The NMJI

In the report by Tandon et al.1 documenting the bone mineral parameters in healthy young Indian adults, the authors focus on the 25-hydroxyvitamin D [25(OH)D] status of the study group and the interrelationships between biochemical and bone mineral density (BMD). In fact, there are no assays available to detect 25-hydroxyvitamin D3 exclusively. The commercially available kit (DiaSorin, Incstar Corporation, Stillwater, MN, USA) used by the authors quantifies 25-hydroxyvitamin D, i.e. 25-hydroxy ergocalciferol {vitamin D2 [25(OH)D2]} and 25-hydroxy cholecalciferol {vitamin D3 [25(OH)D3]}. In this method, the primary antibody which is added reacts equally with 25(OH)D2 and 25(OH)D3.2 Hence, the terminology ‘25(OH)D’ is preferred. In effect, the authors have estimated ‘25-hydroxyvitamin D’ and not ‘25-hydroxyvitamin D3’ as mentioned in the article.1

One should also be cautious about the fact that ‘population-based reference values’ (e.g. derived from blood donors, etc.), developed by the kit manufacturers are depicted as a ‘reference range’ and are usually employed to define vitamin D deficiency. These values are limited by climate, exposure to sunshine, clothing habits, among others, and could therefore vary according to other local conditions. Another problem is that different investigators use different ‘reference populations’. These ‘population-based reference values’ provided by kit manufacturers may lead to fallacious interpretation of the values in different regions of the world. A ‘functional health-based reference value’ based on the levels of vitamin D and parathyroid hormone (PTH) has been proposed by Lips.3 This defines vitamin D deficiency as ‘the critical level of 25(OH)D which prevents secondary hyperparathyroidism’. Staging of vitamin D deficiency is based on 25(OH)D levels, increase in serum PTH and changes in bone histology. Mild vitamin D deficiency is defined as 25(OH)D levels of 10–20 ng/ml, 15% increase in the PTH level, and normal or high turnover in bone histology. Moderate vitamin D deficiency is defined as 25(OH)D levels of 5–10 ng/ml, 15%–30% increase in the PTH level, and high turnover in bone histology; severe vitamin D deficiency as 25(OH)D levels <5 ng/ml, >30% increase in the PTH level, and mineralization defect/incipient or overt osteomalacia in bone histology. These are based on seasonal variations of PTH, which are no longer visible at the corresponding 25(OH)D levels.4,5 This is again based on vitamin D supplementation studies done by various groups and correlation with bone histomorphic studies.6 This classification encompasses the ‘vitamin D–calcium–PTH axis’ and its impact on bone. It is more apt and based on scientific reasoning.

The authors have used ‘population-based reference values’. The reference range given by the manufacture is on a group of 44 midwestern Caucasian volunteers in the age group of 23–67 years.7 On application of a ‘functional health-based reference value’ it can be seen that some of them might come under the category of mild vitamin D deficiency. This might well explain the raised serum alkaline phosphatase and PTH levels in some of them.

Another point of interest is the BMD data. The accompanying editorial8 states that dual-energy X-ray absorptiometry (DEXA) is the ‘gold standard’ for the diagnosis of osteoporosis. In the editorial,8 it has also been argued that Indian norms for BMD have to be developed. However, methodologically, DEXA has its limitations. The software used with the equipment is calibrated for the western population and the same used to measure the BMD of the Indian population. It amounts to propagation of ‘systematic error’. These points should be considered while interpreting the data and also while developing Indian norms. In the Indian context, a low BMD might convey that the bones are osteopenic or osteoporotic, but a normal BMD does not mean that the bones are normal. This is especially true in India where 13 states have been declared endemic for skeletal fluorosis.9 Bone histomorphometry studies of bone biopsies in patients with endemic skeletal fluorosis do not correlate with skeletal X-rays, leave alone BMD.

In the patients studied by Tandon et al., osteopenia by BMD could probably be related to vitamin D levels if the ‘functional health-based reference value’ is used. Osteoporosis in young healthy subjects in their third decade of life warrants further investigations to look for rarer causes. It is well known that haematological disorders can present with severe osteoporosis. An in-depth study of the bone marrow in these patients would be more helpful. Analysing such patients separately from the osteoporotic group may give more information.

Looking back at the data in the background of the ‘functional health-based reference value’ of vitamin D, this paper has documented mild vitamin D deficiency at least in some young, healthy Indian men and women.
15 March 2004