Biologists, chemists and medical scientists (of which I’m none, so bear with me :p ) have worked very hard to understand the effects of different foods on the human body. This process of working out what’s good and what’s bad is, literally, science, and now, since so much work has been done, we call it the science of nutrition
The body is made of cells (biology) which are basically big molecules (chemistry). Those molecules need energy and ‘stuff’ to get built and replicate. We eat food, which gets broken down into tiny atomic bits (physics) in our digestive system. Those tiny bits are sent to where they are needed in the body by our blood to make new molecules and help cell division and growth. We need to make sure we eat enough of the right foods so our cells can reproduce. For example, your blood needs iron, so we need to eat enough food like broccoli (which has got lots of iron in it) to have healthy, effective blood.
We have invented a science (nutrition) which brings together the physics, chemistry and biology of our bodies to tell us what foods are good for us 😀