Physical Activity and Obesity

Physical activity is defined as bodily movement (any form) produced by the contraction of skeletal muscles that increases energy expenditure above the basal level, and can be categorized in various ways, including type, intensity or strenuousness and purpose. Obesity is a condition describing excess body weight in the form of fat, with a body mass index (BMI) of 30 or greater. Obesity is often multifactorial, based on both genetic and behavioral factors. Physical inactivity has been found to associate with obesity because obesity arises from a mismatch between energy intake and energy expenditure that results in a net accumulation of energy stores, mainly as fat, in the body. Persuasive evidence indicates that both increased adiposity and reduced physical activity are strong and independent predictors of death.

Physical Activity and Energy Expenditure

Humans oxidize (metabolize) carbohydrate, protein, and fat (and alcohol) to produce energy that can be consumed in various forms of mechanical and chemical work and heat. The main component of the daily energy turnover in a typical subject is the energy expenditure for maintenance processes (resting energy expenditure). The remaining components are diet-induced energy expenditure and activity-induced energy expenditure. Diet-induced energy expenditure is less than 10 percent of total energy expenditure, depending on the macronutrient composition of the food consumed.

Activity-induced energy expenditure is the most variable component of the daily energy turnover, ranging between an average of 25 to 35 percent, up to 75 percent in extreme situations during heavy, sustained exercise as physical activity levels in free-living humans vary from day to day, seasonally, and in response to environmental factors.

The physical activity involves mechanical work, which must be performed by the muscles to move the center of mass of the segment. During the movement, energy expenditure will increase to propel body segments from one position to another in which mechanical work will be produced proportionally. Thus, work efficiency (the amount of work-/activity-induced energy expenditure) is one of major factors to determine activity-induced energy expenditure. The total amount of energy required by individuals depends on the level of activity, on work efficiency and on their body weight. The more active, heavier, and less efficient they are, the more energy they require. The rates of activity-induced energy expenditure vary depending on intensity, duration, and frequency of the activity and on the body mass and fitness of the person performing the activity.

There is currently a secular trend toward decreased physical activity in work time, which means that activity-induced energy expenditure during leisure time is becoming more dominant in determining total energy expenditure. Thus, physiology, behavior, and lifestyle play major roles in determining energy expended in activity.

Physical Inactivity and Morbidity

Physical activity is a fundamental part of human life. It is a primary link with the environment around us. We can explore, interact with, and change our environment through physical activity. This influences our mental and physical health, which in turn influences our capacity to move.

Regular physical activity is associated with a decreased risk of all-cause mortality. Cross-sectional epidemiological studies and controlled experimental investigations have suggested that physically active persons, in contrast to their sedentary counterparts, tend to develop and maintain a higher level of metabolic fitness.

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