A small study involving 12 very fit older adults (aged 50-80; average age 61) found that, after stopping their exercise routines for 10 days, there was a significant decrease in blood flow to several brain regions, including the hippocampus and regions involved in the default mode network (both the hippocampus and the DMN show deterioration early in Alzheimer’s).

The study participants all had at least 15 years history of participating in endurance exercise and had recently competed in an endurance event; their usual exercise regimens had to entail at least four hours of high intensity endurance training each week. The group had a peak oxygen capacity above 90% for their age. This is a measure of the maximal rate of oxygen consumption of an individual and reflects their aerobic physical fitness.