25(OH)D3-enriched or fortified foods are more efficient at tackling inadequate vitamin D status than vitamin D3

The ability to synthesise sufficient vitamin D through sunlight in human subjects can be limited. Thus, diet has become an important contributor to vitamin D intake and status; however, there are only a few foods (e.g. egg yolk, oily fish) naturally rich in vitamin D. Therefore, vitamin D-enriched foods via supplementing the animals’ diet with vitamin D or vitamin D fortification of foods have been proposed as strategies to increase vitamin D intake. Evidence that cholecalciferol (vitamin D3) and calcifediol (25(OH)D3) content of eggs, fish and milk increased in response to vitamin D3 supplementation of hens, fish or cows’ diets was identified when vitamin D-enrichment studies were reviewed. However, evidence from supplementation studies with hens showed only dietary 25(OH)D3, not vitamin D3 supplementation, resulted in a pronounced increase of 25(OH)D3 in the eggs. Furthermore, evidence from randomised controlled trials indicated that a 25(OH)D3 oral supplement could be absorbed faster and more efficiently raise serum 25(OH)D concentration compared with vitamin D3 supplementation. Moreover, evidence showed the relative effectiveness of increasing vitamin D status using 25(OH)D3 varied between 3·13 and 7·14 times that of vitamin D3, probably due to the different characteristics of the investigated subjects or study design. Therefore, vitamin D-enrichment or fortified foods using 25(OH)D3 would appear to have advantages over vitamin D3. Further well-controlled studies are needed to assess the effects of 25(OH)D3 enriched or fortified foods in the general population and clinical patients.

Vitamin D is usually synthesised in skin that is exposed to UV radiation, which has led to the term 'sunshine vitamin' (1) . Traditionally, the primary role of vitamin D is related to calcium absorption and bone health. Children and adults with vitamin D deficiency have an increased risk of developing rickets or osteomalacia (2) . Recently, a resurgence of childhood rickets has highlighted the need for adequate vitamin D status in many parts of the world (3)(4)(5) . Furthermore, mounting evidence from epidemiological studies indicates that vitamin D status is inversely associated with the risk of CVD, cancers and diabetes (1,6) , although there is some uncertainty about what defines an adequate vitamin D status (7) .
Vitamin D deficiency is prevalent and is considered a serious issue throughout the world (8)(9)(10) , even in sunnier climates such as Australia and New Zealand (11) .
Recently, the Scientific Advisory Committee on Nutrition (7) reported that in the UK, 22-24 % of adults aged 19-64 years, and 17-24 % of those ≥65 years were vitamin D deficient (plasma 25-hydroxyvitamin D 3 (25(OH)D 3 ) <25 nmol/l). There are several factors that have contributed to the low vitamin D status commonly seen today, such as lifestyle changes (increased indoor lifestyle, sun screens use) and human characteristics (e.g. ageing, clothing, increased obesity, low-fat diet trend) (12) . Therefore, foods that contribute to vitamin D intake have become more important than before. However, there are only a few foods naturally rich in vitamin D, such as oily fish and egg yolks (13) .
The aim of this review is first to critically evaluate the existing evidence on whether the vitamin D content of animal-derived foods can be increased by feeding cholecalciferol (vitamin D 3 ) and/or calcifediol (25(OH)D 3 ) supplements to laying hens, fish and cows. Second, the present review summaries evidence from the human randomised controlled trials (RCT), which include the effects of 25(OH)D 3 supplementation on increasing serum/plasma 25(OH)D 3 concentration.

