Today’s news brings a U-shaped curve from a cod liver oil country. The Danish authors found that both too low and too high vitamin D levels are associated with excessive mortality. In particular, they found people with levels above 56 ng/ml (140 nmol/L) had a 40% increased chance of dying compared to those with a level of 30 ng/ml (75 nmol/L), while people with low levels had more than a 200% increased risk of dying over the three years of the study.
Durup D, Jørgensen HL, Christensen J, Schwarz P, Heegaard AM, Lind B. A Reverse J-Shaped Association of All-Cause Mortality with Serum 25-Hydroxyvitamin D in General Practice, the CopD Study. J Clin Endocrinol Metab. 2012 May 9.
Several problems immediately come to mind after reading the study. First, according to a Cochrane review, the preformed vitamin A, as in cod liver oil, is associated with excess mortality. How do I know they were taking cod liver oil to get higher vitamin D levels? Because, consumption of cod liver oil in Norway is about 44% by females, and females tended to have the highest 25(OH)D levels in the above study.
Second, another far northern cod liver oil country, Sweden, also found a U shaped risk.
Michaëlsson K, Baron JA, Snellman G, Gedeborg R, Byberg L, Sundström J, Berglund L, Arnlöv J, Hellman P, Blomhoff R, Wolk A, Garmo H, Holmberg L, Melhus H. Plasma vitamin D and mortality in older men: a community-based prospective cohort study. Am J Clin Nutr. 2010 Oct;92(4):841-8. Epub 2010 Aug 18.
However, eight studies, mostly from non-cod liver oil countries, have found no such association with high levels, but a clear risk of dying with low levels (referenced in first paper), although the number of high levels in many of these eight papers was modest.
Third, the man who started the vitamin D revolution, Professor Reinhold Vieth, questions these far northern studies because he has a theory that intracellular vitamin D enzyme systems harmfully reset when under significant seasonal variations. If the authors of the paper in question sorted their results by season, I cannot find it. We know death rates are lowest in the summer when vitamin D levels are the highest, so without seasonal data, Vieth’s explanation cannot be tested.
Vieth R. Enzyme kinetics hypothesis to explain the U-shaped risk curve for prostate cancer vs. 25-hydroxyvitamin D in nordic countries. Int J Cancer. 2004 Sep 1;111(3):468; author reply 469. No abstract available.
Fourth, the study in question implies that vitamin D levels that the hunter-gatherers in Tanzania easily achieve with sun exposure are dangerous.
I doubt it. I doubt Mother Nature is that dumb.