Total 25-OH Vitamin D Concentrations in Chinese, Malays and Indians

There has been increasing interest in recent years in the prevalence of low 25-hydroxy (OH) vitamin D concentrations in different human populations [1, 2]. There are 2 forms of vitamin D: cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2). Vitamin D3 is manufactured through the photoisomerization of 7-dehydrocholesterol to pre-vitamin D3, which then undergoes a temperature-dependent rearrangement to form vitamin D3. Vitamin D2 is a plant sterol found in foods, such as fatty fish, codliver oil, and eggs. Vitamin D is converted to 25-OH vitamin D, the major circulating form of vitamin D, and then to 1, 25-diOH vitamin D, the active form of vitamin D, by enzymes in the liver and kidney. 25-OH vitamin D assays should be able to measure both the D2 and D3 forms of 25-OH vitamin D as both forms are biologically active after 1-α-hydroxylation. For the majority of patients, total 25-OH vitamin D measurement is suitable for the diagnosis and monitoring of vitamin D deficiency. Quantification of 25-OH vitamin D2 and 25-OH vitamin D3 fractions may be useful in monitoring patients without clinical improvement despite treatment where inadequate dosing, non-adherence, or malabsorption are suspected [3]. High rates of low 25-OH vitamin D concentration have been described in most epidemiological studies, including those from tropical regions with high numbers of sunlight hours per year. However, changes in choice of measurand (25-OH vitamin D2, 25-OH vitamin D3, or total 25-OH vitamin D), analytical methodology, and the cutoffs defining deficiency/insufficiency over time mean that published findings soon become obsolete and require reassessment [4-6]. In 2011, Roche Diagnostics replaced their 25-OH vitamin D3 assay with a total 25-OH vitamin D assay. According to the manufacturer, this new assay shows 98% and 81% cross-reactivity with 25-OH vitamin D3 and 25-OH vitamin D2, respectively. We have previously shown low levels of 25-OH vitamin D3 in Asians using the Roche Diagnostics 25-OH vitamin D3 assay (Mannheim, Germany), particularly in Malays and Indians [7]. However, concerns have been raised that underestimation of 25-OH vitamin D3 concentrations with the Roche 25OH vitamin D3 assay has led to over-reporting of vitamin D deficiency [8]. The new Roche total 25-OH vitamin D assay has been reported as fulfilling present analytical performance requirements with close agreement to other well-established methods for 25-OH vitamin D analysis, such as liquid chromatography-tandem mass spectrometry and high-performance liquid chromatography, making it fit for routine assessment of vitamin D status [9]. This study was designed to assess whether the same pattern of low total 25-OH vitamin D concentrations is seen in the same Asian population using the new Roche total 25-OH vitamin D assay using the 2010 Institute of Medicine guidelines for 25-OH vitamin D interpretation [10]. Total 25-OH vitamin D concentrations were measured on 240 (40 male and 40 female specimens from each race: Chinese, Malay, and Indian) anonymised left-over fasting venous serum samples from apparently healthy ambulatory outpatients undergoing health screening. These were not the same samples or individuals used in the previous 25-OH vitamin D3 study. Singapore is 1.4° north of the equator and has no seasons as such, with a daily difference of less than 30 min of sunlight hours over