Dr Kirsty Pourshahidi is a Research and Teaching Fellow within the Nutrition Innovation Centre for Food and Health (NICHE) at Ulster University, where she specialises in vitamin D, agri-food nutrition and consumer innovation. Her research spans all areas from farm-to-fork, and here she provides an overview of the importance of vitamin D and shares some of her recent dairy research.

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Many will be aware of vitamin D, or the ‘sunshine vitamin’, which has hit global headlines recently as the COVID-19 pandemic sadly maintains its presence in our daily lives.  Now firmly in the depths of the ‘vitamin D winter’ in the UK and Ireland, we should all be routinely taking our daily vitamin D supplement (10 micrograms or 400IU), but this advice is more important than ever this year with more people spending more time indoors during repeated lockdowns, shielding or cocooning2,3.  Indeed, for the first time in most of our lifetimes, we have seen the implementation of a national supplementation programme supported by Government where the most at-risk individuals in our society (those on the clinically extremely vulnerable list) will be provided with free vitamin D pills.  But why the fuss?

Functions and sources of vitamin D

Notwithstanding the role of vitamin D in supporting the normal function of the immune system, highlighted by the ongoing pandemic, vitamin D is also required for the absorption and regulation of calcium and phosphate in the body, and it is therefore essential for musculoskeletal health (i.e. bone health, muscle strength and function)4,5.  Without enough vitamin D, bones can become thin, soft and/or brittle which can lead to rickets in children or osteomalacia/osteoporosis in adults, as well as an increased risk of falls.  Data from our centre also suggests that an adequate vitamin D status may be required for optimal immune function in older adults6 and for better quality of life in respiratory patients7.

Vitamin D comes in two main forms; vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Research suggests that vitamin D3 is more effective at increasing vitamin D status in the body however, if you follow a vegan diet, then vitamin D2 is advised as it is not made from animal-based products8. 

We can obtain the vitamin D we need in three ways: from the sun, dietary sources and food supplements.  Despite its name, vitamin D is actually a pro-hormone as it can be synthesised and stored by humans, animals and fungi.  This endogenous synthesis is our body’s main and preferred source of vitamin D3, which is synthesised in the skin upon exposure to sunlight containing sufficient ultraviolet B (UVB) radiation (i.e. only in the summer months in the UK, approx. April-September).

You can only make vitamin D in the skin when your shadow is shorter than your height!

Natural food sources of vitamin D3 include egg yolk, oily fish, meat and offal, with mushrooms being the main natural source of vitamin D2, but these dietary sources contain relatively low amounts of vitamin D and/or are consumed relatively infrequently.  Our dietary intake of vitamin D can be boosted by fortified food products that are increasingly available within the marketplace, such as ready-to-eat breakfast cereals, margarines and some milks/dairy products, such as yoghurts9.  Except for infant formula, there is currently no mandatory fortification policy for vitamin D in the UK/Ireland so practice can vary considerably across the industry.

Historically, the limited food sources were not a major public health concern as it was assumed that we could make enough vitamin D from summer sunlight to build up our reserves to last over the ‘vitamin D winter’ (October-March), but we now know this isn’t the case.  Furthermore, it is difficult to quantify how much sun exposure is needed for your daily vitamin D boost as it is affected by many inter- and intra-individual factors, e.g. where you live (latitude), your occupation (indoor/outdoor), skin colour, age, body size/shape and the use of sunscreens.  Recently revised dietary requirements for vitamin D now assume minimal sunlight exposure and therefore, dietary sources (whether food or supplemental) are essential.

Current dietary requirements

In order to protect musculoskeletal health, the UK’s Scientific Advisory Committee on Nutrition (SACN) has recently recommended a Reference Nutrient Intake (RNI) for vitamin D of 10µg (400 IU) per day10, all year round, for everyone in the general population aged 4 years and above.  This includes pregnant and lactating women, as well as population groups at increased risk of vitamin D deficiency (such as ethnic minority groups with darker skin, individuals in care homes and those who wear clothing that cover most the skin).

Owing to the difficulties in recommending a safe sun exposure and in getting enough vitamin D from food sources only, it is recommended that everyone over the age of 5 years should consider taking a supplement from October-March (or aligning with key holidays, from Halloween to St Patrick’s Day).  Children (aged 1-4 years), ethnic minority groups with dark skin and all those with no or very little sunshine exposure should continue supplementation throughout the year.  In the Republic of Ireland, similar guidance applies for children and adults, but following a recent review of the evidence (December 2020) the recommendation for older adults (≥65 years) was increased to 15-20µg11.

