This time of year, people are often keen to know if there is a specific food they should eat, or supplement they should take, in order to help strengthen their immune system or improve their overall health. I am a strong believer that people can get the vast majority of their essential nutrients from a balanced and varied diet but there is at least one exception to this -- vitamin D.
In recent years, vitamin D, often referred to as the sunshine vitamin, has been one of the most talked about supplements due to ever-increasing evidence describing relative deficiencies in certain populations, and the benefits of supplementation.
Researchers are still trying to establish exactly how much vitamin D we require in our diet and whether we should be supplementing with it during the winter months. There is no doubt that in certain parts of the world -- due to climate, environmental, dietary and cultural factors -- vitamin D deficiency is common and it should be taken in supplement form. So, what about Ireland: do we need to supplement with vitamin D in the winter months and, if so, how much is enough or too much?
WHAT IS VITAMIN D?
Vitamin D is a fat-soluble vitamin that is also known to act as a lipophilic pro-hormone. It can be obtained from your diet, but the primary source is synthesis in skin when exposed to ultraviolent-B (UVB) radiation from the sun.
When ultraviolet light from the sun hits the leaf of a plant, a molecule known as ergosterol is converted into ergocalciferol, or vitamin D2, whereas when the light hits the cells of our skin, it is converted into cholecalciferol, a form of vitamin D3.
Under normal circumstances, in response to regular sun exposure during the summer, the body will synthesise sufficient vitamin D to meet its needs. However, during the winter, the ultraviolet rays are often not strong enough to allow our body to produce vitamin D. Hence, we rely entirely on dietary sources.
WHY VITAMIN D IS SO IMPORTANT
Vitamin D is required for multiple functions in the human body, including calcium absorption, hormone production, optimum muscle function and the development of healthy bones and joints.
The health-related implications of vitamin D deficiency continue to increase, with a growing body of research linking vitamin D deficiency to certain cancers, high blood pressure, cardiovascular disease, osteoarthritis and lowered immune function to name just a few.
Another hugely important role vitamin D has is in mental health as deficiency has been linked to symptoms of depression and seasonal affective disorder (SAD).
VITAMIN D AND THE ELDERLY
As we age, our muscle mass begins to waste and our strength declines year on year. This is known to be one of the reasons why the elderly are more prone to falls, fractures and certain diseases.
Vitamin D plays an important role in muscle function and strength. Research has shown that not only are elderly people not getting sufficient amounts of vitamin D from their diet but that they are less efficient at producing it from sunlight.
Older people should make a conscious effort to consume greater amounts of foods containing vitamin D and consider a vitamin D supplement in the winter months.
VITAMIN D AND YOUR DIET
Our body is highly-efficient at producing sufficient vitamin D from the sun. For example, one full dose of UVB (15 to 20 minutes) from the sun produces upwards of ~20,000 IU of vitamin D, which equates to consuming well over a kg of wild salmon.
The recommended daily allowance (RDA) for vitamin D from the Institute of Medicine (IOM) has recently been revised upwards to 600 IU. However, many researchers still believe this to be too low.
The foods that naturally contain the richest source of vitamin D include wild-caught oily fish, namely salmon, mackerel, bluefish, tuna, sardines and cod liver oil.
If you happen to be a person that hasn't been exposed to much sunshine over the course of the year; you don't eat vitamin D rich foods; and you don't take a multivitamin; then you are likely to require a vitamin D supplement during the winter months.
The only way you can know for sure if you are deficient in vitamin D is to have your doctor test your blood levels. A practical and safe way to be sure you are getting sufficient vitamin D is by taking a vitamin D supplement containing 1000 to 2000 IU daily until the late spring or early summer. Vitamin D3 is considered the most appropriate form to supplement with.
CAN WE TAKE TOO MUCH?
When we naturally synthesise vitamin D through our skin, the body has an ability to stop production once sufficient amounts have been made.
However, this is not the case when we take it in large doses in supplement form. Having said that, the existence of toxic levels of vitamin D have yet to be proven, but some harmful side effects are reported at mega doses of 40,000 IU daily.
The IOM has set the upper level of intake at one-tenth of this or 4000 IU, but research has shown that taking as much as 10,000 IU daily for six months did not result in adverse side effects.
There is extensive evidence to suggest that a widespread deficiency of vitamin D exists particularly in countries with long winters with limited sunshine -- and Ireland is certainly one of those. If you are someone who doesn't eat foods rich in vitamin D, and you don't get to visit tropical climates during the winter, a vitamin D supplement would be a sensible and practical option.
Daniel Davey BSc MSc, CSCS, NEHS is a performance nutritionist