Does having new life in Christ mean that I should expect and strive for better physical health? How much does God want me to care for my physical body?
Good question. He does want you to care for your physical body. A couple of texts come to my mind.
One is that our bodies are the temple of the Holy Spirit (1 Corinthians 6). And the context there is not giving your body to a prostitute. But the implication is that these bodies are holy and are reverential.
It kept me from smoking as a teenager! It really did! My mother's statement, "Son, your body is the temple of the Holy Ghost, and giving yourself lung cancer for that kind of pleasure would not treat the Holy Spirit rightly." That worked for me! Still does.
But there's another text that comes closer. Earlier in that chapter he is not dealing with prostitutes; he is dealing with food. The slogan in Corinth is—I think it's a slogan in Corinth—"The stomach for food and food for the stomach, and both will be destroyed in hell," which implied in their docetic way, "Eat all you want. It doesn't matter what you eat." And Paul said, "True statement, but I will not be enslaved by anything!" And the context there is food.
The reason people are unhealthy is because they're enslaved. They are enslaved to laziness, and they are enslaved to food. So they eat too much and they exercise too little. And they have heart attacks and get diabetes. And God would consider that a spiritual issue.
So we should strive spiritually. What did Paul mean when he said, "I will not be enslaved by anything?" He meant, "Christ is your master!"
And a third text that comes to mind is, "The fruit of the Holy Spirit is love, joy, peace, patience, kindness, goodness, gentleness, faithfulness, and ..." What? "Self-control," egkrateia (Galatians 5:22-23).
And again, largely sexual self-control is in view; but it's the same thing. The word "self-control" is not the best translation maybe, because it's really a work of the Holy Spirit.
So we should fight against anything that makes us unhealthy. If overeating makes us unhealthy, fight it by the Spirit. If laziness and lack of exercise makes us unhealthy, fight it with the power of the Holy Spirit. That is, believing the promises of God, praying down the Holy Spirit, and then biting the bullet and denying ourselves.
Christianity is self-denial ... for a higher joy. And I don't want my Christian hedonism to be taken to mean that everything is easy. It isn't. Hardly anything worth doing is easy, until we get to heaven. Then it will all be easy.
He cares about our bodies. He gave them to us. He would like them to be healthy and last a long time, until he takes them.