The Epidemiology and Physiology of Sedentary Behavior

Physical inactivity is estimated to account for 6% of global deaths (1), and is associated with risk of Metabolic Syndrome and cancer (2,3,4).

Approximately 100,000 new cases of breast and colon cancer each year are linked to sedentary lifestyles (5). Another study found that taking frequent breaks from sitting is associated with smaller waist circumference and lower levels of C-reactive proteins, both biomarkers associated with elevated risk of some cancers for post-menopausal women (6).

The National Health and Nutrition Examination Survey (7) analyzed data of 4,757 participants and found that even short periods of light activity (standing up and walking for at least a minute) – reduced biomarkers such as large waist circumference, elevated triglyceride levels and increased insulin resistance.

There’s also a risk of heart disease and premature death from any cause increases for those spending more than four hours a day sitting. A study (8) with 4,512 subjects, found a 48% increased risk of all-cause mortality and an approximately 125% increase in risk of cardiovascular events for those spending more than four hours sitting. The risk was found to be independent of other detrimental factors such as smoking, hypertension, BMI, and social class.

Another study from Australia show that prolonged sitting is significantly associated with higher all-cause mortality risk independent of physical activity. From a population study of 222 497 subjects, it was suggest that sitting time sitting was responsible for 6.9% of deaths (9).

Being inactive for more than 23 hours per week had a 64% greater risk of death from cardio vascular disease than being sedentary for less than 11 hours per week (10).  Sitting in one position for too long may develop blood clots, which are estimated to be a cause of death for up to 100,000 people per year (11).

A large Canadian Fitness survey (12) looked at more than 7000 men and nearly 10.000 women over a 12-year period, and the participants ranged from 18 to 90 years. The research found a significant link between time spent seated and mortality, and that being active doesn’t balance out the negative effects.

The physiology

Prolonged sitting disrupts metabolic health, increases plasma triglyceride levels and decreases levels of high-density lipoprotein cholesterol, increases insulin resistance (3,13)  and affects carbohydrate metabolism (3).

Studies have used radioactive triglyceride tracers to examine metabolic effects of not standing on specialized leg muscles, such as the deep red quadriceps, that are designed for postural support (not all muscles have this same function). These muscles quickly lost more than 75% of their ability to siphon off the fat circulating in the lipoproteins from the bloodstream when incidental contractile activity was reduced. This was related to a 90% to 95% loss of lipoprotein lipase (LPL) activity locally in the most oxidative skeletal muscles in the legs, which are specialized for postural support (14).

In addition, sitting appears to lead to about a 20% reduction in high-density lipoprotein, or good cholesterol, increasing the risk of suffering from a cardiovascular disease (15). Remaining sedentary for more than 24 hours impairs the ability of insulin to uptake glucose, raising the risk of diabetes (16).

“We just aren’t really structured to be sitting for such long periods of time, and when we do that, our body kind of goes into shutdown. If there’s a fountain of youth, it is probably physical activity. So the problem isn’t whether it’s a good idea, the problem is how to get people to do more of it” Dr. Toni Yancey.

Sitting trains the body to do nothing and leads to physiological adaptations that reduce functionality.

Non-exercise activity thermogenesis (NEAT)

Sedentary behavior must be reduced, particularly reducing the long, uninterrupted bouts of inactivity. One method is engaging in non-exercise activity thermogenesis (NEAT).

Nonexercise activity thermogenesis (NEAT) is the energy expended for everything that is not sleeping, eating, or sports-like exercise. It includes the energy expended walking to work, typing, performing yard work, undertaking agricultural tasks, and fidgeting (17).

In 2005 Levine et al (18) published detailed results of his analysis of metabolism. Levine tracked food consumption and every activity using motion tracking underwear, he measured their body postures and movements every half-second for 10 days. Those who didn’t gain weight moved more than others, while eating the same, due to a difference of minus 2 hours sitting each day on average. The study suggested that “adopting the NEAT-enhanced behaviors of lean counterparts, might expend an additional 350 calories (kcal) per day.”

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1. World Health Organization. Global Health Risks: Mortality and Burden of Disease Attributable to Selected Major Risks. Geneva, Switzerland:WHOPress; 2009.
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5. Research presented in November, 2011 at the American Institute for Cancer Research's (AICR) annual conference.  The AICR presented data suggesting that about 100,000 new cases of breast cancer and colon cancer per year can be associated with physical inactivity.
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