Vitamin D absorption, synthesis and metabolism
Generally, the term vitamin D refers to both vitamin D 2 and vitamin D 3 . Vitamin D 2 is produced by fungi, while vitamin D 3 is produced by human subjects and animals (14) . Human subjects usually synthesise vitamin D 3 in the skin (15) where 7-dehydrocholesterol in the epidermis is converted to pre-vitamin D 3 when skin is exposed to sunlight. Then, pre-vitamin D 3 undergoes a temperaturedependent isomerisation to vitamin D 3 over a period of approximately 3 d (6) . Vitamin D (vitamin D 2 or vitamin D 3 ) can also be obtained from the diet (15) and it is absorbed with long-chain TAG in the small intestine (16) . It is then incorporated into chylomicrons and transported in lymph to the blood and into the general circulation (17) .
After entering the circulation, there are two hydroxylation reactions to convert vitamin D to the biologically active form (6) . The first hydroxylation reaction is in the liver where vitamin D is hydroxylated to 25(OH)D by the vitamin D-25-hydroxylase enzyme. The second hydroxylation reaction is in the kidney where 25(OH)D is converted to 1,25(OH) 2 D by 25-hydroxyvitamin D-1α-hydroxylase (6) , and the 1,25(OH) 2 D metabolite is the biologically active form of vitamin D (18) .

Foods of animal origin as dietary sources of vitamin D
Vitamin D content of vitamin D-enriched foods can differ considerably between food retailers. One US retail study analysed the vitamin D content of egg yolks collected from twelve individual retail supermarkets across the country and reported a broad range of vitamin D 3 and 25(OH)D 3 concentrations of 9·7-18 and 4·3-13·2 µg/kg, respectively (19) . In addition, our recent UK retail study (20) showed vitamin D 3 and 25(OH)D 3 concentrations of eggs were significantly different depending on the egg production systems. Egg yolks produced by birds kept in indoor systems had much lower concentrations (40·2 (SE 3·1) µg/kg) of vitamin D 3 than the egg yolks produced from outdoor systems (57·2 (SE 3·2) µg/kg), while 25(OH)D 3 concentrations of the eggs were higher in organic eggs only. Similarly, the vitamin D contents of fish have been shown to vary according to the production systems. The study of Lu et al. (21) indicated the vitamin D 3 content of wild salmon to be three times higher than that of farmed salmon; however, the 25 (OH)D 3 content of the salmon was not measured. In addition, other studies (22,23) have shown the 25(OH)D 3 content of several species of marine and freshwater fish to be <0·02 µg/100 g. Therefore, foods generally regarded as rich sources of vitamin D may not be sustainable vitamin D contributors for the general population, due to variability in vitamin D content, which in turn may be influenced by production systems or different species (genotype). Furthermore, the National Diet and Nutrition Survey of the UK (24) reported that the average daily intake of vitamin D for adults was 3·1 µg for men and 2·6 µg for women, which is much lower than the UK vitamin D reference nutrition intake of 10 µg/d (7) . Therefore, vitamin D-enriched or fortified foods are needed to ensure an adequate vitamin D intake for the general population.

