Vitamin D deficiency is recognized as a global health problem at the level of all population subgroups [1
]. It is associated not only with weak musculoskeletal health (e.g., rickets in children, osteopenia, osteoporosis and fractures in adults etc.), but also with increased risk of other illnesses (e.g., common cancers, autoimmune diseases, hypertension, infectious diseases, and some neuropsychological diseases) [2
]. Vitamin D receptors are situated in almost every cell and tissue, evidencing its important role in many body systems (the immune, cardiovascular, and nervous systems etc.). There are two main forms of vitamin D: vitamin D3
(referred as cholecalciferol) and vitamin D2
(referred as ergocalciferol). For human beings, vitamin D3
is synthesized endogenously in the skin during ultraviolet B (UVB) exposure or through diet, whereas vitamin D2
is delivered only through the diet.
Endogenous synthesis may be the main source of vitamin D, but it is influenced by many factors (latitude, season, time of day, ozone layer, air pollution, clouds, surface, time spent outdoors, use of sunscreen, clothing, skin color, age, overweight/obese status, health conditions, and others) [3
]. Vitamin D synthesis ceases between October and March in the Northern Hemisphere at latitudes greater than 40° north. Since the Czech Republic is situated at a latitude of around 50° north, dietary intake becomes an essential source, at least during the winter season [1
Vitamin D is naturally present in a limited number of foods, but fortified foods or dietary supplements could be also good dietary sources. The high content of vitamin D is typical for fatty fish (0.7–19 μg/100 g), because fish accumulates provitamins of vitamin D, vitamin D2
, from phytoplankton and zooplankton. Egg yolks are also good dietary source. The vitamin D content in egg yolk depends mostly on the composition of hen feed and reaches up to 12.6 μg/100 g [4
]. Other animal sources of vitamin D3
are liver, meat and meat products, and milk and dairy products. Mushrooms can also be a source of vitamin D. Vitamin D2
is produced on UVB exposure of the precursor vitamin D2
(ergosterol). Vitamin D2
content ranges from 1 to 30 μg per 100 g fresh weight in wild mushrooms [5
]. Cultivated mushrooms produced indoors do not contain vitamin D without UVB irradiation. New pure yeast or bread (containing ergosterol) treated with UVB light may be also a source.
Due to the pleiotropic effects of vitamin D in organisms as investigated over the last 20 years, Dietary Reference Values (DRVs) for dietary intake have been revaluated recently. The United States Institute of Medicine (IoM US) updated DRVs for vitamin D in 2011. The Estimated Average Requirement (EAR) was set for individuals aged one year and over at 10 μg/day and the Recommended Dietary Allowance (RDA) was set at 15 μg/day for those aged 1–70 years and 20 μg/day for those aged 71 years and older, based on conditions of minimal sun exposure [6
]. Even these increased DRVs are considered too low [7
]. The European Food Safety Authority (EFSA, 2016) set an Adequate Intake (AI) of vitamin D at 15 μg/day for individuals aged one year and older under conditions of assumed minimal cutaneous vitamin D synthesis [3
]. The German-speaking countries (DACH 2015: Deutschland, Austria, Confoederatio Helvetica) neighboring the Czech Republic set the AI of vitamin D for people aged one year and over at 20 μg/day in the case of lack of endogenous synthesis [8
Since there has been an absence of new up-to-date data on vitamin D content in foods marketed in the Czech Republic, relevant intakes and dietary sources in different population groups remain unknown. However, this information is necessary for understanding the situation in the country and for proper decision-making by public health authorities. Thus, it was decided to include vitamin D into the ongoing national Total Diet Study (TDS) because it is widely recognized as an effective and adequately efficient tool to gain reliable data on the occurrence of chemical substances across diet and to estimate dietary exposure or intake in a population [9
]. The aim of this article is therefore to describe the usual dietary intake of vitamin D in 10 Czech population groups (aged 4–90 years) based on currently measured vitamin D content in marketed foods within the national TDS, in order to compare results with available DRVs and to define the main exposure sources of vitamin D in the habitual diet of the Czech population.
Vitamin D in the body originates either from dietary sources or from endogenous synthesis. As mentioned previously, due to its geographical location the Czech population is to a large extent dependent on dietary intake of vitamin D at least during the late autumn, winter, and early spring. Moreover, some population groups can be at risk year-round (i.e., people with minimal sun exposure—e.g., individuals working indoors, homebound individuals, institutionalized people, etc.). That is why the present research was focused on describing vitamin D intake from diet. The results describe usual dietary intake of vitamin D in the Czech population, which was evaluated on the basis of individual food consumption data from the most recent national dietary survey (SISP04) and measured concentrations of vitamin D in marketed food sampled as a part of the national TDS. Such approach was more advantageous compared to the usage of Food Composition Tables (FCTs) for calculation of dietary intake because data on vitamin D content in Czech FCTs is not entirely up to date or is missing for some foods. Intake from dietary supplements is not included into presented results because only limited information on that was available in food consumption data.
