Very high blood concentrations of vitamin D, like very low levels, could be associated with an increased mortality risk, suggests a new Danish study. Experts commented that the results were interesting but far from reality, considering the fact that vitamin D insufficiency is so widespread.
The observational study investigated the association between all-cause mortality and serum concentrations of vitamin D in 247,574 participants for an average of 3 years (1). The study results showed that participants with blood concentrations of less than 10 nmol of vitamin D per liter of serum, mortality was found to be more than two times higher than for those with levels of 50–60 nmol/l, which was linked to the lowest morta-lity risk. On the other hand, blood levels of more than 140 nmol/l were associated with an increased mortali-ty risk of about 1.4 times those of the mid-range.
The researchers commented that while low levels of vitamin D were expectably linked with an increase in mortality, it would have been a surprise if very high levels were also associated with an increased risk of death. There would then be no scientific evidence for a “more is better” argument for vitamin D. As the causes of higher mortality are currently unknown, it is important to conduct further studies in order to understand the relationship, said the researchers.
Experts commented that the findings were interesting but also raised the question who is achieving such high levels. In order to reach serum concentrations of about 140 nmol/l, a daily intake of about 4800 IU would be needed, which is far higher than current recommendations. In the study, 22.5% of participants were vitamin D deficient (below 25 nmol/l), 33.3% had insufficient (25-50 nmol/l) and 27.2% inadequate (50-75 nmol/l) levels. Only 12% had blood vitamin D concentrations of 75-100 nmol/l and 5% even higher ones. In sum-mary, 56% of the participants showed vitamin D levels below the recommendation of the US Institute of Medicine (50 nmol/l) and 83% below the recommendations of the International Osteoporosis Foundation
(75 nmol/l). They suggested that it cannot be entirely excluded that sick individuals or individuals at increa-sed risk for chronic diseasemay have taken vitamin D supplements at higher dose because of a disease-based motivation. However, as the majority of people have levels of vitamin D below the level for optimal health, the problem is more likely that too many people consume too little of the vitamin, and not too much, the experts noted.
With regard to low vitamin D concentrations the study results were consistent with other observational studies in suggesting that mortality may be lowest at 25(OH)D levels between 50 and 80 nmol/l while reporting a marked increased risk in mortality with 25(OH)D levels below 40 to 50 nmol/l.