Measuring vitamin D status

Once consumed or synthesised, vitamin D is transported in circulation bound to vitamin D binding protein, and metabolised (by hydroxylation) in the liver to the storage form: 25-hydroxyvitamin D [25(OH)D].  This primary circulating form of vitamin D is the gold standard for quantifying vitamin D status (Figure 1) as it reflects the availability of vitamin D in the body from both cutaneous synthesis and the diet.

≤25 nmol/l


26-49 nmol/l


≥50 nmol/l


Figure 1. Classification of vitamin D status based on cut-offs for circulating 25(OH)D concentrations10


Data from the most recent UK National Diet and Nutrition Survey (NDNS)12 reports vitamin D deficiency in approx. 10-20% of the population.  Moreover, there has been little change in mean, year-round vitamin D status over the last decade12 and a plethora of evidence now demonstrates that significant proportions of the population have levels of vitamin D that are insufficient for the maintenance of optimal musculoskeletal and immune health.  Research conducted by NICHE has typically shown that even at the end of summer, when blood levels should be at their highest, as many as 50% of adults have an insufficient 25(OH)D concentration, and even more worryingly, that’s before levels naturally start to drop over the winter.

Opportunities for the food industry

From the evidence above, it is clear that alternative food-based strategies are required to help the population meet the revised dietary recommendations for vitamin D.  There is increasing evidence to support the role of vitamin D food fortification in maintaining or improving the vitamin D status of the consumer13, and indeed, both mandatory policies and voluntary fortification practices are in place for a range of staple food products across the world14, including those that cover various fluid milk and other dairy products.  Mandatory fortification of course will offer the greatest benefits to public health, having a bigger impact at the population level.

Although milk and dairy products are sources of naturally occurring vitamin D15,16, without fortification, the vitamin D content of milk is minimal.  ‘The Composition of Foods’ series by McCance and Widdowson (2014)17 provides extensive nutritional data on a number of foods.  Although these tables have been updated several times since their inception, the most recent 7th edition, has reported the lowest vitamin D content for whole, semi-skimmed and skimmed milk (i.e. as trace) compared to earlier editions.  These data, however, are based on limited sampling and owing to advance in laboratory technology, figures may be out-dated. 

One possible strategy to increase vitamin D intakes within the population is through the fortification of fluid milk as a commodity product, which is a staple dietary component for a large proportion of the UK and Irish populations.  Some of my recent research has focused in this area, conducted in collaboration with the Dairy Council for Northern Ireland.

Recent dairy research

Aiming to overcome some of the research gaps noted above, we set out firstly to determine the concentration of vitamin D3 in cows’ milk produced in Northern Ireland.  Secondly, we aimed to simulate (or predict) how the fortification of cows’ milk could theoretically improve overall dietary vitamin D intakes of the UK population using a dietary modelling scenario (Figure 2).   Samples of milk were collected from two processors for 1-year and vitamin D3 concentrations analysed using high performance liquid chromatography (HPLC) at the Agri-Food & Biosciences Institute (AFBI).  Milk consumption data were obtained from Years 1–4 of the NDNS and a theoretical model applied vitamin D3 fortifications of 1, 1.5 and 2 µg vitamin D/100g to simulate improvements in vitamin D intakes.  Consumption of whole, semi-skimmed, skimmed and 1% milks were included in the analysis.

Recently published in the International Journal of Food Sciences and Nutrition1, our research reported an average (±SD), year-round vitamin D3 in raw and pasteurised whole milk of 0.08 (±0.04) and 0.06 (±0.02) µg/100g, respectively.  Although vitamin D3 levels in milk appeared to vary across the year, no significant seasonal differences were noted.  Nonetheless, the higher vitamin D3 measured in these milk samples than that currently in food composition tables, confirms further analysis is warranted.

Figure 2. Vitamin D3 content of cows’ milk produced in Northern Ireland and its efficacy as a vehicle for vitamin D fortification: a UK model (Weir et al. 2020)1


Daily milk intakes (portion size per eating occasion) ranged from 2.5g to 2850g, reflecting the wide and varied consumption patterns of this staple food product.  Although on average, whole milk was consumed in the largest typical portion size, semi-skimmed milk was the most commonly consumed milk, reported in just over half (53%) of the recording days in the total study population (n=4,156 children and adults, aged 1.5–94 years).  A significantly higher proportion of children were consumers of whole milk compared to adults and the opposite was true for the other three milk types (semi-skimmed, skimmed and 1% milks).  

