A new word has been coined to describe a prevailing modern lifestyle condition - sedentarism. And as Peter Dingle PhD points out it's easy to overcome. Just get up and move.
Sitting for any length of time may not be good for us. More and more evidence shows that sedentary behaviours including sitting, watching television, using a computer, and driving a car are risk factors, independent of physical activity, for adverse chronic disease in adults such as obesity, cancer, heart and kidney disease, chronic neck and back pain, as well as premature death.
The act of sitting increases your chances of developing all these conditions independent of how much exercise you do and how active you are outside of sitting. You can do a long run every night, but if you sit too long during the day you still increase your risk of these chronic conditions.
Unfortunately, people have grown more sedentary during the 20th and 21st centuries. On average, adults spend an average of eight hours per day sitting, increasing to 10 or more hours a day in older age and young people between the age of six and 20 spend on average 40 to 60% of the day sitting, often in prolonged and uninterrupted bouts (1).
In Australia, at least according to the ABS, we sit for around 39 hours a week and on average 10 hours at work. But these are average figures - clerical and administration workers have an average of 22 hours sitting at work. Apparently we sit for 13 hours a week in front of television and, in front of a computer for non-work related activities, eight to 24 year olds sit for nine hours a week. Remember, these are just averages and as I don't sit down much (I use a standing desk) someone is sitting a lot more than average. Also however, people who tend to sit at work also tend to sit more in other locations, adding to the increased burden of sitting.Cancer Risk
Studies have linked sitting to a greater risk for colon, prostate, breast and endometrial cancers. In a review of 18 studies, 10 found statistically significant, positive associations between sedentary behaviour and cancer outcomes. Sedentary behaviour was associated with increased colorectal, endometrial, ovarian, and prostate cancer risk; cancer mortality in women; and weight gain in colorectal cancer survivors (2). In a study of 5380 women and 5788 men, a standing/walking occupation was associated with a 32% lower risk of all-cause mortality and a 40% risk reduction in cancer mortality, compared to sitting occupations. Too much sitting is also associated with cancer survival. Sitting is associated with weight gain around the waist, insulin resistance, and markers of inflammation, which may contribute to adverse cancer outcomes (disease progression, recurrence, or death) and to the development of other chronic disease. The daily sedentary time was correlated with the protein levels of inflammatory biomarkers (3,4,5),which is associated with cancer incidence and survival. Initial studies indicate that cancer survivors spend two thirds of their waking hours sitting (6).
Perils of TV
A number of studies have now found that ownership of a car and a TV was associated with an increased risk of heart attack (7,8). Several in-depth studies have reported television viewing time to be detrimentally associated with weight gain, Type 2 diabetes mellitus, some cancers, abnormal glucose metabolism, metabolic syndrome, and other cardiovascular risk factors. According to the American Council on Exercise, every hour you sit in front of the television slashes your life expectancy by 22 minutes. Watching television for six hours a day takes five years off your life.
People who watched the most TV in an 8.5 year study had a 61% greater risk of dying than those who watched less than one hour per day. In a study of 8,800 adults 25 years of age or older, each extra hour of television watching per day was associated with an 18% increase in deaths from heart disease and an 11% increase in overall mortality. People who watched TV for at least four hours a day were 80% more likely to die of cardiovascular disease than those who watched two hours or less, and 46% more likely to die of any cause (9). In a study that included 61,395 men and 73,201 women aged 45 to 75 years, the longest sitting duration, 10 or more hours each day compared to five or less hours a day, was associated with increased all-cause (dying of anything)(11%) and cardiovascular (19%) mortality. The risk of five hours each day compared to one hour each day of sitting and watching TV were 19% in men and 32% for all-cause mortality (10).
A review of prospective studies of screen time (TV and computer) and sitting time has shown that greater sedentary time is associated with an increased risk of fatal and non-fatal cardiovascular disease (CVD). Compared with the lowest levels of sedentary time, risk estimates ranged up to a 68% increase for the highest level of sitting time and 125% increase for the highest level of screen time. In six studies of screen time, for each two hours of sitting there was a 17% higher risk of CVD. However in two studies, the increased risk of CVD was 5% for each extra two hours of sitting in front of a screen (11).
