LET THE SUNSHINE IN!
As a strength and power athlete are you aware of the important role vitamin D may play in your performance?
Firstly, a quick introduction to vitamin D. Synthesized from UVB rays and cholesterol in our skin it is not a true vitamin but a steroid hormone. Whilst you are probably aware that we get it from sunshine, do you know that one billion people including athletes, are reported to be vitamin D deficient or insufficient? (1) But, you may be thinking, I don’t have rickets right, so who cares?! Plus, isn’t it more a concern for bone health in endurance runners? Maybe you vaguely remember something on the female triad from GCSE PE and stress fractures blah blah blah….as a power athlete or a male does this even relate to you?
Well, historically the importance of vitamin D for bone health alone was the major concern, as the principle function of vitamin D was thought to be increasing intestinal absorption of calcium (2), therefore reducing bone stress and bone fractures, both of which impact athlete health.
The role of vitamin D is now being recognised in a wider range of pathologies as it is established to be essential in immune function and inflammatory regulation (3, 4), and there is a vitamin D receptor (VDR) in every cell in our body! But what I want to focus on here is vitamin D’s impact on power athletes. Identification of vitamin D3 receptors in skeletal muscle tissue (5), has led to an increasing interest in the role and action of vitamin D in muscle function, which clearly has other implications other than bone health. And worryingly, vitamin D insufficiency and deficiency amongst athletes is widespread, due to geographical area (northern hemisphere), training predominantly indoors and current advice to stay out of the sun. (6, 7, 8, 9, 10).
In the elderly type IIb fibre atrophy (the explosive fibres recruited for fast movement) has been associated with vitamin D deficiency, musculoskeletal pain or muscle weakness (11, 12, 13).
Conversely in athletes peak sports performance is consistently associated with seasonal highs in serum VD3 levels, (6, 14, 15). If vitamin D has specific effects on type IIb fast glycolytic fibres (those that are recruited first for speed and power) then this hormone could have profound consequences in sports nutrition for power athletes.
As we know, high performance athletes walk a thin line between optimal performance and major injury where the difference between winning and losing can be marginal. Therefore, defining top end factors which contribute to peak performance or reduction in injury are significant to athletes whose livelihood depends on their sports performance. So, through its impact on power output could vitamin D be one of those factors?
What’s Power output?
Muscular power can be defined as the capability of the muscle to rapidly exert force. Power is the product of force x velocity, therefore in this case force is the maximal voluntary contraction of a muscle and velocity is the speed at which the muscle contracts (16).
Muscle hypertrophy can increase force output through new protein synthesis and new muscle fibre growth (myogenesis). Increased velocity can be achieved through enhanced calcium flux which is the chemical mechanism of muscle contraction.
So can vitamin D could enhance either force or velocity increasing muscular power output?
Yes! Vitamin D has been shown to have a genomic (gene transcription) effect on myogenesis and protein synthesis which would affect maximal force output. As well as a rapid (non-genomic) cell signalling effect on calcium flux that could increase speed of contraction.
Research showsconsistent associations between vitamin D levels and a range of muscle measurements seen across a variety of sports (9, 17, 18, 19, 20, 21, 22, 23, 24), including;
· Squat jumps
· Counter movement jumps
· 20m sprints
· Isokinetic contraction of one leg
· Grip strength
· Variables associated with vertical jump execution
And studies have shown post supplement improvements in relevant performance measures (10, 25, 26) including;
· Lean Mass
· 10m sprint time
· Vertical jump
· 1RM bench
· 1RM squat
· Lower limb (quadriceps and hamstrings) isokinetic strength
There are cases when vitamin D seems to have no effect on strength and performance after supplementation. However, a major factor in these contradictory results may be the expression of the VDR in muscles. As the VDR is central to vitamin D’s affect on muscle, its sequential drop during development (27) and with age (28, 29) will inhibit VD3’s actions. This defines a more developmental role for vitamin D in muscle function, which may actually highlight (as opposed to contradict) the central importance of VD3 for power athletes. The type of high intensity resistance training causing micro muscle damage (the sort undertaken regularly by power athletes) leads to muscle regeneration that mimics muscle development, in which VD3 definitely has a role. Which explains how VD3 accelerates strength recovery after a bout of muscle damage inducing exercise and why optimal serum levels of the vitamin reduces muscle damage markers and accelerates the muscle through recovery and myogenesis, (17, 30, 31, 32).
