Vitamin D status in women with rheumatoid arthritis: frequency of hypovitaminosis, associations with disease activity, body composition and comorbidity oleh Olga V. Dobrovolskaya, Natalia V. Toroptsova, Oksana A. Nikitinskaya, Elena Y. Samarkina, Mariia V. Cherkasova, Aleksei Y. Feklistov