WEIGHT REGULATION: HOW IS ENERGY FROM FOOD USED BY THE BODY?
Energy gained from food is used by the body in three ways:
- Maintaining the basal metabolic rate: the energy used to keep bodily functions ticking over; the calories burnt off producing the background hum of involuntary vital activity.
- Thermogenesis: energy used processing and digesting the foods that provide the energy, normally around 10%.
- Energy used in performing additional muscular activity, day-to-day tasks and scheduled exercise.
Around 75% of adults get less physical activity than they should. According to one study, 56% of men believed they were sufficiently active to benefit health, whereas in fact only 36% achieved even moderate activity (BNF 1999). Women are just as bad; 52% believed they did enough whereas only 24% actually did. A sedentary individual will burn off 25% of daily energy expenditure on physical activity, the rest going on thermogenesis and basal metabolism. However, a finely honed athlete will use up to 80% of energy expenditure on physical activity.
Only 20% of men and 10% of women have physically active occupations and the extra physical activity involved in daily living 50 years ago compared with today is the equivalent of running a marathon per week. According to the 1999 Health Survey for England, 25% of us are defined as sedentary because we perform less than one 30-min period of moderate exercise per week, and only 25% of women and 33% of men engaged in regular moderate activity. It has actually been demonstrated that an obese child sitting motionless in front of the TV is so immobile and inert that he or she burns off energy at less than the basal metabolic rate.
Not surprisingly, physical activity levels decline with age and are better in ethnic minorities; social class makes very little difference because activity as part of occupation tends to balance leisure exercise. These figures help to demonstrate why energy expenditure is so important in weight management.
Because we need less energy nowadays for physical pursuits, the number of calories we consume has actually reduced over the years, but it is estimated that the reduction in calories burnt off by physical activity has reduced by the same amount, and overtaken it by 50 calories per day, and that this 50-calorie difference between energy intake and energy expenditure is the reason why we are gaining weight.