Vitamin D-enriched eggs
In general, there are two main methods to enrich the vitamin D content of eggs: increased sunlight exposure and vitamin D supplementation of the birds' diet. Because hens can synthesise vitamin D from natural sunlight exposure, free-range egg production system may be an inexpensive way to increase their vitamin D content. A study by Kuhn et al. assigned laying hens to a free-range treatment or an indoor treatment for over 4 weeks and found that eggs from the free-range group, which were exposed to sunlight, had significantly higher vitamin D 3 content (mean 14·3 µg/100 g DM) than eggs from the indoor group (mean 3·8 µg/100 g DM) (25) . Furthermore, there are several studies which have shown that the vitamin D 3 content of eggs can be enhanced by feeding vitamin D 3 supplements to the hens (Table 1) (26)(27)(28)(29)(30)(31)(32) . The results of all studies revealed that egg yolk vitamin D 3 concentration was efficiently increased by vitamin D 3 dietary supplementation. The study of Yao et al. showed a linear dose-response relationship existed between vitamin D 3 dietary supplementation and vitamin D 3 concentrations of egg yolks (30) . Moreover, as 25(OH)D 3 is a metabolite of vitamin D 3 , the 25(OH)D 3 content in eggs can also be enhanced by supplementing the birds' diet with vitamin D 3. However, the response in 25(OH)D 3 content of egg yolk is much less than that of vitamin D 3. Browning and Cowieson (31) showed that a 4-fold increase in vitamin D 3 , and a 2-fold increase in 25(OH)D 3 in egg yolk resulted from a 4-fold increase in the vitamin D 3 25(OH)D 3 -fortified foods and vitamin D status 283 in the diet (62·5-250 µg/kg). Similarly, evidence from another study showed that the vitamin D 3 in egg yolk was increased approximately 7-fold as a result of feeding a diet with a 3·5-fold higher vitamin D 3 content (from 62·4 to 216 µg/kg), while the corresponding increase in 25(OH)D 3 content was only about 1·5-fold (26) . There are only a few studies (29,31,32) examining the effect of feeding birds with diets supplemented with 25 (OH)D 3. In the EU, 25(OH)D 3 has only recently been authorised for addition to poultry diets, and the maximum content of the vitamin D 3 and 25(OH)D 3 combination for laying hens is 80 µg/kg (33,34) . It is of note that most of vitamin D supplementation studies (27)(28)(29)(30)(31) , summarised in Table 1, had higher vitamin D doses than the EU diet limit (33) , thus, the potential for increasing vitamin D in eggs by adding vitamin D to the diet of laying hens is limited by EU regulations. Browning and Cowieson (31) and Duffy et al. (32) both showed an addition of 25(OH)D 3 to the vitamin D 3 supplement resulted in the elevation of the 25(OH)D 3 content of the egg yolk, but there was no significant increase in the vitamin D 3 content of the egg yolk. Other studies investigated dietary supplementation with 25(OH)D 3 ( 29,32) , and showed that only egg yolk 25(OH)D 3 was increased, but not vitamin D 3 . Therefore, we speculate that 25(OH)D 3 in the diet can be absorbed directly by laying hens without transfer to vitamin D 3 in the circulation.

Vitamin D-enriched fish
There are very few studies on enriching the vitamin D content of fish (Table 2) (35)(36)(37)(38) . Mattila et al. fed rainbow trout with different doses of vitamin D 3 supplements up to 539 µg/kg, but no significant differences in the vitamin D 3 content of the fish fillet were observed (37) . In contrast, the study of Horvli et al. with Atlantic salmon showed a dose-response relationship between the vitamin D 3 in the diet up to 28·68 mg/kg and vitamin D 3 in the fish meat (35) . Similar high vitamin D 3 supplementation doses were reported in another two studies (36,38) , which also showed that elevated vitamin D 3 content of the fish liver or whole fish had been achieved by supplemental vitamin D 3 in the diet. However, 25(OH)D 3 contents of the enriched fish were not measured in these studies (35)(36)(37)(38) , and the lack of evidence on the effects by feeding fish with 25(OH)D 3 on the vitamin D content of the Mattila et al. (26) 26·6 ·7 -Browning and Cowieson (31) 62·5 fish warrants further research. Again, supplement doses of the listed studies (35)(36)(37)(38) in Table 2 were over the EU diet limit for farmed fish of 75 µg/kg (33) , which will limit application in the market.

