Bone--watch out for reversals of fortune.
In order to preserve bone density gains, the weight bearing exercise stimulus must be ongoing. Here is a great example of the old "use it or lose it" adage. And while running itself is great for maintaining bone strength, it may not be good enough. First, the upper body does not benefit at all from the bone-building effects of running. Further, bones may respond better to the overload experienced during weight lifting since bone building occurs when the stress the bone experiences exceeds its previous adaptations. Strength training should be done one to three times a week and the major muscle groups worked to the point of fatigue (after eight to 12 repetitions you can't do another without losing form).
Bone health also includes calcium and vitamin D in your diet. Most Americans find it is difficult to consume adequate amounts of calcium, and supplements can make up the difference. They are absorbed best with food but calcium can interfere with iron absorption and certain medications. So if you're supplementing, don't take iron and calcium at the same time (and check with your doctor if you're taking medication of any kind). Calcium supplements may contain high levels of lead, so look for products marked "purified" or those with the "USP" label (United States Pharmacopeia7, which certifies that testing has been done). Don't forget that vitamin D is needed to utilize the calcium. Your skin produces vitamin D in response to sunlight but using sunscreen reduces the amount of vitamin D your body can make.
(Journal of Bone and Mineral Research, 2000, Vol. 15, No, 12, pp. 2495, The Physician and Sportsmedicine, 2000, Vol. 28, No. 2, pp. 69-84; Nat'! Osteoporosis Foundation, www.nof.org)
DAILY CALCIUM NEEDS Pre-menopausal women 1,000 mg Postmenopausal women on hormone replacement 1,000 mg Premenopausal youth (under age 25) 1,400 mg Postmenopausal women without hormone replacement 1,500 mg Adult men 1,000 to 1,200 mg Daily Vitamin D Needs 400 to 800 international units