## 1 Introduction  
Observational and intervention studies suggest that vitamin D deficiency may increase the risk of developing nutritional rickets, as well as metabolic, autoimmune, infectious, cardiovascular, mental, and other types of disorders [1, 2]. Vitamin D deficiency, however, is a recognized public health problem, especially in Nordic countries [3, 4]. Non-Western ethnic minorities in Nordic countries exhibit especially high vitamin D deficiency rates [5, 6]. Vitamin D deficiency in young non-Western ethnic minority women raises special concerns, as the vitamin D status in young women determines health outcomes not only for the women themselves but also for their potential offspring [7].  
A debate, however, exists regarding whether vitamin D optimal levels are naturally lower in darker-skinned populations. A body of evidence points towards generally lower vitamin D levels in dark-skinned human populations, even those receiving intense sunlight. For example, the vitamin D levels were below 75 nmol/L in 51% of young tanned Hawaiians with 22.4 h per week of unprotected sun exposure [8]; among south Indians, \"agricultural workers starting their day at 0800 and working outdoors until 1700 with their face, chest, back, legs, arms, and forearms exposed to sunlight\", 44% of the men and 70% of the women had less than 50 nmol/L [9]; in Saudi Arabia, levels were below 25 nmol/L in 35%, 45%, 53%, and 50% of normal male university students of Saudi, Jordanian, Egyptian, and other origins, respectively [10]; in a sample of healthy Middle Eastern athletes, 91% had less than 50 nmol/L [11]. A meta-analysis concluded that vitamin D levels were consistently and significantly lower in people of non-European origin than in people of European origin regardless of latitude [12]. Some variations were also shown among Europeans, with lower vitamin D levels in central and southern Europeans than in lighter-skinned Swedes [13]. Some studies have provided evolutionary explanations [14] and argued for genetic differences [15] in human population causing different vitamin D metabolism (e.g., receptors that bind more tightly to the vitamin D molecule) and thus different vitamin D requirements.  
Migration from sunny non-European countries into the north of Europe in any case diminishes exposure to sunshine and negatively affects vitamin D synthesis in the skin. The Danish health authorities therefore recommend vitamin D supplementation for immigrants with darker skin and a sunlight-avoidant lifestyle [6]. The Danish health authorities furthermore are initiating educational campaigns to raise vitamin D deficiency awareness and motivate taking vitamin D supplements for population groups with a non-Western ethnic minority background. For example, in April 2019, the Danish National Board of Health started the campaign called \"Do you get enough vitamin D?\", targeted at some non-Western ethnic minority populations [16]. This campaign consisted of the production of educational material about vitamin D published in Turkish, Arabic, and Urdu. As there have been no studies on vitamin D knowledge in non-Western ethnic minority groups in Denmark or other Nordic countries so far, the aim of this study was to explore vitamin D knowledge, attitudes, and behaviors in Danish women with a non-Western ethnic minority background. We also aimed to assess associations between knowledge, attitudes, and behaviors and women's age, education, and employment.