A closer look at Vitamin D

Vitamin D is an important part of many facets of human health. It is estimated that 85-90% of people are deficient to severely deficient in Vitamin D. Most occupations have indoor jobs when the sun is at the peak.

Table of Contents

  1. Introduction to Vitamin D
  2. Vitamin D: the panacea?
  3. Vitamin D: athletic performance
  4. Vitamin D: Sunlight provision, supplementation, toxicity concerns, knowing your levels
  5. Vitamin D and post workout?
  6. Conclusions
  7. Supplementation

Introduction to Vitamin D

Vitamin D is a hormone (secosteroid) because its structure is derived from the steroid precursor cholesterol like many of the other steroid hormones such as testosterone, estrogen, progesterone, etc.

It is estimated that 85-90% of people are deficient to severely deficient in Vitamin D. Most occupations have indoor jobs when the sun is at the peak. We also get very little from our diets except for fish and in fortified dairy now. When we do go outside we use sunscreen which blocks UVB radiation that allows Vitamin D production to occur in our skin.

Other countries such as Russia take measures to obtain proper levels of Vitamin D. Unfortunately, this is an uphill battle in countries like the U.S. and Britain. Edit 2/2/10 — Finally getting some recognition. This is particularly alarming because of the health benefits of Vitamin D on our bodies. Those with darker skin are often more deficient than others because it takes more sunlight to produce the same amount of Vitamin D as other ethnicities.

We’ll explore the effects of vitamin D in the following sections.

Vitamin D: the panacea?

Vitamin D has a lot of beneficial effects on the human body. I don’t think it is a panacea, but it certainly is an integral part of the functioning of many processes within the body. Those who are deficient may experience issues with the positive effect of Vitamin D on these metabolic processes within the body.

  • Bone health

The most widely known interaction of Vitamin D and bone health is the prevention of rickets (softening of the bones). Supplementation with Vitamin D increases the rate of osteoblast activity leading to faster healing from bone breaks. In addition, like the study above indicates there is evidence in post-menopausal women (because lack of estrogen is correlated with osteoporosis) that it decreases the risk of fractures as well.

Vitamin D supplementation may also help prevent chronic degeneration such as in osteoarthritis as well as rheumatoid arthritisA bit more on fractures anecdotally from an MD plus some additional commentary on blood levels that may or may not be optimal in the comments.

  • Immunomodulation

Vitamin D is also implicated in many upper respiratory tract infections. For example, Vitamin D deficiency is implicated with Influenza (1) (2) and tuberculosis and other viral respiratory tract infections. Also, the recent swine influenza epidemic can also be countered with supplementation of Vitamin D. The largest study on Vitamin D to date showed that Vitamin D deficiency is related to an increased amount of the common cold and other upper respiratory tract infections (URTIs and RTIs).

In addition, asthma especially childhood asthma is related to Vitamin D deficiency as is cystic fibrosis, obstructive lung disease, and pneumonia (2) (3) (4) (5).

The modulation of the immune system by Vitamin D produces anti-microbial peptides such as cathelicidin and defensins. Cathelicidin and other AMPs strongly protect against bacterial, viral, and fungal infections (1, 2, 3, 4). Vitamin D also increases and stimulates activity in monocytes, neutrophils, natural kill cells and other T lymphocytes. It increases IL-5 concentractions (a cytokine signaller) which also helps the immune system ward of bacterial and viral infections. This one article explains it a bit more in depth if you are interested.

Finally, a wiki has been produced on Vitamin D and influenza. It goes into a bit more depth than this article, so feel free to browse it.

My own experience with Vitamin D and influenza was that my fever was obliterated in one day, and I returned to full health within 3 days. Normally, a full course of the flu takes at least 5-7 days.

  • Cancer

Vitamin D is also associated with decreased risk of at least 15 different types of cancers such as colorectal, breast, ovarian, pancreatic, etc. (1), (2), (3). The significant part of this is that supplementation or sun exposure for as little as 1,000-2,000 IU decreased risks of having a lot of the above cancers by as much as 40-77% or more.

The above immunomodulation contributes to this decreased cancer risk. Natural killer cells, for example, help prevent and destroy cancer cells.

