May 05, 2015
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Serum 25-(OH)D thresholds differ based on subgroup, outcomes

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Thresholds for serum 25-hydroxyvitamin D concentration may vary for different outcomes measured and for different populations, according to recent findings.

Moreover, these findings do not justify relatively high recommended levels of serum 25-(OH)D, particularly among older adults, women and those with a high BMI, according to the researchers.

In the study, Natasja M. van Schoor, PhD, of VU University Medical Center in the Netherlands, analyzed data from the Longitudinal Aging Study Amsterdam, on ongoing cohort study of older Dutch individuals. The current study used data from the second cycle of the first cohort, which took place from 1995 to 1996. Participants aged 65 or older (mean age, 75.2 years) on Jan. 1, 1996, were invited for a medical interview, and a study population of 1,164 participants was identified for inclusion.

The researchers defined the following as outcome measures: falls, fractures, parathyroid hormone (PTH) level, blood pressure, cardiovascular disease, BMI, grip strength, physical performance, functional limitations and mortality. These variables were identified due to their association with vitamin D status in previous studies.

Serum 25-(OH)D measurements were derived from morning serum samples. Spline plots were formulated to determine the relationship between serum 25-(OH)D and the outcomes, and the threshold of serum 25-(OH)D was visually estimated. The visually determined thresholds and the statistical best fit of the spline regression models were used to formulate the best estimate of the thresholds.

The researchers found that in the whole sample, thresholds for serum 25-(OH)D concentrations ranged from 46 nmol/L (PTH) to 68 nmol/L (hypertension). Women in the oldest age category (≥ 75 years) and those with high BMI had, on average, lower thresholds compared with men, those in the 65 to 75 year age category and those with BMI lower than 25 kg/m2. Due to lack of clinical relevance or a nonlinearity of the associations, the researchers could not determine thresholds for risk for falling, CVD, systolic and diastolic BP, mean arterial pressure, BMI or fractures.

According to the researchers, these findings will need to be further explored in populations with a wider variety of 25-(OH)D concentrations and in dose-response clinical trials.

“In conclusion, the main finding of this study is that thresholds for serum 25-(OH)D differ according to outcome and subgroup,” the researchers wrote. “This indicates that future guidelines may need to consider more subgroups. In addition, this study does not support relatively high required levels of serum 25-(OH)D, especially not for older persons, women and persons with high BMI.” – by Jennifer Byrne

Disclosure: The researchers report no relevant financial disclosures.