Effects of a healthy Nordic diet on plasma 25-hydroxyvitamin D concentration in subjects with metabolic syndrome: a randomized, placebo-controlled trial (SYSDIET)

Abstract

At northern latitudes, vitamin D is not synthesized endogenously during winter, causing low plasma 25-hydroxyvitamin D (25(OH)D) concentrations. Therefore, we evaluated the effects of a healthy Nordic diet based on Nordic nutrition recommendations (NNR) on plasma 25(OH)D and explored its dietary predictors. In a Nordic multi-centre trial, subjects (n=213) with metabolic syndrome were randomized to a control or a healthy Nordic diet favouring fish (≥300g/week, including ≥200g/week fatty fish), whole-grain products, berries, fruits, vegetables, rapeseed oil and low-fat dairy products. Plasma 25(OH)D and parathyroid hormone were analysed before and after 18- to 24-week intervention. At baseline, 45% had vitamin D inadequacy (<50nmol/l), whereas 8% had deficiency (<25nmol/l). Dietary vitamin D intake was increased by the healthy Nordic diet (P<0.001). The healthy Nordic and the control diet reduced the prevalence of vitamin D inadequacy by 42% (P<0.001) and 19% (P=0.002), respectively, without between-group difference (P=0.142). Compared with control, plasma 25(OH)D (P=0.208) and parathyroid hormone (P=0.207) were not altered by the healthy Nordic diet. Predictors for 25(OH)D were intake of vitamin D, eicosapentaenoic acids (EPA), docosahexaenoic acids (DHA), vitamin D supplement, plasma EPA and plasma DHA. Nevertheless, only vitamin D intake and season predicted the 25(OH)D changes. Consuming a healthy Nordic diet based on NNR increased vitamin D intake but not plasma 25(OH)D concentration. The reason why fish consumption did not improve vitamin D status might be that many fish are farmed and might contain little vitamin D or that frying fish may result in vitamin D extraction. Additional ways to improve vitamin D status in Nordic countries may be needed.