It is widely known that consuming a colourful diet, rich in diversity and flavour, is the best way in which to meet our micronutrient needs and function at our optimum.
However, as the days get shorter and the trees start to change colour, there is one nutrient in particular that becomes difficult to obtain from our external environment.
Vitamin D can be synthesised by our skin when exposed to sunlight (around 30 minutes daily exposure to face, hands and arms without the protection of sun cream is thought to be adequate for the majority of people).
During the winter months in the UK, the levels of ultraviolet B radiation in sunlight are no longer sufficient for this process to take place, so it is a good idea to get it elsewhere.
Vitamin D can be found in some foods
Oily fish, such as salmon and mackerel, is a good source, but this is not eaten regularly by most people, let alone daily. Egg yolks are another, but one egg contains only approximately 5% of your daily vitamin D requirement. Some breakfast cereals may also be fortified in small amounts. How about some liver? Anyone?
It is very difficult to achieve a sufficient intake through food alone, with so few sources. This is why Public Health England and the NHS recommend that everyone takes a 10μg or 400IU supplement daily from the age of 1 between the months of October and April in the UK.
So why is vitamin D important?
Vitamin D is vital for bone health.
Our bones are constantly being both broken down and reformed in a process called remodelling.
During developmental years, bone density should be gradually increasing.
At age 30-35, we reach our individual peak bone mass, a figure largely determined by nutrition and activity levels during childhood and adolescence.
From this age onwards, bone density very gradually decreases as slightly more bone is lost than gained. This process can be slowed through exercise and diet.
Bones and teeth hold 99% of the body’s calcium. Calcium has important uses within the body, such as being a key player in muscle contraction and nerve impulses. If the body has insufficient calcium to carry out bodily functions, it will take from the reservoir we store in our bones. Therefore, adequate calcium intake is necessary to maintain as much bone density as possible as we age.
But where does vitamin D come in?
Vitamin D regulates calcium uptake by maximising the number of transporters and receptors in the gut lining, enabling the absorption of calcium from digested food into the blood stream.
So, adequate vitamin D is necessary to ensure we can uptake enough calcium (obviously dependant on adequate calcium in the diet).
Consequences of insufficient vitamin D
In extreme scenarios, deficiencies in vitamin D can result in rickets and osteomalacia in children and adults respectively.
Both diseases are characterised by soft, weak bones and have scarily seen an increase in prevalence in recent years; potentially because more young people lead sedentary lifestyles and spend increasing amounts of time indoors – the critical years during which much of their bone mass is accrued.
For active adults who spend time in the summer sunshine, vitamin D deficiency throughout the winter and thus reduced calcium absorption may be more likely to display itself later on in life in the form of osteoporosis (yes, this does require some of you to think far into the future – something we should all base our daily actions on if we want to maintain strength, mobility and independence into old age).
Osteoporosis affects 3 million people in the UK and significantly increases fracture risk and subsequent reduced quality years of life. This can be as a result of gradual and consistent depletion of calcium stores from our bones over the years through insufficient vitamin D and/ or calcium intake. It could also be because of a lack of physical activity. Of course, the older we live, the longer we need our bones to support us for too!
Women are at a higher risk of suffering from osteoporosis due to rapid bone loss in the years following menopause, so maintaining as much bone density as possible is advantageous in reducing likelihood of disease later on, ladies!
What about athletes?
As runners and athletes, the activity side of things to maintain strong bones is hopefully in the bag; particularly if we are being good and doing a little strength work to support our running.
But the importance of maintaining bone strength aided by a healthy diet may be escalated due to higher impact and stress we are subjecting our bodies to.
Running increases load through the body and magnifies the pressure our bones are under. Combine that with the prevalence of relative energy deficiency in sport (RED-S) and we have a problem.
Stress fractures are becoming all too common in the running world – likely through a combination of under-fuelling and over-training.
Female athletes, in particular, need to be aware of their micronutrient intake as they typically consume a lower-energy diet, so getting everything they need from their food can be more challenging.
More exercise also equates to more muscle contraction; a mechanism dependant on calcium intake. So, it may be even more important for athletes to supplement in the winter than sedentary individuals.
One particular study in the UK found 61% of 62 athletes to have inadequate vitamin D levels at the start of a supplementation trial.
Regarding performance, multiple studies have found that vitamin D supplementation in deficient athletes improved exercises such as sprinting and jumping and also reduced injury rate. This could be useful in interval training to improve running speed, for example, achieving more adaptations from training with adequate vitamin D levels in the body.
Whilst many studies in the literature demonstrate improvements with vitamin D supplementation, the amount supplemented is way above public health recommendations, likely in order to maximise chances of eliciting an effect. Therefore, they are not particularly relevant when giving generic advice.
These studies do, however, demonstrate that vitamin D can influence these aspects of performance and that deficiency is worth trying to avoid. There is likely also a difference between a minimum intake recommended for health and a potential amount for optimal health or performance.
In summary, a vitamin D supplement during winter months may improve performance and training, but more importantly, will hopefully maintain long term bone health and help keep injury at bay.
Some final points
Supplements containing vitamin D3 are best absorbed and it does not matter which brand you buy providing it provides the recommended 10μg/ day. It may be worth checking if this is already included if you take a multivitamin.
If you are in doubt whether it is safe to take a supplement or are on any medication, please seek further advice from your GP or a dietician.
It is also possible to get a blood test with some doctors if you are interested or concerned about your vitamin levels.
A combination of adequate vitamin D, calcium and weight-bearing activity is essential for optimum bone health. Taking a vitamin D supplement through the winter months is one way to ensure we are satisfying this triad of important considerations, for our today selves and also our future selves.
Izzy Szembek ANutr