Vitamin D content of food and its contribution to vitamin D status: a briefoverview and Australian focus†
Jerry Liu*
Received 14th May 2012, Accepted 20th August 2012DOI: 10.1039/c2pp25150g
There are only a few natural foods that contain nutritionally significant quantities of vitamin D. Theseinclude egg yolk, some fatty fish, and in much lower concentrations, meat. The published values of thevitamin D content of food are summarised here and their contribution to dietary vitamin D intake isdiscussed, with particular reference to Australia. Dietary vitamin D is unlikely to reach the levelrecommended by the US Institute of Medicine. It is therefore concluded that dietary vitamin D will onlybe a component of vitamin D supply, with UVexposure and/or oral supplements continuing to be themain sources of vitamin D to maintain adequate status.
1. Introduction
Vitamin D is needed for calcium and phosphorus metabolism,and hence bone health.1 Generally, healthy people of fair com-plexion are able to cutaneously synthesise sufficient vitamin D3
from solar UV-B irradiation of 7-dehydrocholesterol present inskin, in order to maintain adequate vitamin D status.2 VitaminD2 is produced from the UV irradiation of the sterol, ergosterol,which is only found in fungi and yeast.3 When vitamin D (eithervitamins D2 or D3) enters the circulation, it is transported to theliver to become hydroxylated to 25-hydroxy-vitamin D(25OHD)4 which is secreted back into the circulation. Some25OHD is taken up by the kidney where it undergoes a secondhydroxylation to form the biologically active metabolite 1,25-dihydroxy-vitamin D (1,25(OH)2D) (Fig. 1).
5
In most populations, diet supplies only a minor proportion oftotal vitamin D required to maintain adequate vitamin D status.6
However, certain demographics often have inadequate UVexposure to maintain good vitamin D status (>75 nmol L−1),7
which may lead to vitamin D deficiency. For example, theelderly have lower levels of 7-dehydrocholesterol in their skin,while increased pigmentation of dark skinned individualsalso impede vitamin D3 synthesis.8 It has been estimatedthat one billion people in the world are considered to be vitaminD insufficient or deficient (defined by plasma 25OHD< 75 nmol L−1).9 In Australia, this may be up to almost a halfof the population.10 Thus, an increase in the oral supply ofvitamin D has been advocated by some scientists to ensureadequate vitamin D status of populations.7
There are only a few natural foods that contain nutritionallysignificant quantities of vitamin D. These include egg yolk,some fatty fish (such as salmon, sardines, herring, andmackerel),10 and in much lower concentrations in meat. All theseJerry Liu
Jerry Liu completed his Bachelorof Animal and VeterinaryBiosciences undergraduatedegree with first class honoursat the Faculty of VeterinaryScience, University of Sydney in2007. From there, he went onto commence his PhD in thesame faculty, investigatingvitamin D physiology andanalysis in biological materials.After the completion of his PhDin 2012 he became employed inthe private sector.
Fig. 1 Flowchart outlining vitamin D metabolism.
†Contribution to the Vitamin D Update collected papers.
Faculty of Veterinary Science, The University of Sydney, NSW 2006,Australia. E-mail: [email protected]
animal products contain vitamin D3,11 originating from either
UV exposure or dietary supplementation. Edible mushroomsexposed to UV light have been shown to contain substantialquantities of vitamin D2.12
The 25OHD3 metabolite is also found in some food of animalorigin,13 derived either from the metabolism of vitamin D3
formed in the skin or as a direct dietary supplement fed toanimals. There is some evidence to suggest that 25OHD3 isstored within muscle tissue.14 Therefore, food such as meat maybe a source of 25OHD3. As 25OHD3 contributes to vitamin Dbiological activity, its content in food should not be ignored.Although 1,25(OH)2D3 is more active than vitamin D3 or25OHD3, it has not been detected in significant amounts in foodproducts.13 Consequently, this metabolite is rarely considered infood and nutritional research.
Because of the low concentration of vitamin D in food, itsquantitative analysis has proven to be difficult.15 This isespecially true of the 25OHD metabolites. However, 25OHD infood may be present in nutritionally significant amounts andmay therefore contribute to total dietary vitamin D intake. Onlyin recent years have reliable and accurate analytical methodsbeen developed to determine the vitamin D content of foods.Methods utilising more advanced liquid chromatography techno-logy and internal standards have improved the detection limitand robustness of vitamin D analysis in complex matrices suchas meat.16 The aim of this article is to summarise some pub-lished values of the vitamin D content of some major foods andtheir contribution to dietary vitamin D intake is briefly discussed.In particular, this article will draw attention to Australianexamples and are compared to other countries where possible.
