This study assessed bioavailability and utilisation of vitamin D3 in two feeding trials using young, growing Sprague–Dawley male rats. Trial one fed animals standard AIN-93G diet (casein protein) containing no vitamin D3 and goat or cow skimmed milk supplemented with vitamin D3. Trial two fed animals modified dairy-free AIN-93G diet (egg albumin) containing no vitamin D3 and goat or cow skimmed or full-fat milk supplemented with vitamin D3. Control groups received AIN-93G diets with or without vitamin D, and water. At 8 weeks of age, blood samples were collected for vitamin and mineral analysis, and femurs and spines were collected for assessment of bone mineralisation and strength. In both trials, analyses showed differences in bioavailability of vitamin D3, with ratios of serum 25-hydroxyvitamin D3 to vitamin D3 intake more than 2-fold higher in groups drinking supplemented milk compared with groups fed supplemented solid food. Bone mineralisation was higher in groups drinking supplemented milk compared with groups fed supplemented solid food, for both trials (P<0·05). There was no difference in the parameters tested between skimmed milk and full-fat milk or between cow milk and goat milk. Comparison of the two trials suggested that dietary protein source promoted bone mineralisation in a growing rat model: modified AIN-93G with egg albumin produced lower bone mineralisation compared with standard AIN-93G with casein. Overall, this study showed that effects of vitamin D3 deficiency in solid diets were reversed by offering milk supplemented with vitamin D3, and suggests that using milk as a vehicle to deliver vitamin D is advantageous.