Could sitting in the sun make you fitter?

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.


1. Cannell JJ, et al. Athletic Performance and Vitamin D. Med Sci Sports Exerc. 3rd April 2009 [Epub ahead of print publication]

13 Responses to Could sitting in the sun make you fitter?

  1. Abigail 23 May 2009 at 8:31 am #

    I don’t know what my vitamin D levels are, but suffer with psoriasis which goes into remission when I’m on a sunny holiday. I don’t use sunscreen as I think they encourage one to sit in the sun for far too long, but I am definitely short on sun in the English winter and only get to go away once a year.

    My question is what do you think would be a safe daily dose of D3 if I don’t know what my levels are. A dermatologist I know was scathing about taking D3, advising me it would make no difference and I could cause myself damage by having too much.

    I think she was talking rubbish about it not helping, but I am concerned about overdosing.


  2. Bill Cockerill 23 May 2009 at 12:06 pm #

    I wonder what’s the best way to ensure you have good vitamin D levels: diet, sun, supplementation? Here’s a study that suggests not everyone’s vitamin D levels respond to abundant sun exposure:

    However we chose to increase our vitamin D levels I guess checking it a few times would be wise if we’re trying to change it? Can you test your levels at home? Would a typical Doctor be annoyed with me asking for several vitamin D tests when I’m otherwise healthy?

  3. Jake 23 May 2009 at 12:27 pm #

    Dr Cannell of the the Vitamin D Council agrees that 50 seems to be a adequate Vitamin D level. But he recommends a level of 70 as the Vitamin D levels get used up in fighting illness and he thinks you should have a reserve.

    Dr. Davis, the famed preventative cardiologist, says that he sees the most dramatic reversal of arterial plaque when the Vitamin D level is 70.

    My Vitamin D level is 76 and I need to take 15,000 units a day to achieve it. (I have genetic defect in the Vitamin D processing area)

    I am proof that you have to do testing to find your proper Vitamin D dosage.

  4. Jack 24 May 2009 at 11:04 am #

    Dr. Briffa,

    I am relatively fair-skinned, but I’ve taken to the habit of getting out in the sun when not in vitamin D “Winter.” Initially I started slowly with maybe 5-15 minutes total. Over time, I built up to 30-40 minutes without any sign of burning and now regularly get multip 15-20 minute sessions when at such a tolerance level.

    My Question: In theory, if I only need a fraction of those multiple exposure to get and keep my D levels in the therapeutic range, then are the other sessions without skin protection merely increasing risk from exposure or perfectly fine so long as I always avoid burning?

  5. Thomas Kurz 26 May 2009 at 2:44 am #

    “The evidence as it stands appears to support outdoor exercise as a very worthwhile pursuit in the summer.”

    Why not in the winter?


  6. Su 27 May 2009 at 8:34 pm #

    I recently had my vitamin D level checked due to on-going fatigue and depression (all the usual suspects such as underactive thyroid etc. had been ruled out). The test came back showing I was low. I am now on a prescription of 12mg calcium and 800iu vitamin D3 (taken as two tablets daily). After reading the above, however, I’m concerned that 800iu D3 per day will hardly be scratching the surface, will it?

  7. simona 27 May 2009 at 11:43 pm #

    Unfortunately, Dr. Briffa rarely responds to the comments posted on the site, so I’m just trying to help.
    Jack, first, you need to get your vit D levels tested. From what I understand, just sun exposure doesn’t increase the levels to the optimal level. It’s hard to know how much vit D your skin is able to produce (the ability decreases with age and other factors), if you’re in the sun when the UVB is higher, that is around lunchtime or later.

  8. Trinkwasser 28 May 2009 at 10:37 pm #

    These guys

    will accept UK patients

    I suffer from Atypical Depression with a side order of SAD (“hibernation”): physiologically this ties in nicely with my diabetes (although it comes from the other side of the family).

    Now most of the research I’ve read on this condition suggests the pineal gland (“third eye”) is responsible for adjusting the balance between serotonin and melatonin according to change in day length. There’s a theory that it is still light sensitive, but either way the “treatment” consists of increasing daylength by using high intensity light, either in the morning or the evening. The wavelength is supposedly unimportant, it’s the intensity.

    Here we have another seasonal factor in the fluctuating D3 levels. This would appear to respnd to specific wavelengths, ie. UV, rather than total intensity.

    It makes a lot of sense that humans may have inherited the genetics from hibernatory (and also migratory) species in terms of changing activity levels to conserve energy which would probably have been useful as we migrated out from Africa into temperate regions. It’s entirely plausible that we have inherited different gene sets that do essentially the same job using different inputs.