2. Biological activity
The relative bioactivity of the 25-hydroxy vitamin D metabolitecompared to its parent molecule is uncertain but is an importantconsideration for vitamin D nutrition. If the biopotency of oral25OHD was known, food composition tables would be able toaccurately express vitamin D content as vitamin D3 equivalents,displaying the sum of the biological activities of vitamin D andits metabolites.
It is widely stated that 25OHD3 has five times the biologicalactivity of vitamin D3. Official bodies such as Food StandardsAustralia New Zealand (FSANZ)17 have applied this figure intheir food composition tables. This value was derived fromresearch by Tanaka et al. almost 40 years ago, in rickets-healingrat bioassays.18 This value for relative biopotency has been criti-cised on the grounds that quantitative data from rats does notnecessarily apply to humans. However, a recent paper investi-gating the bioactivity of oral 25OHD3 in relation to oral vitaminD3 in human subjects supported the concept that 25OHD3 hasfive times the biological activity of vitamin D3.
19 Consequently,this will be the activity factor used here.
Much like the debate about 25OHD2 and 25OHD3, there isalso uncertainty about the biological activity of vitamin D2 com-pared to vitamin D3. There is evidence to suggest that these maybe very similar,20 and some official bodies such as FSANZ alsotake the view that dietary vitamin D2 is equivalent to dietary
vitamin D3 and this assumption will be used in this review, forthe calculation of vitamin D equivalents in food.
3. Recommended dietary intake
The recommended intake of dietary vitamin D varies acrossdifferent nations, institutions, and definitions used. The NationalHealth and Medical Research Council of Australia have allocatedan adequate intake (AI) of 5 μg dietary vitamin D per day forindividuals 50 years and under, 10 μg per day for people aged51 to 70 years, and 15 μg per day for those over 70 years old.21
This “adequate intake” figure was proposed in order to maintainserum 25OHD levels above 27.5 nmol L−1, with minimal sun-light exposure. The Institute of Medicine of the US NationalAcademy of Sciences previously stated a recommended dailyallowance (RDA) of 5 μg per day for people aged 0–50 years,10 μg per day for those aged 51–70, and 15 μg per day for thoseaged over 71 years.22 However, various studies and reviewssuggest that these figures are insufficient to maintain vitamin Dstatus at a level where all vitamin D functions are beingmet.23–26 As a result of this, the Institute of Medicine has revisedand increased the above RDA to 15 μg per day for people aged1 to 70, and 20 μg per day for those over 70 years.27 This“recommended daily allowance” was set with the aim of meetingthe needs of “virtually all persons”.27
4. Vitamin D in food
An important factor to note when looking at food compositiondata, is that values from one country may not apply to another.28
There are also many production factors that affect nutrient com-position. Even within an individual animal, vitamin D concen-trations are not homogenous, and vary according to anatomicregion. Therefore, Table 1 provides only a guide for the vitaminD content of some foods, and is by no means comprehensive.The data was collected from a variety of sources from differentcountries and may not be applicable to foods from othercountries.
4.1. Mushrooms
Mushrooms may potentially contain the highest concentration ofvitamin D. It has been shown that wild mushrooms may containvery high levels of vitamin D2, while cultivated varieties willhave undetectable concentrations if they grown in the absence ofUV radiation.34 However, the vitamin D2 content of culturedmushrooms can be significantly increased by artificial UVradiation.35
Although mushrooms may be eaten in significant quantities,the amount of dietary vitamin D is dependent on local mush-room culturing practices. Culturing mushrooms without UVexposure will eliminate their potential as a dietary source ofvitamin D.
4.2. Eggs
Egg yolk is another potentially rich source of dietary vitaminD. However this content is also variable and depends on the
amount and form of vitamin D provided in the feed of layinghens. The vitamin D3 and 25OHD3 content of egg yolk increaseas a result of increasing levels of vitamin D3 in the hen’s feed,36
while the form of vitamin D in egg yolk can be affected by theform of vitamin D fed to the hens.32 This shows that the vitaminD content of eggs is easy to manipulate and may prove to be aneffective delivery mechanism for increasing the dietary intake ofvitamin D3 and/or 25OHD3 of people.
