The Bible states that we are not our own, that we were bought at a price, and therefore ought to honor God with our bodies. My question: If someone blatantly eats a diet of fast food and unhealthy meals, would this person be in blatant sin? -- Dale Gawrychowski
Yes, since 1 Corinthians 6 and many other chapters teach us to be good stewards of what is ultimately the Lord's property. I agree with you: when we eat poorly, refuse to be physically active, and otherwise mistreat our bodies, we are violating the will of God.
However, I think it is important not to legislate ("Thou shalt never eat fried chicken"). It is also important to avoid legalism (Colossians 2:20-23). What is significant are the patterns of our lives. Do we eat healthily most days? One day of binge eating is not going to ruin your health. Do we exercise or engage in some physical activity several times a week? Hopefully yes, but slipping up occasionally will probably have minimal effect on our well-being. What does ruin us, both physically and mentally, is a lifestyle that makes little room for activity, or eating habits involving oversized portions heavy with fat and carbs. Wine will not hurt you (and may even help you -- 1 Timothy 5:23!), but to overindulge is sinful (Proverbs 20:1; 23:19-21, 25-35). Probably the same applies to caffeine.
How can we persuade others to change unhealthy habits? Those of us who are in shape may be tempted to become judgmental of those who are not. There are more effective ways to change them than criticism alone. Personally modeling the healthy lifestyle speaks volumes. Inviting a friend to join you in a healthy meal or activity is another way to effect change. Showing people the scriptures is also very powerful. Films like Supersize Me and books like Fast Food Nation can also open eyes.
But back to the original question. I'm not sure the word "blatant" is always the right adjective--though there are degrees of being out of shape, so in some cases I think the word would be justified. If that person "blatantly eats a diet of fast food," as you put it, I suppose I would be urgent to have a word with him or her. Such a diet can kill you relatively quickly, and at the very least will shave a decade off your life, publicly embarrass your spouse and children, lead to diabetes, and rob you of the joy of living (John 10:10).
Finally, this and other common cultural sins need to be addressed, and in my opinion this is an area in which Christian teachers should speak out.