Surprisingly low levels of Vitamin D in Cyclists

There is growing evidence that for athletes, being replete in vitamin D is important for many key areas of health and performance. For bone health, muscle strength and to support immune function.


At the recent International Association of Dance Medicine conference, in addition to presenting on Dance Endocrinology I also took part in a “duel” on vitamin D to argue the case for vitamin D supplementation, in dance/athletic populations. In fact Vitamin D is a type of steroid hormone. An article in BJSM discussed the synergistic action of steroid hormones, in particular vitamin D and the sex steroids. In dancers who train in studios inside, away from the sun then there is evidence that levels of vitamin D can become low, particularly during winter months. Supplementing with vitamin D in these elite female dancers reduced injuries and significantly improved muscle strength and jump height.

What about athletes that train outside? What about male athletes? You might think that competitive road cyclists would benefit from plenty of time spent outside and that vitamin D levels would be close to athlete recommended level of 90 nmol/L. However, in our recent study of 50 competitive male road cyclists, the majority had low athlete levels of vitamin D. Even some cyclists who reported taking supplementation for this vitamin were found to have low levels, reflecting variations and uncertainties in what dose to take. Conversely some riders taking supplementation had levels that were well above recommended athlete levels. More is not necessarily better in this situation and very high levels can lead to toxicity.

Bone health in road cyclists can be compromised due to 2 factors. In the first instance, being a non weight bearing sport means lack of mechanical osteogenic (bone stimulating) skeletal loading. In addition, road cycling is a gravitational sport where being light weight confers a performance advantage in terms of power to weight ratio. This can lead to restrictive nutrition practices and low energy availability (LEA) in athletes/dancers. LEA is a situation where dietary energy intake is insufficient to support both training demands and the energy requirement to keep healthy.  So LEA has adverse effects on both health and athletic performance described in the clinical model RED-S (relative energy availability in sport). This includes a negative impact on bone health. DXA is regarded as “gold standard” quantification of impact of LEA and RED-S on bone health. In our study a specially designed SEAQ-I (sports specific questionnaire and clinical interview) was found to be the most effective indicator of poor bone health found with DXA. 28% of the cyclists were identified as having LEA with correspondingly low bone mineral density for their age.

What about the effect of vitamin D levels on bone health? In those cyclists assessed as having adequate EA from SEAQ-I, then vitamin D was an important factor in bone health. However, in those 28% cyclists assessed as having LEA, vitamin D did not feature as as such an important factor. Essentially having adequate EA is the top priority for health and performance. The other observation is that many of those cyclists in LEA, although not consuming adequate calories, nevertheless were taking plenty of supplements in the belief that this would reduce any negative effects of restrictive nutrition. This strategy does not work. The reason being that LEA causes dysfunction not just of one hormone in isolation, rather interactive hormone networks become disrupted. Hormones are crucial for supporting bone health, particularly IGF-1, testosterone and vitamin D in males. Furthermore there is evidence to show that there is a synergistic interaction between testosterone and vitamin D in men. In out study those riders with chronic LEA were found to have significantly lower testosterone than the other cyclists. So even if male athletes with LEA have adequate levels of vitamin D, then low levels of other hormones, such as testosterone, will have net negative effect on bone health.

VitD Histogram

So male cyclists are at risk of poor bone health for the following reasons:

  • Cycling is a non-weight bearing sport,
  • Vitamin D can be below athlete recommended levels, even if EA adequate
  • Long term LEA causes clinical consequences of RED-S including disruption of hormones necessary for maintaining bone health

Does this matter? An early warning sign in runners of LEA is stress fracture. In cyclists the first evidence of an issue with bone health could be vertebral fracture from a bike crash, as this is area of skeleton most adversely effected by LEA and most serious in terms of fracture site requiring longest time off bike. Moreover our study found that in some cyclists with chronic, long term LEA cycling performance in terms of 60 minute functional threshold power (FTP) was below that anticipated from training load.

To perform at your full athletic potential you need adequate EA and vitamin D.


Low energy availability assessed by a sport-specific questionnaire and clinical interview indicative of bone health, endocrine profile and cycling performance in competitive male cyclists BMJ Open Sport and Exercise Medicine. Keay, Francis, Hind 2018

How do you identify male cyclist at risk of RED-S? BJSM, Dr N Keay 2018

Fuelling for Cycling Performance Science4Performance 2018

Synergistic interactions of steroid hormones BJSM, Dr N Keay 2018

Raising Awareness of RED-S in Male and Female Athletes and Dancers BJSM, Dr N Keay 2018

2018 UPDATE: Relative Energy Deficiency in Sport (RED-S) BJSM, Dr N Keay 2018

The influence of winter vitamin D supplementation on muscle function and injury occurrence in elite ballet dancers: A controlled study Journal of Science and Medicine in Sport 2014


Synergistic Interactions of Steroid Hormones


The action of the sun on skin is the most effective way of making vitamin D. However, even walking around outside naked for 5 hours every day during UK winter months is not sufficient to make adequate vitamin D. Therefore, much to the relief of the audience at the recent BASEM Spring conference, this was not a strategy recommended by Dr Roger Wolman.

