Thanks.
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There is far too much reliance in the fitness industry on supplements that have little to no evidence in favour of their efficacy. Considering the cost of these supplements and for the most part, a preponderance of short-term studies showing very little benefit compared to the claims made, it would be much wiser to focus on a healthy diet, with sufficient vitamin and mineral levels. This, alongside proper training, hard-work, periodization, and persistence, as you say, is the most effective way to achieve peak muscle growth/strength gains/ athletic performance.
On a side note. With most studies suggesting that 75-90% of people in northern climates (35deg North latitude or further if I recall correctly) being severely vitamin D deficient in the winter months, and with the preponderance of evidence linking vitamin D to muscle growth, skeletal strength, and athletic performance now... Making sure that one is supplementing Vitamin D3 in the winter months is essential to continued gains. More studies are needed to show just what appropriate levels of vitamin D supplement intake are, as the daily recommendation has recently been raised to 800IU per day in America and Canada, although the Endocrine society suggests 1500-2000, and other suggestions are closer to 4000-5000 IU to reach proper 25-Hydroxy Vitamin D levels. Our muscles have vitamin D receptors as does most of our skeletal tissue. Studies have shown that vitamin D acts directly on Muscle to increase Protein Synthesis. Our bodies have an upper intake limit before equilibrium is reached from sunlight absorption and synthesis in summer time of 10,000-15,000 IU, which takes about 20 minutes of full-body exposure for a fair-skinned person. Therefore, it seems to me, that 800IU a day in winter is a ludicrous recommendation and that the safe limit would be up near that summer-time absorption limit, considering there has never been a case of vitamin D toxicity in the blood from UVB creation. Hence my intake of 5000IU per day during the 6-8 months of the year here where Vitamin D is hard to get from sunlight. I get my 25 (OH)D blood level checked once per year in winter to make sure I'm at a minimum of 30 ng-mL, and hopefully closer to 50 which is what I seem to be during summertime each time without supplementing. If you're deficient to the point of being in that 15 ng-mL area, you will probably see significant performance (and health) boost by raising to 30...
Here's a link to some research on the subject.
http://camilleherron.com/wp-content/uploads/2011/12/vitamindimprovesathleticperformance.pdf
And another one, that has separate links to other studies.
http://www.eatmoveimprove.com/2009/10/a-closer-look-at-vitamin-d/