Whole eggs are rarely eaten in large quantities in developednations, possibly because of the public perception that dietarycholesterol in yolk will increase the risk of cardiovasculardisease.37 This is likely to be the major limitation for egg yolk tobecome a dominant contributor of dietary vitamin D.
4.3. Fish
Oily fish such has salmon have long been established as a goodsource of vitamin D3.
38 However, there are many variables thataffect the vitamin D content of fish. It has been found that wildcaught fish tend to have higher, albeit more variable, concen-trations of vitamin D compared to cultured fish.39 Differences invitamin D content also exist between species.40 In the case ofaquaculture, it has been found that the vitamin D content of fishmuscle is not affected by the concentration of vitamin D in fishfeed.41 Thus, it may be challenging to manipulate and increasethe vitamin D content of cultured fish through their diet.
Although fish is frequently eaten in countries such as Japan, itis not consumed in large quantities in other parts of the world,such as the USA.42 Consequently, fish may not be an effective orreliable method of increasing the global supply of dietaryvitamin D.
4.4. Margarine and butter
Foods such as margarine are artificially fortified with vitaminD. This is common practice for countries such as Australia,Canada, and Finland. The level of fortification varies betweencountries. For example, by law, margarine in Australia mustcontain no less than 5.5 μg vitamin D per 100 g.43 The levelvitamin D fortification of margarine is higher in other countriessuch as Canada (13.25 μg per 100 g)44 and Finland (10 μg per
100 g),45 while the USA allows voluntary fortification of up to8.28 μg per 100 g.44 However, despite the relatively high con-centration of vitamin D in margarine, in relation to other food,there is evidence to show that vitamin D fortification of foodmay not benefit population groups with a high risk of deficiency,such as the elderly.46 This is possibly because such foods areoften consumed in small quantities. It has been reported thatfortified margarine was able to increase vitamin D status whenUV exposure was limited. However, in the amount it was con-sumed, it was unable to raise plasma 25OHD to levels of ade-quate vitamin D status.47
Butter is generally not fortified and contains less vitamin Dthan margarine. Regardless, of whether it is fortified or not,butter is unlikely to be a major contributor of vitamin D intake,because it too is rarely consumed in large quantities.
4.5. Meat
Meat such as pork, chicken, and beef contain relatively low con-centrations of vitamin D compared to the other mentioned foods.However, in developed countries such as Australia, meat is gene-rally consumed in large quantities.48 In the UK meat provides asubstantial proportion of total vitamin D intake. It was estimatedthat dietary vitamin D intake of vegetarians and vegans is aroundhalf and a quarter that of meat eaters, respectively.49 This isbecause lacto-ovo vegetarians are able to obtain some dietaryvitamin D from eggs and to a smaller degree, dairy. However,vegans are limited to dietary vitamin D only from mushrooms.
There is some evidence that meat consumption may be protec-tive against rickets and severe vitamin D deficiency.50 The pro-tective effect of meat consumption appears to be independent ofits vitamin D content but the exact mechanism is unknown.
The vitamin D content of meat tends to vary amongst species.This is most likely a result of different feeding practices used bythe animal production industry. There is evidence to suggest thatdietary vitamin D has a direct relationship on muscle vitamin Dcontent.51–52 Species such as chickens and pigs are often inten-sively raised, and are fed a diet high in vitamin D. This is likelyto be the explanation for pork and chicken containing higherlevels of vitamin D compared to beef. Although sheep are alsoraised in similar conditions to beef cattle, it can be seen that the
Table 1 Vitamin D content of various foods expressed as μg/100 g fresh weight
vitamin D content of lamb is higher than that of beef. This maybe explained by the different geographic origins of the samples.The lamb samples were collected from Australia, while the beefsamples were obtained from Finland. Australia (summermaximum UV index of 11 in Sydney) is a more tropical regioncompared to Finland (summer maximum UV index of 5 inHelsinki), and free ranging animals from Australia may have ahigher vitamin D status, and hence increase levels of vitamin Din muscle.
