Exercise strengthens the pumping force of your heart, increases blood flow to your brain, increases exercise tolerance, reduces body weight, lowers blood pressure, reduces bad cholesterol (both LDL and total), increases good cholesterol (HDL), and increases insulin sensitivity, all of which enhance health and reduce the risk for diseases that can affect brain functioning, such as cardiovascular conditions.
Try walking, jogging, hiking, swimming, cycling, or cross-country skiing. Strength exercises such as sit-ups, push-ups, and weight-lifting are also good. Try to get at least 30 minutes of exercise a day for a healthy brain and body.
Sources:
Brain health. Alzheimer's Association. October 18, 2007. http://www.alz.org/we_can_help_brain_health_maintain_your_brain.asp
Shankle, W. R., & Amen, D. G. 2004 Preventing Alzheimer's: Ways to help prevent, delay, detect, and even halt Alzheimer's disease and other forms of memory loss. New York: G. P. Putnam's Sons.

