• 6.1 Hamilton MT, et al. Role of Low Energy Expenditure and Sitting in Obesity, Metabolic Syndrome, Type 2 Diabetes, and Cardiovascular Disease. Diabetes 2007: 56.
• 6.2 Healy GN, et al. Breaks in Sedentary Time. Diabetes Care 2008: 31.

See Combined Summary/Adaptation Below

Combined Summary/Adaptation:

Modern lifestyles allow the avoidance of physical activity as technological and social factors have made prolonged sitting commonplace during work, domestic, and recreational undertakings.  Total sedentary time, which is defined as the absence of whole-body movement, has been associated with obesity, cardiovascular disease, diabetes, and metabolic syndrome.  This has major worldwide clinical and public health implications as the numbers of people engaged in the behavior of sitting too much are growing rapidly in a modern progression of human inactivity. 

A new science, called inactivity physiology, is investigating the effects of sedentary time on our bodies.  Many people in developed nations do not get the recommended amount of daily exercise activity, but most have historically engaged in a sufficient amount of “non-exercise” activity incidentally through various activities of daily living such as routine chores, self-care, and light recreation.  However, because of personal computer use, TV watching, electronic communications, the automation of chores, and motorized transportation, the amount of idle sitting time spent over the 16-hour period that people are awake each day has dramatically increased in recent years.

Interestingly, behaviors involving prolonged periods of inactivity and the absence of whole body movement, which are associated with a greater risk of chronic disease, are independent of exercise activity.  That means people who participate in a structured exercise routine of 30 minutes or more, five days per week as recommended, are no more protected from the damaging effects of an otherwise sedentary existence than those who do not exercise.  General inactivity is not the same as a lack of exercise activity.  For the purposes of avoiding the risk factors associated with being overly sedentary, it is most helpful to perform frequent low-intensity postural and ambulatory non-exercise activity throughout the day. 

A number of prior studies have shown that people who sit a lot, compared to people who sit less often, are at greater risk for cardiovascular disease and generally have a higher death rate.  In fact, the impact of inactivity on health has been found to be on par with the relative risk of smoking and high cholesterol levels. 

A recent Australian study examined the association of breaks in sedentary time with various health risk factors.  One hundred sixty eight people aged 30 to 87 years underwent biochemical, body measurement, and behavioral assessments, including glucose, triglycerides, and HDL cholesterol levels, waist circumference, and resting blood pressure measurements.  An accelerometer was fitted on each person to measure physical activity levels for a continuous period of seven days.  Each minute the accelerometer count was low (less than 100 counts) was considered sedentary time.  A break was considered an interruption from sedentary time when the accelerometer counts rose above 100 per minute (with light activity such as standing from a sitting position or taking a step).

The results showed that study participants spent 57% or 9.12 hours of their waking time (~16 hours) totally sedentary, 39% or 6.24 hours of their time engaged in light intensity activity (standing up, taking a step, fidgeting), and 4% or 36 minutes of their time engaged in moderate or greater activity (continuous movement, manual labor/chores, exercise).  The average break from sedentary time to engage in activity of light or greater intensity was less than five minutes long.  Overall, regardless of total sedentary time, the total number of breaks was associated with significantly lower waist size, Body Mass Index, triglycerides, and 2-hour glucose levels.  In other studies, prolonged inactivity has also been shown to decrease lipoprotein lipase activity, impair triglyceride and HDL cholesterol metabolism, and decrease total energy expenditure.

These findings imply that it is not really the amount of sedentary time so much as the way in which the time is accumulated that is important.  Therefore, frequent breaks to interrupt sustained sedentary time are strongly recommended to avoid the increased health risks of added body fat and altered triglyceride and glucose levels.  What remains unknown at this point is the minimum amount of time and intensity of activity breaks from sustained sedentary time that are needed to avoid any detrimental effects on metabolism and health. 

CONCLUSION: Sedentary time, which accounts for the majority of most people’s waking time, is associated with negative health outcomes among diverse population groups.  Prolonged sitting is a risk factor for altered metabolism of triglycerides, HDL cholesterol, and glucose, blood pressure changes, and increased belly fat─ health indicators that are associated with cardiovascular disease, diabetes and obesity.  While structured exercise activity is important for improved stamina, strength, circulation, and bone density, regular bouts of non-exercise activity are required throughout the day to break up sustained sedentary periods regardless of a person’s exercise activity habits.

• 6.3 Bak E, et al. Are we facing a new paradigm of inactivity physiology? BJSM 2010: 067702.

COMMENTS:

The author points out that current public health guidelines call for 150 minutes or more per week of moderate or greater intensity exercise activity. Also noted is that prolonged sedentary periods characterized by a lack of whole-body movements are associated with obesity, cardiovascular disease, metabolic syndrome, diabetes, cancer, and a greater death risk generally, regardless of physical activity. Therefore, a new emphasis on the study of inactivity physiology is recommended. The important message is that exercise or non-exercise effects on the body should be considered separately from the effects associated with extended sedentary timeframes. Technically and practically speaking these are two different concepts. People that don’t exercise miss out on the proven physiological benefits that come with moderate or greater physical activity, but that doesn’t mean that they are necessarily inactive for long periods of time. The stay-at-home parent may be active nearly all day with domestic chores but never reach exercise level intensity heart rate, weight resistance, or range of motion flexibility. On the other hand, people that exercise and enjoy the benefits thereof, may otherwise be highly sedentary. The weekend warrior or morning gym rat may sit behind the desk eight hours a day in addition to the hour commute each way between home and work. It seems that two body physiologies co-exist within us, one pertaining to exercise activity and the other to prolonged inactivity. The best chance we have at promoting good health will incorporate two concepts: one concerning the pursuit of moderate exercise and the other concerning the moderation of sedentary periods.