▁to ▁and ▁cost ▁of ▁ped iat ric ▁health ▁care ▁and ▁advoc ated ▁for ▁central ▁focus ▁surrounding ▁a ▁few ▁particular ▁dise ases . ▁ ▁Research ▁ ▁Although ▁Park ' s ▁initial ▁research ▁interests ▁revol ved ▁around ▁card i ov asc ular ▁phys i ology ▁and ▁card iac ▁disease , ▁his ▁research ▁later ▁made ▁the ▁most ▁impact ▁in ▁the ▁field ▁of ▁r ick ets ▁and ▁the ▁growth ▁in ▁b one ▁that ▁indicate ▁the ▁ill ness . ▁During ▁Park ' s ▁time , ▁r ick ets ▁had ▁been ▁viewed ▁as ▁an ▁end uring ▁and ▁often ▁perman ently ▁dam aging ▁disease ▁that ▁was ▁especially ▁prominent ▁in ▁the ▁case ▁of ▁inf ants . ▁Before ▁Park ' s ▁initial ▁arrival ▁at ▁Joh ns ▁Hop kins , ▁Dr . ▁How land ▁and ▁Dr . ▁E . V . ▁Mc Coll um ▁had ▁already ▁begun ▁work ▁on ▁studying ▁r ick ets ▁- ▁specifically ▁studying ▁the ▁changes ▁in ▁calci um ▁and ▁pot ass ium ▁content ▁of ▁the ▁blood . ▁ ▁Dr . ▁Park ▁taught ▁and ▁worked ▁in ▁the ▁Hunter ian ▁Lab ▁where ▁he ▁studied ▁hist olog ic ▁changes ▁in ▁the ▁b one , ▁character izing ▁normal ▁versus ▁ab normal ▁changes ▁in ▁b one ▁growth . ▁He ▁did ▁so ▁by ▁varying ▁di et ary ▁def ic ien cies ▁and ▁through ▁his ▁work ▁discovered ▁that ▁vit amin ▁D ▁can ▁be ▁used ▁to ▁prevent ▁r ick ets . ▁This ▁work , ▁along ▁with ▁that ▁done ▁by ▁Dr . ▁E . V . ▁Mc Coll um , ▁Dr . ▁Sim mond s ▁and ▁Dr . ▁Sh