By medwireNews Reporters
Low vitamin D status is a marker of ill health rather than an underlying cause of disease, say the authors of a systematic review published in The Lancet Diabetes and Endocrinology.
The conclusion comes after the team, led by Philippe Autier (International Prevention Research Institute, Lyon, France), found a wide discrepancy between the findings of observational and randomized clinical trials, casting doubt on the purported benefits of vitamin D supplements on non-skeletal outcomes.
“[W]e postulate that inflammation is the common factor between most non-skeletal health disorders and low 25(OH)D concentrations,” they write. “However, increases in 25(OH)D have no effect on inflammatory processes or on disorders at the origin of these processes.”
The team systematically reviewed data from 290 prospective observational studies and 172 randomized trials.
Most of the observational studies assessed disease occurrence or mortality while 11 evaluated cancer characteristics or survival. In general, these studies found moderate-to-strong inverse correlations between 25(OH)D levels and cardiovascular diseases, serum lipid concentrations, inflammation, glucose metabolism disorders, weight gain, declining cognitive function, and impaired physical functioning. High 25(OH)D concentrations were not associated with a lower risk for cancer, except colorectal cancer.
However, when the team assessed the randomized trials, they found a very different picture. These studies showed no effect of vitamin D supplementation on any of the major health outcomes or physiologic parameters studied, including colorectal cancer. This was also the case in a subset of 34 studies in people with low vitamin D levels at baseline.
The team notes that vitamin D supplementation was consistently associated with reduced all-cause mortality, at relative risks ranging from 0.93 to 0.96.
“But gains in survival are mainly in elderly women living independently or in institutional care, who are likely to initially have a very low concentration of 25(OH)D with a substantial risk of falls and fractures,” they remark.
Commenting on the findings in a press statement, Autier explained that the observed discrepancy between observational and randomized trials suggests “that decreases in vitamin D levels are a marker of deteriorating health.”
He added: “Ageing and inflammatory processes involved in disease occurrence and clinical course reduce vitamin D concentrations, which would explain why vitamin D deficiency is reported in a wide range of disorders.”
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