With vitamin D deficiency now firmly on the public health radar in Australia, health experts are now discussing the merits of supplementing the food supply with this ‘sunshine vitamin’. Prof Caryl Nowson of the School of Exercise and Nutrition Sciences delves deeper.
Vitamin D is important in maintaining calcium and phosphate balance in the body, which impacts bone health and muscle function. Severe vitamin D deficiency causes impaired bone mineralisation, resulting in rickets in children and osteomalacia in adults.
In older people, vitamin D deficiency is a predictor of falls, and low vitamin D status is associated with muscle weakness and impaired balance. In addition to these well-established effects, it appears that vitamin D status might impact on risk of a range of diseases such as cardiovascular disease, some types of cancers, brain disorders such as autism and schizophrenia, and reduce immune function. However, it must be acknowledged that more evidence is required to confirm the role of vitamin D deficiency in these conditions.
People were designed to maintain adequate vitamin D status through the regular exposure of bare skin to ultraviolet light. However, lifestyles are such that many people spend much of the day indoors and sensibly, to reduce the risk of developing skin cancer, they cover up and use sunscreen when they go outside. Consequently, many people do not get sufficient exposure to sunlight, particularly in the southern states of Australia and in New Zealand, which are further from the equator. Therefore a dietary source of vitamin D is needed to ensure adequate status for many people.
Very few foods contain significant amounts of vitamin D. Vitamin D is found in small quantities in a few foods such as fatty fish (such as North Sea salmon, herring, mackerel). It is important to note that farmed salmon contains less than half the amount of vitamin D compared to wild salmon. Liver, eggs and fortified foods such as margarine and low fat milk (if fortified) also contain very small amounts of vitamin D and some UV-exposed mushrooms contain vitamin D, but for most people dietary vitamin D is limited. Most adults are unlikely to obtain more than 5-10% of their vitamin D requirement from dietary sources.
In 1975 Canada mandated that that all milk sold in the country must be fortified with vitamin D to prevent the development of frank vitamin D deficiency. In addition to this, vitamin D is routinely added to a number of breakfast cereals and orange juice in both Canada and the United States. This has resulted in the dietary intake from foods alone in Canada being 2-3 times higher than Australian adults. This has been effective in keeping the incidence of rickets low, despite the lack of sunlight that much of the population experience during the extended winter months. Australia needs to adopt a similar vitamin D food fortification strategy.
On a population wide basis, it has been estimated that up to 10.6% of the adult population (approximately 1.9 million adult Australians) may have serum vitamin D levels that place them in the frank deficiency range in winter. It is clear that action is needed now. Mandatory fortification of all milks and probably yoghurt with vitamin D to a level comparable to that of Canada would seem to be the practical approach to reduce the risk of frank vitamin D deficiency.
Should Australia and New Zealand allow more vitamin D into the food supply was the topic of a symposium held last year in Melbourne, to raise the awareness of vitamin D deficiency and highlight the need for the government to urgently consider allowing more vitamin D into the food supply. Prompted by Professor Caryl Nowson, and organised by the Nutrition Committee of the Australian Academy of Sciences and International Life Sciences Institute Australasia, the symposium included presentations by key Australian experts, representation from Food Standards Australia and New Zealand, a video link presentation from a Canadian expert, and a panel discussion.
At the conclusion of they symposium, there was general support for increased vitamin D fortification of the food supply, although no clear consensus on whether fortification should be mandatory or voluntary or what food products to fortify, although there was support for milk products.
Professor Caryl Nowson
School of Exercise and Nutrition Sciences
- Prevalence of vitamin D deficiency and its determinants in Australian adults aged 25 years and older: a national, population-based study