Impairment of vitamin D metabolism and bone mineral content after intestinal bypass for obesity. A longitudinal study.

Ten obese subjects who had undergone intestinal bypass operation (end-to-side jejunoileostomy) were studied longitudinally with respect to vitamin D and other indices of calcium metabolism. Investigations were carried out before operation (t0) and after 6 months (t1), 12 months (t2), and a mean of 54 months (range, 49-58 months) (t3) postoperatively. Serum 25-hydroxyvitamin D (25OHD) was subnormal at t0 but after operation values declined gradually to an extremely low level at t3, possibly because of a loss through malabsorption. Serum 24,25-dihydroxyvitamin D remained normal at t1 and t2 but fell to about half the normal level at t3, probably owing to lack of its precursor, 25OHD. In contrast, serum 1,25-dihydroxyvitamin D (1,25(OH)2D) remained normal throughout the study, indicating a marked stimulation of kidney 1 alpha-hydroxylase activity. Serum calcium fell rapidly to a constant subnormal level, and it is concluded that the serum calcium malabsorption is due to factors other than impaired 1,25(OH)2D activity. Bone mineral content (BMC) was unchanged between t0 and t2, but thereafter (between t2 and t3) the mean BMC fell rapidly to about 90% of preoperative value, possibly due to a defective bone mineralization in the late postoperative period. The findings indicate a high risk of bone disease developing after intestinal bypass operation. Substitution with calcium and vitamin D should be given to these patients, but the optimal vitamin D metabolite (or combination of metabolites) for such treatment is still unknown.