Author contributions
NRS
The authors’ responsibilities were as follows: NR-S and SDRG conceived the project, NR-S and SDRG developed the overall research plan, NR-S conducted the research and analysed the samples, NRS performed the statistical analysis, NR-S and SDRG wrote the manuscript, SDRG had primary responsibility for the final content; and both authors read and approved the final manuscript. The authors would like to thank Mr Chris Grigson for his technical support in the samples analysis and all the volunteers for taking part in the study.