Biblical reasons for maintaining good health
1 Our body is a temple of the Holy Spirit:

Or do you not know that your body is a temple of the Holy Spirit who is in you, whom you have from God, and that you are not your own? (1 Corinthians 6:19 NASB)
2 Our bodies are members of Christ:

Do you not know that your bodies are members of Christ? Shall I then take away the members of Christ and make them members of a prostitute? May it never be! (1 Corinthians 6:15; NASB)
3 We are told to present their bodies to God for service:

Therefore I urge you, brethren, by the mercies of God, to prsent your bodies a living and holy sacrifice, acceptable to God, which is your spiritual service of worship (Romans 12:1; NASB)
4 All that a believer does with his body should glorify the Lord:

Whether, then, you eat or drink or whatever you do, do all to the glory of God (1 Corinthians 10:31; NASB)

According to my easrnest expectation and hope, that I will not be put to shame in anything, but that with all boldness, Christ will even now, as always, be exalted in my body, whether by life or by death (Philippians 1:20; NASB)
God uses the PROMISE of good health and long life to motivate obedience to His Word, so keeping God's Word prolongs life and promotes health:

My son, don not forget my law, but let your heart keep my commands; for length of days and long life and peace they will add to you (Proverbs 3:1-2)

My son, give attention to my words. incline your ear to my sayings. Do not let them depart from your sight; keep them in the midst of your heart. For they are life to those who find them, and healthe to all their whole body (Proverbs 4:20-22)
 
Biblical aids to health:
1 FEARING GOD aids good health: includes trust and the right attitude and thinking about God. A right relationship with the Lord greatly aids good gealth:

Trust in the Lord with all your heart, and lean not on your own understanding; In all your ways acknowledge Him, and He shall direct your paths. Do not be witse in your own eyes; fear the Lord and depart from evil. It will be health to your flesh, and strength to your bones. (Proverbs 3:5-8)

The fear of the Lord prolongs life, but the years of the wicked will be shortened (Proberbs 10:27)
2 Fear of GOd produces OBEDIENCE TO HIM, and Obey God will prolong life and give good health:

so that you and your son and your grandson might fear the Lord your God, to keep all His statutes and His commandents, which I command you, all the days of your life, and that your days may be prolonged (Deuteronomy 6:2)

My son, do not forget my law, but let your heart keep my commands; for length of days and long life and peace they will add to you (Proverbs 3:1-2)
3 RIGHT SPEECH or communication aids health:

There is one who speaks like the piercings of a sword, but hte tongue of the wise promotes health (Proverbs 12:18)

The one who guards his mouth preserves his life; the one who opens wide his lips comes to ruin (destruction-NKJV) (Proverbs 16:24)

Pleasant words are a honeycomb, sweet to the soul and healing to the bones (Proverbs 16:24)
4 HONORING PARENTS prolongs life:

Honor your father and your mother, that your days may be prolonged in the land which the Lord your God gives you (Exodus 20:12)

Children, obey your parents in the Lord, for this is right. Honor your father and mother (which is the first commandment with a promise), that it may be well with you, and that you may live long on the earth (Ephesians 6:1-3)
5 LISTENING TO GODLY WISDOM is physically benefical:

The teaching of the wise is a fountain of life, to turn aside from the snares of death (Proverbs 13:14)

There is one who speaks rashly like the thrust of a sword, but the tongue of the wise brings healing (Proverbs 12:18)
6 THE FAIR TREATMENT OF PEOPLE affects health:

A leader who is a great oppressor lacks understanding, but he who hates unjust gain will prolong his days (Proverbs 28:16)
7 HELPING PEOPLE IN NEED aids good health:

And if you give yourself to the hungry, and satisfy the desire of the afficted, then your light will rise in darkness, and your gloom will become like midday. And the Lord will continually guide you, and satisfy your desire in scorched places, and give strength to your bones; and you will be like a watered garden, and like a spring of water whose waters do not fail (Isaiah 58:10-11)
8 A RIGHT ATTITUDE aids health:

A tranquil heart is life to the body, but passion is rottenness to the bones (Proverbs 14:30)

In general, people with a godly view of life revover faster and are not as sick as those with a sinful view of life.
9 A CHEERFUL OUTLOOK has many benefits:

All the days of the afflicted are bad, but a cheerful heart has a continual feast.... Bright eys gladden the heart; good news put fat on the bones (Proverbs 15:15, 30)

The light of the eyes rejoices the heart, and a good report makes the bones healthy (Proverbs 15:15)

A joyful heart is good medicine, but a broken spirit dries up the bones (Proverbs 17:22)

A cheerful person happily takes care of his body, God's temple. When uncomfortable thins are necesary in that care (such as exercise, dietary restriction, surgery), this person seems beyond the discomfort to God's goals. The body responds better to this view of life.
10 Most biblical references to FOOD are to specific items: to things such as wine or honey, and their proper use. The main emphasis of references dealing with food is on gluttony:

Have you found honey? Eas on as much as you need, lest you be filled with it an dvomit....It is not good to eat much honey; so to seek one's own glory is not glory (Proverbs 25:16, 27)

When you sit down to eat with a ruler, consider carefully what is before you; And put a knife to your throat if you are a man given to appetite....For the drunkard and the glutton will come to poverty, and drowsiness will clothe a man with rags (Proverbs 23:1-2, 21)

~The Christian Counselor's Medical Desk Reference, Robert D. Smith, M.D.