However, in reality, a moderate amount of stress is actually necessary for physical health.
Literal
However, actually moderate [copula-な] stress [topic-は], body [gen-の] health [for-にとって] necessary [gen-の] thing [become-となっています].
しかし 'however' opens the contrastive turn. にとって 'for / from the perspective of' marks the frame of reference. ~となっている '(has) become X / is in the state of being X,' a formal frame for stating a settled fact — slightly weightier than です. 適度な 'moderate / suitable degree of' (適 'suitable' + 度 'degree' + な) is formal vocabulary. A medical/scientific generalization sentence.