Is it time to routinely fortify food or drink with vitamin D in the <scp>UK</scp>?

In its 2016 Vitamin D and Health report, the Scientific Advisory Committee on Nutrition (SACN) concluded that the risk of poor musculoskeletal health was increased at blood 25-hydroxy vitamin D (25[OH]D) concentrations below 25 nmol/L. 25(OH)D is the main blood indicator of total exposure to vitamin D, reflective of UVB exposure from sunlight, dietary intake and biological reserves. Using data emerging from dose–response studies in different age groups and randomised controlled trials (RCTs), SACN set the reference nutrient intake (RNI) for vitamin D at 10 μg (400 IU) per day for the UK population aged 4 years and over (SACN, 2016). This is the average amount needed by most people (97.5%) to maintain a blood 25(OH)D concentration of 25 nmol/L or above when UVB sunlight exposure is minimal. A safe intake of 8.5–10 μg/day was set for younger children. It has been recognised for some time that UK dietary intakes of vitamin D are below recommendations and, as discussed in a 2023 editorial, mandatory fortification of margarine with vitamin D was introduced in 1940, though withdrawn in 2013 (Buttriss & Lanham-New, 2022). Government advice on taking vitamin D supplements (including cod liver oil) directed specifically at groups considered to be at risk (e.g. young children, housebound elderly people) has also been in place for many decades. But advice changed in 2016, following SACN's advice to the government to consider strategies for the entire UK population to achieve the recommended intakes of vitamin D, recognising that it is difficult to achieve the RNI from natural food sources alone (see Box 1). Vitamin D is produced in the skin when it is exposed to sunlight containing ultraviolet B (UVB) radiation. This is the main source of vitamin D for most people. It can also be obtained from food sources and vitamin D supplements. Dietary sources are essential when skin exposure to sunlight containing UVB radiation is limited. From late March and early April (depending on latitude) to the end of September, most people should be able to synthesise all the vitamin D they need from skin exposure to sunlight. But during October to late March/early April, when skin production is minimal, everyone is advised to consider taking a daily vitamin D supplement, since it is difficult to meet the recommendation of 10μg (400 IU) per day in the UK from consuming foods containing vitamin D. During the UK autumn and winter, sunlight exposure is not effective for vitamin D synthesis and serum 25(OH)D concentrations are typically at their lowest. A daily supplement is advised throughout the year for population groups at high risk of having blood 25(OH)D concentrations below 25 nmol/L (e.g. dark-skinned population groups, people with minimal sunshine exposure due to not spending time outdoors and population groups who habitually cover almost all their skin when outdoors). Daily supplements are also advised throughout the year for all children aged 1 to 4 years, and all babies (unless they are having more than 500ml of infant formula a day, which is already fortified by law with the vitamin at the level of 1–2.5 μg/100 kcal). The proportion of the UK population with poor vitamin D status has not decreased according to the National Diet and Nutrition Survey (NDNS) (PHE and FSA, 2020), indicating that government advice on supplementation has so far had limited impact. This suggests that other strategies may be necessary to help ensure the UK population achieves the recommended intakes of vitamin D. In spring 2022, the Department of Health and Social Care (DHSC) launched a review to promote the importance of vitamin D and identify ways to improve vitamin D intake across the population. This included the potential option of fortifying foods and drinks with vitamin D. As part of this review, DHSC asked SACN to provide scientific advice on the potential of mandatory vitamin D fortification for the UK population to meet the dietary recommendations for vitamin D. The terms of reference are in Box 2. The work programme is being undertaken using a phased approach. SACN's review published in May 2024 considers points 1(a) and 1(b) of the terms of reference (SACN, 2024), and it states that other points in part 1 of the terms of reference will be progressed when required by DHSC. Thus, it is unclear whether the work programme will progress, particularly in light of the change in government in July 2024. Consider the potential impact of mandatory fortification of foods with vitamin D, to include: In relation to point 2 of the terms of reference, it is noteworthy that SACN agreed (at their horizon scan meeting in June 2022) to review the evidence on vitamin D requirements of dark-skinned population groups. The RNI for vitamin D, set by SACN in 2016, was based on evidence from predominantly white-skinned groups. Data at that time were insufficient to consider whether requirements differed for dark-skinned population groups but, since then, a number of relevant studies have been published (e.g. Cashman et al., 2022; Darling, 2020; Darling et al., 2018). A decision on when to progress this aspect of the work will be made at a future date. Current UK government advice on vitamin D (Box 1) relates specifically to the protection of musculoskeletal health. Lack of vitamin D can lead to bone deformities such as rickets in children and bone pain caused by a condition called osteomalacia in adults. However, vitamin D has been associated with a wide range of health outcomes although controversies remain for the majority of