Being physically fit could hold back the advance of Alzheimer's disease, University of Kansas researchers have suggested.  Their study, published in the journal Neurology, looked at 121 people aged over 60, around half of them in the early stages of the disease.  Those with Alzheimer's who were less fit had four times more signs of brain shrinkage than those who were fit.  The Alzheimer's Research Trust said other research showed exercise reduced the risk of dementia.  People with early Alzheimer's disease may be able to preserve their brain function for a longer period of time by exercising regularly.  Other studies looking at the relationship between dementia and exercise have tended to focus on whether being active can reduce the risk of the condition developing in the first place.