Vitamin D-enriched milk
A few studies have investigated the longer term effect of supplemental vitamin D 3 on the vitamin D content of the milk; the summary of these studies is presented in Table 3 (39)(40)(41)(42) . Hollis et al. showed a 10-fold enhancement of vitamin D 3 intake from 100 to 1000 µg/d resulted in a 7·5-fold increased vitamin D 3 concentration of the milk and a 2-fold increase in 25(OH)D 3 ( 39) . Moreover, McDermott et al. compared three different doses of vitamin D 3 with a control diet, and showed an increased concentration of vitamin D 3 and 25(OH)D 3 in the milk (41) . However, the relationship between increasing dietary vitamin D 3 doses and milk vitamin D 3 or 25(OH)D 3 concentrations were not linear. Furthermore, the study of Weiss et al. investigated the effect of feeding 450 µg/d vitamin D 3 to pre-calving cows for 13 d which resulted in concentrations of vitamin D 3 and 25(OH)D 3 in the milk ranging from 0·33-0·45 to 0·36-1·02 µg/l, respectively (42) . In addition, the study included a diet treatment of 6 mg vitamin D 3 with a cation-anion difference of −138 mEq/kg daily for 13 d; the concentrations of 25 (OH)D 3 in the milk were increased but the treatment effect disappeared after 28 d. Therefore, evidence from the limited number of studies (39)(40)(41)(42) demonstrated that milk vitamin D concentrations can be increased by feeding dairy cows with vitamin D supplements. However, it is of note that the highest milk vitamin D 3 and 25(OH) D 3 concentrations were 0·47 and 3·69 µg/l, respectively (Table 3), which for one typical milk serving of 200 ml only contributes 0·09 and 0·74 µg vitamin D 3 and 25 (OH)D 3 , respectively, well below the current UK vitamin D reference nutrition intake of 10 µg/d (7) . Furthermore, the doses of vitamin D in those studies (41,42) were much higher than the maximum allowed vitamin D content in EU (0·01 mg/kg diet at 880 g DM/kg approximately equivalent to 2·27 mg/d) (34) , which imposes an even greater restriction on the possibility of increasing vitamin D in milk by adding vitamin D supplements in the diet of dairy cows.  (29) demonstrated that the effect of foods enriched with either vitamin D 3 or 25(OH)D 3 on human vitamin D status depended on their relative effectiveness of raising serum or plasma 25(OH)D concentrations. A previous study (44) indicated that there was no consensus on the relative effectiveness of 25(OH)D 3 compared with vitamin D 3 for raising human serum or plasma 25(OH)D 3 concentrations. Furthermore, UK food composition tables (45) indicate that there is no certainty on the relative potency of 25(OH)D 3 compared with vitamin D 3 , although it was assumed that 25(OH)D 3 had a potency of five times that of vitamin D 3 for calculating the total vitamin D of foods (45) .
Nine studies administered 25(OH)D 3 supplementation only, except two studies which provided a combination supplement of 25(OH)D 3 and calcium (46,49) . Five of the eleven studies (47,(49)(50)(51)(52) supplemented 25(OH)D 3 to generally healthy subjects, whereas the other six studies (46,48,(53)(54)(55)(56) supplemented 25(OH)D 3 to clinical patients. Most studies reported the serum or plasma 25(OH)D concentration at both the beginning and end of the treatment, except one study (55) , which only reported the 25(OH)D concentration at the end of the treatment.
In terms of the vitamin D status measurement, most studies measured total 25(OH)D concentration, except two studies (49,52) , which measured 25(OH)D 3 . For the characteristics of the investigated subjects, five studies included both men and women (46,48,51,53,55) , while the other studies only included men or women. In addition, most studies reported the age and BMI of the subjects, except two studies (46,48) that did not report the BMI range.

Acute pharmacokinetic action of cholecalciferol and calcifediol
An early study provided meals with single doses of 25(OH)D 3 of 1·5, 5 or 10 µg/kg body weight to generally healthy subjects and showed that the peak serum 25(OH)D 3 concentration was reached within 4-8 h after ingestion (57) . A later study by Jetter et al.  (52) . In addition, the maximum plasma concentration of 25(OH)D 3 for 25(OH)D 3 treatment (100 nmol/l) was significantly higher than for vitamin D 3 treatment (44 nmol/l). These results suggest that 25(OH)D 3 was absorbed more quickly than vitamin D 3 possibly because 25(OH)D 3 has higher solubility in aqueous media than vitamin D 3 due to its more polar chemical structure (58) . Furthermore, as this metabolite of vitamin D 3 is produced in the liver, the hepatic metabolism of vitamin D 3 to 25(OH)D 3 is circumvented and consequently the conversion from vitamin D 3 to 25(OH)D 3 would be negligible (59) . In patients with liver disease who had an impaired ability to synthesise 25(OH)D 3 from vitamin D 3 ( 60) , the study of Sitrin and Bengoa (61) verified that 25(OH)D 3 could be absorbed more efficiently than vitamin D 3 after oral supplementation. Therefore, supplementation with 25(OH)D 3 is not only more efficient at increasing vitamin D status in generally healthy people, but may also have a specific role in tackling lower vitamin D status in patients who are suffering from liver diseases.

