With great appreciation, I would like to accentuate the substantial contributions of the following people who made this project possible: To Dr. CMT Fourie (my supervisor), Prof. JM van Rooyen (my co–supervisor) and Prof. AE Schutte (my co–supervisor) whose gracious advise, patient guidance, commitment and support have enabled me to plan, analyse, interpret and write this project in a scientific manner. It has been an educational experience for me, thank you. To Mr. LS Wyldbore for the language editing of this dissertation. I thank all the participants, researchers, field workers and supporting staff of the PURE study. The financial assistance of the National Research Foundation (DAAD–NRF) towards this research is hereby acknowledged. A special thanks to my parents, sister, Albert, family and friends, thank you for the never–ending love, support, patience and understanding that you gave me throughout this project. Last, but not the least, a special thank to God for giving me the opportunity, talent, determination and endurance to complete this project. / Thesis (M.Sc. (Physiology))--North-West University, Potchefstroom Campus, 2012.

With great appreciation, I would like to accentuate the substantial contributions of the following people who made this project possible: To Dr. CMT Fourie (my supervisor), Prof. JM van Rooyen (my co–supervisor) and Prof. AE Schutte (my co–supervisor) whose gracious advise, patient guidance, commitment and support have enabled me to plan, analyse, interpret and write this project in a scientific manner. It has been an educational experience for me, thank you. To Mr. LS Wyldbore for the language editing of this dissertation. I thank all the participants, researchers, field workers and supporting staff of the PURE study. The financial assistance of the National Research Foundation (DAAD–NRF) towards this research is hereby acknowledged. A special thanks to my parents, sister, Albert, family and friends, thank you for the never–ending love, support, patience and understanding that you gave me throughout this project. Last, but not the least, a special thank to God for giving me the opportunity, talent, determination and endurance to complete this project. / Thesis (M.Sc. (Physiology))--North-West University, Potchefstroom Campus, 2012.