Does calcium and vitamin D metabolism differ between people born in Pakistan and Norway?
In the Oslo Health Study 2000-2001 (HUBRO) severe vitamin D deficiency was found among adults born in Pakistan. The levels among people born in Norway were normal. Vitamin D deficiency is often associated with low bone mineral density, so we expected that these Pakistan-born people would have more osteoporosis than ethnic Norwegians. But this was not the case - they had similar bone mineral density in the forearm.
One question is therefore whether there are protective factors in the metabolism of vitamin D in Pakistanis, which may compensate for vitamin D deficiency and protect the skeleton from being broken down.
Lower levels of active vitamin D hormone
Vitamin D is found in several forms in the body. The active form of vitamin D is important to ensure sufficient calcium levels in the blood in order to maintain the bone mineral density in the skeleton. We therefore wanted to examine the levels of active vitamin D in the blood of some of those born in Pakistan and compare these with those with a Norwegian ethnic background.
We also found that the active vitamin D hormone was significantly lower in Pakistanis than Norwegians. This difference persisted after adjustment for body mass index. There was thus no compensatory production of the active vitamin D-hormone. These results as well as measurements of parathyroid hormone in Pakistanis suggest that their skeletal health is not optimal.
Higher levels of calcium
We also examined calcium levels in the blood, in order to explore how the level of the active vitamin D affected the level of calcium. Contrary to expectations, we found that ionised calcium, which is strictly hormonally regulated within a narrow range, was higher in Pakistanis than Norwegians.
Our findings suggest that the balance between hormones involved in calcium metabolism in the body may differ between Pakistanis and Norwegians. It has not previously been shown that the level of calcium in the blood is high while the active vitamin D hormone is low. The observed ethnic difference in calcium levels may reflect a true difference, or it may result from a difference in acidity (pH) in the blood. The causes, or long-term consequences, for the skeleton of a low vitamin D level and a high calcium level in Pakistanis are not known, but we want to study this further.
The project was a collaboration between the Norwegian Institute of Public Health, the Institute of General Practice and Community Medicine at the University of Oslo, and Aker University Hospital.
Kristin Holvik, Haakon E. Meyer, Anne Johanne Søgaard, Egil Haug og Jan A. Falch:
Pakistanis living in Oslo have lower serum 1,25-dihydroxyvitamin D levels but higher serum ionized calcium levels compared with ethnic Norwegians. The Oslo Health Study. BMC Endocrine Disorders 2007, 7: 9