In its primary meaning akal (to eat) is used in the sense of taking food by animals; this needs no illustration. It was afterwards observed that eating includes two processes -- (1) the loss of the food, i.e. the destruction of its form, which first takes place; (2) the growth of animals, the preservation of their strength and their existence, and the support of all the forces of their body, caused by the food they take. (Part I, Chapter XXX, opening paragraph)From there he illustrates how the first process caused "eating" to become symbolic of destroying ("A land that eats the inhabitants thereof"), and the second caused "eating" to become symbolic of things that cause growth and strength ("Come, buy and eat ...").
When it comes to the Lord's Supper, Jesus knew full well the shades of meaning of "eating", when he told us to take and eat the bread, his body given for us. When he said his body was "given for us," he focused on giving. So he may have meant for us to think of it as a gift rather than focusing on the first part of eating as destruction. After all, his body was not destroyed; "given for you" gives us permission to remember not only his death but also his resurrection. But he probably did mean for us to understand the second meaning of eating: what we "take and eat" causes our growth and gives us strength. It causes growth in our spirit, and strength in our faith. When he called up the imagery of food, I think he meant for us to understand that "my body, given for you" preserves us, and supports all the forces and attributes of our faith. Like any wholesome food, it renews our strength and vitality.