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The New York Times (8/12, Reynolds) “Well” blog reported that a study published last month in PLoS One suggests that “overall, ‘a small dose of exercise’ may be sufficient to improve many aspects of thinking, and more sweat may not provide noticeably more cognitive benefit,” although it will improve aerobic fitness. The study of “101 sedentary older adults, at least 65 years of age,” revealed that “briskly walking for 20 or 25 minutes several times a week” appeared to be enough to help keep “brains sharp as the years pass.”
Blogfinger Medical Commentary: By Paul Goldfinger, MD, FACC
There have been so many studies about the value of excercise, that I have come to the conclusion that this therapeutic modality exceeds every other that is advocated to prevent all sorts of medical problems.
Whenever exercise is used for prevention, such as in cardiology, the mechanisms are never clearly elucidated. But all doctors and most people in general believe in the benefits of exercise. At a basic common sense level, it makes sense. Our bodies are so complex and with so many moving parts that it seems logical that “use it or lose it” is a reasonable mantra.
I recall other studies in the past that I raised my eyebrows over when they said that even small doses of exercise can be helpful, but I see now that it is likely to be true. You can benefit even if all you do is take a walk. Exercise makes the heart work harder and increases blood flow throughout the body, and from an evolutionary point of view, it makes sense that more blood is good for survival of the fittest, even if the fittest only get up from their computers and walk around the block.
The effects on the brain, as noted in this trial, are fascinating and believable although I don’t know why. Here is what we said about exercise in our book (2011): 
Exercise: Physical inactivity is associated with increased risk for coronary disease (CHD) and is considered a major risk factor. Increased physical activity results in a reduced risk for CHD. Advantages of exercise include a lower risk of diabetes, less tendency for blood to clot, improved lipids including higher HDL levels, improved sleep patterns, and reduced anxiety and depression. Other benefits reported include reduced colon cancer, reduced gallstones, reduced arthritis symptoms, lower blood pressure, less prostate enlargement and less osteoporosis (weak bones in the elderly).
A regular aerobic exercise program is a necessary adjunct to a good diet. It is very difficult to lose weight without exercise, but don’t be discouraged if the weight loss is slow. Exercise does not burn a lot of calories. Walking or running a mile will burn only about 100 calories, but exercise improves fitness and thus allows you to do more exercise and burn more calories. Also, the calories that are burned are more likely to be from fat than from carbohydrates, and the ability to keep weight off after losing is easier with regular exercise.
The amount of exercise necessary is controversial; however, a brisk walk 4-5 times per week can offer some protection. In the Nurses Health Study from Harvard, women who regularly engage in brisk walking reduced their risk of heart disease to the same degree as women who engaged in vigorous exercise. Some studies suggest that strenuous efforts are probably better.
An exercise prescription based on your heart rate is a good technique for judging how hard to exercise and your doctor can give you advice about this. Pulse monitors, (e.g. by Polar at polarusa.com or FitBit) are available in sporting goods stores. If you are healthy, you can get a fitness evaluation at the local YMCA and receive advice regarding an exercise program, or, if there were concerns about coronary risk, then a formal exercise stress test ordered by your physician would be appropriate. The Centers for Disease Control suggests that people should get at least 30 minutes of moderate activity on most, or preferably all, days of the week.
If someone has underlying heart disease, vigorous physical exercise can be risky, especially if the patient is not accustomed to regular exercise. Sometimes people who have no awareness that they have heart trouble can be at risk for complications during exercise. Individuals with heart disease or who are at risk for heart disease should speak to their doctors before engaging in strenuous forms of exercise. According to Barry Maron, MD, an expert on the subject of cardiac events during exercise, “The balance of the evidence supports the value and importance of participation in regular exercise regimens (NEJM.11/9/2000).”
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