In Leviticus 7:30, God commands the children of Israel to abstain from the consumption of blood. וְכָל־דָּם֙ לֹ֣א תֹאכְל֔וּ בְּכֹ֖ל מוֹשְׁבֹתֵיכֶ֑ם לָע֖וֹף וְלַבְּהֵמָֽה Commenting on this verse, Rashi notes that this prohibition includes not only the Land of Israel but also any other lands the Jews might dwell in. Rashi here does not discuss the reasons for such prohibition but Sforno in his commentary on Leviticus provides us with a more detailed explanation.
He writes that since “the life essence of living creatures is contained in it while the animal was alive” then “it is liable to transfer some of these animalistic characteristics kind to the persons ingesting it as food. The Torah wants to protect the Jewish people from absorbing more animalistic tendencies than we already possess.”
The phrase “You are what you eat” does not originate in Torah. It was first coined by the originator of culinary blogging, and the inventor of the low-carb diet, the French gastronome Jean Anthelme Brillat-Savarin.
In 1826, he wrote in his Physiologie du goût (The Physiology of Taste), “Dis-moi ce que tu manges, je te dirai ce que tu es.” [Tell me what you eat and I will tell you what you are].
Torah, however, has invented this principle a long time ago. Our dietary practices, as noted by Sforno, are not mere fads nor they relate just to the sustenance of physical health.
Kashrut and other prohibitions are introduced to make the Jews a holy nation, far removed from their animal origins.