Cristina Vassalle ^*

• Gabriele Monasterio Tuscany Foundation (CNR), Pisa, Italy

attention towards the measurement of serum 25(OH)D (main circulating form and recognized biomarker of vitamin D status) in laboratory medicine as well as the requirements of accuracy and speed of testing. However, despite the growth and refinement of analytical methods, the measurement of vitamin D still represents a challenge; immunoanalytical techniques still retain great variability in inter-laboratory comparisons, whereas mass spectrometry presents many different difficulties (e.g costs, time and complexity, matrix effects, derivatization step) (1).Interestingly, other vitamin D metabolites may have biological roles, and are expected to be assessed in the serum in the future (2).In any case, at present the high prevalence of vitamin D deficiency is a worldwide major public sanitary issue in every stage of life, including children and adolescents, where an inadequate status can have implications on future health and wellbeing (3,4). In this regard, as many as five contributions focusing on different children issues are included in this special issue (see Table ).Moreover, available dosing recommendations for vitamin D supplementation may considerably vary in the literature depending on the clinical setting and specific cohort evaluated. Importantly, there is still no international general consensus on how to define an optimal vitamin D status, mainly defined based on the inverse relationship of parathyroid hormone and 25(OH)D. For it concerns bone health, the Endocrine Society defines as adequate serum 25(OH)D levels higher than 75 nmol/L (30 ng/mL) and values higher than 50 nmol/L (20 ng/mL) as deficient (5); instead, the Institute of Medicine (IOM, now National Academy of Medicine) definition suggests vitamin D sufficiency for values higher than 50 nmol/L (20 ng/mL), insufficiency between 30 and 50 nmol/L (12-20 ng/mL); deficiency for levels lower than 30 nmol/L (12 ng/mL) (6). In addition, differences may also be due to methodological issues (e.g seasonal sampling, variability in the vitamin D assays), characteristics of the studied populations, (e.g age, sex, calcium intake), and life-style habits (physical activity, outdoor activities and sun exposure) (1). Moreover, the majority of evidence derived from studies conducted in adults, and even the threshold used to define vitamin D deficiency or insufficiency may not be appropriate when applied to children and/or adolescents. Indeed, currently, there are no specific guidelines and no clear consensus on targets for optimal vitamin D status and supplementation in most extra-skeletal conditions (1). Thus, for non-classical actions, more research is needed concerning the optimal 25(OH)D levels to maintain and clarify the suitability of 25(OH)D reference levels (in turn useful to decide the recommended vitamin D intake to maintain most people above this threshold, providing adequacy) and the role of vitamin D as key indicator of health in different clinical settings. We hope that manuscripts included in this research collection, which highlight the role and levels of vitamin D in different pathophysiological conditions, may contribute to the actual discussion in this field and may be appreciated by readers of Frontiers in Nutrition, facilitating further studies stimulated by the data reported and by the many challenges still to be solved in this important research area.