Kinesiology Lecture Week 3

KIN2000 Physical Activity & Health Lecture Notes: The Perils of Inactivity

Overview of Physical Inactivity

Definition of Physical Inactivity

Physical inactivity is defined as the insufficient level of physical activity in accordance with established guidelines. The following guidelines are specified:

  • Adults should engage in at least 30 minutes of physical activity per day, at least 5 days a week, totaling a minimum of 150 minutes of activity per week.

  • Strength training should be performed at least twice a week.

Impact of Physical Inactivity

Health Implications

Despite the well-documented health benefits of physical activity, physical inactivity remains a significant global health concern. Key facts include:

  • It is a leading contributor to premature mortality.

  • Over 30% of adults worldwide are classified as physically inactive.

Reasons for Physical Inactivity

Several factors contribute to higher levels of physical inactivity, primarily:

  • Technological Advances: Innovations have significantly diminished the necessity for human movement in daily life.

  • Transport Behavior: There's a noticeable decline in active transport methods such as walking or bicycling.

  • Work Environment Changes: In the 1960s, more than 50% of all workers held jobs that demanded moderate to vigorous physical activity. Currently, less than 20% of jobs require such levels of activity.

  • Youth Activity Levels: A combination of less active commuting options and longer school days leads to increased sedentary behavior in youth.

Quantifying Physical Inactivity

Human motion has drastically declined, moving from a lifestyle that necessitated substantial movement for survival to a contemporary lifestyle where movement is significantly reduced. On average, adults spend nearly 8 hours per day in sedentary activities and less than 30 minutes daily engaged in moderate to vigorous physical activities.

Consequences of Physical Inactivity

Inadequate physical activity and excessive sitting are linked to a multitude of health issues:

  • Obesity

  • Cardiovascular diseases

  • Diabetes

  • Cancer

Understanding Sedentary Behavior

Definition of Sedentary Behaviour

Sedentary behavior refers to waking activities that occur while sitting or reclined, resulting in an energy expenditure of 1.5 METs (Metabolic Equivalent of Task).

Sedentary Behaviour and Mortality

A meta-analysis by Wilmot et al. (2012) with over 700,000 participants revealed that sedentary behavior correlates with an increased risk of mortality:

  • 49% increase in the overall risk of mortality.

  • 90% increase in the risk of cardiovascular-related mortality.

  • Individuals who spend more than 12 hours sedentary each day have a 21% increased risk of death compared to those spending less than 6 hours.

  • Those who are highly sedentary (over 11 hours/day) and least active (less than 9 MET-h/day) face a 75% increase in risk of death compared to the least sedentary (under 7 h/day) and most active (over 20 MET-h/day).

Additional Studies on Sedentary Behaviour

According to another meta-analysis by Biswas et al. (2015):

  • Sedentary behavior is linked to a 13% increase in cancer mortality.

  • Sedentary individuals with higher physical activity levels showed a 30% reduced risk of all-cause mortality compared to those with lower activity levels.

  • The relationship between sedentary behavior and mortality can be moderated by physical activity levels.

Longitudinal Studies

The National Institutes of Health and American Association of Retired Persons Diet and Health Study (Keadle et al, 2015) involving 221,426 individuals aged 50-71 followed for 14 years showed that higher TV watching correlates with increased risk of mortality from various diseases including cancer, heart disease, and diabetes.

Dose-Response Relationship

Some studies indicate a linear dose-response relationship between sedentary time and mortality, while others propose a nonlinear relationship, recommending a limit of 9 hours on sedentary behavior. Reducing sedentary time to less than 6 hours/day yields additional benefits.

Sedentary Behaviour and Cardiovascular Disease (CVD)

A historical study on London bus workers (Morris et al, 1953) compared bus drivers (more sedentary) and conductors (more active), highlighting that drivers had double the risk of fatal coronary heart disease. Moreover, each additional 2 hours per day of screen time correlates with a 17% increase in cardiovascular risk.

Meta-Analyses Findings

A meta-analysis involving 1.5 million participants and almost 48,000 cases of CVD concluded that screen time of 5-6 hours/day and total sedentary time of 10-11 hours/day are linked to an increased risk of CVD incidence and mortality, showing significant risks associated with exceeding 11 sedentary hours/day compared to 8 hours, with protective effects from moderate to vigorous physical activity.