Vitamin D is a fat soluble steroid hormone. The majority of which is synthesised in the skin when exposed to ultraviolet B in sunlight, with a small contribution from dietary sources: this vitamin D3 molecule is then hydroxylated twice in the liver and then kidney to produce the metabolically active form of vitamin D. This activated steroid hormone binds to vitamin D receptors in various tissues to exert its influence on gene expression in these cells. The mono hydroxylated form of vitamin D is measured in the serum, as this has a long half life.

Does it matter having low levels of circulating vitamin D during winter months? What are the solutions if moving to warmer climates during the winter is (unfortunately) not feasible? What are the other hormones interact with vitamin D?

What are the beneficial effects of vitamin D, particularly in the athletic population?


Rickets and osteomalacia are conditions where vitamin D deficiency results in bone deformities and radiographic appearances are characterised by Looser zones, which in some ways are similar in appearance to stress fractures.

In a large prospective study of physically active adolescent girls, stress fracture incidence was found to have an inverse relationship with serum vitamin D concentrations. In adult female Navy recruits monitored during an 8 week training programme, those on vitamin D supplementation had a 20% reduction in stress fracture. However, oestrogen status was a more powerful risk factor at 91% in those recruits reporting amenorrhoea. Vitamin D is, itself, is a steroid hormone with range of systemic effects. As will be discussed below, its interaction with the sex steroid oestrogen has an important effect on bone turnover.


Although sanatoriums, for those suffering with tuberculosis, were based on providing patients with fresh air, any beneficial effect was probably more due to vitamin D levels being boosted by exposure to sunlight. Certainly there are studies demonstrating the inhibitory effect of vitamin D on on slow growing mycobacteria, responsible for TB. What about the influence of vitamin D on other types of infection? In a recent publication, evidence was presented that supplementation with vitamin D prevented acute respiratory tract infections. This effect was marked in those with pre-existing low levels of vitamin D. In a study of athletes a concentration of 95 nmol/L was noted at the cut off point associated with more or less than one episode of illness. In another randomised controlled study of athletes, those supplemented with 5,000IU per day of vitamin D3 during winter displayed higher levels of serum vitamin D and had increased secretion of salivary IgA, which could improve immunity to respiratory infections.


There is evidence that supplementing vitamin D3 at 4,000IU per day has a positive effect on skeletal muscle recovery in terms of repair and remodelling following a bout of eccentric exercise. In the longer term, dancers supplemented with 2,000IU over 4 months reported not only reduction in soft tissue injury, but an increase in quadriceps isometric strength of 18% and an increase of 7% in vertical jump height.

Synergistic actions of steroid hormones

No hormone can be considered in isolation. This is true for the network interaction effects between the steroid hormones vitamin D and oestrogen. In a study of professional dancers, there was found to be significant differences in serum vitamin D concentrations in dancers from winter to summer and associated reciprocal relationship with parathyroid hormone (PTH). In situations of vitamin D deficiency this can invoke secondary hypoparathyroidism. Although low levels of vitamin D were observed in the dancers, this was not a level to produce this condition. However, there was an increase in soft tissue injury during the winter months that could, in part, be linked to low vitamin D levels impacting muscle strength.

The novel finding of this study was that female dancers on the combined oral contraceptive pill  (OCP) showed significant differences, relative to their eumenorrhoeic counterparts not on the OCP, in terms of higher levels of vitamin D and associated reductions of bone resorption markers and PTH. The potential mechanism could be the induction by the OCP of liver enzymes to increase binding proteins that alter the proportion of bound/bioactive vitamin D.

This interaction between steroid hormones oestrogen and vitamin D could be particularly significant in those in low oestrogen states such as postmenpoausal women and premenarchal girls. Menarche can be delayed in athletes, so is there a case for vitamin D supplementation in young non-menstruating athletes? What is the situation for men? Do testosterone and vitamin D have similar interactions and therefore implications for male athletes with RED-S, where testosterone can be low?

Vitamin D is not simply a vitamin. It is a steroid hormone with multi-system effects and interactions with other steroid hormones, such as sex steroids, which are of particular relevance to athletes.


BASEM Spring Conference 2018 “Health, Hormones and Human Performance”

BASEM Spring Conference 2018 Part 2 “Health, Hormones and Human Performance”

Calcium and Vitamin D Supplementation Decreases Incidence of Stress Fractures in Female Navy Recruits JBMR 2009

Vitamin D, Calcium, and Dairy Intakes and Stress Fractures Among Female Adolescents Arch Pediatr Adolesc Med 2012

A Single Dose of Vitamin D Enhances Immunity to Mycobacteria American Journal of Respiratory and Critical Care Medicine 2007

Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data BMJ 2017

Influence of vitamin D status on respiratory infection incidence and immune function during 4 months of winter training in endurance sport athletes Exerc Immunol Rev. 2013

The effect of 14 weeks of vitamin D3 supplementation on antimicrobial peptides and proteins in athletes J Sports Sci. 2016

A systems-based investigation into vitamin D and skeletal muscle repair, regeneration, and hypertrophy American Journal of Physiology 2015

The influence of winter vitamin D supplementation on muscle function and injury occurrence in elite ballet dancers: A controlled study Journal of Science and Medicine in Sport 2014

Vitamin D status in professional ballet dancers: Winter vs. summer J Science and Medicine in Sport 2013