Vitamin D: deficiency or no deficiency?

To the Editor: Vitamin D deficiency is a common disorder that can have adverse effects on bone metabolism. There is no universal agreement on the levels of serum 25-hydroxyvitamin D [25(OH)D] that define vitamin D deficiency. Several cut-off points were suggested, including levels less than 12.5 ng/mL (30 nmol/L), 20 ng/mL (50 nmol/L) and 30 ng/mL (75 nmol/L). (1,2) Levels higher than 30 ng/mL (75 nmol/L) have been recently recommended to achieve better bone and dental health and lower the risk of fractures. (3) Vitamin D deficiency is generally associated with increased levels of parathyroid hormone (PTH), and the level of 25(OH)D at which PTH concentration starts to rise has been postulated to define vitamin D deficiency.

This study was undertaken to determine the level of 25(OH)D that defines vitamin D deficiency in a population with year-round sun exposure. The study population consisted of ambulatory individuals residing in the eastern province of Saudi Arabia. Enrolled patients were aged 15 years or more, attending primary care clinics for conditions not related to metabolic bone disease, not taking medications known to affect bone metabolism and who were suspected to have vitamin D deficiency based on a quantified history of sun exposure, dietary intake and musculoskeletal symptoms. Levels of serum 25(OH)D, calcium, and PTH were measured. To determine the cutoff point to define vitamin D deficiency, a multivariate adaptive regression analysis of PTH on 25(OH)D level was performed. A total of 71 patients were evaluated (87% females, 13% males), with a mean age of 41 years. Mean 25(OH)D concentration was 7.75 ng/mL (18.6 nmol/L); 95% confidence interval, 15.9 to 21.3. Serum 25(OH)D was less than 30 ng/mL (75 nmol/L) in 100% and less than 20 ng/mL (50 nmol/L) in 97.2% of patients. Gender and age were not significant factors in determining vitamin D status. Normal PTH concentrations were found in 35.7% of patients with severe deficiency [serum 25(OH)D, below a level of detection of 5 ng/mL (12 nmol/L)]. PTH levels correlated negatively with 25(OH)D [r = -0.23; P = 0.04], but we could not identify a threshold level for serum 25(OH)D at which PTH would have started to rise.

Even though vitamin D deficiency is a very common condition worldwide, identifying a criterion for its definition remains unresolved. Some suggested certain levels of 25(OH)D based on data correlating with PTH concentrations, while others proposed a population-based estimate, considering factors that can affect the definition such as geographical location, age, race and time of the year. (4)

Our study revealed that in a population with abundant sun exposure, PTH concentrations were not helpful to define the cutoff level that defines vitamin D deficiency; this confirms findings from a study of vitamin D status in a Finnish population. (5) Larger scale population-based studies are suggested to delineate individual characteristics that define vitamin D status.

Mohsen Eledrisi, MD, FACP, FACE

Reem Alamoudi, MD

Buthina Alhaj, RN

Rifat Rehmani, MD

Division of Endocrinology

Department of Internal Medicine

National Guard Medical Center

Al-Ahsa, Saudi Arabia

References

1. Holick MF, Siris ES, Binkley N, et al. Prevalence of Vitamin D inadequacy among postmenopausal North American women receiving osteoporosis therapy. J Clin Endocrinol Metab 2005;90:3215-3224.

2. Plotnikoff GA, Quigley JM. Prevalence of severe hypovitaminosis D in patients with persistent, nonspecific musculoskeletal pain. Mayo Clin Proc 2003;78:1463-1470.

3. Bischoff-Ferrari HA, Giovannucci E, Willett WC, et al. Estimation of optimal serum concentrations of 25-hydroxyvitamin D for multiple health outcomes. Am J Clin Nutr 2006;84:18-28.

4. Levis S, Gomez A, Jimenez C, et al. Vitamin d deficiency and seasonal variation in an adult South Florida population. J Clin Endocrinol Metab 2006;90:1557-1562.

5. Valimaki VV, Alfthan H, Lehmuskallio E, et al. Vitamin D status as a determinant of peak bone mass in young Finnish men. J Clin Endocrinol Metab 2004;89:76-80.