It was found that median usual intakes of vitamin D from foods ranged from 2.5 to 4.9 μg/day in Czech children and adolescents (aged 4–17 years) and from 2.8 to 5.1 μg/day in Czech adults (aged 18–90 years). The highest dietary intake of vitamin D (9.9 μg/day, UB, 95th percentile) was observed in men aged 18–64 years.
When comparing the Czech results with those from other European countries the situation in terms of vitamin D dietary intake is similar. According to an EFSA report (2012) comparing results from 14 European countries, the mean intake without dietary supplements in children and adolescents varied from 1.4 to 4 μg/day, in adult women from 1.1 to 6.0 μg/day, and in adult men from 1.5 to 8.2 μg/day. The highest intake from foods was identified in men from Finland (16 μg/day, 25–74 years, 95th percentile) [24
]. In this context it is necessary to mention that there was a large diversity in the methodologies of the described studies.
Vitamin D intake, excluding food supplements, was also explored in the EPIC (the European Prospective Investigation into Cancer and Nutrition) study (1992–1999) in 10 European countries using 24 h dietary recall and a standardized nutrient database. When all countries were combined, the mean vitamin D intake (the EPIC mean) was determined in adults (35–74 years) as 3.3 μg/day for women and 4.8 μg/day for men, and the highest level of vitamin D intake was observed in Sweden [25
Viñas (2011) compared vitamin D intake in European adults (19–64 years) and the elderly (>64 years) from 11 European countries. Mean daily vitamin D intakes were between 2 and 5 μg/day in most investigated countries, but the highest (up to 9 μg/day) was in Nordic countries (Sweden, Finland) [1
]. Mensink et al. (2013) reanalyzed raw data from eight European national surveys (Belgium, Denmark, France, Germany, the Netherlands, Poland, Spain, and the United Kingdom). The mean intakes ranged in children and adolescents (4–17 years) from 1.5 to 4.8 μg/day, and in adults from 0.8 to 6 μg/day [27
]. The most recent data from a Spanish study (ANIBES) showed the total mean intake of vitamin D in the population was 4.4 ± 0.1 μg/day [28
], and a similar Belgian study (VITADEK) showed the mean intake of vitamin D from natural and fortified foods (without supplements) was 3.37–4.41 μg/day in men aged 3–64 years and 3.18–3.48 μg/day in women aged 3–64 years [29
In most of the above referenced surveys the highest levels of vitamin D intake were described in Nordic countries. The difference between Nordic countries and the Czech Republic can be attributed especially to much lower fish consumption among Czech people. Moreover, a much wider range of products in Nordic countries is mandatorily fortified (milk, milk products, margarine) [1
] when compared to the situation in the Czech market.
A comparison with available DRVs indicated that the dietary intake of vitamin D is too low in the majority of the Czech population (>95%). Unfortunately, there was only a limited possibility for comparison of the present findings with results on vitamin D status based on biomarker measurements such as 25-hydroxyvitamin D (25(OH)D). The main marker of vitamin D saturation in the body is 25(OH)D, which reflects dietary intake as well as endogenous synthesis. Some data were produced as a part of the Czech biological monitoring performed by the NIPH, but its status has only been described recently in children aged 5 and 9 years (a study for adults is ongoing). In the spring about 50% of children (n
= 124) had 25(OH)D levels lower than 50 nmol/L; in summer this was the case for about 10% of children (n
= 85), and in winter for more than 30% of children (n
= 66) [30
]. Even those these results are not extensive, they support our findings that dietary intake is low and, particularly in the winter season, it is not enough to cover vitamin D requirements.
The foods that contribute substantially to dietary intakes of vitamin D differ across countries according to habitual dietary patterns and fortification policies. In the Czech population the following main sources of vitamin D exposure were identified: hen eggs (contribution to the usual intake: 21–28%), fine bakery wares excluding biscuits (11–19%), cow’s milk and dairy products (7–23%), fortified margarines (7–18%), fish and fish products (6–20%), and meat and meat products (4–12%).
