A proverb-like expression meaning 'the body is honest' — your body reveals the truth about your health or feelings; it shows fatigue or what you truly need.
睡眠不足で顔に疲れが出ている。やっぱり体は正直だね。
Lack of sleep is showing on your face. Indeed, the body is honest.
運動をサボるとすぐに調子が悪くなる。体は正直だから手を抜けない。
If you skip exercise, you quickly feel worse. The body is honest, so you can't slack off.
二日酔いのときは体は正直に『休め』とサインを出す。
When you have a hangover, the body honestly signals you to rest.