these relationships, including whether causal pathways exist (SACN, 2016). It is noteworthy that there are vitamin D receptors in many tissues and organs, and since SACN's 2016 review, research evidence concerning non-skeletal roles of vitamin D has been building, as was discussed at a workshop funded by the Medical Research Council (MRC) (see Lanham-New et al., 2022). Regulation of immune function is the most recognised extra-skeletal action of vitamin D from a mechanistic perspective (Bishop et al., 2020). The role of nutrition in immune health was catapulted into the spotlight during the coronavirus pandemic (Lanham-New et al., 2020), and a systematic review and meta-analysis of data from RCTs suggest that vitamin D supplements reduce the risk overall of acute respiratory infections, though the effect was small (Jolliffe et al., 2021). Subgroup analysis found that benefit was associated with daily doses of 10–25 μg compared to placebo. However, a review by SACN concluded that the evidence on vitamin D supplementation and respiratory infection risk was inconsistent and generally did not show a beneficial effect of vitamin D supplementation on infectious disease risk (SACN, 2020). The MRC-funded workshop concluded that large-scale randomised controlled trials are required to investigate whether maintaining vitamin D adequacy reduces the incidence and/or severity of infections and autoimmune diseases and, if so, to establish the vitamin D requirements and plasma 25(OH)D levels required for optimal immune function (Lanham-New et al., 2022). A major focus of the 2024 review from SACN is the relative efficacy of the two major forms of vitamin D: vitamin D3, which is produced in the skin and can also be obtained from the diet; and vitamin D2, which can be obtained only from the diet (SACN, 2024). There are relatively few naturally rich sources of vitamin D in the UK diet. It is present as D3 in foods of animal origin, such as oily fish (5–16 μg/1000 g), egg yolks (12.6 μg/100 g) and meat (0.1–1.5 μg/100 g). Voluntarily fortified foods, such as breakfast cereals and spreading fats, typically contain D3, although D2 is also sometimes used. Vitamin D2 can be obtained from mushrooms and yeast if these have been exposed to UVB light but is typically present in low amounts in people's diets. Data from the NDNS show that mean vitamin D intakes from dietary sources have remained below the RNI (10 μg or 400 IU/day) in all age groups (SACN, 2024). Despite revised government advice on vitamin D supplementation issued in 2016, the proportion of adults who reported taking vitamin D supplements was 17% for ages 19 to 64 years; 34% for ages 65 to 74 years; and 28% for ages 75 years and over. The low uptake of vitamin D supplements in the UK suggests that recommendations for vitamin D supplement intake are either not reaching the UK population or are not being acted upon. According to the NDNS, the proportions in each age group with plasma 25(OH)D concentrations below 25 nmol/L (taking account of seasonal variation) were 2% of children aged 4 to 10 years; 19% of children aged 11 to 18 years; 16% of adults aged 19 to 64 years; and 13% of adults aged 65 years and over (PHE and FSA, 2020). The proportions with plasma 25(OH)D concentrations below 25 nmol/L were higher during January to March: 19% of children aged 4 to 10 years; 37% of children aged 11 to 18 years; and 29% of adults. It is timely that SACN has proposed a review of the evidence on vitamin D requirements of dark-skinned population groups. A secondary analysis of UK Biobank data suggests that low vitamin D status is almost universal in the UK South Asian population (Darling, 2020; Darling et al., 2018): 92% <50 nmol/L; 55% <25 nmol/L; 20% <15 nmol/L. The authors comment that the UK Biobank cohort are likely to be relatively health conscious, so vitamin D status in the general South Asian population may be even lower. Fortification appears to help. A 12-week RCT in Denmark among women of Pakistani origin reported that vitamin D-fortified foods, together providing an intake of 20 μg/d, reduced the prevalence of 25(OH)D < 30 nmol/L from 34% to 3% (Grønborg et al., 2019). Historically, it has been assumed that dietary requirements do not differ between ethnic groups. However, it has recently been suggested that intakes of 24 μg/day might be necessary to maintain status above 25 nmol/L among dark-skinned population groups (Cashman et al., 2022) (i.e. more than twice the RNI of 10 μg/day established by SACN using data from predominantly white population groups). Cashman et al used data from RCTs with vitamin D supplements/fortified foods to undertake a meta-regression using individual participant data for dark-skinned children and adults residing at a latitude >40 degrees north. This approach using individual participant data avoids some of the limitations intrinsic to standard meta-regression based on aggregate data. If this higher requirement is confirmed, it will carry implications for modelling studies and for policy decisions regarding fortification or supplementation. Cashman et al. concluded that much more work is needed in dark-skinned populations, both in terms of the dose–response relationship and the risk characterisation for health outcomes. Following introduction of a vitamin D policy in the 1940s, the vitamin D deficiency disease, rickets, was more-or-less eradicated in the UK in the 1950s (see Buttriss & Lanham-New, 2022) but then began to re-emerge in some ethnic minority groups as the 20th century progressed, and the condition continues to be diagnosed in the 21st century, as elaborated by Dr Suma Uday