These effects may be only small gains from day to day, but a faster recovery from one session means a higher quality of training in the next. Over a year, you can complete more high intensity, high quality training sessions leading to increased muscle strength or power output potentially improving your related sports performance.
Possibly even more importantly this mechanism could also have impact on regeneration after acute muscular injury (as opposed to training induced muscular injury), which is obviously a huge issue for us as athletes. Accelerating the process of regeneration of muscle fibres clearly enhances rehabilitation as well as reducing length of time lost through that injury. That’s a huge advantage in my mind, and one which we shouldn’t ignore!
Interestingly, one large dose given toVD3 deficient athletes showed increased strength after just one week (10). Whilst this is more likely due to increasing the function of muscle contraction through enhanced calcium flux (as opposed to hypertrophy), the faster an injured muscle can be returned to normal function then the threat of a re-injury, or a compensatory injury is reduced, win win! Additionally, this could also impact on competition performance if such short-term gains can be seen from merely correcting a deficiency…something to mull over!
Ok great – but what should my levels be?
If you had a blood test at your GP today anything over 25nmol/l VD3 and you would be considered sufficient – why? Because below this level you are at risk of developing rickets.
Hang on though, have we not established that VD3 plays a vital role in much more than just bone health? Current deficiency/sufficiency categories are based solely on markers related to bone health and avoiding disease (33). Unfortunately, these markers show no correlation with vitamin D levels and its effect on muscle (9, 34, 35). And like I have mentioned before, simply avoiding disease seems like setting the bar pretty low! Therefore, using the same categories to define optimal muscle function is inherently flawed for performance.
In actual fact, peak neuromuscular performance is associated with VD3 levels of 125nmol/l, with evidence that the only athletic demographic to achieve levels in this range are lifeguards, following a full summer of sun exposure, (36). Are any of us ever getting a full summer of sun exposure?! “Optimal” VD3 has been argued to be higher still, with levels of up to 250 nmol/l reported as being optimal and safe. (37, 38).
Despite this, in Britain no recommended intake exists for populations between the ages of 4-65yrs as “it is assumed that the action of summer sunlight is enough to provide adequate vitamin D” (39) and supplementation is recommended at a level of 400IU/day only in vulnerable groups, of which athletes are not considered to be one. I would argue on both counts – what summer sunlight are we really getting? And I think athletes are a vulnerable population, we ask a lot more of our body than jo public.
Therefore, the evidence makes me believe vitamin D levels should be a concern for all power athletes and something injured athletes should pay immediate attention to.
Whilst the debate around what is truly optimal continues research suggests that maintaining a VD3 level over 100nmol puts you in the range where the vitmain can positively impact muscle function and performance. And studies using a supplementation regimen of between 4-5000IU of vitamin D3/day for 4-8 weeks resulted in levels in the hypothetical “optimal” range accompanied by significant improvement in muscular performance. With no “true” cases of toxicity <350nmol/l reported in sports science referencing guidelines (40, 41), then this protocol is a safe and efficacious place to start.
So, maintaining optimal VD3 levels may be an overlooked but vital companant in your training, performance and injury reduction. If you believe you may be in danger of being deficient because you;
o spend most if your time working/training indoors
o cover up when in the sun
o live in the northern hemisphere
o have darker skin
(you may notice that at least one, if not more of these points probably applies to all of us!)
then it may be worth getting your vitamin D levels checked with your GP. Following which I would recommend seeking advice from a professional in regards to supplementation as not all supplements are equal!
· Optimal VD3 levels can support increased hypertrophy or speed of muscle contraction that may improve power output and therefore sports performance.
· Vitally, optimal VD3 may also support a quicker recovery of maximal function and strength after high resistance/intensity training sessions, increasing training capacity and quality whilst reducing injury risk from fatigue.
· Optimal VD3 levels may enhance muscle regeneration and reduce recovery times after muscular injury
· Optimal VD3 levels for muscle function appear to be >100nm/l
· Vitamin D toxicity is reported at serum levels >350nm/l, which would require daily doses of VD3 at between 40,000-100,000IU (not recommended!)
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