Picture of Domhnall MacAuleyDomhnall MacAuley is a CMAJ Associate Editor and professor of primary care in Northern Ireland, UK


Is being sedentary the new smoking? Many have posed this question and there are some parallels between how our knowledge evolved about smoking and how it is evolving regarding sitting too much. While the hazards of physical inactivity are now well known, however, there hasn’t yet been the enormous culture change that we have seen in our attitudes towards smoking. When smoking cessation was primarily a medical issue there were modest reductions in smoking rates but it was only with societal change, political will and legislation that we see major impact. There is increasing awareness of the influence of social, cultural and environmental factors in encouraging physical activity but we have yet to see the same political will and change in population mind-set. In the meantime, research continues to underline the benefits of physical activity and the hazards of a sedentary lifestyle, and here are seven recent research studies that made a significant media impact.

How much physical activity is needed? In April 2015, a paper in JAMA Internal Medicine looked at all cause mortality compared to the 2008 US Physical Activity Guidelines that recommend a minimum of 75 vigorous-intensity or 150 moderate-intensity minutes per week (7.5 metabolic-equivalent hours per week) of aerobic activity. They pooled leisure time physical activity data from 6 studies including 661 137 men and women (median age, 62 years; range, 21-98 years) with 116 686 deaths in a median follow-up time of 14.2 years and found that, even among those who were less active than the guidelines, there was 20% lower mortality risk compared to those who were inactive. There was 31% lower risk at x1-2 times the recommended minimum and a dose response relationship until the upper threshold for mortality benefit of 39% lower risk at 3 to 5 times the recommended level. It seems that the greatest gain is by undertaking any activity and one might argue that the incremental benefit above the recommendations is modest. There is a great dose response graph- their figure and the paper is supported by a nice editorial that points out that “a lot of the mortality reductions were seen in people only one step away from doing no leisure time physical activity.” The message seems to be “Just do something!”

When can you stop? The bad news for middle aged couch potatoes, comes from a study of middle aged Australians published just a few days later.

The Active Australia study was a prospective cohort study of 204 542 adults aged 45 to 75 years in New South Wales, with a follow-up of 6.52 years (1 444 927 person-years of follow-up) with 7435 deaths, looking at leisure time physical activity. Compared to those with no moderate or vigorous activity the hazard ratio was 0.66 for those active from 10 to 149 min/week; 0.53 from 150 to 299 min/week and; 0.46 for 300 min/week or more of activity. But, there was dose-response (inverse) relationship between proportion of vigorous activity (of all moderate and vigorous) and mortality. Their take home message was that vigorous activities should be endorsed by activity guidelines.

Is there a link with cancer? Most epidemiological research on the benefits of physical activity relate to cardiovascular disease and all cause mortality but, in keeping with our theme, could there be a relationship between physical inactivity and cancer? A major systematic review published in the Journal of the National Cancer Institute in 2014 included 43 observational studies with 68936 cancer cases and looked specifically at sedentary activity. They found an increased risk for colon cancer of 1.54 for TV viewing time, 1.24 for occupational sitting time, and 1.24 for total sitting time. For endometrial cancer the risk was 1.66 for TV viewing time and 1.32 for total sitting time and, for lung cancer was 1.21 with overall sedentary behaviour. Sedentary behaviour was unrelated to cancers of the breast, rectum, ovaries, prostate, stomach, oesophagus, testes, renal cell, and non-Hodgkin lymphoma.

More recently, in March 2015, a study published in JAMA Oncology of 13 949 men attending the Cooper Clinic, compared baseline fitness with subsequent incidence of lung, prostate, and colorectal cancer from 1999 to 2009. The hazard ratios for lung, colorectal, and prostate cancer incidence among men with high cardiorespiratory fitness were 0.45, 0.56, and 1.22 respectively. It was interesting, but difficult to explain, the inverse association between midlife cardiorespiratory fitness and lung and colorectal cancer but not with prostate cancer. Of those diagnosed with cancer, midlife fitness was associated with 32% risk reduction in all cancer-related deaths and a 68% reduction in cardiovascular disease mortality following a cancer diagnosis.

There are other reasons why being sedentary is harmful. In January 2015, in the Journal of Physical Activity and Health, a study of 14,631 people, of whom 2,910 were aged 60 or older, from National Health and Nutrition Examinations Surveys (NHANES) looked at the association between disability in activities of daily living and the daily percentage of sedentary time using accelerometers. They found a strong relationship between greater time spent in sedentary behaviour and disability, independent of the time spent in moderate or vigorous activity. It’s not just activity- it’s sitting too. In the context of our increasingly inactive lifestyle, we also know from a CMAJ study about the link between ownership of common household devices, such as televisions, cars and computers and obesity and diabetes.

What is the role of doctors? As with smoking, epidemiologists identify the risks and clinicians see the results. Evidence supports the guidelines that suggest that even moderate intensity physical activity at even a minimum of 150 minutes a week is effective (equivalent to about 30 minutes of physical activity most days comprising, for example, three 10 minute walks). We know the answer: “Move, Now.” Like smoking cessation, however, medical advice is not enough and increasing population physical activity requires more than doctors alone.