at a workshop funded by RANK (seeButtriss et al., 2022). Currently, in the UK, there is no mandatory provision for vitamin D fortification except for fortification of infant formulas. In the UK, vitamin D fortification of food is voluntary for food producers. Foods commonly fortified with vitamin D include fat spreads (such as margarines and fat spreads made from plant oils) and breakfast cereals. The EU-funded ODIN project has explored fortification options for Europe (see Buttriss et al., 2022 for a summary). Randomised controlled trials (RCTs) conducted during the winter months demonstrated that several fortified foods were effective at preventing serum 25(OH)D concentrations falling below 30 nmol/L in the treatment group. These fortified foods included UV-exposed mushrooms (providing around 100 μg D2/100g fresh weight) (Cashman et al., 2016), eggs biofortified with vitamin D3 and 25(OH)D3 (4–5 μg per egg) (Hayes et al., 2016) and fortified low-fat Gouda cheese (about 6 μg D3/d) (Manios et al., 2017). Fortification of bread with vitamin D3 has also been shown to be effective in enhancing vitamin D status (Natri et al., 2006). Overall, fortification of foods with vitamin D was deemed to be technologically feasible, and sensory data indicated a good level of consumer acceptability. Dietary modelling by the ODIN consortium (Cashman et al., 2015) has explored the potential of biofortified and fortified foods to raise total vitamin D intakes. A combination of foods biofortified with vitamin D (beef, pork and eggs) or fortified with the vitamin (milk and cheese), when substituted into the modelled diet, increased vitamin D content from 3.3 μg/d to 8.0 μg/d, thus almost achieving the target intake of 10 μg/d. An integrated predictive model, which accounted for changes in UVB availability during the year, suggested that the prevalence of a 25(OH)D concentration <30 nmol/L during winter in Irish adults (18.1%) could be reduced to 6.6% by the stepwise introduction of an increasing number of fortified foods (Cashman et al., 2015). In the ODIN modelling, consideration was also given to whether consumption of fortified foods could increase the risk of exceeding the upper intake level for vitamin D in adults of 100 μg/d (4000 IU/d) (see Box 3). Consumption of a combination of fortified and biofortified foods did not pose a risk at the 99th percentile of the distribution (Cashman et al., 2015). Consuming too much vitamin D can lead to hypercalcaemia (above normal concentrations of calcium in the blood), which can weaken bones and damage the kidneys and heart. The recommended upper levels per day for vitamin D, set by the European Food Safety Authority based on its scientific opinion on the tolerable upper intake level for vitamin D (and endorsed by the UK Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment), are as follows: 100 μg (4000 IU) for ages 11 years and over; 50 μg (2000 IU) for ages 1 to 10 years; 35 μg (1400 IU) for ages 7 to 11 months; and 25 μg for ages 0 to 6 months. Both vitamin D2 and vitamin D3 increase blood 25(OH)D concentrations, and both forms prevent and treat vitamin D deficiency. However, there is ongoing debate about their relative efficacy, since some studies have reported lower increases in blood 25(OH)D concentrations following consumption of vitamin D2 compared with vitamin D3. SACN's 2024 review focuses on a systematic review and meta-analysis that included data from 21 studies of adults from 12 countries (Balachandar et al., 2021). SACN found evidence that vitamin D3 is slightly more effective than vitamin D2 for increasing blood 25(OH)D concentrations. However, due to the limitations in study design identified, the difference in estimated effect between the two forms may not be reliable (SACN, 2024). There was greater consistency in studies that administered vitamin D daily at doses between 5 and 25 μg (200 and 1000 IU), which are more representative of the vitamin D intakes that are likely to be achieved through food fortification. SACN concluded that, taken together, these studies comparing the efficacy of D2 versus D3 indicate an advantage of vitamin D3 over D2 in raising blood 25(OH)D concentration of about 8 nmol/L. Although 25(OH)D concentrations were generally higher in the vitamin D3-supplemented groups, these studies confirm that both vitamins D2 and D3 are effective in raising blood 25(OH)D concentrations. SACN has reviewed the experiences of countries with existing vitamin D fortification programmes, and their impact on vitamin D intakes and blood levels of vitamin D (SACN, 2024). Mandatory fortification policies were identified in Australia, Canada and Sweden. Voluntary policies exist in Finland, Norway and the United States. Policies were also identified in Belgium, Chile, Ethiopia and Pakistan, but information about these four was limited. The most common foods or drinks fortified with vitamin D were milk, breakfast cereals, yogurts, fat spreads and edible oils. Vitamin D fortification levels in the different products varied between countries. For example, fortification levels in margarine and fat spreads range between 5.5 μg (220 IU) per 100 g (Australia) and 19.5 to 21 μg (780 to 840 IU) per 100 g fortification levels in range from μg IU) per 100 g to μg IU) per 100 g from countries that have data on intakes of vitamin D-fortified foods and blood 25(OH)D concentrations suggests that have both higher intakes and status compared with of fortified has the impact of its vitamin D fortification policy on vitamin D intakes and vitamin D status of the population. 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