Results from the dietary modelling exercise were particularly encouraging.  Overall, fortification of cows’ milks with 1, 1.5 and 2 µg vitamin D/100g, theoretically increased median vitamin D intakes from 2.0 µg/day to 4.2, 5.1 and 5.9 µg/day, respectively.  At the highest level (2 µg /100g), the proportion of the population meeting the new dietary recommendations would increase to just over 12% (from <1%) and the greatest impact was apparent in children.  Importantly, this strategy could translate into a beneficial effect on consumer’s vitamin D status, without putting anyone at risk of exceeding the tolerable upper limit (UL) for the vitamin, set by the European Food Safety Authority (EFSA)18.  Based on the results from this dietary modelling scenario, fortification of all types of milk (whole, semi-skimmed, skimmed and 1% milks) is recommended to maximise the impact to consumers of all ages and make progress towards eradicating vitamin D deficiency among the UK population.


Together with our data, other emerging research on the updated vitamin D content of many animal-derived foods, as well as results from novel biofortification studies are promising, and this is an area of research that I am continuing in collaboration with AFBI and other industry partners.  Novel biofortification (or bio-enrichment) techniques applied on farm have allowed u to improve the total vitamin D content of milk and other animal foods, and publications on this will hopefully be available soon!  

At the time of writing this article, whilst we don’t yet have sufficient data to know that vitamin D will help in the fight against COVID-193, one thing nutrition and health professionals do agree on is that we all must avoid vitamin D deficiency and insufficiency for optimal musculoskeletal and immune health. It is likely that a combination of fortification and biofortification strategies will be required to help consumers meet the revised requirements and tackle the problem of vitamin D deficiency at the population level, but this is a challenge that I am sure the dairy industry as a whole can help to overcome.


To keep up to date with the vitamin D research being conducted at NICHE, follow them on Twitter (@NICHE_ULSTER), Instagram (@niche_ulster), Facebook (@UlsterUniversityNICHE) & LinkedIn (NICHE Ulster) using #NICHEvitd.



  1. Weir RR, Johnston M, Lowis C, et al. 2020. Vitamin D3 content of cows' milk produced in Northern Ireland and its efficacy as a vehicle for vitamin D fortification: a UK model. Int J Food Sci Nutr. doi:10.1080/09637486.2020.1837743.
  2. Lanham-New SA, Webb AR, Cashman KD, et al. 2020. Vitamin D and SARS-CoV-2 virus/COVID-19 disease. BMJ Nutr Prev Health, 13;3(1):106-110.
  3. National Institute for Health and Care Excellence (NICE). 2020. COVID-19 rapid guideline: vitamin D. NICE guideline [NG187]. Available at:
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  5. Martens PJ, Gysemans C, Verstuyf A, Mathieu AC. 2020. Vitamin D’s Effect on Immune Function. Nutrients. 12(5):1248.
  6. Laird E, McNulty H, Ward M, et al. 2014. Vitamin D deficiency is associated with inflammation in older Irish adults. J Clin Endocrinol Metab. 99(5):1807-1815.
  7. Carson EL, Pourshahidi LK, Madigan SM, et al. 2018. Vitamin D status is associated with muscle strength and quality of life in patients with COPD: a seasonal prospective observation study. Int J Chron Obstruct Pulmon Dis. 13:2613-2622.
  8. Tripkovic L, Lambert H, Hart K, et al. 2012. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 95(6):1357-1364.
  9. British Dietetics Association (BDA). 2019. Vitamin D: Food Fact Sheet. Available at: 
  10. Scientific Advisory Committee on Nutrition (SACN). 2016. SACN vitamin D and health report. Available at:
  11. Food Safety Authority of Ireland (FSAI). 2020. Scientific Recommendations for Food-Based Dietary Guidelines for Older Adults in Ireland older adults. Available at:
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  13. Jääskeläinen T, Itkonen ST, Lundqvist A, et al. 2017. The positive impact of general vitamin D food fortification policy on vitamin D status in a representative adult Finnish population: evidence from an 11-Y follow-up based on standardized 25-hydroxyvitamin D data. Am J Clin Nutr. 105:1512–20.
  14. Itkonen ST, Erkkola M, Lamberg-Allardt CJE. 2018. Vitamin D Fortification of Fluid Milk Products and Their Contribution to Vitamin D Intake and Vitamin D Status in Observational Studies-A Review. Nutrients. 10(8):1054.
  15. McCance RA, Widdowson E. 2002. McCance and Widdowson’s the composition of foods. 6th ed. Cambridge: Royal Society of Chemistry.
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  18. European Food Safety Authority (EFSA) Panel on Dietetic Products, Nutrition and Allergies (NDA). 2012. Scientific opinion on the tolerable upper intake level of vitamin D. EFSA J. 10(7):2813.


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