Adequate vitamin D status plays an important role in bone health and may also protect against Type 1 Diabetes (T1D), multiple sclerosis and certain cancers. Vitamin D is obtained from two sources; diet and through skin synthesis through the action of ultraviolet (UV) light. Dietary intakes of vitamin D are low in New Zealand (NZ) and the majority of our vitamin D comes from UV exposure. The NZ population may be at risk of low vitamin D status because of low dietary intakes, the country�s latitude (35-46 �S), and high proportion of darker skinned Maori and Pacific People. While case reports have described the occurrence of rickets, predominantly in immigrant groups, there are currently no national data on the vitamin D status of the NZ population. Reports of low vitamin D status in countries of similar latitude to NZ justify an examination of New Zealanders� vitamin D status. The best method to assess of vitamin D status is to measure circulating 25-hydroxyvitamin D concentrations. This thesis comprises three main studies. The first two had the following aims: to measure 25-hydroxyvitamin D concentrations and their determinants in a national sample (n=1585) of NZ children aged 5-14 y and to measure serum 25-hydroxyvitamin D concentrations and their determinants in a national sample (n=2948) of New Zealanders aged 15 y and over. The 2002 Children�s Nutrition Survey CNS02 was a year long (December, March-November) cross-sectional survey of a nationally representative sample of NZ school children 5-14 y. Over-sampling of Maori and Pacific children allowed ethnic specific analyses. The 1997 National Nutrition Survey (NNS97) participants were recruited over one year according to an area-based sampling frame with a 3 stage stratified design consisting of primary sampling units, households within each unit, and one randomly selected respondent from each household. Mean (99% CI) serum 25-hydroxyvitamin D concentrations were similar in children and adults (both 50 nmol/L). Among Maori, Pacific and NZEO children respectively, prevalence (%, 99% CI) of serum 25-hydroxyvitamin D deficiency (< 17.5 nmol/L) was 5% (2, 12), 8% (5, 14), and 3% (1,7). Based on a cutoff of < 37.5 nmol/L, prevalence of insufficiency was 41% (29, 53), 59% (42, 75) and 25% (15, 35), respectively. Based on a cutoff of 50 nmol/L, 56% of children were insufficient. Three percent of adult New Zealanders had serum 25-hydroxyvitamin D concentrations indicative of deficiency ([less than or equal to] 17.5 nmol/L); 48% and 84% were insufficient based on cutoffs of [less than or equal to] 50 and [less than or equal to] 80 nmol/L The main determinants of vitamin D status in NZ children were season, ethnicity and sex. After adjustment for other factors and covariates, boys had an adjusted mean (99% CI) 25-hydroxyvitamin D concentration 5 (1, 9) nmol/L higher than girls, Maori children were 7 (2, 11) and Pacific children 15 (11, 20) nmol/L lower than NZ European and Other (NZEO) children. Obese children were 7 (2, 11) nmol/L lower than overweight or �normal� weight. Children�s mean 25-hydroxyvitamin D concentrations (adjusted for other variables) peaked in March (69 nmol/L) and was at its lowest in August (36 nmol/L). In adults, there were effects of a similar magnitude of ethnicity and season on serum 25-hydroxyvitamin D concentrations. Obesity, latitude and age were determinants of vitamin D status in women but not men. Obese (BMI > 30) women had an adjusted mean vitamin concentration 6 (3, 10) nmol/L lower than women with BMI < 25. Women living in the South Island were 6 (3, 9) nmol/L lower than women living in the North Island. Additionally, adjusted mean serum 25-hydroxyvitamin D was 13 (8, 18) higher in women 15 -18 y than women 65 y or older. The third and final study aimed to determine whether the higher rates of vitamin D inadequacy reported in the winter than summer months in NZ also result in higher PTH concentrations, which would provide evidence for functional effect of inadequate vitamin D status. We also aimed to objectively explore the effect of natural skin colour on vitamin D status, given the higher prevalence of vitamin D insufficiency in dark-skinned groups living far from the equator. Skin colour measurements were taken with a hand-held light reflectometer (Datacolor Mercury[TM] 1000 colorimeter, Lawrenceville, NJ). In the 342 residents of Invercargill and Dunedin, mean serum 25-hydroxyvitamin D concentrations were lower in the late summer versus early spring (79 vs 51 nmol/L; P< 0.001). The lower serum 25-hydroxyvitamin D in early spring versus summer was associatedwith a 2 pg/mL (P< 0.001) higher parathyroid hormone (PTH) concentration. Interestingly, no significant effect of natural skin colour, based on light reflectance at the inside of the upper arm, was discovered, though there was a positive effect of tanning, based on light reflectance at the upper forearm, on serum 25-hydroxyvitamin D concentrations. Ethnicity and season are major determinants of serum 25-hydroxyvitamin D in New Zealanders. There is a high prevalence of vitamin D insufficiency in NZ children and adults, which may contribute to increased risk of osteoporosis and other chronic disease. While there is a pressing need for more convincing evidence with regards to the health risks associated with the low vitamin D status in children, evidence from the study of adults, where higher PTH concentrations were found during spring versus summer, suggests that the low 25-hydroxyvitamin D concentrations are having an adverse effect on bone health of adults. The high prevalence of vitamin D insufficiency in New Zealanders, warrants serious consideration of strategies such as fortification, to improve the vitamin D status of the population.

