Exercise can reduce cancer risk

Everybody knows that aerobic exercise produces endurance, strength training builds muscles, and stretching exercises result in better flexibility.  But did you know that exercise creates new brain cells?  Or that it can boost insulin sensitivity?  That it can curb your risk of developing cancer?

Here are facts about exercises that may surprise you.

Exercise creates new brain cells

“We know that older adults on average perform slower and less accurately on cognitive tests than younger people,” says Charles Hillman, director of the Neurocognitive Kinesiology Laboratory at the University of Illinois at Urbana-Champaign.  “But what we also see is that older individuals who are more fit perform better to the point where they’re sometimes no different from young adults.”

Hillman and his co-workers asked 240 people between the ages of 15 and 71 to quickly pick out the direction of an arrow that was embedded among distracting elements. “The older adults were generally slower,” he notes. “But the more days a week they exercised, the faster their reaction time and the more accurate their responses.”

How does exercise boost your brain?  In laboratory animals, it increases blood flow to the brain and stimulates the growth of new cells, and new capillaries to distribute the blood and its nutrients.

In a 2006 study by University of Illinois researchers, 30 healthy but sedentary men and women aged 60 to 79 were put on an aerobic exercise training program.  After six months, their brain volume — the amount of grey and white matter — had increased.  That meant more brain cells and more connections between them.  Brain volume didn’t increase in 30 similar people who participated in a toning and stretching program.

 â€œAerobic exercises increase the supply of a protein called brain-derived neurotrophic factor, or BDNF, which protects brain neurons and promotes the growth of new nerve cells and synapses that are related to learning and memory,” says Hillman.  BDNF is active in the hippocampus, an area deep in the brain that is essential for memory, especially for what is called relational memory (see diagram on Page D-2).  That’s the ability to make logical connections among pieces of information and to navigate in space.  The hippocampus tends to shrink as people age, and it’s one of the first parts of the brain to be damaged in Alzheimer’s disease.

In rats and mice, aerobic exercise increases the size of the hippocampus and improves spatial memory.  The same appears to be true for older humans.  Hillman’s colleagues recruited 165 healthy men and women between the ages of 59 and 81, and measured how long they can last on a treadmill.  The more fit they were, the larger their hippocampus was on MRI scan, and the better they did on a test for spatial memory.

Impaired spatial memory — having trouble finding your way or knowing what goes where — is a major reason older people lose their independence, points out one of the researchers, Art Kramer.  “This study shows that there’s a significant and substantial relationship between how fit you are and how good  your spatial memory is — a type of memory that we need all the time,” he notes.

Aerobic exercise also appears to boost executive function.  That’s the ability to plan or make decisions, correct errors, or react to new situations.  In 2004, Kramer and his colleagues put 29 healthy adults, aged 58 to 77, on either an aerobic walking or a stretching program three times a week.  After six months, the walkers performed better on a test of executive function.  “We know now that you can teach an old dog, new tricks,” says Hillman.  “Older brains can grow nerve cells, too.”

Strength training like weightlifting helps build muscles and strengthen bones, but it’s not yet clear whether it affects the brain to the same degree as aerobic exercise, he notes.
 

Exercise boosts insulin sensitivity

“One of the most consistent effects of both aerobic and strength training is that they improve insulin sensitivity,” says Ben Hurly, a professor and exercise physiologist in the Department of Kinesiology at the University of Maryland in College Park.

Insulin is a hormone that allows blood sugar to enter the body’s cells, where it’s stored or used as fuel.  However, as we age or when we put on weight, our bodies don’t respond as well to insulin and can become insensitive, or resistant, to the hormone.  Insulin resistance increases the risk of heart disease and, if blood sugar levels keep rising, type 2 diabetes.

“There are dozens of studies showing that when people do aerobic exercise training, their insulin sensitivity improves within a short time,” says Hurley.  “We and others have now shown that strength training can also improve insulin sensitivity by 20 percent within a matter of months.”

