Harvard Medical School – February Newsletter
We hear so much about the physical benefits of regular exercise. But what effects do exercise and staying fit have on our cognitive function? Research has looked at how well people perform mentally, both while exercising and immediately after an exercise session. Other research has examined the association between fitness level and age-related cognitive decline.
Unlike physical measurements that can be taken with some precision, defining tests of mental performance and exercise to get reliable outcomes is a much greater challenge. Despite the obstacles, researchers have made some headway.
Mental performance tasks influenced by exercise
During a session of moderately intense aerobic exercise, mental performance improves in several measurable ways:
- reaction time
- perception and interpretation of visual images
- executive control processes
Of these, exercise exerts the most positive influence on tasks of executive control, such as:
- coordination of people, places, events, etc.
- working memory—the brain’s ability to temporarily store and manage the information required to carry out complex mental functions
- inhibition—the ability to block out unnecessary distractions
How long does improvement take?
Improved cognitive function begins to show at about 20 minutes of moderate intensity aerobic exercise and will be maintained for about another 40 minutes. Beyond 60 minutes of exercise, fatigue is likely to become a factor. For very fit individuals, enhanced mental performance could continue beyond an hour.
Once fatigue sets in, you start to lose the mental edge you have gained. If exercise continues, then mental performance actually will decline to a level lower than where you started.
These positive cognitive effects of exercising for 20 to 60 minutes are primarily related to increase in blood flow to the brain and stimulation of nerve cells to release more neurotransmitters (chemicals that send signals between brain cells). These positive effects will be maintained for a short time after the exercise session as long as you have not become overly fatigued. If you expect to have a long and hard workout, don’t plan on doing any important decision making or complex mental functions immediately afterward.
Influence of exercise intensity
Moderate intensity aerobic exercise that keeps you breathing a little faster and makes you sweat is probably the optimal intensity level to get the mental boost. If you monitor your heart rate to guide your intensity level, you want to strive for about 75% of your maximal heart rate.
With moderate intensity exercise, your body is activating the sympathetic nervous system and raising levels of adrenalin. These are likely the two main factors driving improvements in mental performance.
At high intensity of exercise, you will perceive your level of exertion and this sensation will likely interfere with concentration and ability to perform mental tasks.
Fluid intake and exercise
Dehydration is associated with a marked reduction in mental performance, independent of whether it is exercise-induced or caused by other factors. Researchers have shown that the decreased cognitive function immediately after exercise-induced dehydration can be quickly reversed once fluids are given to return body weight to the pre-exercise level.
One study found that hyper-hydration, extra fluid beyond what is lost, improved mental performance more than just replacing fluid losses. This principle should not be taken to extreme since over-hydration with water during prolonged exercise can dangerously lower blood levels of sodium (a condition called hyponatremia).
Water and sports drinks are equally effective at maintaining hydration during exercise. Sports drinks that contain simple carbohydrates (sugar) may provide a mental advantage for other reasons.
Carbohydrates to feed the brain
The brain needs a constant supply of glucose to function normally. During exercise, the body preferentially uses glucose as the main energy source for contracting muscles, including the heart and the muscles used to expand the lungs. At moderate intensity exercise of 20 to 60 minutes, there is still plenty of sugar available to the brain to allow the improved mental performance noted above. If exercise is more prolonged, especially at a high intensity level, then the amount of blood sugar available to the brain may be an issue.
Studies have shown that cognitive function is better when fluids are replaced with a sugar-containing solution rather than a drink without any calories. However, when blood glucose levels are measured, they are not low enough to say that hypoglycemia is the explanation. More likely, the sugar-containing solutions improve endurance and lessen the perceived level of exertion. Improving both of these factors positively impacts mental performance.
Boosting cognition now and for the future
In the short run, each session of aerobic exercise on a stable piece of equipment such as a stationary bike, treadmill, or elliptical machine has the potential to give you a double benefit for your time spent. Not only will you be improving your fitness, your ability to concentrate on and perform mental tasks also will likely be enhanced.
In the long run, physical activity appears to be at least as important in staying mentally sharp as keeping your mind active and maintaining strong social connections. Multiple studies have shown that people who exercise regularly will have less age-related cognitive decline and lose less brain tissue, as seen on MRI and PET scans.