Chronic effects and relative effectiveness of cholecalciferol and calcifediol treatments
Regarding the expected higher biological effect of 25(OH)D 3 in raising serum or plasma 25(OH)D level after long-term administration, several studies have confirmed that oral consumption of 25(OH)D 3 is highly effective in raising serum or plasma 25(OH)D level (Table 4) (46)(47)(48)(49)(50)(51)(52)(53)(54)(55)(56) . However, the majority of the evidence in support of a higher impact of 25(OH)D 3 supplementation compared with vitamin D 3 on serum or plasma 25(OH)D 3 level is from only four studies (51,52,54,56) where both 25(OH)D 3 and vitamin D 3 treatments were included in the same study ( Table 5). The study of Barger-Lux et al. (47) provided three different doses of vitamin D 3 (25, 250, 1250 µg/d) or 25(OH)D 3 (10, 20, 50 µg/d) to the participants for 8 and 4 weeks, respectively. However, the effects of 25(OH)D 3 and vitamin D 3 treatments were not directly comparable as the interventions were not at the same dose or treatment time. Thus, the study of Barger-Lux et al. (47) was excluded from the relative effectiveness analysis. In order to compare the relative effectiveness of 25(OH)D 3 and vitamin D 3 supplementation on raising serum or plasma 25 (OH)D concentrations, a dose-response factor was calculated for each μg of orally consumed 25(OH)D 3 or vitamin D 3 in four studies (51,52,54,56) . The dose-response factors of 25(OH)D 3 and vitamin D 3 were calculated by using endpoint 25(OH)D concentration minus baseline 25(OH)D concentration, divided by the dose of the supplementation (dose-response factor = Δ serum/ plasma (mmol/l)/dose (μg)). Then, the relative  { Same study of (Jetter et al. (52) ) and (Bischoff-Ferrari et al. (62) ). § Study has measured vitamin D status as 25(OH)D 3 .