We are told to avoid sunlight because of the damage to our skin and potential for developing skin cancer. This is partially true. There is an increase in two skin cancers from sunlight: basal cell carcinoma (BCC) and squamous cell carcinoma (SCC). However, risk of melanoma decreases with increased exposure to sunlight. The key point here is that BCC and SCC are “benign” cancers in that they rarely metastasize. Melanoma easily metastasizes — becomes malignant and spreads — and subsequently makes up approximately 90-95% of all skin cancer deaths. Melanoma risk is increased substantially by sunburns, especially in younger populations.

Thus, it is important to increase sun exposure without sunscreen if at all possible while avoiding sunburns, or to supplement with Vitamin D or obtain the necessary amounts in your diet.

  • Cardiovascular disease

Vitamin D is found to decrease the risk of cardiovascular disease. For instance, one study indicates that there is an increased risk of hypertension, increased VLDL, impaired insulin metabolism, and lipoprotein lipase activity with Vitamin D deficiency. Vitamin D deficiency is associated with atherosclerosis (accumulating plagues), coronary artery disease, high cholesterol levels, and increased risk of myocardial infarction (heart attack) during the winter.

The theory on Vitamin D deficiency and increased risk of cardiovascular disease is because of cholesterol. Remember in the beginning when we examined that Vitamin D is produced through reactions involving cholesterol? Well, if you are Vitamin D deficient, your body is going to try to up your levels of cholesterol to try to synthesize more Vitamin D from the little amount of time we spend in the sun as modern humans. Thus, higher amounts of cholesterol being transported various places may lead to increased deposition of cholesterol to places like the arteries, especially if there are co-factors like obesity, diabetes, chronic inflammation, poor nutrition, and so on. One study may corroborate this hypothesis as it is shows there is a cholesterol reduction during summer months.

  • Inflammation

Vitamin D is linked with inhibiting proinflammatory cytokines and preventing low extracellular calcium levels which modulate endothelial inflammation. As you may well know, our diets in the modern age are pro-inflammatory because of the large amount of carbohydrates we eat. We also get very little omega 3s with disproportional increases in omega 6s (omega 3s are anti-inflammatory, omega 6 are pro-inflammatory). For example, meat and chicken such as grass fed vs. grain fed show improved omega 3 vs. omega 6 ratios for the former rather than the latter. A pro-inflammatory diet is associated with poor healing, and a lot of the illnesses above.

  • Intestinal / gut health

Likewise, with the point above about inflammation which is a major player in any of the inflammatory bowel diseases such as Celiac’s, irritable bowel syndrome, etc. (2) Part of the problem is that the inflammation in the gut prevents proper absorption of nutrients, one of which is Vitamin D. If Vitamin D levels are low as we already noted it cannot help much with down regulation of pro-inflammatory cytokines. Thus, malabsorption of Vitamin D and the ensuing increased inflammation leads to a progressively worse condition.

Note 1: Both inflammation and instestional gut health are actually sub-sets of immunomodulation (as is cardiovascular health and cancer risk to some extent). All of these are interconnected.

  • Autism?

One meta-study review indicates that Vitamin D deficiency may be indicated in cancer, heart disease, multiple sclerosis, diabetes, autism, and a host of other illnesses.

There has been some debate about Vitamin D deficiency and autism. There may be a link as low Vitamin D levels in either the mother during pregnancy and/or the child during development. Further studies are obviously required.

The Vitamin D council has put together a nice comprehensive article on the topic. Here is another study and an article in Scientific American about Vitamin D and its potential link to autism.

  • Depression / Seasonal Depression / Schizophrenia

There is some indication that Vitamin D deficiency is also associated with depression and seasonal depression. (1), (2), (3), (4), (5). If you are prone to moody swings especially with the seasons, it may be a good idea to supplement with Vitamin D.

Vitamin D deficiency may also be linked up with schizophrenia (2) (3) because of abnormal brain development. This goes along with depression and autism as both of these are also potential neurological disorders.

  • Other autoimmune and chronic conditions

Vitamin D deficiency has been implicated in “multiple sclerosis, lupus, and psoriasis, and chronic conditions such as osteoporosis, osteoarthritis, metabolic syndrome, fibromyalgia and chronic fatigue syndrome.“ Additionally, besides the aforementioned fibromyalgia and CFS, there is some indication of improving outcomes of general musculoskeletal pain and myositis (3) (4).

