An alien visitor to our planet would be perplexed by modern human life, particularly our approach to physical activity. After millions of years as hunter-gatherers, humans now often reside in overheated rooms in winter and icy-cold rooms in summer, accentuating our disconnect from nature.

Humans now lounge on comfortable seats in front of glowing screens, effortlessly moving between floors via mechanical staircases, and travelling across continents in cosy, mobile compartments. Strangely, these same humans can be seen later spending their leisure time running outside in all types of weather for no apparent reason, or even paying money to a place called 'the gym' to repeatedly lift heavy objects or run on a revolving mat until they become red and sweaty.

How would we explain this paradoxical behaviour?

We could begin by discussing the global obesity epidemic, noting that inactivity is responsible for an estimated 9% of premature deaths worldwide. We might also highlight the benefits of exercise for physical health and mental well-being. Despite understanding this, many people who promote these benefits spend most of their day sitting in an office chair.

In recent years, studies have suggested that prolonged sitting may be as detrimental to health as smoking. This comparison highlights the severe impact of a sedentary lifestyle on overall well-being.

Here’s an in-depth look at why sitting for extended periods is harmful, drawing on various aspects of biochemistry, metabolism, physical health, and mental well-being.

The Hidden Dangers of Prolonged Sitting

Research has connected prolonged sitting with numerous health concerns, including obesity and a cluster of conditions such as high blood pressure, high blood sugar, excess body fat around the waist, and unhealthy cholesterol levels, collectively known as metabolic syndrome. Studies have also indicated that overweight and obesity increase the risk of metabolic syndrome, but metabolic risk factors can also appear in individuals with a normal BMI. This suggests that sedentary behaviour may play a pivotal role in metabolic disorders.

Extended periods of sitting — whether at a desk, behind the wheel, or in front of a screen — can be detrimental. Researchers reviewed 13 studies examining sitting time and activity levels, revealing that those who sat for more than eight hours a day with no physical activity faced a mortality risk similar to that posed by obesity and smoking. However, unlike some studies, an analysis involving over 1 million people found that 60 to 75 minutes of moderate-to-intense physical activity daily mitigated the negative effects of excessive sitting. Other research has shown that highly active individuals have minimal mortality risk attributable to sitting.

Furthermore, sedentary behaviour doesn’t just affect physical health but also mental well-being. Prolonged sitting has been associated with increased risks of depression, anxiety, and cognitive decline. It can also exacerbate musculoskeletal problems such as lower back pain and neck stiffness.

The Sedentary Epidemic

Physical inactivity has become the fourth leading risk factor for mortality. People with sedentary behaviours have a 20-30% higher risk of all-cause mortality than those who regularly engage in moderate-intensity physical activities. Globally, physical inactivity is associated with 3.2 million deaths and 32 million disabilities per year.

Effects on Obesity and Metabolic Syndrome:

A systematic review and meta-analysis published in the American Journal of Preventive Medicine established that half of the studies show a correlation between prolonged sitting and a higher risk of obesity, type 2 diabetes, cancer, and metabolic syndrome. The studies highlighted that individuals who sit for long periods without breaks have a significantly increased likelihood of developing these conditions compared to those who are more active throughout the day[1]. However, researchers concluded that more research was needed, citing that the heterogeneity of study designs, measures, and findings makes it difficult to draw definitive conclusions.

Another study in the same journal (10,000+ participants) found that obesity was associated with more time spent watching TV[2] in individuals declaring low physical activity levels. Additionally, the risk of metabolic syndrome and abnormal triglycerides concentration was higher compared to those declaring high physical activity, regardless of BMI, meaning that a sedentary lifestyle can lead to metabolic syndrome and, in turn, push people to be less active and snack on ultra-processed junk while watching more TV.

This is important because, in overweight or obese participants, low physical activity was associated with a higher risk of abdominal obesity (visceral fat, or “brown” fat), increased glucose concentration, and elevated blood pressure. In participants with a normal BMI, these associations did not occur.

One further study found that longer sitting time was only associated with abdominal obesity and that excessive sitting time and a sedentary occupation correlated positively with metabolic syndrome[3], potentially echoing other researchers’ findings: sitting for long periods slows down metabolism and individuals may exceed their calorie intake by snacking on ultra-processed sugar-rich manufactured food products.

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1. van Uffelen, JG. Wong, J. Chau, JY. et al. (2010). Occupational sitting and health risks: A systematic review. American Journal of Preventive Medicine. 39(4), pp. 379-388. doi:10.1016/j.amepre.2010.05.024. PMID: 20837291

2. Pulsford, RM. Stamatakis, E. Britton, AR. et al. (2013). Sitting behavior and obesity: evidence from the Whitehall II study. American Journal of Preventive Medicine. 44(2), pp. 132-138. doi:10.1016/j.amepre.2012.10.009

3. Nam, JY. Kim, J. Cho, KH. et al. (2016). Associations of sitting time and occupation with metabolic syndrome in South Korean adults: A cross-sectional study. BMC Public Health. 16(1), 943. doi:10.1186/s12889-016-3617-5

Health Risks of Prolonged Sitting

Research has linked sitting for long periods with numerous health concerns, including obesity and metabolic syndrome, including high blood pressure, high blood sugar, excess body fat around the waist, and unhealthy cholesterol levels; and certain cancers. Prolonged sitting is also associated with an increased risk of cardiovascular disease and cancer.