In the United Kingdom, the major dietary sources in children aged 4–10 years are milk and milk products (13%), meat and meat dishes (25%), fat spreads (21%), cereal and cereal products (20%), fish and eggs (8%). In adults the main dietary sources of vitamin D are meat and meat products (27% for women, 34% for men), fat spreads (19% for men, 20% for women), fish and fish dishes (15% for men, 18% for women), eggs (14% for men, 12% for women), cereal and cereal products (13% for men, 12% for women), and milk and milk products (5% in both groups) [31
]. In Irish adults, the significant contributors are meat (30% in people aged 18–64 years, 22% in people ≥65 years), fish (12% in people aged 18–64 years, 16% in people ≥65 years), and spreads (10% in people aged 18–64 years, 13 % in people ≥65 years) [32
]. In Denmark, in the population aged 7–39 years, the main contributors of vitamin D are fats (36%), meat and meat products (20%), fish and shellfish (8%), and fine pastries (7%) [21
]. In France, fish contributes 31% of dietary vitamin D intake in children and 38% in adults. Eggs contribute 10% and 9%, and cheese 9% and 7% of dietary vitamin D intake in children and adults, respectively [33
]. In Spain, the situation is similar to that of France. The main source of vitamin D is fish (68% of vitamin D intake), eggs (20%), and cereals (4%) [34
]. In Finland, the major food sources of vitamin D are fish, dietary fats, and fortified liquid milk and dairy products. Dietary fats have been fortified with vitamin D since the 1950s and milk since 2003 [35
]. In Norway, the most important sources of vitamin D are fatty fish, fortified fats (margarines, butter), and supplements with cod liver oil, a similar situation to that in Iceland. In Sweden, the main vitamin D sources are dietary fat, oil-rich fish and fish products, and fortified milk and dairy products [1
As we can see, fatty fish are an important source of vitamin D and they contribute up to 68% of the total dietary intake in some countries. However, in the Czech Republic fatty fish contribute 20% at maximum (in the group of women aged 18–64 years). Although the Czech Nutrition Society (2012) recommends a weekly intake at least 400 g of fish and fish products [38
], fish consumption is still low in the long term. Fish consumption was only 5.5 kg (raw) per capita per year in 2015 [39
] i.e., about 100 g weekly. This is probably due to the Czech Republic’s historical development, cultural traditions, and inland location. There are many attempts to increase fish consumption in the Czech population, for example, recommendations from professional authorities or media campaigns, but changing consumer habits is a long-term process. A wide range of fish and fish products is available on the Czech market, but it seems the specific fish characteristics (smell, relatively large numbers of small bones, higher price especially in sea fish etc.) could have a significant impact on the previously mentioned low consumption, mainly among children. Despite the described features, an increase in fish consumption in the Czech population is publicly supported because it is favorable not only for vitamin D but also for intake of other positive substances like omega 3 fatty acids. It should be mentioned that public health interventions to increase intake of vitamin D are not new. Children regularly received fish oil in schools during 1950s and early 1960s as a form of prevention of rickets.
Hen eggs, which are the major contributor to total vitamin D intake in the Czech Republic (up to 28%), accounted for 20% at maximum in other compared countries. The reasons for the higher contribution in the Czech Republic are the flat fortification of feed for laying hens and a relatively high egg consumption rate.
There are several possibilities for how to improve vitamin D intake in a population. One of the most commonly used solutions is fortification of foods. For example, fortified fats are an important source of vitamin D in many countries (Denmark, Nordic countries, and the Czech Republic). However, the policy of vitamin D fortification is not harmonized across Europe. In the Czech Republic, mandatory food fortification applies only for nutrition for infants and young children. Other foods (margarines, milk, instant cocoa drinks etc.) are fortified voluntarily. At present, there are not a lot of vitamin D fortified products on the Czech market; those are mostly produced by global food companies. The introduction of mandatory fortification of certain products or voluntary fortification of a larger range of products (e.g., milk, dairy products, breakfast cereals, pasta etc.) could contribute substantially to higher vitamin D intake on a population-wide basis in the Czech Republic. However, the remaining issue is affordability of such products. Currently, vitamin D fortified milk is often twice as expensive as non-fortified milk, and therefore not cost-attractive for consumers.
Another possibility how to increase dietary intake is introduction of foods (mushrooms, yeast, pastry with yeast e.g., bread), which naturally contain the vitamin D precursor (ergosterol) and are treated with UVB light. Therefore, food producers could be encouraged to use UVB in food production. Since this measure is a relatively new method for the increasing vitamin D content in foods, such products are referred to as novel foods and their placement on the market is subject to special procedure.
Dietary supplements can be also a means to improve vitamin D intake. There are differences in the usage rate of dietary supplements across Europe. Higher consumption is typical for Nordic countries compared to the south of Europe and there are differences also between genders. The consumption is higher in women than in men [1
]. In the Czech Republic there is compulsory supplementation of vitamin D in infants and small children in order to prevent of rickets. In other population groups it is voluntary. According to limited information from national survey (SISP04) we can assume that the frequency of days of consumption of dietary supplements containing vitamin D is about 6% in adult men and about 11% in adult women.