  • Obesity

Of note is this post from Whole Health Source. Great blog, highly recommended.

Vitamin D in particular plays an important role in immune responses (including inflammation), and also appears to influence body fat mass. Vitamin D status is associated with body fat and insulin sensitivity in humans (14, 15, 16). More convincingly, genetic differences in the vitamin D receptor gene are also associated with body fat mass (17, 18), and vitamin D intake predicts future fat gain (19).

  • Improvement of sleep quality

Unfortunately, there is nothing in pubmed at the moment about this that I’ve seen so far, but I feel it must be mentioned. A bunch of people have e-mailed me since the posting of this article and have mentioned that Vitamin D improves sleep quality. I have found this to the case as well. My sleep drastically improved (first time I was able to sleep through the night well in months) after starting to take Vitamin D.

If you sleep is crappy why not try it? Even though it’s just anecdotal evidence so far.


The reason why Vitamin D results in such a widespread effects in the body are for two reasons.

  1. As previously mentioned, Vitamin D is derived from cholesterol and is “hormonal-like” in its effects on the body. Hormones are some of the strongest chemicals in the body and significantly alter cell growth, differentiation, metabolism and immune function.
  2. Vitamin D receptors are ubiquitous within the body and appear on almost every type of tissue. This will be discussed later in section 4.

Vitamin D deficiency is so widespread in the world’s population (especially American) and correlates well with the rapidly declining health of the populations. Though it is clearly not the only reason because of things like processed foods and overconsumption, it is one reason to look towards this readily available simple and cheap solution to improving health and certain risk factors. Of course, this does by no way mean that Vitamin D is a panacea that cures all ills. Rather, it should be used as one part of the plan to improve many areas of health simultaneously in accordance with healthy nutrition and exercise. Only if one is deficient.

Note 2: There are many more studies than just the ones I listed here. I just chose a few to get you rolling with Pubmed. If you are interested in any of these topics, then search “vitamin D” with any of the bolded terms above and you will find a lot of studies on those topics.

Note 3: The prevalence of increased risk of cardiovascular disease and other such illnesses among those with darker skin color may by explained by Vitamin D deficiency because they are getting less from the sun than the rest of the fairer skinned ethnicities. Higher blood pressure, higher cholesterol, etc. result.

Note 4: This great article, this article, and this study discuss the health benefits above. This other take on similar information (with some different references) may interest you.

Vitamin D: athletic performance

There is a large amount of evidence from studies implicating optimal Vitamin D levels and athletic performance. Fortunately, most of these studies which are from the old Soviet Union and Eastern Bloc countries (who knew something about increasing performance during the ’50s to the ’70s) are compiled into this PDF file (now defunct). This study has the data.

While it is well recognized that vitamin D is necessary for optimal bone health, emerging evidence is finding that adequate vitamin D intake reduces risk for conditions such as stress fracture, total body inflammation, infectious illness, and impaired muscle function. Studies in athletes have found that vitamin D status is variable and is dependent on outdoor training time (during peak sunlight), skin color, and geographic location. Although research has found that athletes generally do not meet the U.S. dietary reference intake for vitamin D, inadequate endogenous synthesis is the most probable reason for insufficient/deficient status. Given the recent findings, it is imperative that sports dietitians and physicians routinely assess vitamin D status and make recommendations to help athletes achieve a serum 25(OH)D concentration of >or=32 and preferably >or=40 ng.mL(-1). Further research is needed to determine the effect of vitamin D status on injury, training, and performance in athletes.

There has also been some anecdotal articles on the subject such as this one from T-nation. Some RCTs show positive effects on muscle strength and enhanced recovery of strength.

Suffice to say according to the research it seems that approximately ~50 ng/mL is optimal for athletic performance. Vitamin D does scale somewhat to weight so the heavier you are you may need to take a bit more or less depending. The performance benefits are increased muscle mass, increased strength and explosiveness, and ability to recover. These results range from very significant to noticable. A lot of it depends on how deficient you were in the first place getting up to 50 ng/mL, and how Vitamin D responds through receptors in your muscles.

Sunlight provision

If you choose not to supplement, you must at least know how much Vitamin D production you are receiving from the sun and from other sources.

