Vitamin D (25OHD) Serum Seasonality in the United States

Abstract

Background: Vitamin D is an important micronutrient for health. Hypovitaminosis D is thought to play a role in the seasonality of a number of diseases and adverse health conditions. To refine hypotheses about the links between vitamin D and seasonal diseases, good estimates of the cyclicality of serum vitamin D are necessary.

Objectives: The objective of this study is to describe quantitatively the cyclicality of 25-hydroxyvitamin D (25OHD) in the United States. We provide a statistical analysis with weekly time resolution, in comparison to the quarterly (winter/spring/summer/fall) estimates already in the literature.

Methods: We analyzed time series data on 25OHD, spanning 287 consecutive weeks. The pooled data set comes from 3.44 million serum samples from the United States. We statistically analyzed the proportion of sera that were vitamin D sufficient, defined as 25OHD [Formula: see text] ng/mL, as a function of date.

Results: In the United States, serum 25OHD follows a lagged pattern relative to the astronomical seasons, peaking in late summer (August) and troughing in late winter (February). Airmass, which is a function of solar altitude, fits the 25OHD data very well when lagged by 8 weeks.

Conclusions: Serum vitamin D levels can be modeled as a function of date, working through a double-log transformation of minimal solar airmass (easily calculated from solar altitude, retrievable from an online solar altitude/azimuth table).

Figure 1. Weekly log(airmass) Rochester, Minnesota versus log(airmass) Miami, Florida (on log scale, thus double-log).

Colors signify month. January–June, squares; July–December, triangles.

Figure 2. Time series plot of 25OHD data and covariates.

Top panel plots model covariates. Left axis: logarithm of minimum solar airmass, 8-week lag (light red, on log scale, thus double-log); right axis: Oceanic Niño Index (green). Bottom panel plots proportion of serum samples testing ng/mL (blue), and model (black), 95% confidence bounds (gray), from model 3 in table 1.

Figure 3. Time series plot of 25OHD data, four categories.

Each color represents one 25OHD category, as indicated. Models (black) and 95% confidence bounds (gray) from models in table 3 are also shown. All four series total 100%.