Researchers analysed 13 studies on sitting time and activity levels, finding that those who sat for more than eight hours a day without physical activity had a mortality risk similar to that posed by obesity and smoking. (Biswas, A. et al. 2015)

Impact on Metabolism and Physical Health

When you sit, your body uses significantly less energy than when you stand or move. This reduction in energy expenditure has profound effects on metabolism and overall physical health.

1. Reduced Energy Expenditure

The human body is designed for movement. Physical activities, even as simple as standing or walking, engage muscles and stimulate various physiological processes. When you sit for extended periods, your muscles, particularly those in the lower body, become inactive. This inactivity reduces the number of calories burned, leading to a lower overall energy expenditure.

2. Slowed Metabolism and Caloric Surplus

A sedentary lifestyle slows down your metabolism, the process by which your body converts food into energy. When you burn fewer calories than you consume, the surplus is stored as fat. Over time, this can lead to weight gain and obesity, even if your calorie intake remains constant. Additionally, prolonged sitting affects the body's ability to regulate blood sugar and blood pressure, contributing to metabolic syndrome.

3. Development of Metabolic Syndrome

Metabolic syndrome is a cluster of conditions that occur together, increasing your risk of heart disease, stroke, and type 2 diabetes. These conditions include:

• Increased Blood Pressure: Sitting for long periods can lead to poor circulation and blood pooling in the legs, contributing to higher blood pressure.

• High Blood Sugar: Physical activity helps regulate insulin, the hormone responsible for blood sugar control. Inactivity can lead to insulin resistance, a precursor to type 2 diabetes.

• Excess Body Fat Around the Waist: Central obesity, characterised by fat accumulation around the abdomen, is a key component of metabolic syndrome.

• Unhealthy Cholesterol Levels: Lack of movement can decrease levels of high-density lipoprotein (HDL) cholesterol (the "good" cholesterol) and increase levels of low-density lipoprotein (LDL) cholesterol (the "bad" cholesterol), as well as triglycerides.

4. Increased Risk of Type 2 Diabetes

Insulin is crucial for glucose uptake into cells. Prolonged sitting impairs insulin sensitivity, making it harder for your body to manage blood glucose levels. This can lead to chronic hyperglycaemia (high blood sugar), eventually resulting in type 2 diabetes. Studies have shown that each hour of sitting per day is associated with a 22% increased risk of developing type 2 diabetes.

5. Cardiovascular Diseases

The cardiovascular system is significantly impacted by prolonged sitting. Physical inactivity leads to poor circulation, which can cause blood to pool in the legs and increase the risk of blood clots. It also contributes to endothelial dysfunction, where the blood vessels can't dilate properly, leading to hypertension (high blood pressure) and atherosclerosis (plaque build-up in the arteries). These conditions are major risk factors for heart attacks and strokes.

6. Effects on Lipid Metabolism

Sitting for extended periods affects how the body metabolises fats. Lipoprotein lipase (LPL) is an enzyme that breaks down triglycerides into free fatty acids for energy. Physical inactivity reduces LPL activity, leading to higher levels of triglycerides in the blood. Elevated triglycerides are a risk factor for cardiovascular diseases.

7. Bone Health and Muscle Atrophy

Regular physical activity is essential for maintaining bone density and muscle mass. Prolonged sitting can lead to muscle atrophy, particularly in the lower body. It also decreases the mechanical loading on bones, which is necessary to stimulate bone formation and maintain bone density. Over time, this can increase the risk of osteoporosis and fractures.

8. Inflammatory Response

Chronic physical inactivity is associated with a low-grade inflammatory response. Prolonged sitting increases inflammatory markers such as C-reactive protein (CRP) and interleukin-6 (IL-6). Chronic inflammation is linked to various health conditions, including cardiovascular diseases, diabetes, and cancer.

9. Gut Health

Sitting for long periods can negatively affect digestion and gut health. Physical activity helps stimulate intestinal peristalsis. Prolonged sitting can slow this process, leading to constipation and other digestive issues. Additionally, physical inactivity is associated with alterations in the gut microbiome, which can affect overall health and contribute to metabolic diseases.

Cardiovascular and Digestive Systems

The human body is designed to function optimally when it is in an upright and active state. Prolonged sitting, however, disrupts these natural functions, particularly affecting the cardiovascular and digestive systems.

— Cardiovascular System

When seated for extended periods, blood flow becomes sluggish, potentially leading to cardiovascular issues. One of the primary concerns is the increased risk of cardiovascular disease (CVD). Studies have demonstrated that sitting for more than four hours a day, particularly while watching television, significantly raises the risk of all-cause mortality. Specifically, individuals who sit for more than four hours daily in front of a TV have a 50% higher risk of dying from any cause compared to those who sit for less than two hours.