The maximum amount of Vitamin D produced from the sun is approximately 10,000 IU per day. This study also shows that it can be achieved by full body exposure for 20 minutes of UVB radiation (no sunscreen) in fair skinned individuals. If exposure is less than full body, it may take longer to achieve maximum levels. Similarly, another study indicates that darker skinned individuals may require 3-6x more time in the sunlight to achieve the same amount of Vitamin D production as fair skinned individuals.

Prolonged exposure to sunlight does not cause Vitamin D toxicity. Maximum production levels are approximately 10,000 IU per day because the precusors to Vitamin D3 reach an equilibrium. Further production is degraded within the skin. This correlates well with the proposed higher safety limit discussed in toxicity concerns.


If you are severely deficient from never going out in the sun and never drinking or eating dairy, it seems that supplementing 10,000 IU for about 4-5 weeks will bring up levels to around 50 ng/mL.

From there, maintenance of Vitamin D levels are about 100 IU for every 1 ng/mL. Thus, to maintain these levels, approximate supplementation of 5,000 IU/day is necessary. Please read this one on why 10,000 IU is recommending to bring up levels in healthy adults. For a more conservative estimate, this 2016 review of exercise performance strategies suggests that 4,000 IU is the maximum dosage for exercise enhancement.

Before you supplement, read the next section on toxicity concerns.

Toxicity concerns

Toxicity concerns have been raised with such high supplementation given the federal government’s current recommendation 400 IU for adults and less for children. Although there is talk about changing the proposed upper limit to 10,000 IU in healthy adults in a study that is supposed to be released in the spring. Another risk assessment confirms the 10,000 IU upper level safe limit in healthy adults.

However, some of these studies have limitations which are pointed out in this article. In particular, the author of that said article is concerned about hypercalcemia, upper limit dosages, and the fact that the level testing might not accurately indicate effectiveness of dosage. Said author recommends 800-1200 IU/day as the max limit if you want to be super safe.

The other thing to keep in mind is that vitamins and minerals don’t act alone in the body. If you are increasing vitamin D in the diet, you should make sure you also have enough vitamin A, K, and magnesium to ensure that calcium levels don’t get out of whack. Cod liver oil is not the way, as it has been shown that it can give someone vitamin A toxicity. Ideally, get the sources from whole foods like spinach.

A recommended range of the super safe limit of 1000 IU to approximately 4000 IU for all of the benefits of enhanced athletic performance.

Knowing your levels

The only way to know your true levels are through blood tests. If you have biannual or annual visits to the doctor for blood work then you can request a 25-hydroxy Vitamin D test. The link above contains some guidelines to follow if you are to request one. Alternatively, there are self tests you can use although I cannot vouch for the accuracy or veracity of such tests.

This article discusses the accuracy of some lab tests as well as other toxicity concerns of megadosing. It’s worth a read.

The scale for Vitamin D deficiency fairly variable, but later research suggests:

  • 0-14.9 ng/mL = Severely deficient
  • 15.0-31.9 ng/mL = Mildly deficient
  • 32.0-100.0 ng/mL = Optimal
  • >100.0 ng/mL = Toxicity possible

As stated, optimal levels for athletic performance should be at approximately 50 ng/mL. However, for normal health people who do not exercise it is generally best to aim for the 50-80 ng/mL range.

Vitamin D and post workout

As most of us know, post workout it is almost unanimous that you are supposed to take in only carbohydrates and protein to maximize protein synthesis.

However, there is this one study that always bothered me.

Milk ingestion stimulates net muscle protein synthesis following resistance exercise.

