The value and significance of 25(OH) and 1,25(OH) vitamin D serum levels in adult coeliac patients: A review of the literature

Abstract

Within the wide spectrum of symptoms and alteration of systems that characterizes CeD, several studies indicate a low-level of vitamin D, therefore recent guidelines suggest its evaluation at the time of diagnosis. This review examines the data from existing studies in which vitamin D has been assessed in CeD patients. Our review indicates that most of the studies on vitamin D in adult CeD report a 25 (OH) vitamin D deficiency at diagnosis that disappears when the patient goes on a gluten-free diet, independently of any supplementation. Instead, when the calcitriol, the active 1,25 (OH) vitamin D form, was evaluated, it resulted in the normal range at the time of CeD diagnosis. A strict and lifelong gluten-free diet can help recover vitamin D level without any supplementation.

Introduction

Coeliac disease (CeD) is a chronic autoimmune systemic multi-organ disease triggered by the ingestion of gluten in genetically-predisposed individuals [1]. Within the wide spectrum of symptoms and alteration of systems that characterizes CeD, several studies indicate a low-level of vitamin D, therefore recent guidelines suggest its evaluation at the time of diagnosis [1]. Vitamin D3 is a prohormone produced in the skin. It is biologically inert and must be metabolized to 25-hydroxyvitamin D3 [25 (OH) vitamin D] in the liver, and then to 1,25-dihydroxyvitamin D3 [(1,25 (OH) vitamin D, called calcitriol], which is the active form, in the kidney. It must be noted that the plasma concentration of the deposit form, the 25 (OH) vitamin D, is at least 1000 times higher than calcitriol, the circulating active but short-lasting form. The half-life of the former is quite long and represents the storage form of vitamin D, whereas that of calcitriol is approximately 15 h, and it is quite variable, depending on other factors, including the status of calcium. Because of its longer half-life, 25 (OH) vitamin D can therefore be considered a better measure of vitamin D storage. As is widely known, calcitriol regulates calcium absorption and homeostasis. It promotes calcium absorption from the gut, enabling mineralisation of newly formed osteoid tissue in the bone, also playing an important role in muscle function.

Vitamin D deficiency is considered one of the main causes of low bone mineral density (BMD). More than 50% of CeD patients show low BMD at the time of diagnosis [2], which has been also described as a unique pathological sign in asymptomatic patients [3]. Consequently, CeD patients present a high risk of fractures [4], [5], [6]. It is known that in Northern Europe the hazard ratio for fragility fractures in CeD patients has doubled [5], however no clear data exist on the relationship between vitamin D levels and fracture risk in CeD.

This paper reviews the existing studies that assess vitamin D in adult CeD patients to understand whether a real deficiency exists. Moreover, the review aims at understanding whether a routine evaluation of vitamin D should be recommended at diagnosis and follow-up, and eventually if a supplementation is needed.