In recent years international research on Sedentary Behaviour (SB) has increased dramatically and there is growing evidence that a multitude of serious health risks may be connected to "prolonged" and "excessive" sitting. Sedentary Behaviour science is still emerging, however, the evidence to date is compelling. A variety of studies warn that sedentary lifestyles are likely to be causing as many deaths as smoking.
1. Cardiovascular Health
There is evidence for an association between greater sedentary behaviour and an increased risk of cardiovascular disease among adults, schoolchildren, and youth. Two hours per day of screen time and sitting time were linked with a 5% and 17% increased risk of cardiovascular events, respectively.
Increases in sedentary behaviour have been associated with increases in blood pressure among children, teenagers, and adults.
Interrupting sitting time has been associated with improved vascular function in young girls and with improved endothelial function among adults.
Both light intensity and moderate-to-vigorous physical activity breaks, but not standing, might reduce inflammatory responses in adults.
Pulmonary embolism is a common cause of cardiovascular morbidity and mortality and it is typically a consequence of deep vein thrombosis in the lower extremities. Risk factors for deep vein thrombosis/pulmonary embolism include age, limb immobilisation, malignancy, pregnancy, surgery, and acquired or inherited thrombophilia.
Physical inactivity has been associated with incident pulmonary embolism in women, while prolonged work- and computer-related seated immobility was associated with an increased risk of venous thromboembolism.
Regarding older adults, although there might be an association between overall sitting and TV viewing and a higher risk of blood pressure, the evidence is still inconclusive.
Sedentary behaviour has been associated with an increased risk of cancer, endometrial cancer, colon cancer, and breast cancer.
Although both body-mass index (BMI) and physical activity might be attenuating the link between sedentary behaviour and some types of cancer, researchers still suggested adults (aged 50-71) who watched TV for at least 7 hours a day had a 22% increase risk of cancer mortality relative to those who watched TV less than one hour a day.
It has been suggested that an important underlying mechanism by which sitting increases cancer risk is adiposity (excess body weight) which might facilitate carcinogenesis through insulin resistance, chronic inflammation, increased level of sex hormones, and altered secretion of adipokines.
Regarding older adults, the data on the association between sitting and cancer remains insufficient.
A positive association between sedentary behaviour and type 2 diabetes has been reported among adults, independent of physical activity. People who watched TV for more than 2 hours a day had a 20% increased risk of type 2 diabetes.
Regularly interrupting sitting with activity bouts of slightly more than one minute every 30 minutes of sitting might be more effective than a single 30 minute walking in lowering postprandial glucose and insulin levels.
Light-intensity physical activity breaks, but not standing, was associated with significant reductions in postprandial (after meal) blood glucose and insulin levels.
4. Weight Gain
In 2015 researchers found a positive association between sitting and body composition, heart fat, liver fat, visceral fat, and waist circumference independent of physical activity. Interruptions of sitting time with standing and stepping were associated with lower BMI scores and waist circumference, but the evidence remains unclear.
Researchers suggested that watching TV might lead to obesity among children and teenagers. However, factors such as unhealthy eating and physical inactivity might contribute to this association. Among adults, there is limited evidence for a longitudinal relationship between sedentary behaviour, weight gain and obesity risk.
There is currently weak evidence for an association between sitting and obesity and waist circumference among older adults.
5. Metabolic Syndrome
Metabolic syndrome has been defined as central obesity (waist circumference) plus any two of the following four risk factors: raised blood pressure, raised triglycerides, reduced high density lipoprotein (HDL) cholesterol and raised fasting plasma glucose.
Approximately 25% of European, American and Canadian adults have metabolic syndrome.
Men and women who sit more might have up to a 73%-76% increased risk of developing metabolic syndrome compared with those who sit little, regardless of activity and cardiorespiratory fitness. A similar association has been reported for children and youth.
Being physically active might reduce the risk. Among older adults (over 60 years old), each hour of TV watching has been associated with a 19% increased risk of having metabolic syndrome.
