The importance of Vitamin D in our diet, with Dr Kevin McCarroll
Vitamin D has become the focus of a huge amount of interest and research in nutritional science and medicine in recent years. The discovery of vitamin D in the 1920s came from the search for a cure for rickets, a disease prevalent in northern European cities in the late 19th Century.
Studies examining the effects of sunlight and cod liver oil in both the prevention and treatment of rickets led to the discovery of the ‘sunshine vitamin’ and the early fortification of foods that eradicated a disease once reaching epidemic proportions. Much has been learned since about vitamin D that has fuelled a burgeoning level of research into it’s potential affect on several diseases and possible use as simple and yet inexpensive treatment option.
Vitamin D is not really a vitamin at all but in fact a precursor steroid hormone that is activated by our liver and kidney as well as other organs. Furthermore, apart from supplements 90% of vitamin D is obtained from synthesis in our skin from a cholesterol derivative following exposure to ultraviolet light in sunshine.
The effects of vitamin D are mediated by the vitamin D receptor found in most tissues including the heart, kidney, gastrointestinal tract, muscle and brain. In this way, it is believed to regulate hundreds of genes and their protein products including hormones.
The well-established role of vitamin D is in maintaining bone health by promoting the adequate absorption of calcium from the gut. Deficiency in children leads to rickets and in adults to osteomalacia or softening of the bones, as well as contributing to the development and exacerbation of osteoporosis. In recent years, sedentary lifestyle and reduced sun exposure has led to the emergence of a small number of cases of rickets and osteomalacia in Ireland.
Beyond bone health, the role of vitamin D has not been definitively proven but an increasing body of research at a biological and epidemiological level supports a role in a host of diseases including those of the cardiovascular system (heart disease, peripheral arterial disease), cancer, depression as well as neurological and autoimmune disorders. There is also strong evidence which suggests that vitamin D can improve muscle strength and reduce falls in older adults.
However, most studies are observational and simply show a higher risk in those with lower vitamin D levels with few intervention trials to date to confirm whether supplementation is beneficial.
In animal models, vitamin D has anti-inflammatory properties, up-regulates neurotrophc factors that help to preserve neurons, down-regulates amyloid (a protein that builds up in Alzheimer’s) and increases neurotransmitters important in cognition. Lower vitamin D levels have been associated with an increased risk of developing Alzheimer’s and other neurological conditions such as multiple sclerosis and Parkinson’s disease.
The association between vitamin D and cancer appears to be most significant with bowel cancer, where pooled analyses of large studies shows a greater likelihood in those who are deficient. This may be mediated by its effect on programmed cell death, inflammation and blood vessel proliferation. Lower vitamin D levels have also been associated with a greater risk of depression, diabetes, respiratory and cardiovascular disease and mortality.
Whilst there is a strong biological plausibility that underpins many of these associations, causality has not been proven and large interventional studies using vitamin D now need to be performed. Several factors that lower vitamin D status such as reduced sun exposure, low levels of physical activity and poor diet are of themselves negatively associated with several diseases making study interpretations less clear.
What constitutes a healthy vitamin D level has been defined only with regard to optimal bone health. From October to March little or no vitamin D can be made in response to sunlight and levels typically drop by about 30%.
However, in the summer exposure of the face and lower arms/legs to about 15 minutes of sunshine three to four times per week will provide one with all the vitamin D they require. Indeed, exposure beyond this will result in little or no extra vitamin D as excess is broken down in the skin.
There are few sources of vitamin D in the diet but the richest of these include oily fish and cod liver oil with smaller amounts contained in Vitamin D-fortified milk and breakfast cereals.
Vitamin D requirement is higher in pregnancy, those who are breastfeeding and in the presence of gut malabsorption syndromes and liver disease. The risk of deficiency is higher in such groups as well as in older adults who have up to 75% less capacity for cutaneous vitamin D synthesis and where consideration should be given to checking vitamin D levels.
In the absence of disorders of calcium metabolism, it is very hard to overdose on vitamin D unless taken at very high doses. The Institute of Medicine recommends a minimum daily intake of 600 IU (International Units) daily for those aged between 1-70 years and rising to 800 IU in those who are older. For adults, a conservative safe upper intake of 4000 IU per day has also been set. In those with brittle bones, a dose of a least 800 IU daily is routinely advised. Supplements are available on prescription and over the counter and are inexpensive.
Dr Kevin McCarroll is a consultant physician in geriatric medicine in St James’s Hospital, Dublin.