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Item Nutritional status, exercise, and insulin sensitivity : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Public Health, Massey University, Manawatu, New Zealand(Massey University, 2011) Green, Jackson GeorgeThe insulin-glucose system in lean-healthy people adapts its normal function in the face of challenging metabolic conditions. To improve understanding of these adaptations, I exposed subjects to periods of starvation, high-protein-low-carbohydrate diet (HPLC) and overfeeding. In six lean-healthy men, dietary carbohydrate was eliminated but gluconeogenic substrate supply was maintained by three-day HPLC diet, compared with three-day starvation and three-day mixed-carbohydrate diet. Insulin sensitivity, vastus lateralis intramyocellular lipid (IMCL) and fasting glucose were unaffected by HPLC diet, although they were significantly altered after starvation. These results indicate that dietary carbohydrate restriction does not trigger metabolic adaptations, although total metabolic carbohydrate supply remains likely to be important. Six lean-healthy men underwent two three-day periods of starvation with either no exercise or daily endurance exercise (80 min.day−1 at 50% VO2Max) and a three-day mixed diet without exercise. Compared to mixed diet, starvation elevated fasting FFA and IMCL and decreased insulin sensitivity and fasting glucose. Exercise during starvation prevented the elevation of IMCL but did not prevent other metabolic disruption, in contrast with exercise during lipid infusion. Maintaining high physical-activity may prevent the metabolic disruption associated with overfeeding, while insulin sensitivity may predict partitioning of fuel between tissues during overfeeding. Nine endurance-trained athletes maintained their normal physical activity while consuming a diet providing 90 kJ.(kg body mass)-1.day-1 above their normal dietary intake for four weeks. Subjects’ body-mass, fat-mass and body fat% increased while insulin sensitivity tended to decrease (14.5 ± 5.9 to 9.5 ± 4.1 min-1.mU.l-1, p = 0.08). Change in insulin sensitivity was correlated with change in body fat % (r = -0.77, p < 0.023). Initial insulin sensitivity was correlated with change in body fat% (r = 0.90, p < 0.009) and the proportion of mass gained as lean tissue (r = 0.86, p < 0.024). Maintenance of already high physical-activity cannot prevent metabolic disruption associated with overfeeding. These results also suggest that insulin sensitivity influences energy partitioning between tissues. The results in this thesis suggest important interaction effects between exercise and diet. I propose that carbohydrate availability is a key modulator of the effects of exercise on metabolism in lean-healthy men.Item The role of vitamin D in metabolism and bone health : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Nutritional Science at Massey University, Albany, New Zealand(Massey University, 2009) von Hurst, Pamela RuthBackground Hypovitaminosis D is becoming recognised as an emerging threat to health, even in countries like New Zealand which enjoy plentiful sunshine. The evidence for a role for vitamin D deficiency in the aetiology of a plethora of diseases continues to accumulate, including type 2 diabetes, and the preceding insulin resistance. Objectives The primary objective of the Surya Study was to investigate the effect of improved vitamin D status (through supplementation) on insulin resistance. The secondary objectives were to investigate the vitamin D status and bone mineral density of South Asian women living in New Zealand, and to investigate the effect of vitamin D supplementation on bone turnover as measured by biochemical markers of bone resorption and formation. Method Women of South Asian origin, ≥20 years old, living in Auckland (n = 235) were recruited for the study. All were asked to complete a 4-day food diary, invited to have a bone scan, and were screened for entry into the intervention phase which required insulin resistance (HOMA-IR >1.93) and serum 25(OH)D < 50 nmol/L. Eighty-one completed a 6-month randomised controlled trial with 4000 IU vitamin D3 (n = 42) or placebo (n = 39). Primary endpoint measures included insulin resistance, insulin sensitivity (HOMA2%S), fasting C-peptide and markers of bone turnover, osteocalcin (OC) and collagen C-telopeptide (CTX). Ninety-one of the 239 had a bone scan and bone mineral density (BMD) was measured in the proximal femur and lumbar spine. Results Adequate serum 25(OH)D concentrations (>50 nmol/L) were observed in only 16% of subjects screened. Median (25th, 75th percentile) serum 25(OH)D increased significantly from 21 (11,40) to 75 (55,84) nmol/L with supplementation. Significant improvements were seen in insulin sensitivity and insulin resistance (P = 0·003, P = 0·02 respectively), and circulating serum insulin decreased (P = 0·02) with supplementation compared to placebo. There was no change in C-peptide with supplementation. Insulin resistance was most improved when endpoint serum 25(OH)D =80 nmol/L. In post-menopausal women OC and CTX levels increased in the placebo arm but CTX decreased from 0.39±0.15 to 0.36±0.17 (P = 0.012) with supplementation. Osteoporosis (T score <-2.5) was present in 32% of postmenopausal, and 3% of premenopausal women. Women 20 – 29 years (n=10) had very low BMD, calcium intake and serum 25(OH)D Conclusions Improving vitamin D status in insulin resistant women resulted in improved insulin resistance and sensitivity but no change in insulin secretion. Optimal 25(OH)D concentrations for reducing insulin resistance were shown to be ≥80 nmol/L. The prevalence of low 25(OH)D concentrations in this population was alarmingly high, especially in younger women. In post-menopausal women, vitamin D supplementation appeared to ameliorate increased bone turnover attributed to oestrogen deficiency.