**1. Introduction**

Vitamin D is of the highest importance for the skeletal system, promoting calcium and phosphorus absorption in the gut, maintaining their adequate concentrations, and for bone growth and remodeling by osteoblasts and osteoclasts [1]. Studies also sugges<sup>t</sup> an association between vitamin D status or intake and various diseases and disorders. Recent meta-analyses show that vitamin D deficiency is associated with a higher risk of sleep disorders [2] and may be related to autoimmune thyroid disease [3], whereas high vitamin D serum levels have a protective e ffect on breast cancer in premenopausal women [4]. They also indicate that vitamin D supplementation significantly reduces cancer mortality [5], as well as the rate of asthma exacerbations [6]. Taking it into account, scientists agree that vitamin D goes beyond influencing bone health only [7,8].

However, vitamin D status is a global problem. Based on protocols of the Vitamin D Standardization Program (VDSP) by National Institutes of Health (NIH) [9], the analysis of data for 14 European countries indicated the prevalence of vitamin D deficiency of 13% [10], while defined by the United States (US) Institute of Medicine (IoM) and the United Kingdom National Osteoporosis Society (NOS) as serum 25-hydroxyvitamin D (25(OH)D) level lower than 30 nmol/L (12 ng/mL, 1 nmol/L = 0.4 ng/mL) [11,12]. At the same time, for the alternate threshold for vitamin D deficiency of serum 25(OH)D level lower than 50 nmol/L, which is suggested by the US Endocrine Society [13], European Food Safety Authority (EFSA) [14] and Polish recommendations [15], the prevalence was 40.4% [10]. Among healthy adolescents, the prevalence of vitamin D deficiency (25(OH)D level lower than 50 nmol/L) sometimes reaches even 42% [16] and in teenage girls living in northern Europe, in the winter season, even 92% [17]. In young women aged 18–29 years living in France, the prevalence of vitamin D deficiency (25(OH)D level lower than 50 nmol/L) amounts to 48.4% and is the highest among female groups [18].

The main source of vitamin D is the skin synthesis of cholecalciferol from 7-dehydrocholesterol [19] as a result of UVB radiation (290–320 nm) from sunshine exposure, which depends on the latitude, the season, and the time of the day [7]. In countries such as Poland, which are situated in a moderate climate, endogenous vitamin D synthesis is possible from April to October only [20]. The other source of vitamin D is dietary intake—mainly from animal products, especially from fatty fish [21]. However, the vitamin D content in fish species differs significantly—in Poland, from 30 μg/100 g for eel, 19 μg/100 g for herring, and 13 μg/100 g for salmon to 1 μg/100 g for cod and 0.8 μg/100 g for flounder [22].

Due to a low fish intake in most European countries [23], inadequate vitamin D intake has been observed for years [19]. In Poland, it has been reported that for women the average vitamin D intake is 3.3 μg [24], whereas the most prominent recommendations indicate at least 10 μg as the reference intake of vitamin D for adults [11,25,26]. The problem of inadequate vitamin D intake is most serious when it comes to women under 30 years of age, since this is the group in which the intake of vitamin D is the lowest according to the 2003–2006 National Health and Nutrition Examination Survey (NHANES) [27]. Moreover, until the age of 30, the maximum bone density and peak bone mass are reached [28], which is influenced by vitamin D, so obtaining adequate vitamin D intake and status would be especially important in that vulnerable group of young women.

In recent years, intervention studies with the aim of improving vitamin D status through fish intake increase have been conducted in various European countries [29]. Their efficacy differed depending on the studied group, the type of intervention (fish species, dose, frequency), and intervention duration. However, no effective and recommended intervention has been defined in order to improve vitamin D intake and status in young women. Taking this into account, the aim of the presented study was to assess the efficacy of salmon intake intervention on vitamin D status in young Polish women during autumn.
