The church has a very strange relationship with bodies. It likes to specify precisely what parts of them can be displayed in public. It is very uncomfortable with talk about some of the more common things that bodies do. It would prefer that you not look at this painting. It would like to forget that God himself had a body for about 33 years. Simultaneously, spend 5 minutes listening to the conversations of churchgoers and you will hear 4.9 minutes of conversation about bodies, namely what is wrong with those of distant relations. The body is hard to ignore.
I exaggerate. The body is of course a part of God's creation declared "good" prior to the fall of man, is capable of great good and of great evil, and is a great part of oneself. To paraphrase Terry Eagleton, we may never own our bodies because we did not ask for them and cannot choose to rid ourselves of them save by suicide. Christians can acknowledge this fact and recognize the body as a gift of God. We also recognize that the effects of sin have made our bodies imperfect, subject to decline, disease, and death.
What them to do with this gift? Between neglect and obsession lies the way of care. I think that Christians have, if not a duty, then extremely strong reasons to care physically for themselves and for one another. Let us suppose that there is such a thing as proper bodily function. By striving to maintain this function, one then honors God's intention for the body. Suppose this reason is invalid. Good functioning of our bodies still makes all other tasks in life, even those that we think are primarily mental and spiritual, easier. Try getting work done when you're sick.
Practical steps realizing this duty: healthy diet, regular exercise, abstention from smoking, authentic encouragement of those working toward better health, consideration of careers in health, advocacy for a fair and efficient political arrangement of healthcare (whatever that may be).
I recognize that this is a very easy virtue for some, very hard for others. So it is with kindness, charity, prudence, hope and all the others. Moreover, I recognize that for some people, no efforts can produce good health, and that eventually, all bodies fail. But I'm advocating care, not immortality.