Why getting fit this summer may be easier than you think.

Over the last decade or so, there has been increasing interest in the role that sunlight plays in health. It’s well known for its importance in bone health, but evidence also supports its role in various processes, including mood regulation, muscle function, and immune system efficiency.

The summer offers us an opportunity to reap the benefits of vitamin D, but it’s also a time when people may find it easier to be more active and perhaps regain some fitness lost during the darker, colder, wetter months. What I find interesting, though, is evidence linking light exposure and vitamin D with increased fitness and athletic performance.

In one review, published online in the journal Medicine and Science in Sports and Exercise, several lines of evidence were examined [1]. For example, research shows that vitamin D can influence the manufacture of muscle in the body (alluded to above). It also cites studies showing that vitamin D supplementation increased measures of ‘fast twitch’ muscle fibres without physical training.

The review also cited evidence linking 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). This sort of evidence (referred to as ‘epidemiological evidence’) may show an association, but does not prove that vitamin D improves these functions. However, the authors of the review go on to present evidence that vitamin D supplementation improved measures of several functions, including muscular strength, balance, and reaction time.

This last line of evidence, in the form of what are called ‘intervention’ studies, strongly suggests that ensuring optimal levels of vitamin D may help optimise athletic performance. If this is the case, we would expect to see evidence linking fluctuations in physical performance to 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 the things I found fascinating about this review is that the authors went back to research from several decades ago, in which impact of ultraviolet light treatment on physical performance was assessed. For example, in one study from 1944, German researchers treated adults with UV light twice a week for 6 weeks [2]. 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 [3]. Physical fitness increased by more than 19 per cent in these students compared with a similarly trained group not exposed to UV therapy.

Another study cited by the authors examined the effect of UV therapy in children [4]. 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 children who had not been exposed to the light therapy with a single dose of 250,000 IU of vitamin D. This led to a dramatic improvement in their fitness, and 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 [5-7].

WHAT WORKS

For decent vitamin D production, the sun needs to be relatively high in the sky. A good rule of thumb is that, when standing, if our shadow is shorter than our height, we can make vitamin D in our skin. In practice, what this means is that in the UK, vitamin D can only be made from around October to March, and only in the middle of the day. Short bursts of exposure to large areas of skin in the middle of the day is one tactic. I don’t see much potential for this when working in an office. However, we might consider incorporating this tactic into our routine when working from home and on weekends.

Taken as a whole, the research suggests that sunlight, through vitamin D production, has the ability to enhance fitness and physical performance. It’s not a replacement for physical activity and exercise. However, some of us may like the idea that simply getting more sunlight during the summer is likely contributing to our health and fitness.

References:

1. Cannell JJ, et al. Athletic performance and vitamin D. Med Sci Sports Exerc. 2009;41(5):1102-10

2. Lehmann G, et al. Ultraviolet irradiation and altitude fitness. Luftfahrtmedizin. 1944;9:37–43 [Article in German]

3. Allen R, et al. Effects of ultraviolet radiation on physical fitness. Arch Phys Med. 1945;10:641–4

4. Ronge HE. Increase of physical effectiveness by systematic ultraviolet irradiation. Strahlentherapie. 1952;88:563–6 [Article in German]

5. Cheatum BA. Effects of a single biodose of ultraviolet radiation upon the speed of college women. Res Q. 1968;39(3):482–5

6. Rosentsweig J. The effect of a single suberythemic biodose of ultraviolet radiation upon the strength of college women. J Assoc Phys Ment Rehabil. 1967;21(4):131–3

7. Rosentswieg J. The effect of a single suberythemic biodose of ultraviolet radiation upon the endurance of college women. J Sports Med Phys Fitness. 1969;9(2):104–6