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Roger Hallett/The Globe and Mail

Low vitamin D levels are a symptom of poor health – not the cause, according to a systematic review of previously published scientific studies. The new finding casts further doubt on the role of vitamin D supplements in disease prevention, even as millions of Canadians pop pills of the sunshine drug each day.

The promise of vitamin D supplementation was not borne out in a systematic review of 462 studies looking at the effects of vitamin D levels on health outcomes including cancer, cardiovascular disease, type 2 diabetes and Parkinson's disease.

In the review published Thursday in The Lancet Diabetes & Endocrinology, high vitamin D levels detected in 290 observational studies – including reduced risk of cardiovascular events (up to 58 per cent), type-2 diabetes (up to 38 per cent) and colorectal cancer (up to 34 per cent) – were not confirmed in 172 randomized trials, which are the gold-standard method for determining a cause-effect relationship, according to lead author Philippe Autier from the International Prevention Research Institute in Lyon, France.

When Autier and colleagues took a closer look at the randomized trials, they found that higher vitamin D supplementation had no effect on disease occurrence, severity or disease course.

"What this discrepancy suggests is that decreases in vitamin D levels are a marker of deteriorating health. Aging 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," Autier said in a statement.

An accompanying editorial pointed to the need for large clinical trials to evaluate the effects of vitamin D on health outcomes. "It would be a real boon to patients if the results are positive," the authors noted, "but unless effect sizes for clinically important outcomes are large, the results will only confirm the neutral effect reported by most clinical trials thus far."

These are dark times for vitamin D.

In October, a large review published in the Lancet found that vitamin D did little to improve bone loss.

"Most healthy adults do not need vitamin D supplements," Dr. Ian Reid, the lead author of that study, concluded.

In November, a major review published in the Annals of Internal Medicine concluded that "limited evidence supports any benefit from vitamin and mineral supplementation for the prevention of cancer or cardiovascular disease."

Back in 2010, when prominent researchers boasted about taking 6,000 to 8,000 IU of vitamin D per day, others offered a word of caution.

Dr. Roger Bouillon, a professor of medicine at the Catholic University of Leuven, Belgium, noted that early studies on vitamin D were not backed by experiments using actual doses. Proving that a nutritional supplement offers a therapeutic benefit would require rigorous drug-style clinical trials, he said at the time.

Meanwhile, evidence is growing that supplements, such as fish-oil capsules, may be less beneficial than food sources of specific nutrients.

Health Canada's recommended dietary allowance for vitamin D is 800 IU per day, with an upper limit of 4,000 IU per day. In addition, Health Canada recommends that everyone over the age of 50 take a daily vitamin D supplement of 400 IU.

Natural food sources of vitamin D include fatty fish and egg yolks. But until research on vitamin D is more conclusive, for most Canadians, taking vitamin D supplements remains a gamble.