Role of foods of animal origin as dietary sources of vitamin D

Mounting evidence suggests lower vitamin D status is associated with increased risk of cardiovascular disease (CVD) and type 2 diabetes (T2D), which are the leading causes of morbidity and mortality in the world. Findings from the Caerphilly Prospective Study (CAPS) (n=452 men), showed a higher dietary vitamin D intake was associated with lower fasted plasma triacylglycerol concentration, an independent risk marker for CVD, after over 20 years follow-up. Over the past decade hypovitaminosis D of the general population has become a concern throughout the world due in part to limitations in the endogenous vitamin D synthesis from ultraviolet radiation, which has increased the importance of dietary vitamin D intake. There are only a few foods naturally rich in vitamin D, such as oily fish and egg yolk, however the vitamin D content and form can vary and oily fish is only regularly consumed by a small section of the UK population. To address this, a retail study was conducted to investigate the vitamin D content and form (vitamin D3 and 25-hydroxyvitamin D3 (25(OH) D3)) in eggs from different production systems (indoor, organic and free-range) and supermarkets (n=3) between July to November of 2012. Vitamin D3 was significant higher in free range and organic, compared with indoor eggs, while 25(OH) D3 was only higher in organic eggs. Total vitamin D content (vitamin D3 and 25(OH) D3) of each egg was approximately 2 µg, which would contribute 20% of vitamin D recommended dose of 10 µg/day. However, there is debate over the possible detrimental effect on human health of the relatively high cholesterol content of eggs. Further findings from CAPS demonstrated that higher egg consumption was not associated with incident of CVD, T2D or all-cause mortality, but a higher egg consumption (up to 1 egg per day) was associated with a higher risk of stroke and elevated fasting glucose in subjects with impaired glucose tolerance (fasting glucose ≥6.1 mmol/L) and/or T2D. Due to the relatively low natural enrichment of vitamin D in foods, fortification has become a recognised strategy to increase dietary vitamin D intake. Milk is used successfully as a vehicle for vitamin D fortification in a few countries, but there remains some uncertainty about the effects of milk consumption on risk of CVD. Thus, an updated dose-response metaanalysis which included all of the published prospective cohort studies up to May 2016 was conducted. It was found that milk was not associated with CVD or all-cause mortality, and suggested a beneficial role of fermented dairy or cheese by lowering the risk of CVD and mortality. Vitamin D fortified milk and dairy are not available in many countries such as the UK. Furthermore, limited evidence suggests that supplementation of 25(OH) D3 has a greater efficacy for improving vitamin D status, than vitamin D3. Thus, a further study was designed with the aim to increase the vitamin D content of milk by a food chain approach by feeding vitamin D (vitamin D3 or 25(OH) D3) supplements to dairy cows. This study showed feeding dairy cows with 25(OH) D3 either pre-calving or post-calving was more effective in raising plasma 25(OH) D3 concentration than vitamin D3 supplementation, but vitamin D concentration in the milk was not affected by treatments. The mean 25(OH) D3 concentration of the enriched milk was 0.88 µg/L. Thus, fortification was favoured as a strategy for increasing dietary vitamin D intake. A randomised, controlled, cross-over and double-blinded 24-hour acute intervention study was conducted in 17 men with sub-optimal vitamin D status (mean plasma 25(OH) D concentration was 31.7 ± 3.4 nmol/L) to compare the effects of 20 µg 25(OH) D3 with 20 µg vitamin D3 fortified dairy drink and a control dairy drink on vitamin D status (plasma 25(OH) D3) and CVD risk markers. Consumption of 25(OH) D3 fortified dairy drink was found to be more effective and faster at raising plasma 25(OH) D3 concentrations postprandially. In summary, vitamin D fortified foods are needed to address the high prevalence of low vitamin D status within population. Fortification using 25(OH) D3 would appear to have advantages over vitamin D3.