25(OH)D 3 -fortified foods and vitamin D status
effectiveness of 25(OH)D 3 to vitamin D 3 was calculated by dividing the dose-response factor of 25(OH)D 3 by that of vitamin D 3 . The highest relative effectiveness was found in the study by Catalano et al. (54) . Weekly treatment of 140 µg 25(OH)D 3 or 140 µg vitamin D 3 supplements was provided to osteopenic and dyslipidaemic postmenopausal women for 24 weeks. Supplementation with 25(OH)D 3 raised serum 25(OH)D from a baseline of 56-126 nmol/l, while vitamin D 3 treatment increased serum 25(OH)D to a lower extent, from baseline 51 to 61 nmol/l. Thus, the relative effectiveness factor derived from this study was 7·14, i.e. dietary 25(OH)D 3 was 7·14 times more effective at increasing serum 25(OH)D than dietary vitamin D 3 .
Vitamin D dietary recommendations are generally between 10 and 20 µg/d (10) (52) . They found that for the 25(OH)D 3 treatment, plasma 25(OH)D 3 increased to 173 nmol/l from a baseline of 31 nmol/l, whereas for the vitamin D 3 treatment, plasma 25(OH)D 3 increased to 77 nmol/l from a baseline level of 35 nmol/l. The relative effectiveness factor of each μg of 25(OH)D 3 was 3·40 compared with vitamin D 3 in raising plasma 25(OH)D 3 level. A similar low relative effectiveness factor was found in another study where post-menopausal osteoporotic women were given either 20 µg vitamin D 3 or 20 µg 25(OH)D 3 over 6 or 12 months (56) . The serum concentration of 25(OH)D for the 25(OH)D 3 treatment reached 161 and 188 nmol/l from a baseline of 37 nmol/l after 6 or 12 months administration, respectively, while the comparable values for the vitamin D 3 treatment were an increase to 80 and 86 nmol/l from a baseline of 41 nmol/l. So the relative effectiveness factor of 25(OH)D 3 relative to vitamin D 3 treatment at 6 and 12 months were 3·13 or 3·29, respectively.
In summary, of the studies reviewed, the relative effectiveness of 25(OH)D 3 to vitamin D 3 for raising vitamin D status (Table 5), ranged from 3·13 to 7·14. Previous studies have demonstrated that the season may have influences on vitamin D status (13,14) . There were two studies conducted during the winter which may have minimised any confounding influence of cutaneous vitamin D synthesis from UV radiation (47,51) . Other studies have longer intervention periods of 6 months or more, which could not have avoided some cutaneous synthesis. Furthermore, baseline status may be another factor that influences the relative effectiveness factor. The study of Catalano et al. had the highest factor of 7·14 in the present review, and the baseline concentration of 25(OH)D of the study participants was higher (>50 nmol/l) than the others (54) . Therefore, the different relative effectiveness seen in different studies may be due to the different characteristics or genotypes of the subjects, or different study designs.
Overall, evidence suggests that dietary 25(OH)D 3 can more effectively increase serum 25(OH)D concentrations than vitamin D 3 and may also be absorbed faster reaching a serum or plasma 25(OH)D plateau earlier than vitamin D 3 supplementation. Furthermore, supplementation with 25(OH)D 3 may also have more benefits to human health compared with vitamin D 3 in a general healthy population. Bischoff-Ferrari et al. reported that 20 µg 25(OH)D 3 supplementation over 4 months led to a 5·7 mmHg decrease in systolic blood pressure and improvements in several markers of innate immunity in healthy postmenopausal women (62) .
For patients with different diseases and receiving longterm medication, studies (63)(64)(65) showed that several drugs (e.g. antiepileptic agents, glucocorticoids, antiretroviral or anti-oestrogen drugs) interfered with vitamin D metabolism, which resulted in patients being more likely to have low vitamin D status. Thus, it is not only important to increase vitamin D status in the generally healthy population but also in patients with specific illnesses and receiving certain medication. Therefore, the studies using 25(OH)D 3 treatments in patients were also summarised in Table 4 (46,48,(53)(54)(55)(56) , and those studies consistently reported that chronic 25(OH)D 3 supplementation effectively increased serum 25(OH)D concentrations. For example, Ortego-Jurado et al. showed a lower daily dose of 8·85 µg 25(OH)D 3 to be more effective than a 20 µg dose of vitamin D 3 for increasing vitamin D status in patients with autoimmune disease who were treated with a low dose of glucocorticoids throughout the year (55) . Similarly, the study of Banon et al. showed that a monthly dose of 400 µg 25(OH)D 3 was safe and effective at improving vitamin D status of HIV-infected patients throughout the year (53) . Furthermore, supplementation with 25(OH)D 3 may have additional benefits on patients' health. Previously, 25(OH)D 3 was recommended for patients with kidney disease since 25(OH)D 3 has a direct action on bone metabolism (66) . Hahn et al. provided a daily 40 µg 25 (OH)D 3 and 500 mg calcium supplement to patients who had glucocorticoid-induced osteopenia for 18 months (46) . The treatment markedly increased vitamin D status from 39 to 205 nmol/l. In addition, this study showed that the 25(OH)D 3 treatment improved mineral and bone metabolism. Jean et al. also offered haemodialysis patients who suffered from vitamin D deficiency with a daily dose of 16 µg 25(OH)D 3 for 6 months; vitamin D status reached 126 nmol/l from 30 nmol/l, at the same time 25(OH)D 3 supplementation corrected the excess bone turnover (48) . Similarly, a study by Catalano et al. (54) provided 140 µg 25(OH)D 3 supplements for 24 weeks to osteopenic and dyslipidaemic postmenopausal women, and results showed that 25(OH)D 3 improved plasma lipid levels (increased HDL-cholesterol (P = 0·02) and decreased LDL-cholesterol (P = 0·02)) in osteopenic and dyslipidaemic postmenopausal women when added to an ongoing atorvastatin treatment.
As an alternative to vitamin D-enriched foods, vitamin D fortification of foods may also be an option for tackling vitamin D deficiency throughout the world. In general, fortification of foods refers to mandatory and voluntary fortification. The contribution of vitamin D-fortified foods to vitamin D intake by the public varies considerably between countries as there are different food standard policies (10) , and in practice, vitamin D 2 or vitamin D 3 are used for fortification. Evidence from one previous meta-analysis of RCT showed that vitamin D 3 supplementation is more effective at raising vitamin D status than vitamin D 2 ( 67) . However, a further comprehensive systematic review and meta-analysis of thirtythree RCT (68) showed that the effect of vitamin D 3 supplement on serum 25(OH)D 3 response was limited by the supplemental dose, duration, age of subjects and baseline level. In addition, the meta-analysis showed a greater serum or plasma 25(OH)D increase when the intervention study used a dose of 20 µg/d vitamin D 3 or even higher, with subjects aged >80 years and an administration period of at least 6-12 months or subjects had lower baseline 25(OH)D status (<50 nmol/l) than subjects aged <80 years, administration period <6 months or subjects had higher baseline 25(OH)D status (≥50 nmol/l) (68) . Therefore, better strategies are needed to raise vitamin D status of the public throughout life, and 25(OH)D 3 -fortified foods warrant further research.

