Defining physiologically "normal" vitamin D in African Americans

Abstract

The relationship between serum 25(OH)D and intact parathyroid hormone (iPTH) was evaluated in the Multicenter Osteoarthritis Study (MOST). No further change in iPTH was observed for African Americans with 25(OH)D levels above 20 ng/ml, suggesting that compared to Caucasians, lower vitamin D targets for sufficiency may be appropriate for African Americans.

Introduction: Vitamin D levels ≥30 ng/ml are commonly considered "normal" based upon maximal suppression of iPTH; however, this has recently been challenged and the optimal 25(OH)D level among non-Caucasians is unclear. We evaluated the cross-sectional relationship between serum 25(OH)D and iPTH in a sample of Caucasian and African American adults.

Methods: We used baseline serum samples of participants from the Multicenter Osteoarthritis Study (MOST) for this analysis and used three methods to model the relationship between 25(OH)D and iPTH: ordinary least squares regression (OLS), segmented regression and Helmert contrasts.

Results: Among Caucasians (n = 1,258), 25(OH)D and iPTH ranged from 4 to 51 ng/ml and 2 to 120 pg/ml and from 3 to 32 ng/ml and 3 to 119 pg/ml in African Americans (n = 423). We observed different thresholds between African Americans and Caucasians using each analytic technique. Using 25(OH)D as a categorical variable in OLS, iPTH was statistically higher at lower 25(OH)D categories than the 24-32 ng/ml referent group among Caucasians. However, in African Americans, the mean iPTH was only significantly higher at 25(OH)D levels below 15 ng/ml. Using segmented regression, iPTH appeared to stabilize at a lower 25(OH)D level in African Americans (19-23 ng/ml) compared to in Caucasians (>32 ng/ml). Helmert contrasts also revealed a lower threshold in African Americans than Caucasians.

Conclusion: Among MOST participants, the 25(OH)D thresholds at which no further change in iPTH was observed was approximately 20 ng/ml in African Americans versus approximately 30 ng/ml in Caucasians, suggesting optimal vitamin D levels in Caucasians may not be applicable to African Americans.

Figure 1

The Relationship between 25(OH)D and iPTH by Race – LOWESS Plot

Figure 2

The Relationship between iPTH and 25(OH)D by Race – Results from Segmented Regression

Figure 3. The Relationship between iPTH and 25(OH)D by Race – Results from Helmert Contrast*

* The level of 25(OH)D where the Helmert contrast p-value first transitions below the reference line at p=0.05 shows the point at which the mean iPTH of all the higher 25(OH)D observations combined is significantly different than the mean iPTH of all the lower 25(OH)D observations. The 25(OH)D value at which this transition occurs is consistent with a threshold.

Figure 3. The Relationship between iPTH and 25(OH)D by Race – Results from Helmert Contrast*

* The level of 25(OH)D where the Helmert contrast p-value first transitions below the reference line at p=0.05 shows the point at which the mean iPTH of all the higher 25(OH)D observations combined is significantly different than the mean iPTH of all the lower 25(OH)D observations. The 25(OH)D value at which this transition occurs is consistent with a threshold.