We have known for a long time that exercise helps keep our bodies fit. Now, however, there is more and more evidence that exercise also promotes brain fitness. For example, a study recently published in Archives of Neurology showed that moderate-intensity exercise reduced the odds of developing mild cognitive impairment, which often precedes Alzheimer's disease, by 30% to 40% in the 1,324 study participants (median age 80).

But what type of exercise does the best job of strengthening the brain, and how much is needed for optimal effect?

What you need to know…

The Aging Brain

After age 40, we lose about 5% of our brain cells (neurons) per decade-a process that often accelerates in those who are age 70 and older.

Since the average person has about 100 million neurons, his/her cognitive reserves-that is, the brain's healthy cells that help compensate for damage by recruiting other brain areas to assist with tasks-may be sufficient to maintain mental agility...but not always.

The risk: Millions of Americans who are middle-aged and older start to "slip" in their mental capacities. Even if they have no signs of dementia, it may be harder for them to remember words, names or people than it once was. Or they may struggle to learn new information or take longer to think through problems and find solutions.

Why does this gradual mental decline affect some people much more than others?

Age-related loss of neurons, which affects all of us as we grow older, is just one factor. There's also a decline in dopamine, a neurotransmitter that controls motivation and motor function. This decline interferes with the electrical signals in the brain that allow the remaining neurons to communicate, which is necessary for memory, speech and other key brain functions.

Stronger Body, Bigger Brain

Scientists now know that the brain has plasticity, the ability to form new neurons and connections between neurons. This process can increase the brain's ability to take in information, process it and remember it.

What few people realize: Researchers have now identified a molecule-brain-derived neurotrophic factor (BDNF)—that's largely responsible for plasticity, and its levels increase dramatically with exercise. In animal studies at the University of California, Irvine, mice that exercised regularly were found to have BDNF levels that were about four times higher than those in sedentary mice. Many researchers think that humans show a similar increase.

The BDNF molecule could explain, in part, why people who exercise tend to have less memory loss, are less prone to anxiety and depression, and have up to a 50% lower risk of developing Alzheimer's disease or other forms of dementia than those who are sedentary.

Best exercises for the brain…

The Aerobic Formula

For overall fitness, the Centers for Disease Control and Prevention recommends doing some form of aerobic exercise, such as walking, for 30 minutes at least five days a week. But that's not enough for brain fitness.

Walking at an easy pace might increase your heart rate to about 50% of its maximum. But this has little effect on the brain. For optimal brain benefits, you need to exercise hard enough so that your heart is pumping at 70% to 75% of its maximum rate. Many treadmills have built-in heart-rate monitors, and heart-rate monitors that you wear are available at most pharmacies.

*To calculate your maximum heart rate, subtract your age from 220. The goal is to exercise at an intensity that raises your pulse to 70% to 75% of your maximum heart rate. The average 65-year-old man, for example, will need to raise his heart rate to about 108 to 116 beats per minute.

Good brands of heart-rate monitors: Garmin, Polar and Timex.

Important finding: One study published in Archives of Neurology found that people who walked or jogged on a treadmill for 35 minutes at a moderate intensity had improvements in cognitive flexibility (the ability to think flexibly and creatively, rather than merely repeating information) after just one session.

My advice: Exercise at a moderate intensity for 45 minutes to an hour, six days a week.

Remember, a moderate-intensity aerobic workout means elevating your heart rate to 70% to 75% of its maximum capacity. At this rate, you will most likely break a sweat and/or have difficulty carrying on a conversation. You can achieve this by jogging, bicycling, swimming or walking briskly and then pushing yourself harder when the exercise starts to feel easy.

Example: Once you're comfortable walking for 45 minutes to an hour at the pace described above, increase the intensity by walking faster, swinging your arms or holding hand weights.

If a moderate intensity is too much for you, exercising at 60% of your maximum heart rate has also been shown to offer some improvement in cognitive health.

Cross Training

To add variety to your aerobic exercise regimen, try some form of cross training. It combines different forms of exercise to target various parts of the body. Circuit training, in which you move quickly from one exercise machine to the next without pausing, is one form of cross training, Another is swimming followed by fast walking.

Cross training is useful because it generally results in a prolonged elevation in heart rate, the critical factor for generating BDNF. Cross training is desirable because it challenges not only your aerobic capacity and strength but also calls upon parts of the brain that govern coordination, planning, etc.

My advice: Whenever possible, incorporate some form of cross training into your regular workouts. In addition, balance exercises are a good way to round out your regimen. Try to work balance exercises, such as tai chi or even any fast-paced form of dancing, into your schedule once or twice a week. These exercises are especially good because they increase your heart rate and require you to think about what you're doing.

Bonus: The social interaction that occurs in tai chi or dance classes and other group activities increases serotonin, a neurotransmitter that reduces anxiety and depression, both of which can impair cognitive functions.

The Power of Mood Workouts

Research shows that the hippocampus (the brain's memory center) is 15% smaller in depressed individuals than in those without depression. Exercise may be one of the most effective ways to reverse depression-perhaps because it influences the same neurochemicals that are affected by antidepressants.

My advice: If you suffer from depression, be sure to follow the exercise guidelines described above. This may allow you to reduce or even eliminate antidepressant medication.

Don’t Forget Mental Workouts

Many different studies have shown that higher levels of education are associated with a decreased risk for dementia. But it doesn't matter where you went to school—or even if you went to school. The key factor is continued learning.

Like physical activity, mental workouts increase the number of connections between neurons that enhance memory and cognitive functions.

Perform mental workouts as often as possible.

Good choices: Try vocabulary quizzes, read books on subject matters you're not already familiar with or do any activity that requires you to push yourself intellectually.

Vitamin B-12—Now Available in A Nasal Spray

Studies show that up to 15% of older adults are deficient in vitamin B-12. Oral supplements and injections have long been used to treat this deficiency. The new, slightly pricier nasal spray is inhaled into both nostrils and absorbed directly into the bloodstream-but is made with a less bioavailable form of B-12 than injections and oral forms. A common side effect with frequent spray use is nasal irritation.

Insulin May Fight Alzheimer's

An insulin nasal spray improved or maintained cognitive abilities in Alzheimer's patients. Insulin might regulate the beta-amyloid proteins associated with the disease.

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