Conclusions
Vitamin D insufficiency has become a world problem, especially where sunlight exposure is limited by geographic reasons (latitude), personal characteristics (skin pigmentation, ageing) or behaviour (sunscreen use, cultural reasons). However, there are a few natural foods rich in vitamin D. Thus, vitamin D-enriched foods produced through a food chain approach such as feeding animals vitamin D supplements or vitamin D-fortified foods are needed to guarantee an adequate dietary intake of vitamin D by the general population.
The present review summarised the available and limited number of RCT investigating the effect of 25(OH)D 3 supplementation on serum or plasma 25(OH)D concentration. We concluded that it is difficult to get consensus on the effectiveness of 25(OH)D 3 supplementation relative to vitamin D 3 for raising vitamin D status, due to various influencing factors such as different person characteristics (age, BMI), baseline vitamin D status and time of the year. However, it is unquestionable that 25(OH)D 3 supplementation is more efficient at raising serum 25(OH)D concentrations and also appears to be absorbed faster by than the same dose of vitamin D 3 . Second, by reviewing available evidence on vitamin D-enriched eggs, fish or milk, it is practical and possible to increase the vitamin D content of eggs, fish or milk by addition of vitamin D supplements to the diet of poultry, fish or dairy cows. However, the limitations of adding vitamin D to animal feed should be considered in future enrichment studies. Furthermore, there are a few RCT investigating the impact of these vitamin D-enriched foods on improving vitamin D status. Therefore, 25(OH)D 3 -enriched or fortified foods should be further explored in the future, and additional RCT should be conducted to investigate the effect of 25(OH)D 3 -enriched or fortified foods on vitamin D status of the general population and patients with long-term health conditions.

Financial Support
This review received no specific grant from any funding agency in the public, commercial or not-for-profit sectors. J. G. was supported by the Barham Benevolent Foundation studentship.

Conflicts of Interest
None. 25(OH)D 3 -fortified foods and vitamin D status 289