The Complexities of Vitamin D
Two very different vitamin D research studies demonstrate the complexities of nutrition. After identifying 20 eligible studies involving 9,209 participants, Zhang and colleagues report vitamin D deficiency is associated with increased risk of gestational diabetes mellitus (GDM). Qi and colleagues report that carrying a specific vitamin D polymorphism, the T allele of DHCR7 rs 12785878, may benefit individuals with greater improvements in insulin resistance (vs non-carriers) when following a high-protein weight-loss diet.
In the GDM meta-analysis, the odds ratio of GDM increased by 1.53 in women who were vitamin D deficient. Of the 20 studies, 8 were conducted in Asia, 7 in North America, 4 in Europe and 1 in Australia. Approximately eighteen per cent of the participants were diagnosed with GDM and the average age at pregnancy ranged between 24-35y. Vitamin D deficiency was defined as 50 nmol/L. According to the Institute of Medicine, this is the cut-off for insufficiency with 20 nmol/L being the cut-off for deficiency. Women diagnosed with GDM had, on average, serum 25(OH)D concentrations that were 4.9 nmol/L lower than those without GDM.
The genetics study studied obese and overweight individuals without type 2 diabetes. The authors identified 3 vitamin D metabolism genes: DHCR7 which helps make cholesterol – a building block of vitamin D; CYP2R1 – which encodes the enzyme that converts vitamin D to the circulating active form; and GC – which codes for vitamin D binding protein. A meaningful relationship was only observed with the DHCR7 polymorphism.
These two new studies emphasize the complexities of our bodies. Nutrition-health relationships are complex. Both nutrient intake (both energy balance and macronutrient source, eg protein) and physiological state (pregnancy) interact to affect vitamin D status and health.
Zhang M-X, Pan G-T, Guo J-F, Li B-Y, Qin L-Q, Zhang Z-L. Vitamin D deficiency increases the risk of gestational diabetes mellitus: A meta-analysis of observational studies. 2015 Nutr doi: 10.3390/nu7105398
Qi Q, Zheng Y, Huang t, Rood J, Bray GA, Sacks FM, Qi L. Vitamin D metabolism-related genetic variants, dietary protein intake and improvement of insulin resistance in a 2 year weight-loss trial: POUNDS Lost. 2015 Diabetologia doi: 10.1007/200125-015-3750-1
Dorjgochoo T, Shi J, Gao YT, Long J, Delahanty R, Xiang YB, Cai Q, Shu XO. Genetic variants in vitamin D metabolism-related genes and body mass index: analysis of genome-wide scan data of approximately 7000 Chinese women. 2011 Int J Obes doi: 10.1038/ijo.2011.246