investigation due to performance in the in vitro assays together with results achieved by others in the larger project (working on H. pylori and associated gastric inflammatory markers). Manuka honey, already widely known to posses antimicrobial (UMF™) and wound-healing properties, was shown to increase probiotic growth and decrease pathogen growth in mono-cultures of those organisms, and to partake in synergistic interactions with some of the other ingredients tested.

Manuka honey and BroccoSprouts® were then tested in a small animal in vivo model to confirm that the result that was observed in the test tube was also observed in the whole body. Results of this animal trial (and a previous pilot clinical trial by other researchers) failed to demonstrate any changes to the parameters examined (macrophage numbers of phagocytosis activity, bacterial populations, SCFA concentrations) by manuka honey. This was attributed, in part, to the likelihood that the manuka honey would be digested prior to reaching the colon (or caecum of the mice in the animal trial) where effects were being sought. Nevertheless, the efficacy of the manuka honey in vitro was undeniable, and the active constituent(s) and antimicrobial mode of action were still unknown.

Subsequent work focused on these elements, with a fortuitous discovery by researchers overseas, identifying the putative active UMF™ compound, MGO, at the time that this

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exploratory part of the thesis was barely underway. During the course of this work it was established that the main contributors to the observed in vitro manuka honey activity against the test organism E. coli were MGO and the osmotically active sugars, both modulated by the honey acidic pH, in a complex web of interacting, and perhaps overlapping, activities. A common honey antimicrobial factor, hydrogen peroxide, was eliminated, whilst honey phenolics were not investigated here due to lack of support in the literature and the need to focus this research on the factors which appeared to play greater antimicrobial roles.

Further work examining the response of the bacterium E. coli to the manuka honey and primary antimicrobial factors revealed for the first time that manuka honey appears to act primarily in a bacteriostatic fashion, not necessarily bactericidal as had been previously implied by the existing literature.

8.2 Implications

Firstly, the discovery of MGO by Prof. Henle’s food chemistry group at the University of Dresden has revolutionised New Zealand’s UMF™ manuka honey industry. The Active Manuka Honey Association, which monitors and maintains the status and value of the UMF™ brand, now contends with alternate branding, such as Manuka Health’s MGO™ brand, and associated publicity, based upon quantification of the MGO in the honey by HPLC (http://www.manukahealth.co.nz/main.cfm?id=105).

Secondly, manuka honey research has been strengthened by the addition of another bioactive compound to investigate, and an existing field of research already focusing on antimicrobial MGO has been incorporated into the manuka honey research area (Adams et al., 2008; Mavric, 2008).

Finally, this thesis, and the larger project of which this work forms a part, has revealed that manuka honey appears to confer no in vivo large intestinal benefits. This thesis suggests that manuka honey may not kill pathogenic bacteria, but merely delay the onset and/or extent of growth of bacteria. The duration and extent of this effect depends upon the concentration of the manuka honey, and is mitigated by the density of the bacterial biomass, the respiratory state and/or redox environment of the bacteria, the pH of the environment, and sundry other factors.

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The rich, diverse and numerous gut microbiome (or metabolome, as the host must be taken into account), with its robustness, redundancy, competition for nutrients and environmental niches, is likely to suffer little impact from ingested manuka honey. The sheer numbers and mass of the microflora coupled with this extreme competition and the adaptability of the microbiome, confounded by the host’s own digestive processes, means that manuka honey was always unlikely to both reach the colon at concentrations capable of exerting an effect, and to actually induce significant changes. The results in this thesis support this hypothesis. Thus, from a functional food perspective, manuka honey possesses little value in terms of managing the gut microbiota, given consumption of reasonably normal manuka honey portions, unless the honey is formulated in a way that protects it from digestion.

However, it is still not known what, exactly, is the effect of a honey solution (which may or may not reach the colon at an adequate concentration to exert effects) upon a complex and robust ecosystem, or even a biofilm of a single strain. Furthermore, manuka honey is likely to still exert effects in the comparatively early regions of the GI tract, such as the mouth, oesophagus and stomach, with potential roles against dental caries and/or gastric

Helicobacter or other more opportunistic pathogens. Finally, manuka honey is likely to still

retain its value as an antibacterial agent during topical wound healing, where bacterial numbers are comparatively fewer, inhibition of bacterial growth may oppose microbial prevention of the wound healing process, and the manuka honey can be controlled, maintained and refreshed at effective concentrations. Moreover, investigators are still uncovering various immunostimulatory and wound healing properties (Timm et al., 2008; Tonks et al., 2001; Tonks et al., 2003; Tonks et al., 2007) and antioxidant activities (Henriques et al., 2006; Inoue, 2005a) of manuka honey, which continue to stimulate research, generate unanswered questions, and provide potential health benefits to the consumer.

8.3 Future Work

This thesis has opened some new avenues for exploration of the effects of manuka honey, osmotically active sugars, and MGO, on bacteria such as E. coli. The precise mechanisms of action, optimum conditions for activity, apparently contradictory responses of the bacterium, and structural integrity of the bacterial membrane (and nucleic material) still require investigation.

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Furthermore, additional work on the characterisation of the manuka honey constituents, apparently bioactive and inactive, is still required. The source of the extremely high MGO concentrations in manuka honey, and the reason why it is unique to this variety, has only very recently started being elucidated (Adams et al., 2009). The role that honey phenolics, proteins and sugars may play in stabilising or binding this reactive electrophile, and contributing to its activity in the complex honey matrix still requires substantial investigation. The antioxidant and immunostimulatory properties are still essentially uncharacterised and unexplained, and the role of the newly discovered MGO in those functions, if at all, requires further investigation.

8.4 Conclusion

In conclusion, manuka honey is a complex matrix of bioactive compounds which exert an inhibitory, but not necessarily lethal, effect on pathogenic bacteria, by mechanisms still to be clarified, and no doubt to be identified. Manuka honey also possesses a number of other bioactive functions derived from its numerous constituents, and much further investigation of its composition, formation, storage and activities awaits.

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