For a while now I have been interested in the role that vitamin D plays in health. My interest was recently heightened when I discovered earlier this year that my own vitamin D levels were very deficient. As I revealed here, I since commenced on a programme of supplementation (3000 IU per day). I’ve also redoubled my usual efforts to get as much sunlight exposure as possible, whilst avoiding sunburn. I am, for instance, writing this outside in the intermittent sun we’re getting here in London right now.
Back in March, one of my blogs explored the notion that vitamin D might have some role to play in obesity. The idea is that as winter sets in, drops in vitamin D signal to the body to down-tune the metabolism in an effort to conserve energy. What sparked my interest in this concept was a meeting with someone who complained that she felt she was accumulating weight during the winter while training for a marathon. While exploring the potential mechanism for this experience, I also got side-tracked into looking for any evidence which links vitamin D levels with sporting or exercise performance.
I recently came across a study which reviewed relevant evidence in this particular area [1]. The review, published on-line in the journal Medicine and Science in Sports and Exercise, looks at a number of lines of evidence. For example, it reviews some science which shows that vitamin D influences the manufacture of muscle in the body. It also cites studies in which vitamin D supplementation was found to increase measures of type 2 (fast twitch) muscle fibres, without any physical training.
The review also cited evidence which links higher vitamin D levels in the body with improved physical performance such as reaction time, balance and timed tests of physical performance (tests of how long it takes to complete a particular task). More than this though, the authors also present evidence which vitamin D supplementation has been found to improve measures of several functions including muscular strength, balance and reaction time.
This last line of evidence, in the form of intervention studies, strongly suggests that ensuring optimal levels of vitamin D may help optimise athletic performance. If this is the case, then we would expect to see evidence linking fluctuation of physical performance with seasonality. Again, here, the story fits in that several studies have found peak performance and fitness in the late summer, when vitamin D levels are at their highest.
One of things I found fascinating about this review is that the authors went back to old research (some of it 60 or more years old) in which researchers assessed the impact of ultraviolet (UV) light treatment (which has the capacity to raise vitamin D levels) on physical performance. For example, in 1944 German researchers treated adults with UV light twice a week for 6 weeks. Performance on a stationary bike increased by 13 per cent. However, in non-treated individuals performance was unchanged.
In another study, this one from 1945, UV treatment was tested in a group of college students undergoing physical training. Physical fitness increased by more than 19 per cent in these students, compared to a similarly-trained group not exposed to UV therapy.
Another study cited by the authors examined the effect of UV therapy in children. UV lights were installed in the classroom of 120 schoolchildren and used for 9 months of the year. After this, fitness was assessed using a stationary bicycle. Compared to children who had not undergone UV therapy, treated children were 56 per cent fitter.
This study was particularly interesting because it went on to treat some of the untreated children, not with light, but vitamin D. 250,000 IU of vitamin D was given to 30 children in February, and this led to a dramatic improvement in their fitness. A month after treatment, their fitness levels approached those of children who had received the light therapy.
The authors of the review also cite evidence from the late 1960s which demonstrated that even a single dose of UV irradiation tended to improve the strength, speed and endurance of college women.
Taken as a whole, the research suggests that vitamin D plays a critical role in physical performance. This may have relevance for those of us who like to engage in sporting pursuits. And it also got me thinking about the conditions that would be optimal for sport training. I am a big rugby union fan, and am acutely aware of somewhat of a gulf that exists between northern and southern hemisphere rugby. I’m wondering whether the fact that southern hemisphere supremacy in the sport has anything to do with climate, and sunshine availability in particular.
There is no consensus on what represents optimal vitamin D levels in the body. However, the review presents epidemiological evidence which appears to show that performance increases with increasing vitamin D levels, but only up to a threshold of about 40-50 ng/ml. Beyond this level, further increases in vitamin D levels do not appear to confer benefit.
The evidence as it stands appears to support outdoor exercise as a very worthwhile pursuit in the summer. However, it’s a comforting thought that just being in the sun sat on my arse may well being doing something positive in terms of my physical fitness.
References:
1. Cannell JJ, et al. Athletic Performance and Vitamin D. Med Sci Sports Exerc. 3rd April 2009 [Epub ahead of print publication]
