Vitamin D strengthens your bones and immune function, helps reduce high blood pressure, and can even help you lose weight.
By improving muscle recovery after exercise, vitamin D just might make you a better athlete. Or so we’ll discuss in today’s review.
Vitamin D is involved in so many physiological functions and maintains our health in so many different ways that, once you start to count its benefits, you might begin to wonder if there’s anything this fat-soluble vitamin doesn’t do.
First, it helps us absorb calcium – which, in turn, affects bone development and growth, nerve signaling, immune function, blood pressure, and even muscle strength and mass, especially as we get older.
Vitamin D may also play a role in decreasing our risk for certain types of cancer, as well as diabetes, rheumatoid arthritis, and multiple sclerosis. It can even help us lose or maintain weight.
For more about vitamin D, see All About Vitamin D.
Vitamin D and human skeletal muscle
Our bodies produce vitamin D naturally if we spend time in the sun. Our diets can also provide us with limited amounts of this vitamin.
Yet vitamin D deficiency is rampant throughout the world, particularly in the northern latitudes, where it can be hard to get enough sunshine, especially in the winter months.
Recently, scientists have discovered vitamin D deficiencies even in the sunny Middle East, Australia, and Hawaii (1, 2).
In other words, no matter where we live, it may not be easy for us to produce as much of this important vitamin as we need.
Currently, clinical vitamin D levels are defined as follows:
- deficient (<20 ng/mL)
- insufficient (20 to 32 ng/mL)
- sufficient (>32 ng/mL).
In children, Vitamin D deficiencies result in weak, soft bones and delayed growth – a condition known as rickets.
Meanwhile, older people – who often unwittingly remain indoors and reduce their dietary sources of vitamin D – put themselves at risk for osteomalacia, a disease involving muscle weakness, pain, and a greater likelihood of falls.
The most reliable indicator of vitamin D intake and storage is 25-hydroxyvitamin D (25(OH)D), which can be found in the liver. This form moves on in the body and is converted in the kidneys to its active form: 1,25-dihydroxyvitamin D3
Vitamin D and athletic performance: Sunlight and supplements
Osteomalacia may not get as much press as its sister disease, osteoporosis, but it’s a serious concern. And, as a matter of fact, until quite recently, most studies on vitamin D and muscle weakness involved the elderly.
But lately, scientists have started to recognize that even healthy, young athletes can be affected by vitamin D deficiencies, and they’re looking into the role that this vitamin might play in performance and injury.
One interesting 2009 study (3) reviewed several lines of evidence on the effects of vitamin D on athletic performance.
As long ago as the 1940s and 50s, German research suggested that ultraviolet light improved athletic performance and that athletes performed better in the summer months (when exposed to more UV light) than in the winter.
Researchers at the time proposed that vitamin D is a performance-limiting factor when it is deficient, and performance-enhancing when in abundance.
However, as the authors of the 2009 review noted, the earlier German studies didn’t look specifically at serum 25(OH) concentrations and athletic performance, which made it tough to form any absolute conclusions, and even harder to identify an ideal level of vitamin D for peak athletic performance.
More recent controlled experiments have produced equally confusing results.
In one study, elite ballet dancers took 2000 IU vitamin D3 (in supplement form) for four months. They performed better and suffered fewer injuries.
But since no one actually checked the dancers’ serum vitamin D concentrations (4), it’s unclear whether the supplements were responsible for the improvement.
Meanwhile, in another study (5) recreational athletes took high doses of vitamin D (20,000 and 40,000 IU) for twelve weeks. Researchers found no improvement in their performance measures (leg press and vertical jump).
Based on these studies, it’s impossible to say definitively whether vitamin D helps athletic performance.
Vitamin D receptor (VDR) and skeletal muscle
As research on vitamin D has evolved, we’ve gained some insight into the complex mechanisms by which vitamin D and its metabolic pathways may affect muscle function (2, 6, 7).
In particular, the discovery of a vitamin D receptor (VDR) on skeletal muscle tissue has provided some interesting leads.
And while study results have been conflicting, mounting evidence suggests that vitamin D levels affect both performance and risk of injury in athletes.
We don’t fully understand how – or why – this works. Nor do we know whether increased vitamin D helps only those athletes who are deficient, or whether higher levels of vitamin D could improve performance across the board.
This week’s study tries to investigate some of those questions.
This week’s review considers whether pre-exercise serum concentrations of 25(OH) D concentrations can predict muscular weakness immediately following and several days after intense exercise.
Researchers also wanted to see if levels of circulating cytokines could be used to predict levels of 25(OH)D after intense exercise. They expected to find an inverse relationship.