6. Mental Health
High amounts of sitting might be associated with a higher risk of psychological distress. Similarly, among overweight/obese adults, decreasing sedentary time and increasing moderate-to-vigorous-intensity physical activity were associated with a reduced risk of depression.
Inactive mid-aged women who sat more than 7 hours a day were three times as likely to have depressive symptoms compared with women who sat up to 4 hours a day AND were physically active.
Screen time has also been associated with a higher risk of depression when exceeding 2 hours/day and a lower risk when screen time was less than 2 hours a day among children and adolescents.
A 31% increased risk for developing a mental disorder has been linked to adults who engage in 42 hours of watching TV and/or using the computer per week when compared with those who do so for less than 10.5 hours per week.
Relative to those who were sedentary (4 hours or more/day), older adults who were moderately (2–4 hours/day) and least sedentary (<2 hours/day) were 38% and 43% more likely to age successfully, respectively.
Less convincing evidence is available for the association between sedentary behaviour and anxiety. However, moderate evidence suggests a link between total sitting and a greater risk of anxiety.
Although there seems to be a positive association between sitting and depressive symptoms in children, teenagers and adults, the evidence remains inconclusive.
The type and context of the sedentary behaviour is important, as activities such as computer use, reading, playing board games and craft activities might be associated with a reduced risk of dementia among older adults (over 60 years old).
7. Back & Neck Pain
In terms of occupational health, lower back pain is the number one cause of disability and one of the major risk factors for absenteeism.
Sitting time has been positively associated with both low back pain and neck-shoulder pain intensity among blue-collar workers. When asked to interrupt their sitting at work every 30 min during the day, overweight/obese office workers showed a 32% reduction in lower back discomfort, compared to seated work.
8. Muscle Degeneration
Age-related declines in muscle mass and strength (sarcopenia) often have a negative impact on the independence of the elderly, increasing their risk of falling.
When sedentary our locomotor muscles are inactive most of the day.
Older adults watching TV at least 6 hours a day might have poorer grip strength compared to those who engage in less than 2 hours of TV watching a day.
Stair climbing, for example, results in greater muscle activity than brisk walking, as do standing and walking compared with sitting.
Weight bearing activities such as standing, walking and running stimulate hip and lower body bones to grow thicker denser and stronger. Scientists partly attribute the recent surge in cases of osteoporosis to lack of activity.
Osteoporosis is particularly important in the context of an ageing population. Prolonged sitting might be a risk factor for bone health in women, even in those who are physically active. How sedentary time is accumulated might also be important: the longer (rather than the more frequent) the sitting bouts, the higher their deleterious effects on bone mineral density (BMD).
Some specific sedentary activities (screen-based) are negatively associated with bone health in youth. This relationship between screen-based time and bone health is independent of the total amount of physical activity, but not independent of the frequency of muscle strengthening and vigorous activities. Frequent, short bouts of such activities (muscle strengthening and vigorous activity) interspaced with periods of sedentary behaviours appear to be beneficial.
Sedentary behaviour has been associated with an increased risk of all-cause mortality of up to 24%-49%.
Whether physical activity attenuates the relationship, it remains unclear. The risk of all-cause mortality might be lowest in standing/walking occupations, especially among women.
Both maintaining sitting time low and reducing it was associated with a reduced risk of all-cause mortality in American post-menopausal women. Increasing standing time among Australians aged 45 and above seemed to lower the risk of all-cause mortality.
Watching TV for more than 2 hours per day was associated with a 13% increased risk.
The association between sitting and all-cause mortality among older adults (over 60 years old) has also been supported by evidence.
Replacing one hour of sitting with low exercise activities such as household chores, garden work and daily walking were sufficient to reduce all-cause mortality by 30%. Exercise reduced the same risk by 48%.
Replacing sedentary time with equal amounts of sleeping (in people who sleep less than 7 hours/day) and standing had beneficial effects.