Characteristics of Serum Ratios of 1,25-Dihydroxyvitamin D to 25-Hydroxyvitamin D for Assessment of Bone Metabolism

Vitamin D is obtained in the body by food intake or by production from 7-dehydrocholesterol by exposure of the skin to ultraviolet B radiation. It is first metabolized in the liver to 25-hydroxyvitamin D (25D), which is a major circulating metabolite. In the kidney, 25D is subsequently metabolized to the hormonally active form, 1,25-dihydroxyvitamin D (1,25D), via 1α-hydroxylase encoded by the CYP27B1 gene. 1,25D has a cellular effect through the vitamin D receptor, which leads to calcium absorption in the gut, bone metabolism, and parathyroid function. A recent study showed that a low vitamin D status is common worldwide and is associated with various diseases including kidney, heart, and liver failure, secondary hyperparathyroidism, osteomalacia, inflammatory bowel disease, granuloma-forming disorders (sarcoidosis and tuberculosis), and cancer. Vitamin D deficiency also increases the risks of falls, fractures, bone loss, sarcopenia, leading to worse outcomes of illness severity, morbidity, and mortality. The 1,25D/25D ratio is considered to be a useful tool for diagnosis of ocular sarcoidosis; however, its clinical utility and relevance to pathophysiology of evaluation of the ratio 1,25D/25D which indicates vitamin D activation have remained unknown. To clarify the clinical usefulness of markers for vitamin D activation, 87 patients in whom serum 25D and 1,25D level was measured were retrospectively reviewed in the present study. Data for 79 patients (33 males and 46 females) were analyzed after exclusion of 8 patients taking vitamin D. The median serum 1,25D/25D ratio was significantly lower in males than in females: 4.1 (IQR: 2.3–5.8) x 10−3 versus 6.8 (3.0–9.8) x 10−3. However, individual levels of 25D and 1,25D were not different in males and females. The major categories of main disorders were endocrine (30.6 %), inflammatory (18.5 %), and bone-related (16.7 %) disorders. The ratios of serum 1,25D/25D had significant negative correlations with femoral dual energy X-ray absorptiometry % young adult mean (DEXA %YAM) (R=-0.35) and lumbar DEXA %YAM (R=-0.32). Significant correlations were found between 1,25D/25D ratio and serum levels of inorganic phosphate (R=-0.34), intact parathyroid hormone (R=0.64) and alkaline phosphatase (R=0.46) in all patients. Of interest, the 1,25D/25D ratio had gender-specific characteristics: the ratio had a significant correlation with age in males (R=0.49), while it had a significant correlation with body mass index (BMI) in females (R=0.34). Collectively, the results revealed that the ratio of serum 1,25D/25D as a marker for activation of vitamin D had relevance to clinical parameters, especially bone turnover, with gender-specific features. It is suggested that the existence of a gender-specific difference of aging males and obese females regarding the activation of vitamin D that is functionally linked to bone metabolism.