Sedentary Behaviour and Obesity

Mechanisms of Weight Gain

Long-term weight gain leading to obesity arises from a positive energy imbalance, which occurs when caloric intake from food surpasses caloric expenditure. Energy expenditure consists of:

  1. Basal/resting metabolic rate (65-70%)

  2. Thermic effect of food (10-15%)

  3. Activity thermogenesis (20-30%)

Types of Activity Thermogenesis

  • Exercise Activity Thermogenesis (EAT)

  • Non-Exercise Activity Thermogenesis (NEAT): All physical activities excluding structured exercises, such as active transport, work-related activity, and leisure activities. NEAT accounts for a greater total energy expenditure than EAT and can differ by up to 2000 kcal/day between individuals.

Relationship with Adiposity

There exists a well-established connection between sedentary behavior, adiposity, and obesity. A linear dose-response relationship indicates that for each additional hour spent watching TV, there is a 13% increase in the risk of childhood obesity. Sedentary adults show susceptibility to weight gain, while overweight individuals are likely to become increasingly sedentary over time.

Impact of Reduced Sedentary Time

A decrease of 1.5 hours/day in sedentary time is associated with a 1.4 kg reduction in body mass (Golubic et al, 2015).

Sedentary Behaviour and Metabolic Disease/Diabetes

Insulin Resistance and Diabetes Development

Physical inactivity fosters the onset of insulin resistance, and an increase in sedentary time correlates with diabetes risk:

  • A 3% increase in diabetes risk for each additional hour of sedentary activity, particularly from TV watching.

Explanation of Diabetes

Diabetes is characterized by excessively high blood sugar (glucose) levels. Carbohydrates intake is converted into glucose, an essential energy source. Glucose enters the bloodstream and must reach body cells with the assistance of insulin, a hormone produced by the pancreas. Insulin is critical for glucose uptake; insufficient insulin production or ineffective insulin usage results in hyperglycemia (high blood glucose).

Types of Diabetes

There are several recognized types of diabetes:

  1. Type 1 Diabetes

  2. Type 2 Diabetes

  3. Prediabetes

  4. Gestational Diabetes

Insulin Resistance Explained

Insulin resistance occurs when muscle, fat, and liver cells do not respond appropriately to insulin:

  • Glucose fails to enter cells efficiently, leading to an accumulation of glucose in the bloodstream.

  • The pancreas compensates by producing excess insulin, possibly leading to hyperinsulinemia and ultimately prediabetes or Type 2 diabetes if not addressed.

Causes of Insulin Resistance

Key contributors to insulin resistance include:

  • Excess Body Fat: Particularly abdominal fat, which releases proinflammatory cytokines that disrupt insulin function.

  • Physical Inactivity

  • Dietary Choices

  • Genetics and Family History

  • Conditions like Prediabetes, Type 2 Diabetes, Polycystic Ovarian Syndrome (PCOS), and certain medications.

Overview of Diabetes in Canada

In a global context, Metabolic Syndrome is characterized by a cluster of conditions increasing the risk of heart disease, stroke, and Type 2 diabetes. It includes:

  1. Central Obesity

  2. Insulin Resistance

  3. Hypertension

  4. Dyslipidemia

Epidemiology of Metabolic Syndrome

Globally, about 25% of individuals meet the criteria, with variations:

  • United States: 40%

  • Increases with age (20-40 years: 22%; >60 years: 56%)

  • China: 24%

  • Africa: 32%

  • Globally, 3% of children and 5% of adolescents exhibit metabolic syndrome.

Diagnosis of Metabolic Syndrome

To be diagnosed, an individual must meet 3 out of 5 criteria:

  1. Waist circumference > 40 inches in men and > 35 inches in women

  2. Serum triglycerides level ≥ 150 mg/dL

  3. Decreased high-density lipoprotein (HDL) cholesterol < 40 mg/dL for men and < 50 mg/dL for women

  4. Fasting glucose ≥ 100 mg/dL

  5. Blood pressure values of systolic ≥ 130 mm Hg or diastolic ≥ 85 mm Hg

Signs and Symptoms of Metabolic Syndrome

Key indicators include:

  • Central Adiposity

  • Hyperglycemia: Symptoms include increased urination, thirst (polydipsia), fatigue, blurred vision, and skin changes (acanthosis nigricans).