    Anecdotally when I was a truck driver those sunny February days when I was blasted in my cab to the extent I would start to get brown on my face and right arm would always switch me into high energy mode to the extent I would actually enjoy having to offload six tonnes of biscuits by hand! In winter I’d just be moaning at the idiot who broke the forklift.

    Usually the SAD needs treatment to start around August/September, when the days become noticeably shorter although still often bright. This year instead of the light and increased venlafaxine I think I’ll try early and robust D3 supplementation.

  9. Chris 2 June 2009 at 4:28 am #

    Reading this and Trinkwassers’ comments has just reminded me how those relatively infrequent haze free ‘blue sky days’ can really infuse one with a sense of well-being to the extent that they exceed the context even of being at work – I too drive wagons. Such days are all too infrequent as haze usually develops on even cloud free days. Seemingly, the occurrence of such days in any given year can be counted on the fingers of one hand.
    Moving on, I recall that all air traffic was grounded in the USA for a succession of days following the twin towers attack and that accordingly some folks noted that the skies were refreshingly different with less haze. I think the effect was reported in the UK media and the topic was subject to a TV programme (Horizon?) sub-titled ‘Global Dimming’. My detailed memory might not be so reliable but I think folks were quoted as saying, nostalgically, skies had been restored to how they were remembered in childhoods from 40 or 50 years ago. I don’t think I am mis-representing the programme content to surmise that exhaust from fossil fuel burning transportation was cited as a likely cause of much haze and that this instance was specifically cited as being significantly attributable to commercial aviation. No doubt that figures quantifying the reduction in light (or component wavelengths) can be found somewhere.
    It would be interesting to attempt to quantify what likely effect ‘global dimming’ may have upon the ability of humans to synthesise vitamin D and whether any such shift is significant in relation to some healthy threshold.
    But moreover, the phenomenon of ‘global dimming’ serves as an illustration of something else and that is the ability of human nature not to notice slow but consistent and progressive change until some significant event highlights a marked contrast. In this regard are there not similarities to human behaviour in relation to the development of chronic diseases. (Some) People gain weight progressively to the extent of 10, – 20, even 30kg overweight without noticing how obesity is diminishing their QOL; and it only becomes apparent to them in the light of a significant event. But the similarities don’t end there; while it may be easy to convince you that ‘global dimming’ is a consequence of human relationship with, activity and exploitation of, ‘habitat’ it would likely take more than one sentence to convince you that causality for obesity, and likely several more of the chronic illnesses of our age, can be linked likewise. That’s radical, and I anticipate your sceptism that a comment from a truck driver should be accorded gravitas, but as Ben Goldacre makes clear in ‘Bad Science’, money, effort, and PR is directing the search away from causality. As they say in the Army, ‘Bullshit baffles brains!’ Consider this, there is more money to be made from selling a succession of mop 99p sponges to people to mop there flooded kitchens than there is to point to the dripping tap and sell them a 50p tap washer.

  10. Trinkwasser 7 June 2009 at 10:03 pm #

    Yes small changes in the environment can go unnoticed until they have become a rather large change. On the one hand the “ozone holes” have increased the levels of potentially carcinogenic radiation at ground level and on the other not only aircraft exhausts but probably vehicle exhausts at ground level have increased haze, blocking different wavelengths.

    Looking out from the top of the South Downs across one of the most populous parts of the country, containing huge levels of traffic and a major airport, you could actually watch a layer of crud spreading across the land. If you looked back at the bowl of hills containing Bristol sometimes you could hardly see the city for the fumes. Here in thinly populated Suffolk (and in many other areas) you can see several times the density of stars at night. suggesting we are still getting a higher level of solar radiation at ground level. Would be very interesting to see a national study of D3 levels and their geographical variation.

  11. Mike 10 June 2009 at 8:33 am #

    The Germans discovered this a long time ago and exposed their atheletes to UV to immprove their performance. Take a look at for some good summaries.. The site also offers a good newsletter

  12. Pat 28 July 2009 at 11:34 pm #

    I find it amazing that German researchers had time and volunteers to do a research on vitamin d in 1944. One would think that they were a little busy, what with WWII going on in their backyard.

  13. KARIMAH BINT DAWOUD 22 April 2012 at 1:43 pm #

    thank you dr john for this, really interesting , i also wonder if things like reaction time and balance are improved how this has an affect on the learning’s of children, i remember being little and when it was summer sometimes we had classes out side, which we adored, i know this doesn’t suit all types and temperaments but its worth a thought…..

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