In general, you can't expect exercise to really stop an underlying, inexorable brain condition such as Alzheimer's disease. However, there are grounds for hope. At least part of the problem in Alzheimer's seems to be vascular. Exercise can help reduce your chances of developing the full-blown condition. Also, exercise (including mental exercise) is likely to improve your cognitive reserve, delaying the time when the manifestations of conditions such as Alzheimer's disease become evident. While this may seem to be a very poor second prize, it's an important consideration. Conditions such as Alzheimer's gradually chip away at our brain's abilities (sometimes for 20 years or more). If you can resist or repair this erosion, even for a little while, you might be able to have a better quality of life. Even if you just gain an additional year or two, this can be very meaningful, for both you and the people around you.