You are what you eat." This phrase is true metaphorically, and makes sense when one thinks about it logically, in a deeper stage.
Food that we consume affects our image, which refers to the 'you" in said phrase. The food that we eat does not represent us literally, but our image in the long run. If we were to eat a burger, we will not turn into a burger physically, nor will be became unhealthy instantly. This phrase describes the effects of our food consumption over time. If we continuously take in unheathly food, we will get unhealthy eventually.
For starters, we will definitely not turn into ice-cream upon eating them. If we want to be healthy, we have to take in good food, so as to achieve the image we want. I'm not the only one who thinks so as Anthelme Brillat-Savarin said in 1826 that, "Dis-moi ce que tu manges, je te dirai ce que tu es." which meant "Tell me what you eat and I will tell you what you are." this proves that he agrees to that statement as he wrote it in, Physiologie du Gout, ou Meditations de Gastronomie Transcendante.