The form of vitamin D may also differ depending on theedible tissue. Some studies have shown that vitamin D3 is inhigher concentrations in fat compared to lean meat.29,53 It ispossible that this is because the strongly lipophyllic nature ofvitamin D3 molecule traps it in the triacyl glycerol of adiposetissue.29 However, any vitamin D in fatty tissue has only limitednutritional significance in developed countries such as Australia, asit is usually removed by both retailers and consumers, in line withdietary recommendations to limit consumption of dietary fat.54
4.6. Other fortified foods
Fortification practices differ from country to country dependingon local legislation. In Australia, Margarine is the only food thatis subject to mandatory vitamin D fortification.55 However, incountries such as Finland, vitamin D insufficiency anddeficiency are major concerns. This has led to government inter-vention in an attempt to improve vitamin D status, especiallyduring winter.56 Such programs have shown promise in signifi-cantly improving the nation’s vitamin D status. However, it wasalso concluded that despite improvements, a substantial portionof the population still remain insufficient.45 Data from the USAand Canada have found that fortification of food is generally noteffective, especially for high risk demographics during winter,and supplementation may be more effective.44 The reason forthis may be attributed to highly variable consumptions offortified foods and inadequate levels of fortification.44 Thesefactors are also likely to be applicable to other countries. Forvitamin D fortification to be effective, governing bodies need toinvest largely into research, in order to determine the optimumlevel of fortification across a variety of commonly eaten foods.This increases the likelihood of each demographic, particularlyhigh risk groups, consuming adequate vitamin D, while alsominimising the risk of vitamin D intoxication.
5. Vitamin D from food compared to UV radiation
Despite the presence of vitamin D in some foods, it is stilldifficult to achieve dietary recommendations. The vitamin Dconcentration of foods differs greatly depending on region andproduction methods, which may make dietary vitamin D fromnatural foods unreliable. Furthermore, it has been estimatedthat the total vitamin D intake is only up to 3 μg per day inAustralia,10 4.2 μg per day in Britain,57 and 5.9 μg per day in theUSA.58 It has been reported that the vitamin D data fromAustralia17 and the USA,15 at the time of writing those articles,are incomplete or inaccurate. These estimates of total vitamin Dintake may therefore also be inaccurate. However, it is likely thateven with more accurate food composition data, that it will still
be difficult for individuals to reach the 15 μg per day RDA, asrecommended by the Institute of Medicine.
It is evident that it may be difficult to achieve adequatevitamin D from food alone. For populations living in tropical orsubtropical regions, UV exposure may be the simplest way toobtain adequate vitamin D. There is sometimes resistance toadvice about increased UV exposure because of increased risk ofskin cancers. However only 10 to 15 minutes of morning sun-shine on the face, arms, and hands during spring in Australia,would produce the equivalent of 15 μg of dietary vitamin D.59
Although this amount of UV exposure is unlikely to be harmfulfor most people, it should be noted that this approach may not besuitable for those with increased risk of skin cancer. Anadditional factor to consider is that UV intensity, and hencepotential for vitamin D synthesis decreases with increasingdistance from the equator.60 Therefore, individuals living furtherfrom the equator are subject to longer winter periods, in whichthey are unable to synthesise vitamin D in the skin. The use ofartificial UV irradiation had once been explored as a means toincrease vitamin D synthesis. For the institutionalised elderly, itis possible to use UV-B emitting lights to increase UV exposurefor limited periods. However, such a method has been criticisedfor showing inconsistent results as well as being linked tocomplications including skin burns and cataracts.61 The pro-vision of oral vitamin D supplements and vitamin D fortificationof food may therefore be the most effective and practicalmethods of avoiding vitamin D insufficiency. However, it shouldbe noted that taking an oral vitamin D supplement may bedifficult for some elderly people when they are already receivingseveral other medications.62 In such cases, adequate monitoringbecomes necessary.
6. Conclusion
Mushrooms, eggs, and oily fish may potentially contain highconcentrations of vitamin D. However, these foods may providean unreliable source of vitamin D, or are not eaten in largeenough quantities to have a significant impact vitamin D status.Although meat contains relatively low concentrations of vitaminD, it is often eaten in large enough quantities to be a significantcontributor to total dietary vitamin D intake. Regardless of thepattern of food consumption, dietary vitamin D is unlikely toreach the level recommended by the US Institute of Medicine. Itis therefore concluded that dietary vitamin D will only be a com-ponent of vitamin D supply, with UV exposure and/or oral sup-plements continuing to be the main sources of vitamin D tomaintain adequate status.
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