Inactivity: A Hidden Cancer Risk

According to The American Institute for Cancer Research (AICR) expert report shows that physical activity lowers the risk of several cancers, both independently and by preventing weight gain. Yet if activity can help prevent cancer, can inactivity increase the risk? It's quite possible, suggests the emerging field of sedentary behavior.

Recent studies suggest that sitting for prolonged periods of time leads to unique physiological effects – independent of a person's structured activity time – that play a role in cancer and other chronic diseases. The research is still in its early phases, experts say, yet it may soon change the traditional view of healthy activity.

"There has been an explosion of research in [sedentary behavior] and health, and the preliminary evidence is pretty encouraging to make us think this is an important area of work to focus on for cancer risk," said Brigid Lynch, PhD, an epidemiologist at Canada's Alberta Health Services and author of a review on cancer risk and sedentary behavior.

"People who are active are getting health benefits of physical activity, and that's great, but what the research is finding is that it's not enough. We need to think about how people are spending the bulk of their day."

Possible Mechanisms

One of the first series of animal studies providing evidence that prolonged inactivity can lead to harmful physiologic effects came from findings with lipoprotein lipase (LPL), a protein produced in fat cells that plays a key role in cholesterol and other metabolic risk factors. "LPL, triglyceride metabolism and HDL are some of the most sensitive responses to inactivity we see," said Marc Hamilton, PhD, an inactivity physiologist at Pennington Biomedical Research Center and one of the leading scientists on the physiological effects of inactivity. Much of the data stem from animals, where scientists can clearly measure and control inactivity. The research has recently expanded to humans.

In one study, Hamilton and his colleagues found that fit men and women at a healthy weight experienced altered insulin action after only one day of inactivity. The participants acted as their own controls, sitting for long stretches at a time over one 24-hour period; sitting the same amount and eating more over a second 24-hour period; and moving about over the third 24-hour trial. Insulin activity was measured by giving participants an infusion of glucose after each trial.

"The [effect] appears to be something that doesn't take weeks or months or years to kick in, and that means that's independent of altering body fat," said Hamilton. Hamilton and his colleagues have also identified dozens of genes in our muscles whose expression depends upon activity or inactivity. The goal now is to show a clear cause and effect, he says.

Shifting Studies

In order to measure sedentary habits, researchers commonly ask study participants the amount of TV they watched and/or their occupation. Screen time, the amount of time people spend looking at their computer, watching TV or playing online games– is becoming a standard measure of sedentary behavior.

Advances in accelerometers, devices that measure movement, have also led to more affordable and accurate measures of activity levels, says Christine Friedenreich, PhD, Senior Research Scientist at Alberta Health Services-Cancer Care and part of the program committee for AICR's upcoming Research Conference.

The compelling data on sedentary behavior "has absolutely changed the way I do my research," says Friedenreich. Her research on physical activity and cancer risk now incorporates measurements of sedentary behavior as well as physical activity. Last year, for example, Friedenreich's study looking at lifetime physical activity found that endometrial cancer risk increased with sedentary occupational activity by 28 percent.

Excerpted from ScienceNow.

Issue 72 – June 15, 2011

American Institute for Cancer Research