Brief thoughts: Sunlight, vitamin D, and seasonal variability in performance

I was reading some scientific papers on nutrition and bone health which led my attention off in an interesting direction.  The topic I had originally been researching was the role of calcium in maintaining bone health (spoiler: calcium is important!), but what really sparked my interest was the emerging science of vitamin D.  Vitamin D is something I've long thought of as just an adjunct to calcium intake; much like vitamin C boosts iron absorption, so too does vitamin D boost calcium absorption.  But as it turns out, there is much more to vitamin D.  For one, it's really more of a hormone than a nutrient, and strictly speaking it's not a "nutrient" at all—even an extremely healthy and nutritious diet hardly contains any vitamin D at all.  Most of the vitamin D in your body is synthesized by exposure to sunlight. 

With some interesting chemistry, your skin is able to capture some of the energy from the sun's ultraviolet light, using it to kick-start the reaction that synthesizes vitamin D from a cholesterol-like precursor.  Since this is the primary source of vitamin D for most people, blood levels of vitamin D tend to vary by season, peaking in the late summer and dropping sharply by mid-fall.  Additionally, people who spend a lot of time indoors (including indoor athletes like gymnasts), live in northern climates (like Minnesota, my home state), and who have dark skin are less likely to be able to synthesize enough vitamin D through their day-to-day sun exposure.

Our understanding of the role of vitamin D in the body when it comes to health and overall wellness is developing rapidly, but there is still much to be learned.  Vitamin D appears to play a role in a whole host of health issues: A 2009 review article by Cannell et al. reports that overall death rates are 26% lower in people with the highest blood levels of vitamin D than in those with the lowest levels,¹ a paper by Halliday et al. reports that college athletes with lower levels of vitamin D suffered from illness more frequently,² Judd and Tangpricha report that vitamin D deficiency is associated with cardiovascular disease and diabetes,³ and Plotnikoff and Quigley report that vitamin D deficiency is an extremely common finding among inner-city residents who complain of persistent, nonspecific musculoskeletal pain.⁴  The science is developing so rapidly that the recommended daily intake amount for vitamin D is well under what it should be, according to some of the authors of these papers.  But what interests me is not supplementation and RDIs—that will surely be pounced upon by the vitamin and supplement industry.  What interests me most  in the context of running is the seasonal variance of vitamin D levels and performance. 

I came across a review study which cites some fascinating studies done in Germany in the mid-20th century that connected exposure to ultraviolet light with performance gains in an astounding range of tasks, ranging from swimming time-trials in trained athletes to general fitness tests and reaction time in schoolchildren.¹  These studies also reportedly found that performance in the control groups varied with a stark seasonality that tracks very closely with blood levels of vitamin D. 

Unfortunately, modern studies on the effects of vitamin D levels or supplementation on athletic performance are practically non-existent.  Research trends in physiology seem to come and go, though why these promising German studies were not followed up with larger and more rigorous ones is beyond me.  Cannell et al. succinctly sum up what is still unknown when it comes to vitamin D and athletic performance, and what kind of study could address these questions:

If an effect [of vitamin D on performance] exists, what is its magnitude? Which athletic performance variables (reaction time, muscle strength, balance, coordination, or endurance) improve the most? What is the optimal 25(OH)D [form of vitamin D found in the blood] level for peak athletic performance? Do higher levels impair it? Only direct interventional studies in vitamin D–deficient athletes will answer the athletic performance questions. A double-blind, placebo-controlled, multiple-dose crossover study with long washout periods using variable but relatively high physiological doses, such as 2000, 4000, and 6000 IU of vitamin D·d^-1[per day], combined with periodic 25(OH)D levels, might answer the question of whether peak performance levels exist for any particular serum 25(OH)D.¹

While supplementation is certainly an attractive treatment for those who are in-the-know when it comes to vitamin D deficiency, what interests me more is the natural seasonal variability in performance that is associated with blood levels of vitamin D.  In an admittedly brief survey of the physiology literature I could find, I found one study which explicitly addressed seasonal variance in aerobic fitness in the laboratory.  Kuipers et al. reported no seasonal effect on VO₂ max or maximum workload.  Still, this study was done in the Netherlands, which is warmer (though more northern) than some of the more landlocked cities where, presumably, the residents get less sun exposure overall.

Seasonal variability in blood vitamin D levels and a representative performance test (sort of).  Changes in wrist strength aren't as informative as more relevant running-related tests, but these two graphs show an interesting association.  Could sunlight and vitamin D be behind some of the frustrating drops in running performance in the late fall? Graphics from Cannell et al.

 Until there's more research out, we'll be left musing on whether vitamin D has an effect on performance, and if so, how significant it is.  Should there be more super-fast invitational track meets and road races in the late summer to take advantage of naturally higher levels of vitamin D? Is your team's poor performance at the end of the cross country season the result of poor peaking or simply less sun exposure? I'll leave you with this graphic that shows the seasonal variability in blood vitamin D levels and the training response measured from a wrist-flexor strengthening protocol.  While wrist strength is a long ways from distance running performance, it's nevertheless a good springboard for future research questions.

References

1.         Cannell, J. J.; Hollis, B. W.; Sorenson, M. B.; Taft, T. N.; Anderson, J. J. B., Athletic Performance and Vitamin D. Medicine & Science in Sports & Exercise 2009, 41 (5), 1102-1110.

2.         Halliday, T. M.; Peterson, N. J.; Thomas, J. J.; Kleppinger, K.; Hollis, B. W.; Larson-Meyer, D. E., Vitamin D Status Relative to Diet, Lifestyle, Injury, and Illness in College Athletes. Medicine & Science in Sports & Exercise 2011, 43 (2), 335-343.

3.         Judd, S.; Tangpricha, V., Vitamin D deficiency and risk for cardiovascular disease. Circulation 2008, 117 (4), 503-511.

4.         Plotnikoff, G. A.; Quigley, J. M., Prevalence of severe hypovitaminosis D in patients with persistent, nonspecific musculoskeletal pain. Mayo Clinic Proceedings 2003, 78 (12), 1463-1470.