Newfoundland, Vitamin D Synthesis, and Diabetes Risk
Members of the Canadian Society for Nutrition are meeting in St John’s Newfoundland. “The Rock” is known for its grey (Canadian spelling intended), overcast, damp maritime climate. Fog is common. President Robert Bertola aptly described the weather in opening remarks, “If the landing wasn’t instruments only where you feel the ground before seeing the ground from the plane, you haven’t truly experienced Newfoundland!”
Vitamin D is the sunshine vitamin. Serum 25-hydroxyvitamin D3 concentrations fluctuate seasonally, increasing with sun exposure during the summer and falling in the winter. Low serum 25(OH)D3 is associated with insulin resistance, metabolic syndrome and diabetes. Oosterwerff and colleagues studied the effect of vitamin D supplementation (1,200 IU daily vs placebo) for 16 week in 130 non-Western individuals living in the Netherlands with prediabetes and suboptimal serum 25(OH)D levels (< 50 nmol/L) . 11% of participants had diabetes, 40% had impaired glucose tolerance and 49% were glucose tolerant (measured by oral glucose tolerance test at baseline). Baseline serum 25(OH)D3 levels averaged 21-25 nmol/L so many, but not all, were vitamin D deficient (< 20 nmol/L). After 2 months of supplementation, mean 25-hydroxyvitamin D levels increased to ≥58 nmol/L. The primary outcomes of insulin sensitivity and β cell function were not affected by treatment. Post hoc analysis restricted to subjects without diabetes who achieved serum 25(OH)D3 levels ≥ 60 nmol/L had statistically significant increases in insulinogenic index.
In a population of insulin resistant women with baseline 25(OH)D3 concentrations < 50 nmol/L, Von Hurst and colleagues reported that achieving vitamin D concentrations between 80-119 nmol/L improved insulin resistance. The Australian Diabetes, Obesity and Lifestyle study identified 6,537 individuals who developed diabetes and found higher serum 25(OH)D3 levels were associated with a significant reduced risk of diabetes. Those who developed diabetes had lower vitamin D concentrations (58 vs 65 nmol/L). Each 25 nmol/L increase in serum 25(OH)D3 was a associated with a 24% reduced risk of diabetes.
The evidence linking low vitamin D status with diabetes risk is emerging but not definitive. Still it is enough to make me want to get outdoors in the sunshine. But there is the balancing act between vitamin D synthesis and risk of skin melanoma to consider. Nutritionists always encourage variety, balance and moderation. Therein may be the solution: some sun exposure and a vitamin D supplement.
Oosterwerff MM, Eekhoff EMW, Schoor NMV, Boeke AJP, Nanayakkara P, Meijnen R, Knol DL, Kramer MHH, Lips P. Effect of moderate-dose vitamin D supplementation on insulin sensitivity in vitamin D-deficient non-Western immigrants in the Netherlands: a randomized placebo-controlled trial. 2014 AJCN doi: 10.3945/ajcn.113.069260
Harris SS, Dawson-Hughes B. Seasonal changes in plasma 25-hydroxyvitamin D concentrations of young American black and white women. 1998 AJCN 67(6):1232-1236
Von Hurst PR, Stonehouse W, Coad J. Vitamin D supplementation reduces insulin resistance in South Asian women living in New Zealand who are insulin resistant and vitamin D deficient – a randomized, placebo-controlled trial. 2010 BJN doi: 10.1017/S0007114509992017
Gagnon C, Lu ZX, Magliano DJ, Dunstan DW, Shaw JE, Zimmet PZ, Sikaris K, Grantham N, Ebeling PR, Daly RM. Serum 25-hydroxyvitamin D, calcium intake, and risk of type 2 diabetes after 5 years: Results from a national, population-based prospective study (the Australian Diabetes, Obesity and Lifestyle study). 2011 Diab Care doi: 10.2337/cd10-2167