There is no doubt that exercise yields benefits across a spectrum of health: reduced blood pressure, reduced inflammation, reduced blood coagulation, better weight control, stronger bones, less depression, reduced risk for heart attack.
Exercise also influences blood sugar. Diabetics understand this best: Exercise reduces blood sugar 20, 30, 50 or more milligrams. A starting blood sugar, for instance, of 160 mg/dl can be reduced to 80 mg/dl by jogging or riding a bicycle. (I recently had brunch at an Indian restaurant with my family. Blood sugar one-hour postprandial: 134 mg/dl. I was sleepy and foggy. I got on my stationary bike and pedalled at a moderate clip for 60 minutes. Blood sugar: 90 mg/dl.)
Could the reduction of blood sugar with exercise be THE reason that exercise and physical activity provide such substantial benefits?
Think about it. Reduced blood sugar:
1) Reduces risk for future cardiovascular events.
2) Reduces glycation of proteins, i.e., reduced glucose binding to proteins like the ones in artery walls and the lenses of your eyes.
3) Reduces blood coagulation
4) Reduces endothelial dysfunction (abnormal artery constriction that leads to atherosclerosis)
This might explain why it doesn't require high levels of aerobic activity to derive benefit from exercise, since even modest efforts (e.g., a 15-minute walk after eating) reduce blood sugar substantially.
The incredible 33-year, 18,000-participant Whitehall study tells us that a postprandial (after-eating) blood sugar of an impossibly-difficult 83 mg/dl is required to erase the excess cardiovascular risk of blood sugar. Could this simply be telling us that physical activity or exercise is required to suppress blood sugars to these low levels?
It makes me wonder if an index of the adequacy of exercise is your post-exercise blood glucose.