This is an age-old saying in Aurveda, the ancient Indian science of life - however, like most age-old sayings, no school really teaches it, few follow it, but plenty think they have it all figured out. For someone trying to break out of a lifelong imbalance of focusing squarely on developing and maintaining the mind while completely neglecting the body, this can be quite a challenge - believe me, at least that was my situation ... for 30 years!
Yes, school does teach a lot about the body - parts of the body, function of each organ, what exotic juices each uses and produces, how food makes its way through our system, etc. - despite all this "knowledge", why do we keep graduating children with minds that have absolutely no understanding of how to take good care of the body? You can attribute it to taste of the tongue, ease of access or convenience, peer pressure, what you see around you at home and at restaurants and fast food joints, .... the list of reasons goes on endlessly - the consequence is the same: we have way too many people who are too unaware of the importance of what they eat and how it shapes their body....and their lives ultimately.
The upshot here is that for most people, there is probably a recipe out there wherein they can eat food that they find tasty, which they can eat in good quantity, and get their bodies to start becoming fit. You really don't need to go on that next diet fad, exercising all that will power, depriving yourself of your cravings, becoming unhappy and eventually dropping out of the diet. In my experience, the key is to find the formula that works for you, and make the lifestyle change to use that formula for yourself - you can take it one day at a time with the confidence that you can change at any point and get on the right track slowly but surely.
As well educated as we may all be, I would submit that there is room for us to become more aware of what we do to our bodies each day - through what we eat and the physical activities we undertake. My observation is that people are a lot more aware and conscious of the importance of exercise - whether they choose to act on it or not is a different matter - but most people will assert or grudgingly agree that exercise is good / necessary for a healthy body. When it comes to food, however, I believe the story is quite different....
There are broadly three categories of individuals:
a) Those who get it
b) Those who think they get it
c) Those who don't get it
I started out my journey clearly in bucket (c), did a bunch of things that moved me to (b), and over the last 6-8 months I have been endeavoring to move to bucket (a) .... although I think that's still quite a ways off - but the cool thing is that I am motivated more than ever before to take as much care of my body as my mind. I feel great, energetic, and finally feel good about trying sincerely to take care of my health. My weight is down about 60 pounds - from 226 to 165 - if you can picture that, I hope you will agree that its a striking transformation.
The tenet of "You are what you eat" is at the core of what has enabled me to make this journey - its all about Awareness, Choices and Time - the rest of it seems to take care of itself.