In other words, they hypothesized that a higher inflammatory response would correspond to lower serum levels of vitamin D.
Do vitamin D levels affect muscular recovery and inflammation following exercise?
Barker, T., Henrikesne, V.T., Martins, T. B., Hill, H. R., Kjeldsberg, C. R., Schneider, E. D., Dixon, B. M., & Weaver, L.K. (2013, May). Higher serum 25-hydroxyvitamin D concentrations associate with a faster recovery of skeletal muscle strength after muscular injury. Nutrients. 5:1253-1275; doi:10.3390/nu5041253.
In this study, all subjects completed the same exercise-testing protocol.
Researchers collected data at baseline, during exercise, and several times after the exercise protocol.
Each participant served as his or her own control. All were informed about the testing protocol at the start of the study.
Because subjects were not randomly assigned, this study falls into the category of “quasi-experimental design”. While studies of this kind can suggest trends and correlations, they can’t establish cause and effect, and we need to interpret them with caution.
Subjects were “recreationally active”, meaning they had participated in at least 30 minutes of continuous exercise three times a week in the year before the study began.
Participants were carefully chosen. If would-be participants suffered from any type of chronic disease, metabolic disorder, nutrient or hormone deficiency, lower leg injury, or organ dysfunction, they were excluded. If they were pregnant, morbidly obese, taking any type of medication, planning to increase or decrease their sun exposure, planning to travel north, or planning to travel south, they were also ruled out.
And that’s not all. Believe it or not, there were even more exclusion criteria than this! But by now, you’re probably getting the idea. Researchers wanted active subjects free from any type of health problem.
In the end, they chose fourteen non-smoking subjects (9 males and 5 females) with an average age of 32 + 1 year. Average body mass index was 26 (normal is 18.5-24.9) and none of the participants exercised for more than an hour each day.
Data collection took place during the winter (December to March) in Salt Lake City, Utah, USA. Since for most people, the sun is the primary source of vitamin D, blood levels tend to vary by season. It’s safe to assume that natural levels would be at their lowest in this region during the winter months.
Researchers explained the study protocol and asked subjects to keep their diets consistent with their regular habits during the previous year.
To test whether vitamin D improved muscle repair in humans, the researchers measured markers of inflammation and vitamin D levels before, during, and after an intense exercise protocol.
The testing site(s) were the participants’ legs.
Yup. You read that right. One leg was the test leg, and one leg was the control. That’s why the researchers had ruled out people with leg problems!
In fact, unless peak isometric force in both legs was approximately equal, volunteers were not accepted for this study.
Single-leg strength testing
One leg was randomly assigned as the control and the other one was the test leg.
The test leg performed an intense-stretch shortening contraction (SSC) protocol.
This protocol was designed to result in muscular deficits. In other words, it was intense enough to produce an inflammatory response that would last for a couple of days. DOMS, here we come.
Subjects performed 10 sets of 10 repetitive single-leg jumps on a horizontal Plyo-press (it looks like a leg press machine) with a 20 second rest between each set, at 75% of body weight on one leg only (i.e. the test leg).
Subjects were allowed to drink as much water as they wanted during and after the protocol.
I’ll bet they were happy about that.
Researchers obtained objective data by analyzing the subjects’ blood for concentrations of a number of clinical chemistries.
Each subject gave 8 fasting blood draws.
The first was taken 28 days before the exercise protocol, to provide information about the seasonal decrease in serum 25(OH)D concentrations.
Blood was also drawn at baseline, immediately post-exercise protocol, and again at specific hourly intervals: 1, 24, 48, 72, and 168 hours.
The researchers wanted to find out if pre-exercise serum 25(OH)D concentrations could predict muscular weakness (as measured by peak isometric force) immediately, 48 hours, and 72 hours post intense exercise.
They also wanted to identify if markers of inflammation in the blood might predict alterations in serum 25(OH)D concentrations.
What they discovered in this particular study population was that yes, pre-exercise serum concentrations of serum 25(OH)D did influence the rate of recovery of skeletal muscle strength after an acute bout of intense exercise.
Here are some further discoveries connected with individual markers:
Serum 25(OH)D concentrations
When the participants enrolled in the study, the majority of them didn’t have enough vitamin D stores.
- Only 5 of them had “sufficient” vitamin D stores (>32 ng/mL).
- 8 were “insufficient” ( 20 to 32 ng/mL).
- 1 was considered “deficient” at <10 ng/mL.
At baseline, all concentrations decreased significantly by 10% — consistent with seasonal declines.
Immediately post-exercise, researchers noted a transient, but significant, increase in vitamin D concentrations, which then leveled off at post-exercise when measured at 1, 24, 48, 72, and 168 hours post-exercise.
