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.
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.