In a study of 2,761 women and 2,103 men without clinically diagnosed diabetes, sitting time was detrimentally associated with waist circumference, BMI (body mass index), weight gain, blood pressure, fasting blood fats, HDL cholesterol, two hour post load plasma glucose, and fasting insulin in both men and women. TV viewing time was detrimentally associated with all metabolic measures in women and all except HDL cholesterol and blood pressure in men (12). An analysis of 18 studies including a total of 794,577 participants found that those who sat for long periods of time compared with the lowest sitting time was associated with a 112% increase in the risk of diabetes, a 147% increase in the risk of cardiovascular events, a 90% increase in the risk of cardiovascular mortality and a 49% increase in the risk of all-cause mortality (13). In a study of 222,497 individuals 45 years or older, the risk of all-cause mortality (any type of death) increased by two percent for those sitting for four to eight hours, 15% for eight to 11 hours and 40% for those sitting more than 11 hours a day compared to less than four hours each day and was independent of physical activity levels (14,15,16). Gaining Weight
Sitting is also a risk factor for weight gain and diabetes. That is, the more you sit the greater the risk of developing these conditions independent of other activities. In a meta-analysis using 48 studies, a consistent relationship of self reported sedentary behaviour with mortality was found with weight gain from childhood to the adult years (17). That is, the greater the sedentary time in childhood, the greater the weight gain. In a cross-sectional study of 8,357 adults aged 35 years or more and free from diabetes, time spent watching television for women was positively associated with increased 2 hour plasma glucose, fasting insulin, and insulin resistance and pancreas Beta cell function. These findings highlight the unique negative relationship of sedentary behaviour of television viewing time and glycaemic measures, independent of physical activity time and weight, and suggest an important role for reducing sedentary behaviour in the prevention of Type 2 diabetes and cardiovascular disease, especially in women (18).
In theory, this may be in part because non-exercise activity is a much greater component of total daily body energy expenditure than exercise or because any type of brief, yet frequent, muscular contraction throughout the day - such as standing or moving - may create healthy molecular signals which positively alter the body's biochemistry and metabolism. One of these is a particular muscle chemical, lipoprotein lipase (LPL), a protein enzyme that has been studied in depth because it has a central role in several aspects of lipid (fat) metabolism (19,20). LPL controls plasma triglyceride (fat) breakdown (burning the fat into energy), shifting the cholesterol from LDL to HDL and other metabolic risk factors that decrease when we stand or are involved in physical activity.
Experimentally reducing movement by sitting had a much greater negative effect on LPL regulation than a positive effect of adding vigorous exercise training on top of the normal level of non exercise activity (21). Within a couple hours of sitting, HDL cholesterol drops by 20% (22). This suggests that, at a minimum, we don't need to exercise regularly but we do need to be breaking up our sitting time, probably every 20 or 30 minutes. In support of this, rat studies also show that the amount of time we are sedentary influences how our bodies process fats, given that leg muscles only produce the lipase lipoprotein (LPL) fat-processing molecule when they are being actively flexed, either standing or moving. The importance of producing enough LPL cannot be underestimated as a meta-analysis of 29 separate studies with 20,903 participants who have a specific genetic type (polymorphism) that creates less LPL was associated with a five-fold increase in the risk for death and greater chronic heart disease (23). The production of LPL is therefore extremely beneficial to us.Standing Up
A growing body of evidence suggests that simply standing up (independent of total sedentary time and moderate-to-vigorous intensity activity) reduces the risk of coronary heart disease (24). By limiting the time spent sitting, the risk of diabetes, heart disease and death can be reduced dramatically. It is important to take breaks from long periods of sitting down, such as taking a walk during your lunch break and taking a break from work at the computer or having a standing desk (25). Just taking breaks or increasing standing time has a significant benefit on blood biochemistry. Researchers found levels of C-peptide, which is involved in the synthesis of insulin, were significantly lower during interrupted sitting compared with prolonged sitting. In another study independent of total sedentary time and moderate-to-vigorous intensity activity time, increased breaks in sedentary time were beneficially associated with waist circumference, body mass index, triglycerides, and 2-hour plasma glucose (26). While in a study of 70 adults involving sitting for nine hours, regular activity breaks lowered plasma insulin levels and lowered plasma glucose when compared with prolonged sitting, even when compared with physical activity. While physical activity lowered plasma triglyceride more with regular activity breaks, activity breaks were more effective than continuous physical activity at decreasing negative blood sugar and insulin levels in healthy, normal-weight adults (27).
Overall, there is a compelling case for sitting reduction to be included in clinical preventive advice as a key component of "active living," where adults and children are encouraged to "stand up, move more and sit less" across different settings and locations throughout the day. Just standing up every 20 or 30 minutes can have a remarkable health benefit reducing your risk of many chronic illnesses. How simple is that?
DISCLAIMER: Dr Peter Dingle is a researcher, educator and public health advocate. He has a PhD in the field of environmental toxicology and is not a medical doctor.