PURPOSE: Previous studies have examined the response of muscle protein to resistance exercise and nutrient ingestion. Net muscle protein synthesis results from the combination of resistance exercise and amino acid intake. No study has examined the response of muscle protein to ingestion of protein in the context of a food. This study was designed to determine the response of net muscle protein balance following resistance exercise to ingestion of nutrients as components of milk. METHOD: Three groups of volunteers ingested one of three milk drinks each: 237 g of fat-free milk (FM), 237 g of whole milk (WM), and 393 g of fat-free milk isocaloric with the WM (IM). Milk was ingested 1 h following a leg resistance exercise routine. Net muscle protein balance was determined by measuring amino acid balance across the leg. RESULTS: Arterial concentrations of representative amino acids increased in response to milk ingestion. Threonine balance and phenylalanine balance were both > 0 following milk ingestion. Net amino acid uptake for threonine was 2.8-fold greater (P < 0.05) for WM than for FM. Mean uptake of phenylalanine was 80 and 85% greater for WM and IM, respectively, than for FM, but not statistically different. Threonine uptake relative to ingested was significantly (P < 0.05) higher for WM (21 +/- 6%) than FM (11 +/- 5%), but not IM (12 +/- 3%). Mean phenylalanine uptake/ingested also was greatest for WM, but not significantly. CONCLUSIONS: Ingestion of milk following resistance exercise results in phenylalanine and threonine uptake, representative of net muscle protein synthesis. These results suggest that whole milk may have increased utilization of available amino acids for protein synthesis.

Why exactly is whole milk superior increasing amino acid uptake and rates of protein synthesis in this study even in isocaloric milk (significant in 1 category, more in 2, and slightly below in 1) plus totally dominating the FM?

This question bugged me for a while because the “butter fat” in milk doesn’t have any special qualities on its own. It sat at the back of my mind recently until I started researching more on Vitamin D. As it turns out, Vitamin D is a fat soluble hormone that is best absorbed in the intestines with other fat. Fat is absorbed as micelles in the intestines to the lymphatic ducts. From there it is transported up the thoracic duct into the left subclavian vein.

Could it be that Vitamin D was acting on muscles post workout to boost protein synthesis OVER that of post workout carbohydrate and protein alone?

It turns out that muscles, along with most other tissues in the body have vitamin D receptors (VDRs). Most of which are located on the nuclear envelope and will modulate transcription of DNA. It has been shown that VDR binding activates tyrosine kinase cascades in muscle cells. For you lay-people the activation of tyrosine kinase cascades are usually seen in cell growth and differentiation. Similarly, other studies (2), (3), (4), (5) have confirmed this finding and also shown involvement of VDR activity with calcium modulation and MAPK cascades in muscles (which are also responsible for cellular growth and differentiation). Ca2+ modulation by genomic and non-genomic mechanisms may account for potential increases in strength as Ca2+ binding is part of the process of myosin head binding to actin myofibrils before contraction.

In addition, research also demonstrates that Vitamin D regulation in involved with increasing the amount of insulin-like growth factor binding proteins (receptors) while IGF-1 can also elevate Vitamin D3 levels. Follow up studies (1) (2) have confirmed this and narrowed down the association of VDRs to the regions with IGF binding proteins 1, 3 and 5. These processes and their anabolic effects on muscles are thoroughly discussed in Vitamin D, Volume 1 by Feldman, Glorieux, and Pike from pages 885-898 (google preview does not allow all the pages to be seen).

The production of the IGF family (peptide hormones) is stimulated by growth hormone. They are responsible themselves for most of growth hormone’s anabolic effects on the body. Increases in IGF binding proteins is similar in nature to insulin sensitivity. We often talk about insulin sensitivity and how insulin resistance causes type II diabetes. For example, increases in insulin sensitivity occur through exercise and regulation of carbohydrate intake. This means the amount of receptors are increasing, which means the body needs less insulin to deal with blood sugar levels. The opposite occurs in insulin resistance – receptors are down regulated and your tissues have less and less receptors until you get diabetes because your pancreas cannot produce anymore insulin.

In this case, increased amounts of IGF receptors leads to more sensitivity to IGF’s effects on the body. Thus, you will have more potent anabolic growth per amount of hormones produced in your body. We can all see why this may be potentially big for muscle gains. It is like taking steroids without taking steroids. In addition, there is the tyrosine kinase and MAPK cascades promoting muscular growth. Unfortunately, there has been no studies done specifically on Vitamin D supplementation with and contrasted against carbohydrates and protein post workout. Although the whole milk vs. isocaloric skim milk does show indirectly that there is some benefit of probably Vitamin D absorption acting on increasing protein synthesis and growth within the muscle cells.

So what does this mean?

Honestly, I do not know what it means until there is an official study on it. The milk in the study was only 237g which is approximately 1 cup. This is 25% of the recommended daily dose based on government nutrition standards. The current nutritional standard is 400 IU per day for 100%. Thus, the study participants were getting 100 IU. If they were deficient or severely deficient this may have made a big impact on their protein synthesis rates.

