Energy for healthy muscle and bone
Food provides energy for physical activity as well as giving us the nutrients we need to develop, grow and stay healthy through life. A healthy diet and regular exercise throughout life can help reduce the risk of developing diseases of the musculoskeletal system such as osteoporosis (weak bones) and sarcopenia (muscle loss) that affect many of us as we get older.
All tissues in the body need a steady supply of energy and for most tissues, energy metabolism changes very little throughout the day. The process that releases energy for use by cells is called respiration (a word which is sometimes mistakenly used for the process of breathing, which is properly called ventilation). In aerobic respiration, carbohydrates, fats and proteins in the diet supply the body with smaller carbon-based molecules that combine with oxygen to create carbon dioxide and water, with the release of energy. When the energy is not used immediately to create heat or to do work, it is stored in cells as chemical energy that can be called upon when it is needed. One of the most important energy storage molecules in cells is adenosine triphosphate (ATP), which we will learn more about later this week.
Skeletal muscle has a relatively low resting energy metabolism (basal metabolic rate) compared with other tissues, if we consider it on a per pound or per kilogram basis. Weight for weight, the liver uses about fifteen times as much energy as resting skeletal muscle, while the heart and kidneys use about thirty times as much as skeletal muscle in the resting state.
But if we consider how much skeletal muscle there is in the body compared with these other tissues, overall it accounts for a considerable part of the body’s resting energy metabolism. In non-obese adults, skeletal muscle accounts for around 40% of body weight and this accounts for 20 - 30% of resting oxygen consumption, while the liver, heart, kidneys and brain account for most of the rest.
When we consider energy metabolism during heavy physical activity, muscle tissue can account for up to 90% of the body’s oxygen consumption. The largest part of the energy we need as adults over and above our basal metabolic needs is required for physical activity. Our body’s daily needs for energy are therefore closely linked to our habitual daily physical activity level and to our basal metabolic rate.
Nutritionists and dietitians express physical activity level (PAL) as a number, calculated by dividing total energy expenditure over 24 hours by the basal metabolic rate. Basal metabolic rate (BMR) accounts for between 45% and 70% of total energy expenditure in most adults, and it is dependent on gender, body size, body composition (lean vs fat) and age.
Strictly, BMR can only be measured under laboratory conditions, but it can be simply estimated based on measurements such as body weight and height. Physical activity levels are categorised by the intensity of habitual physical activity into sedentary or light, active or moderately active, and vigorous or vigorously active lifestyles. An office worker getting very little or no habitual exercise would be classified as sedentary, and would typically have a PAL of between 1.4 - 1.69. An agricultural worker, spending eight hours a day doing non-mechanised work such as planting and weeding, would be classified as vigorously active and would typically have a PAL of between 2.0 - 2.4.
How would you rate your physical activity level - sedentary, light, active, moderately active, vigorous or vigorously active?
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