Recker says that researchers speculate, however, that it has to do with exercise triggering osteocytes (the most mature bone cells) to instruct bone-building cells called osteoblasts to increase bone formation.
Wards off diabetes
According to Gerald Shulman, a cellular and molecular physiologist at Yale University School of Medicine in New Haven, Conn., exercising may prevent and even reverse type 2 diabetes.
Diabetes type 2 is a disease in which the body begins to ignore or fails to produce enough insulin (a condition called insulin resistance). If muscles and other tissues cannot absorb glucose from the blood, nerve and blood vessel damage ensues, paving the way for heart disease, stroke and infections.
"We've shown that in insulin-resistant individuals… build up of fat leads to biochemical reactions that interfere with the glucose-transport mechanism [leading cells to block the activity of insulin]," Shulman says. But physical activity helps reverse this process. He notes that when someone runs, cycles or does other vigorous exercise, muscle contractions ramp up production of adenosine monophosphate-activated protein kinase (AMPK), an enzyme that promotes the breakdown of the fats interfering with the cells' glucose transporters.
"It is very likely that there are differences in the extent to which individuals respond to exercise, just as there are in responses to medications," says Ronald Sigal, a clinical epidemiologist at the Ottawa Health Research Institute in Canada. Leon agrees, pointing to research demonstrating that exercise leads to varying decreases on visceral body fat (the fat surrounding organs), one of the key risk factors for developing type 2 diabetes.
Makes you smarter
Researchers have long believed that exercise boosts smarts but there was not any hard scientific evidence until a few years ago. Now, says Fernando Gomez-Pinilla, a neurosurgery professor at the University of California, Los Angeles, it's known that exercise increases levels of some molecules in the brain that are very important for cognition.
One such chemical is brain-derived neurotrophic factor (BDNF), a molecule that promotes the growth and survival of brain cells as well as communication between them. Studies in rats show that physical exercise boosts BDNF levels in the hippocampus, a brain structure critical for learning and memory formation, which in turn helps them remember how to navigate their way through underwater mazes. "The more exercise, the more changes in the brain; we found almost a linear relationship," Gomez-Pinilla says. "If we block the BDNF gene, we block this capacity of exercise to help learning and memory."
Numerous studies suggest that fitness enhances cognition in humans as well. A randomized clinical trial published recently in the Journal of the American Medical Association found that people 50 years and older with memory problems scored higher on cognitive tests after a six-month workout regimen. Those study participants assigned to exercise programs scored 20 percent higher than their sedentary peers at the end of the six months, and maintained a 10 percent edge one year after the trial ended.
But skeptics warn that not enough research has been done to confirm a link between exercise and human brain power. A recent review of studies on cognition in older adults (primarily those age 65 and older) by Dutch scientists published in the Clinical Journal of Sport Medicine concluded that "beneficial effects of various exercise programs on aspects of cognition have been observed in studies among subjects with and without cognitive decline. The majority of the studies, however, did not find any effect."
The relationship between exercise and weight loss is complicated. Contrary to popular belief, working out at the gym every day will not necessarily lead to weight loss. "It is reasonable to assume that persons with relatively high daily energy expenditures would be less likely to gain weight over time, compared with those who have low energy expenditures," write the authors of the American College of Sports Medicine (ACSM) and the American Heart Association's (AHA) 2007 guidelines. "So far, data to support this hypothesis are not particularly compelling."