Peak isometric force
As expected, in the test leg, peak isometric force decreased significantly immediately after exercise and remained impaired throughout the post-exercise testing period of 168 hours.
There was a significant difference between the test leg and control leg, with the control leg exerting similar force throughout the testing protocol.
Now, here is the interesting stuff: Pre-exercise serum 25(OH)D concentrations significantly predicted muscular weakness immediately, post-exercise, and at 48, 72, and 168 hours.
Higher serum 25(OH)D concentrations predicted a lower deficit in muscle strength at each of the measured time intervals except at 24 hours post-exercise.
In other words, participants with optimal serum vitamin D concentrations bounced back better after intense exercise.
Plasma AST and ALT
Both these values were significantly higher post-exercise, indicating that skeletal muscle damage had occurred. AST was significantly higher at 72 hours, and ALT at 24, 72, and 168 hours.
Serum cytokine concentrations INF-γ and IL-4
Serum INF-γ increased significantly after exercise, while IL-4 increased, but not significantly
Plasma calcium, PTH, and albumin concentrations
Total plasma calcium concentrations were significantly higher immediately following, and one hour after exercise. PTH concentrations were significantly higher right after exercise, and significantly lower an hour later. Plasma albumin concentrations were significantly higher, post-exercise.
The researchers compared all of the measured values between sexes to identify any differences. They discovered that the female subjects had significantly lower circulating calcium and albumin levels than the nine male subjects.
This study offers interesting new information about how vitamin D status in humans relates to muscular recovery from exercise.
More pre-exercise vitamin D meant less post-exercise muscle weakness and better recovery through the entire recovery process. Less pre-exercise vitamin D meant more weakness and worse recovery.
The various blood chemistries they measured gave them a fairly accurate picture of this process. Some of those markers supported their theory while others were less compelling.
For instance, they expected that the inflammatory cytokine INF-γ would increase after intense exercise and impair production of 25(OH).
INF-γ did show a significantly inverse correlation to serum 25(OH)D, but given the small sample size, it’s too early to draw firm conclusions about this.
Meanwhile, the cytokine IL-4 (also thought to be involved in vitamin D metabolism) didn’t appear to be affected in this particular study.
The other parameter that predicted serum 25(OH)D concentrations in this study was albumin.
Albumin – a protein synthesized in the liver – facilitates the increase of blood volume when we exercise. Albumin binds to vitamin D metabolites, and it is believed that as albumin increases with exercise, it will in turn bind to vitamin D in the blood.
Here, the results supported that theory. But again, due to the study’s small size, we can’t really draw any firm conclusions.
Finally, in this study, while calcium and PTH increased after exercise, they didn’t predict changes in serum 25(OH) D after exercise. Since both are believed to regulate circulating 25(OH) D concentrations, this finding is difficult to interpret.
What’s more, calcium concentrations were significantly different between genders in this study, and we don’t really know what that means.
This study was based on a small sample; to trust its findings, we’d need to see them replicated.
And while the research measures were a reasonable match for the research questions, it’s always possible to make findings stronger and more complete by measuring other factors.
For instance, muscle biopsies might have given more direct evidence of local muscle damage.
Finally, we should never underestimate the importance of diet – particularly when the topic of study can be obtained that way! Here, researchers simply asked the subjects to maintain their previous diet. They could have done a much better job of controlling dietary vitamin D.
What does any of this mean for you?
First, based on this study along with larger-scale studies on vitamin D levels in big populations, there’s a good chance you’re deficient in vitamin D. This will affect overall health as well as your athletic performance and exercise recovery.
Whether you’re an athlete or not, have your vitamin D levels checked the next time you visit your doctor. Your doctor can then provide you with some appropriate next steps.
Current guidelines suggest that you should consume 600 IU of dietary vitamin D if you are between the ages 1-70 and 800 IU if you are over 70 (8). This recommendation may be too low for many people.
(Many folks at PN supplement around 1000-3000 IU daily, depending on the season.)
Food sources of vitamin D are somewhat limited, but you should still aim to include vitamin D-rich foods every day. Some examples are:
- mushrooms (although you need to eat a lot of them)
- cod liver oil
- vitamin D fortified dairy products
- fortified cereals
- fortified orange juice.
If you can’t get enough vitamin D from your diet and the sun – and most of us can’t – then it’s probably a good idea to supplement.
In fact, if you have the choice, you might even go with the supplement over fortified cereals, milk, and orange juice. That way you get all the vitamin D without the extra sugar, lactose, etc.
And while further research will enrich our understanding of how and why this works, in the meantime, ensuring that your vitamin D levels are optimal appears to support athletic performance and recovery.
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