Context: Children with calcium deficiency rickets have high 1,25-dihydroxyvitamin D values.

Objective: To determine if vitamin D increased calcium absorption.

Design: Experimental study.

Setting: Teaching hospital.

Participants: 17 children with nutritional rickets.

Intervention: Randomized to oral vitamin D₃ (n=8) or vitamin D₂ (n=9) 1.25 mg.

Main outcome measure: Fractional calcium absorption 3 days after vitamin D administration.

Results: Mean baseline 25-hydroxyvitamin D concentrations were 20 ng/ml (range 5–31 ng/ml). The increase in 25-hydroxyvitamin D was equivalent after vitamin D₃ (29±10 ng/ml) or vitamin D₂ (29±17 ng/ml). Mean 1,25-dihydroxyvitamin D values increased from 143±76 pg/ml to 243±102 pg/ml (P=0.001), and the increase in 1,25-dihydroxyvitamin D did not differ between vitamin D₂ and vitamin D₃ (107±110 and 91±102 ng/ml, respectively). The degree of rise in 1,25-dihydroxyvitamin D was explained almost entirely by the baseline 25-hydroxyvitamin D concentration (r²=0.72; P<0.001). Mean fractional calcium absorption did not differ before (52.6±21.4%) or after (53.2±23.5%) vitamin D, and effects of vitamin D₂ and vitamin D₃ on calcium absorption were not significantly different. Fractional calcium absorption was not closely related to concentrations of 25-hydroxyvitamin D (r=0.01, P=0.93) or 1,25-dihydroxyvitamin D (r=0.21, P=0.24). The effect of vitamin D on calcium absorption did not vary with baseline 25-hydroxyvitamin D values or with the absolute increase in 25-hydroxyvitamin D or 1,25-dihydroxyvitamin D values.

Conclusions: Despite similar increases in 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D with vitamin D₂ or vitamin D₃, fractional calcium absorption did not increase, indicating that rickets in Nigerian children is not primarily due to vitamin D-deficient calcium malabsorption.