Literally: medicine – food – same – source
Alternately: Just as medicine is used to cure diseases, eating well is key to maintaining one’s health. The best defense against ill health is a proper balanced diet (as an every-day practice).
Notes: Some versions of this saying use 薬 (yaku), “medicine,” instead of 医. It seems that the 薬 version is the original Chinese idea, while the 医 version was created and used in Japan based on it. Apparently some people use 衣 (also pronounced i, but meaning “clothing”) in place of 医. This is plainly an error.