Similarly, an alarming correlation has been found between the total amount of time spent sitting each week and the risk of cardiovascular disease-related mortality. Those who sit for over 23 hours a week are 64% more likely to die from cardiovascular diseases than those who sit for less than 11 hours weekly. This substantial increase in risk is due to several factors, including reduced calorie burning, slower metabolism, and increased blood pressure, which all contribute to the development of CVD.

Mechanisms Behind Cardiovascular Risks

1. Reduced Blood Flow:

   Prolonged sitting reduces the effectiveness of the muscle pumps in the legs, which aid in returning blood to the heart. This can lead to blood pooling in the lower extremities, increasing the risk of deep vein thrombosis (DVT) and varicose veins.

2. Increased Blood Pressure:

   Extended periods of inactivity can cause blood pressure to rise, which is a significant risk factor for heart disease and stroke. The lack of physical movement means the heart and blood vessels are not exercised, leading to a stiffer, less flexible cardiovascular system.

3. Elevated Cholesterol Levels:

   Sitting for long periods is associated with an increase in LDL (bad) cholesterol and a decrease in HDL (good) cholesterol. This imbalance can lead to the development of plaque in the arteries, further increasing the risk of heart attacks and strokes.

4. Insulin Resistance:

   Prolonged sitting can lead to decreased insulin sensitivity, making it harder for the body to manage blood sugar levels effectively. This can result in higher blood glucose levels, contributing to the development of type 2 diabetes and associated cardiovascular complications.

— Digestive System

The digestive system also suffers from extended periods of sitting. When you sit for too long, the abdominal muscles become compressed, and the digestive organs are forced into an unnatural position. This can lead to several digestive issues:

1. Slower Digestion:

   An upright position helps food move more efficiently through the digestive tract. Sitting for prolonged periods slows digestion, leading to discomfort, bloating, and constipation.

2. Increased Risk of Gastroesophageal Reflux Disease (GERD):

   Sitting, especially after eating, can increase the risk of acid reflux and GERD. The pressure on the abdomen while sitting can cause stomach acids to move back up into the oesophagus, leading to heartburn and other symptoms of GERD.

3. Impact on Gut Microbiome:

   Physical activity helps maintain a healthy gut microbiome. Sedentary behaviour can negatively impact the balance of gut bacteria, essential for digestion, immune function, and overall health and mental well-being.

4. Obesity and Related Disorders:

   Prolonged sitting reduces the number of calories burned, contributing to weight gain and obesity. Obesity is a significant risk factor for numerous digestive disorders, including fatty liver disease, gallstones, and various forms of gastrointestinal cancer.

Mental Health

Prolonged sitting affects physical health and has profound implications for mental well-being. The connection between sedentary behaviour and mental health issues such as anxiety and depression is increasingly supported by scientific research.

— Anxiety and Depression

Sedentary behaviour has been strongly linked to increased risks of anxiety and depression. A comprehensive study conducted in Australia found that employees who sit for more than six hours a day are significantly more likely to develop symptoms of anxiety and depression compared to those who sit for less than three hours a day. This study particularly highlighted that the risk is higher among female employees, suggesting potential gender differences in how sedentary lifestyles impact mental health.

Research showed that Australians spend the most time sitting — about 57% of their waking time.

Several mechanisms explain this connection. Extended periods of inactivity can lead to feelings of lethargy and low energy, contributing to a depressive mood. Furthermore, the lack of physical activity can reduce the production of endorphins and other neurotransmitters that promote feelings of happiness and well-being. The social isolation that often accompanies sedentary lifestyles, especially in remote working scenarios, can exacerbate feelings of anxiety and depression.

— Cognitive Decline

In addition to anxiety and depression, sedentary behaviour is associated with cognitive decline. Physical inactivity can lead to structural changes in the brain, particularly in the medial temporal lobe, which is crucial for memory formation and storage. Studies have shown that a thinner medial temporal lobe is associated with an increased risk of dementia and Alzheimer’s disease.

Brain imaging research has revealed that individuals with sedentary lifestyles have less grey matter density in areas of the brain responsible for memory and executive function. This suggests that prolonged sitting can accelerate the brain ageing process, leading to an earlier onset of cognitive decline.

— Sleep Disturbances

Physical inactivity and extended screen time, both common in sedentary lifestyles, can disrupt sleep patterns. Poor sleep quality and insufficient sleep are risk factors for numerous mental health disorders, including depression and anxiety. The blue light emitted by screens can interfere with the production of melatonin, the hormone that regulates sleep-wake cycles, making it harder to fall asleep and reducing sleep quality.

— Social Isolation and Mental Health

Sedentary behaviour often involves solitary activities, such as watching television or working on a computer, which can lead to social isolation. Social connections are vital for mental health, and isolation can increase the risk of depression and anxiety. Engaging in physical activities often involves social interaction, whether it is a team sport, a gym class, or a walk with friends, which can provide emotional support and improve mental well-being.

References:

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Ekelund, U. Steene-Johannessen, J. Brown, WJ. et al. (2016). Does physical activity attenuate, or even eliminate, the detrimental association of sitting time with mortality? A harmonised meta-analysis of data from more than 1 million men and women. Lancet. 388(10051), pp. 1302-1310. doi:10.1016/S0140-6736(16)30370-1

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