Adequate vitamin D status plays an important role in bone health and may also protect against Type 1 Diabetes (T1D), multiple sclerosis and certain cancers. Vitamin D is obtained from two sources; diet and through skin synthesis through the action of ultraviolet (UV) light. Dietary intakes of vitamin D are low in New Zealand (NZ) and the majority of our vitamin D comes from UV exposure. The NZ population may be at risk of low vitamin D status because of low dietary intakes, the country�s latitude (35-46 �S), and high proportion of darker skinned Maori and Pacific People. While case reports have described the occurrence of rickets, predominantly in immigrant groups, there are currently no national data on the vitamin D status of the NZ population. Reports of low vitamin D status in countries of similar latitude to NZ justify an examination of New Zealanders� vitamin D status. The best method to assess of vitamin D status is to measure circulating 25-hydroxyvitamin D concentrations. This thesis comprises three main studies. The first two had the following aims: to measure 25-hydroxyvitamin D concentrations and their determinants in a national sample (n=1585) of NZ children aged 5-14 y and to measure serum 25-hydroxyvitamin D concentrations and their determinants in a national sample (n=2948) of New Zealanders aged 15 y and over. The 2002 Children�s Nutrition Survey CNS02 was a year long (December, March-November) cross-sectional survey of a nationally representative sample of NZ school children 5-14 y. Over-sampling of Maori and Pacific children allowed ethnic specific analyses. The 1997 National Nutrition Survey (NNS97) participants were recruited over one year according to an area-based sampling frame with a 3 stage stratified design consisting of primary sampling units, households within each unit, and one randomly selected respondent from each household. Mean (99% CI) serum 25-hydroxyvitamin D concentrations were similar in children and adults (both 50 nmol/L). Among Maori, Pacific and NZEO children respectively, prevalence (%, 99% CI) of serum 25-hydroxyvitamin D deficiency (< 17.5 nmol/L) was 5% (2, 12), 8% (5, 14), and 3% (1,7). Based on a cutoff of < 37.5 nmol/L, prevalence of insufficiency was 41% (29, 53), 59% (42, 75) and 25% (15, 35), respectively. Based on a cutoff of 50 nmol/L, 56% of children were insufficient. Three percent of adult New Zealanders had serum 25-hydroxyvitamin D concentrations indicative of deficiency ([less than or equal to] 17.5 nmol/L); 48% and 84% were insufficient based on cutoffs of [less than or equal to] 50 and [less than or equal to] 80 nmol/L The main determinants of vitamin D status in NZ children were season, ethnicity and sex. After adjustment for other factors and covariates, boys had an adjusted mean (99% CI) 25-hydroxyvitamin D concentration 5 (1, 9) nmol/L higher than girls, Maori children were 7 (2, 11) and Pacific children 15 (11, 20) nmol/L lower than NZ European and Other (NZEO) children. Obese children were 7 (2, 11) nmol/L lower than overweight or �normal� weight. Children�s mean 25-hydroxyvitamin D concentrations (adjusted for other variables) peaked in March (69 nmol/L) and was at its lowest in August (36 nmol/L). In adults, there were effects of a similar magnitude of ethnicity and season on serum 25-hydroxyvitamin D concentrations. Obesity, latitude and age were determinants of vitamin D status in women but not men. Obese (BMI > 30) women had an adjusted mean vitamin concentration 6 (3, 10) nmol/L lower than women with BMI < 25. Women living in the South Island were 6 (3, 9) nmol/L lower than women living in the North Island. Additionally, adjusted mean serum 25-hydroxyvitamin D was 13 (8, 18) higher in women 15 -18 y than women 65 y or older. The third and final study aimed to determine whether the higher rates of vitamin D inadequacy reported in the winter than summer months in NZ also result in higher PTH concentrations, which would provide evidence for functional effect of inadequate vitamin D status. We also aimed to objectively explore the effect of natural skin colour on vitamin D status, given the higher prevalence of vitamin D insufficiency in dark-skinned groups living far from the equator. Skin colour measurements were taken with a hand-held light reflectometer (Datacolor Mercury[TM] 1000 colorimeter, Lawrenceville, NJ). In the 342 residents of Invercargill and Dunedin, mean serum 25-hydroxyvitamin D concentrations were lower in the late summer versus early spring (79 vs 51 nmol/L; P< 0.001). The lower serum 25-hydroxyvitamin D in early spring versus summer was associatedwith a 2 pg/mL (P< 0.001) higher parathyroid hormone (PTH) concentration. Interestingly, no significant effect of natural skin colour, based on light reflectance at the inside of the upper arm, was discovered, though there was a positive effect of tanning, based on light reflectance at the upper forearm, on serum 25-hydroxyvitamin D concentrations. Ethnicity and season are major determinants of serum 25-hydroxyvitamin D in New Zealanders. There is a high prevalence of vitamin D insufficiency in NZ children and adults, which may contribute to increased risk of osteoporosis and other chronic disease. While there is a pressing need for more convincing evidence with regards to the health risks associated with the low vitamin D status in children, evidence from the study of adults, where higher PTH concentrations were found during spring versus summer, suggests that the low 25-hydroxyvitamin D concentrations are having an adverse effect on bone health of adults. The high prevalence of vitamin D insufficiency in New Zealanders, warrants serious consideration of strategies such as fortification, to improve the vitamin D status of the population.