Applied nutritional investigationVitamin D: Can fish food–based solutions be used for reduction of vitamin D deficiency in Poland?
Graphical Abstract
Introduction
Vitamin D is a group of fat-soluble secosteroids that exhibit a wide range of physiological activities, mainly with regard to calcium and phosphorous homeostasis, and to the maintenance of a normal skeleton function and structure.
There are two physiologically important forms of vitamin D, which differ in their side-chain construction. They are ergocalciferol (vitamin D2), which naturally occurs in plants, and cholecalciferol (vitamin D3), which is found in animal organisms.
Although cholecalciferol can be synthesized by skin cells exposed to sunlight (290–315 nm), there are several factors that affect the skin's production of vitamin D. The widespread use of sunscreen or wearing protective head gear and clothing, as well as other strategies meant to protect the body against the harmful effects of ultraviolet (UV) radiation, can lead to significantly lower vitamin photosynthesis [1], [2], [3], [4]. Dark skin pigmentation is also correlated with a risk for vitamin D deficiency, because the melanin present in dark skin competes with 7-dehydrocholesterol for absorption of UVB radiation. Such a competition prevents cholecalciferol formation [2], [5]. Older individuals are at higher risk for vitamin D deficiency as a result of decreased quantities of skin 7-dehydrocholesterol, lower renal production of calcitriol (active form of vitamin D), and limited sun exposure [2]. Another factor enhancing cholecalciferol deficiency or insufficiency is latitude. From October to March, individuals living above a 35-degree latitude are deprived of UVB radiation capable of cholecalciferol photosynthesis [6], [7]. At higher latitudes (e.g. in Poland), this period of deprivation is even longer.
All of the just-cited factors lead to the conclusion that a dependence on sun exposure as the sole source of vitamin D may cause significant cholecalciferol deficiency. This deficiency may in turn result in a number of diseases and dysfunctions such as rickets, osteoporosis, muscle weakness, bacterial and viral infections, various autoimmune diseases (e.g., type 1 diabetes, multiple sclerosis, psoriasis), hypertension, cardiovascular disease, and many types of cancer [3], [4], [5], [6], [8], [9].
The multitude of diseases promoted by vitamin D deficiency explains why providing the human organism with a constant and sufficiently high supply of this compound should be treated as a high priority, especially in light of the low vitamin D status observed in several regions of the world [10], [11]. One of the regions where significantly lower vitamin D status was observed is northern Europe. This effect was strongly pronounced in Poland [7], [12], [13], [14]. Vitamin D status is determined as the blood concentration of serum 25-hydroxyvitamin D [S-25(OH)D] [15]. It is considered that the blood concentration of S-25(OH)D should at least be 50 nmol/L [16]. Levels of S-25(OH)D ranging between 25 and 50 nmol/L are considered insufficient; range <25 nmol/L is vitamin D deficiency [2], [17].
Study carried out in northern Europe demonstrated that 87% of Polish girls (n = 61) and 91% of Polish women (n = 65) were characterized by insufficient (<50 nmol/l) S-25(OH)D levels [12]. It was also reported that 33% of examined girls and 25% of women in Poland had S-25(OH)D levels <25 nmol/L, which qualified them as vitamin D deficient [12]. Percentage of insufficiency observed in postmenopausal women in Poland (n = 152) was the highest of all 25 studied countries and exceeded 45; 12.5% of postmenopausal women were suffering from vitamin D deficiency [14]. According to a study of newborns, insufficiency and deficiency of this nutrient was observed respectively, in 46.3% and 53.7% of the studied infants (n = 41) [13].
Strategies that may help to prevent cholecalciferol deficiency are increased exposure to UVB radiation, vitamin D supplementation, fortification of food products, and sufficient dietary intake. An excessive exposure to UV light may result in skin cancer, so an increased dietary intake should be promoted, as safe and fairly efficient. Intestinal absorption of 1 μg of cholecalciferol daily increases S-25(OH)D by 1 nmol/L [1], although this effect may be influenced by several factors, such as total vitamin D intake or the initial S-25(OH)D concentration [18]. No differences were observed for various dietary sources of vitamin D [19].
It is well established that the best supplementation sources available are fish, fish liver, and fish oil [20], [21], [22], [23]. In Europe, the lowest levels of vitamin D deficiency occurred in Norway, a country with a traditionally high intake of fish and fish oils [24]. In Japan, fish contributes as much as 90% of dietary cholecalciferol [25].
Because Poland is a country with one of the lowest vitamin D statuses in Europe [7], [12], [13], [14], it seems important to determine the content of vitamin D in fish species popular in the Polish market. Based on these updated data, as well as on data regarding fish consumption, it would be possible to verify the extent to which fish present in the Polish diet can satisfy the body's requirement for cholecalciferol. The obtained data also would enable an evaluation of whether a diet rich in fish may be a solution for eradication or reduction of vitamin D deficiency.
Section snippets
Fish samples
Content of vitamins D3 and D2 in muscle tissues of 11 popular fish species present in the Polish market were determined. The species taken into consideration were: Baltic cod (Gadus morhua callarias), Baltic herring (Clupea harengus membras), Baltic salmon (Salmo salar), mackerel (Scomber scombrus), Alaska pollock (Theragra chalcogramma), sole (Limanda aspera), farmed trout (Oncorhynchus mykiss), farmed carp (Cyprinus carpio), farmed Norwegian salmon (Salmo salar) as well as farmed sutchi
Results
Table 1 presents the average content of vitamin D in the muscle tissue of examined fish. The study showed that significant interspecies and intraspecies variation in vitamin D content characterizes all the examined fish species. Alaska pollock and sutchi catfish were characterized by a low average content of vitamin D3. The values noted for those species were close to the LOQ of the method used. Baltic cod, containing 0.7 μg of cholecalciferol/100 g of muscle tissue, also cannot be considered a
Discussion
It is often maintained that fatty rather than lean fish are a better source of vitamin D [5], [23], [24]. Our results, in the case of vitamins D3 and D2, do not support this opinion (Fig. 1). It is true that pollock, cod, and sutchi catfish, which are characterized by low amounts of fat, contain small amounts of vitamin D. On the other hand, tilapia, which is also a lean fish (1.98% ± 0.55% fat), is characterized as having the highest content of vitamin D from all the studied species. The same
Conclusion
The disproportion between requirement and supply of vitamin D seems too vast to enable total eradication of its deficiency by fish food–based solutions. Still, when answering the question asked in the title of this article, one must say that increasing fish consumption or changing consumption patterns could result in noticeable growth in vitamin D status.
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This research project was financed by the Polish Ministry of Science and Higher Education using 2008 to 2011 funds for scientific studies and statutory activities of National Marine Fisheries Research Institute. The Polish Ministry of Science and Higher Education had no role in the study design, analysis, and interpretation of data, or in the writing of this article. MM-C designed the study, carried it out, analyzed the data, and wrote the manuscript. ZU conducted statistical analyses and contributed to writing the manuscript. Both authors revised and approved the final version of the paper for publication. The authors of the manuscript affirm that the submission represents their original work which has not been published previously, and which is not currently being considered by another journal. The authors have no conflicts of interest to declare.