Vitamin D can be Measured: Has Your 25-hydroxyvitamin D3 been Assayed?
When it is summertime and beaches beckon, we aren’t very likely to reflect upon vitamin D status. But scientists will. Bhagatwala and colleagues report results from a randomized-controlled trial (RCT) evaluating blood vitamin D responses to vitamin D3 supplementation in overweight/obese African Americans with suboptimal vitamin D status (<50 nmol/L).
The Institute of Medicine recommended daily intake of 600 IU for adults was insufficient to achieve serum 25(OH)D concentrations of 75 nmol/l in this group whereas 2,000 and 4,000 IU doses daily were adequate. The 4,000 IU dose increased serum 25(OH)D concentrations more rapidly. In all cases, vitamin D supplementation had a dose- and time-dependent effect increasing blood vitamin D status.
Because vitamin D status changes seasonally, it is important to increase vitamin D intake during the winter months. I spend most of my time indoors. For the first time in my life, I had my serum 25(OH)D concentration measured (in February). My vitamin D status was 49 nmol/L, in other words suboptimal. This is despite regularly taking either 1,000 or 2,000 IU vitamin D3 per day for the past 2 years.
While my lack of skin pigmentation may help maintain a higher vitamin D status, my supplementation practices are not that different from recommendations of Ng and colleagues for African-Americans. Still, they reported tremendous variability among individuals to vitamin D supplementation. The median serum 25(OH)D concentrations were 75 nmol/L (29.7 ng/mL) and 87 nmol/L (34.8 ng/mL), with most values ranging between 25-100 nmol/L and 25-125 nmol/L, respectively for 1,000 and 2,000 IU vitamin D3 daily. My serum vitamin D concentrations are less than the median level measured in African Americans taking either 1,000 or 2,000 IU daily. Why?
Genetics influence serum 25(OH)D responses to diet and ultraviolet solar exposure. Four common genetic variants rs10741657 and rs10766197 in 25-hydroxylase (CYP2R1) and rs842999 and rs4588 in vitamin D binding protein (GC) predict serum vitamin D concentrations. Nissen and colleagues reported a mean 20.9 nmol/L difference in serum vitamin D concentrations between carriers of no- risk alleles and carriers of all 4 risk alleles. I don’t know if I have am a carrier of zero or 4 risk alleles. I do (now) know my serum 25(OH)D3 status; it isn’t optimal.
Experts can define categories according to age and sex when setting dietary recommendations with a serum 25(OH)D3 concentration goal above 50 nmol/L. However, the ONLY way for me (or anyone else) to know my vitamin D status is to have my serum 25(OH)D3 concentration measured at a lab. With this information, I can adjust the amount of time spent in the sun or the IUs of vitamin D3 consumed daily. Hopefully, my physician will also measure my status again next year. With this information, I may be able to maintain an optimal vitamin D status.
Bhagatwala J, Zhu H, Parikh SJ, Guo D-H, Kotak I, Huang Y, Havens R, Pham M, Afari E, Kim S, Cutler C, Pollock NK, Dong Y, Raed A, Dong Y. Dose and time responses of vitamin D biomarkers to monthly vitamin D3 supplementation in overweight/obese African Americans with suboptimal vitamin D status: a placebo controlled randomized clinical trial. 2015 BMC Obesity doi: 10.1186/s40608-015-0056-2
Ng K, Scott JB, Drake BF, Chan AT, Hollis BW, Chandler PD, Bennett GG, Giovannucci EL, Gonzalez-Suarez E, Meyerhardt JA, Emmons KM, Fuchs CS. Dose reponse to vitamin D supplementation in African Americans: results of a 4-arm, randomized, placebo-controlled trial. 2014 AJCN doi: 10.3945/ajcn.113.067777
Nissen J, Vogel U, Raven-Haren G, Andersen EW, Madsen KH, Nexo BA, Andersen R, Mejborn H, Bjerrum PJ, Rasmussen LB, Wulf HC. Common variants in CYP2R1 and GC genes are both determinants of serum 25-hydroxyvitamin D concentrations after UVB irradiation and after consumption of vitamin D3-fortified bread and milk during winter in Denmark. 2015 Am J Clin Nutr doi: 10.3945/ajcn.114.092148