For example, researchers at the University of Maryland in Baltimore put nine overweight or obese men in their 60s on a program of treadmill walking or jogging and 13 similar men on a strength-training program.  After working out three times a week for six months, the aerobic exercisers increased their cardio-respiratory fitness by 16 percent, the resistance exercisers increased their arm and leg muscle strength by 45 percent, and both groups improved their insulin resistance by 20 to 25 percent.  “The average physician who deals with insulin resistance and diabetes recognizes that aerobic training is helpful, so they’ll prescribe it,” says Hurley.  “But hardly anyone will prescribe strength training, even though it works just as well if done properly.”

Contrary to what many people believe, says Hurley, the reason strength training is effective isn’t because it builds muscle mass.  “Both aerobic exercise and strength training increase the number of proteins called glucose transporters — particularly GLUT4 — which move glucose from the blood into muscles and fat cells,” he notes.  The more GLUT4 you have, the better your cells respond to insulin.  “The effect seems to last at least 24 hours, if not 48 hours,” says Hurley.  “That’s long enough if you exercise at least every other day.”
 

Never too late to start

“Physical activity is central to reducing your risk of cancer,” says Michael Thun, MD, vice president emeritus of epidemiology and surveillance at the American Cancer Society.  “There are two ways that regular exercise can potentially do that,” he explains.  It can indirectly lower your risk by keeping off excess weight, or it can work directly on cancer risk.

“With respect to colon cancer, the direct effect is very well documented.  Studies show that even light to moderate regular activity is associated with lower risk compared with inactivity.” In 2009, researchers at the Washington University School of Medicine in St. Louis reviewed 52 studies of exercise and colon cancer. The most active people were about 21 percent less likely to be diagnosed than their least active counterparts. Researchers aren’t sure though, how activity may protect the colon.  “Bowel motility is one possible mechanism,” says Dr.Thun.  “Just moving things through the bowel is better than having them sit there.”  But, he cautions, “It’s much easier to propose mechanisms than to prove them.”

In contrast, it may take more than brisk walking or other moderate activity to lower the risk of breast cancer.  “With colon cancer, just not being sedentary is a good thing,” says Dr. Thun.  “With breast cancer, there’s fairly consistent evidence that the reduced risk occurs only with greater, moderate-to-vigorous activity.”  And it may never be too late to start.  Researchers tracked some 119,000 women in their 50s and 60s for seven years.  Those who reported more than an hour a day of moderate-to-vigorous activity — even if they hadn’t exercised earlier in their lives — were 16 percent less likely to be diagnosed with breast cancer than those who rarely did moderate-to-vigorous exercise.

“It looks like women who are more aerobically fit are less likely to die of breast cancer,” says Steven Blair, an exercise epidemiologist at the University of South Carolina.  Again, researchers aren’t sure why. One possibility: “Physical activity reduces the exposure of tissues to insulin-like growth factor (IGF),” notes Dr. Thun. If IGF promotes cancer, having less in the bloodstream may help.

Exercise may also indirectly prevent cancer by keeping the pounds off.  While “the direct effects of exercise are most clearly on large bowel and breast cancer,” says Dr. Thun, the impact of weight gain is broader.  “Weight gain is associated with quite a few cancers, notably postmenopausal breast cancer, colorectal cancer, endometrial cancer, pancreatic cancer, and adenocarcinoma of the lower esophagus,” he notes.  “And the list isn’t complete yet because there is accumulating evidence that obesity is a risk factor for several of the blood-forming leukemia and lymphoma.”

That’s why medical organizations and scientists now recommend at least 30 minutes of moderate-to-vigorous physical activity at least five days a week, although many say that 45 to 60 minutes is preferable.  “Just how much exercise is enough and how large is the benefit are still gray areas,” says Dr. Thun.  But the overwhelming reality is that most people are too sedentary, and the issue is not what the perfect amount is, but how we can all increase the amount of exercise we’re getting!