Chontita RattanakulMahidol UniversityCommission on Higher Education2018-06-112018-06-112012-12-01International Journal of Mathematical Models and Methods in Applied Sciences. Vol.6, No.7 (2012), 873-884199801402-s2.0-84871630856https://repository.li.mahidol.ac.th/handle/123456789/14387A mathematical model proposed by Rattanamongkonkul et al. [1] is modified here to study the effects of both vitamin D and time delay. Hopf bifurcation theorem is then applied to derive the conditions on the model parameters for which a periodic so lution exists. Computer simulation is also carried out in order to support our theoretical result. Both theoretical and numerical results show that a periodic behavior observed clinically in the level of vitamin D can be expected in our model. Moreover, different kinds of dynamic behavior are investigated numerically.Mahidol UniversityMathematicsArticleSCOPUS