Vitamin D in the spot light...

Vitamin D is causing quite a stir in the news. In the summer of 2016, Public Health England (PHE) recommended that to protect bone and muscle health, everyone needs vitamin D equivalent to an average daily intake of 10 micrograms. PHE also advised that in spring and summer, the majority of the population get enough vitamin D through sunlight on the skin and a healthy, balanced diet.
 
This advice is great, but the requirements for people who exercise regularly differ greatly from that of general population.
 
What is Vitamin D
 
Vitamins are organic compounds (meaning they contain carbon) and can be characterised into water soluble vs fat soluble. Fat soluble vitamins are stored and sequestered into fat stores, and therefore have a much longer bioavailability than water soluble vitamins. In contrast water soluble vitamins are excreted out in urine if consumed in too large a quantity and are used by the body fairly quickly. Vitamin D ​is a fat soluble vitamin. 
 
Where does vitamin D come from?
 
Our bodies cannot produce vitamins except for one exception, vitamin D. The term vitamin D is, unfortunately, an imprecise term referring to one or more members of a group of steroid molecules. Vitamin D3, also known as cholecalciferol is generated in the skin of animals when light energy is absorbed by a precursor molecule 7-dehydrocholesterol. If someone has adequate exposure to sunlight, theorectically they do not require dietary supplementation.
 
How does my body synthesis Vitamin D?
 
Vitamin D, as either D3 or D2, does not have significant biological activity. Rather, it must be metabolised within the body to the hormonally-active form known as 1,25-dihydroxycholecalciferol by a 2 step process in the liver and then kidney.
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Optimal Vitamin D levels can reduce the risk of debilitating stress responses in bone

What does vitamin D do in the body?
 
Vitamin D is well known as a hormone involved in mineral metabolism and bone growth. Its most dramatic effect is to facilitate intestinal absorption of calcium, although it also stimulates absorption of phosphate and magnesium ions. In the absence of vitamin D, dietary calcium is not absorbed at all efficiently. Vitamin D stimulates the expression of a number of proteins involved in transporting calcium from the lumen of the intestine, across the epithelial cells and into blood. Not only does vitamin D assist in growth and maintenance of the bone, but it also aids in regulation of electrolyte metabolism, protein synthesis, gene synthesis and immune function.

What happens if I exercise with low vitamin D levels?
 
Although there is not yet enough convincing evidence to support vitamin D as a direct performance enhancer, obtaining optimal Vitamin D levels can reduce the risk of debilitating stress responses in bone which may lead to stress fractures. In addition, because of its active role in muscle and protein synthesis, recovery and fatigue may also be improved, all impacting your performance. Newer studies are also suggesting that as immune function is boosted, active people are less likely to suffer from common colds (1).
 
What should I do to boost my vitamin D?
 
1. Sunlight exposure 

During the summer months, training or playing sport outdoors can have a massive impact in boosting your vitamin D stores. This can be further enhanced by maximizing skin exposure to sunlight e.g. by wearing shorts and tee-shirts. Your body can't make vitamin D if you are sitting indoors by a sunny window because ultraviolet B (UVB) rays (the ones your body needs to make vitamin D) can't get through the glass. Sunscreen prevents sunburn by blocking UVB light. Theoretically, that means sunscreen use lowers vitamin D levels. But as a practical matter, very few people put on enough sunscreen to block all UVB light, plus the risks of unprotected sun exposure far outweighs any additional vitamin D you might receive!
 
Remember, the bioavailbility of your replenished vitamin D stores during the summer months will last for weeks to months and therefore stand you in good stead for the sun deprived winter.
 
2. Managing obesity 

A recent study funded by the British Heart Foundation has shown that for every unit increase in BMI (1kg/m2) was associated with a 1.15% reduction in the level of vitamin D in the blood (2). As we now know, Vitamin D is a fat soluble vitamin. This means the more vitamin we have ‘locked away’ in our fat stores, the lower the level circulating in our blood will be. This means we cannot effectively use the vitamin D in our body, the more fatty tissue we have. However, it also means, that we need adequate amounts of fat in our diet to absorb the Vitamin D in the first place. This again highlights the importance of a balanced healthy diet.
For every unit increase in BMI, there is a 1.15% reduction in blood vitamin D levels
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3. Dietary intake
 
During autumn and winter, PHE recommends that everyone will need to rely on dietary sources of vitamin D, as sunlight is not enough. Vitamin D3 is also found in a small number of foods. Good food sources are:

• oily fish – such as salmon, sardines, herring and mackerel
• red meat
• liver
• egg yolks
• fortified foods such as most fat spreads and some breakfast cereals

 
The plant form of vitamin D is called vitamin D2 or ergosterol. In the UK, cows' milk is generally not a good source of vitamin D because it isn't fortified, as it is in some other countries. 

4. Supplementation

Another source of vitamin D is dietary supplements. Recent guidance from PHE has suggested that the  Vitamin D recommendation from consuming foods naturally containing or fortified with vitamin D, people should consider taking a daily supplement containing 10 mcg (micrograms) of vitamin D in autumn and winter. Ethnic minority groups with dark skin, from African, Afro-Caribbean and South Asian backgrounds, may not get enough vitamin D from sunlight in the summer and therefore should consider taking a supplement all year round.
 
What does the mean for me?
 
There is currently no guidance for people who exercise regularly on vitamin D, but in my medical opinion, we should be counted as a high risk group. This means as well as taking the above measures including optimising sunlight exposure and diet, supplementation should be taken over the winter months. If you exercise and train all year round, you will be doing no harm by taking 10 mcg Vit D supplementation all year round. Bear in mind however that even this dose may not be enough for high level exercisers and it is always worth asking for your Vitamin D levels to be checked by your doctor.
If you exercise regularly, you will be doing no harm by taking 10 mcg of Vitamin D once a day all year round. However, in some cases this may still not be enough.  
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What are normal values of vitamin D?
 
So you go to your doctor and get your vitamin D level checked. He tells you it is normal. Great.
 
No, not great. It is always worth finding out the value of your Vitamin D level as what is considered ‘normal’ can vary greatly. There is consensus that levels below 25nmol/Lqualify as ‘deficient’, but beyond this there is currently no standard definition of ‘optimal’ Vitamin D levels. Some sources suggest that levels above 50nmol/L are ‘sufficient’, while 70–80nmol/L is ‘optimal (3). It is worth noting that normal ranges also differ depending on which laboratory you get your blood processed in. Most sporting bodies will advise their athletes keep their levels above 90-100 nmol/L .
 
In other words, don’t be afraid to challenge your doctor if you feel your vitamin D levels are not being taken seriously enough! Or if he tells you they are in the normal range but doesn't take into account the fact you run 5 times a week. Optimising your Vitamin D intake is any easy way to improve your health and performance. 
 
 
References
 
1. Nutrients. 2013 Jun; 5(6): 1856–1868. 2013 May 28. doi:  10.3390/nu5061856. Vitamin D and the Athlete: Risks, Recommendations, and Benefits D, Ogan,  K, Pritchett.
2. Vimaleswaran KS, Berry DJ, Lu C, et al. Causal Relationship between Obesity and Vitamin D Status: Bi-Directional Mendelian Randomization Analysis of Multiple Cohorts. PLoS One Medicine. Published online February 5 2013
3. NICE. Vitamin D position statement. 2010.