Insulin-Mediated Glucose Uptake

Canonical Pathway

The canonical pathway for insulin-mediated glucose uptake involves the interaction of insulin with receptors, leading to complex cellular responses facilitating glucose transport via GLUT4.

Muscle Contraction-Mediated Glucose Uptake

Additionally, muscle contraction enhances glucose uptake through non-insulin-dependent mechanisms, crucial in diabetes management.

The Importance of Muscle and Activity

Muscle is the largest organ by mass and plays a vital role in glucose regulation, responsible for about 80% of postprandial glucose uptake. Insulin resistance can emerge decades before the development of clinical complications.

Role of Exercise

Physical activity significantly enhances glucose metabolism and is therapeutic in managing insulin resistance and Type 2 diabetes by promoting glucose uptake independent of insulin mechanisms. Regular vigorous activities improve insulin sensitivity and overall health through:

  • Enhanced β-cell function

  • Improved insulin sensitivity

  • Better vascular function

Sedentary Behavior's Influence on Diabetes

Sedentary behavior elevates Type 2 diabetes risks; for individuals engaging in 9 hours of sedentary behavior, each additional hour correlates with a 22% increase in type 2 diabetes risk. Interrupting prolonged sedentary periods with light activity may help manage glucose and insulin levels effectively.

Sedentary Behaviour and Cancer Risk

High levels of sedentary activity are associated with heightened cancer risk. Key findings include:

  • Sitting for 13 hours/day doubles cancer risk compared to 8 hours/day.

  • Specific cancer types are notably affected:

    • Colorectal Cancer: Increased sedentary behavior elevates risk.

    • Endometrial Cancer: The highest levels of sedentary activity correlate with a 30% increased risk.

    • Breast Cancer: Higher prevalence found among individuals with significant sedentary lifestyles.

Sedentary Behaviour and Bone Health

Impacts on Bone Dynamics

Sedentary behavior negatively impacts Bone Mineral Density (BMD), leading to a higher risk of fractures and osteoporosis.

Bone Remodelling Insights

BMD is quantified through the balance of resorption (osteoclast activity) and formation (osteoblast activity) processes. Childhood and adolescence are crucial for maximizing bone density, as 90% of peak bone mass is achieved by age 18 in females and 20 in males. Utilization of weight-bearing exercises, along with adequate nutrition (calcium and vitamin D), is essential for bone health.

Age-Related Bone Loss and Osteoporosis

Aging leads to inevitable bone loss. Maintaining adequate calcium and vitamin D intake, and engaging in weight-bearing exercises, can mitigate these losses. Osteoporosis risk increases if the peak bone mass developed during formative years is low.

Proper Nutrition and Activity Recommendations

Minimum daily intake guidelines include:

  • Calcium: 1,200 mg

  • Vitamin D: 800-2,000 IU daily for individuals over 50 or at high risk.

BMD and Physical Activity

Physical activity plays a crucial role in maintaining BMD across different regions such as the lumbar spine and femoral neck. Specifically, high-impact and resistance exercises provide adequate mechanical stimuli to prevent demineralization.

Role of Screen Time and Sedentary Guidelines

Recommendations

Guidelines suggest:

  • Adults should limit sedentary time to < 8 hours/day, with screen time limited to < 3 hours.

  • Children (0-5 years) should not be restrained for more than 1 hour at a time and no screen time is encouraged.

  • Children aged 5-17 should be limited to < 2 hours of recreational screen time.

Adverse Health Outcomes Associated with Excessive Screen Time

Higher screen time correlates with:

  • Decreased physical activity

  • Increased rates of anxiety and depression

  • Poor sleep quality

  • Social withdrawal and cognitive decline.

Conclusion

In conclusion, sedentary behavior poses significant health risks. Strategies to reduce sedentary time and integrate physical activity into daily routines are essential for improving overall health outcomes, particularly concerning obesity, diabetes, cardiovascular diseases, and certain cancers. Regular participation in physical activity, especially weight-bearing and high-impact exercises, is fundamental for maintaining bone health and physical fitness across all age groups.