There Are Two Sides to Each Coin: Vitamin D Increases Insulin Sensitivity in Obese, Yet Decreases it in Lean Mice

I am probably repeating myself, but I cannot emphasize enough that a common fallacy of medical research is the focus on pathologies. A recent example with respect to the "omnipotency" (that's what the Internet news could make you believe) of vitamin D comes from researchers at George Town University (GU. Press Release).

The scientists were able to replicate the results of previous studies, where high dose vitamin D supplementation had "significantly reduced development of estrogen receptor-positive (ER+) breast cancer", but for estrogen receptor-negative (ER-) breast cancer they found either no effect (lean mice) or even an increased rate of cancerous growth in the group of obese mice.

What's yet even more interesting that a similar contradiction was evident with respect to vitamin D's widely perpetuated beneficial effects on insulin resistance. In the pathologic model of the obese mice, vitamin D @ 15-25k IU per day was in fact able to ameliorate insulin resistance, in the lean, naturally insulin sensitive mice, however, insulin sensitivity was reduced by supplemental vitamin D.

As I've discussed it in the context of the most recent fish oil study (and will probably repeat for other "super nutrients"), in 99.9% of the cases the effect of - especially high dose - supplementation with ostensibly harmless "vitamins" or "nutrients" vary enormously depending on the subjects current nutritional and health status. To derive one-size-fits-it-all recommendations from individual studies and to transfer results obtained from a pathological model without further scientific investigations to a healthy one is careless and may turn out to be very dangerous.

9.600 IU Vitamin D Required to Get 97.5% of a Study Population to Serum 25(OH)D Levels of At Least 40ng/ml

I refrained from posting each and every study on vitamin D that has been published within the past weeks - nothing new or exciting there + way too much hype, if you want my opinion... BUT now, finally, there is some additional large-scale scientific data on the issues of how much is enough and how much is too much of supplemental vitamin D!

In the latest issue of Anticancer Research Garland et al. (Garland. 2011) published a study in which self-reported supplemental vitamin D intake in a cohort of 3,667 men and women from different ethnic backgrounds was assessed and corresponding serum vitamin D levels were measured.

Figure 1: Reported daily intake of vitamin D vs. measured serum 25(OH)D levels in study cohort of 3,667 subjects (Garland. 2011)

Unsurprisingly, "serum 25(OH)D rose as a function of self-reported vitamin D supplement ingestion in a curvilinear fashion", but other than some fear mongers would have us believe, ...

no intakes of 10,000 IU/d or lower producing 25(OH)D values above the lower-bound of the zone of potential toxicity (200 ng/ml).

While you, as a regular visitor of the SuppVersity, already knew that doses up to 10,000 IU are generally save, this is probably the first time you read a reliable number on how much supplemental vitamin D it takes to bring 25(OH)D levels to >=40ng/ml (which is even below the concentration of 60-80ng/ml, where scientists believe the anti-cancer effects of vitamin D set in):

The supplemental dose ensuring that 97.5% of this population achieved a serum 25(OH)D of at least 40 ng/ml was 9,600 IU/d.

In view of the authors conclusion that even "universal intake of up to 40,000 IU vitamin D per day is unlikely to result in vitamin D toxicity", and under the assumption that your vitamin D levels, as measured by blood tests, are low, you better invest in some high dose vitamin D supplements if you do not want to pop dozens of pills everyday.