Now, compare this to supplementation with potentially 1,000 to 4,000 IU of Vitamin D post workout with carbohydrates, protein, and fat. There may be a HUGE effect on muscle protein synthesis.

My personal story: I started supplementing with Vitamin D at 10,000 IU for about a month. It just so happens that I usually workout then eat my dinner and take my supplements along with dinner. In this month I have been eating a bit less, but yet I have gained approximately 3-4 lbs of lean body mass and dropped a bit of fat. Now, the muscle gain could have occurred just from increased performance with my workouts, but I am almost positive that some of it has to do with increased protein synthesis post workout from the Vitamin D supplementation. Thus, I am continuing on with 10,000 IU post workout transitioning into 5,000 for maintenance.

Obviously, I was deficient. If you are not this may not work. Also, this is very old data, so if I had to go back I would only do 4,000 IU maximum.

If you are interested in maximizing your muscle gains, it may be a good idea to see if this can potentially work for you just like BCAAs/leucine have shown to have potent anabolic effects. If you do try this, feel free to comment (1) whether it works or not. Also, post (2) whether you were deficient before or not, (3) when you started taking it PWO, (4) how many days/weeks it has been, and (5) your results compared to your normal PWO. I’d love to know if this works for you and get some data.

Note 5: This may also explain the “steroid-like” effects of the recommended gallon of milk a day (GOMAD) associated with such programs as Starting Strength and powerlifting. Vitamin D deficiency being corrected besides the calories and potential bovine growth hormones (although most of the peptide hormones should be hydrolyzed by enzymes in the stomach/intestines which is why I don’t think bovine GH plays a big role).


Most people are deficient or severely deficient of Vitamin D. If you are not working an outdoor job with significant sun or taking in significant amounts of dairy then supplementation may be for you. Those with darker skin are more at risk for Vitamin D deficiency.

Only supplement if you are deficient. Get a test from your doctor or the above testing links to find out if you are.

  • There are significant health benefits to supplementing Vitamin D if you are deficient which 85-90% of people are.
  • For those who want to be super safe, the 800-1200 IU/day recommendation is the safest bet. Other conservative estimates are 2,000 IU as the super safe limit.
  • Supplementing Vitamin D up to 4,000 IU/day can increase your athletic performance.
  • Supplementing Vitamin D with fat post workout coupled with carbohydrates and protein may lead to increased muscle gains over just post workout carbohydrates and protein.

For those deficient, although 10,000 IU is a proposed safe upper limit, most of the benefits coming out of deficiency seem to be in the 1,000 IU up to 4,000 IU range. Hence, I would not suggest exceeding 4,000 IU/day.

A small portion of the population is hypersensitive to exogenous vitamin D supplementation, even from as little as what you get from dairy intake. They may not need any supplementation at all. and instead they may get adverse reactions from intake of any food with vitamin D. Obviously, consult your doctor before taking any supplements, and if you are experiencing adverse side effects do not continue to take it and notify your medical professional. Hence, why it is important to supplement only if you are deficient.


Here’s some supplements that work at 1000, 2000, and 4000 IU levels.

I suggest getting the vitamin D test added to your next doctor visit so you can check your vitamin D status, or use some of the above links to order a test for yourself. Remember to each enough whole foods to get the right amounts of vitamin A, K, and minerals such as magnesium along with any supplementation. If you experience any adverse side effects, back off.

This article was originally published October 10, 2009 on Eat Move Improve. Updated Dec 2016. 

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Author: Steven Low

Steven Low, author of Overcoming Gravity: A Systematic Approach to Gymnastics and Bodyweight Strength (Second Edition), is a former gymnast who has performed with and coached the exhibitional gymnastics troupe, Gymkana. Steven has a Bachelor of Science in Biochemistry from the University of Maryland College Park, and his Doctorate of Physical Therapy from the University of Maryland Baltimore. Steven is a Senior trainer for Dragon Door’s Progressive Calisthenics Certification (PCC). He has also spent thousands of hours independently researching the scientific foundations of health, fitness and nutrition and is able to provide many insights into practical care for injuries. His training is varied and intense with a focus on gymnastics, parkour, rock climbing, and sprinting.