It is often expressed in kilojoules per day. It takes energy to metabolise foods. This means that, to metabolise kilojoules of carbohydrate, 5 kilojoules is needed, leaving the body with 95 kilojoules of useful energy. In comparison, to metabolise kilojoules of protein, 30 kilojoules is needed, leaving the body with 70 kilojoules. A relatively simple way of determining resting metabolic rate is to use an equation known as the Mifflin equation.
It is based on body mass w , height h , age a and sex. Although it does not take ethnicity or amount of fat tissue to lean tissue into account, it is still a good predictive measure. Balancing energy intake by eating food with output through resting metabolism, growth, exercise and digesting food is one of the key components of maintaining health and wellbeing.
Healthy children and adolescents until they stop growing are in a state of positive energy balance — weight gain. The extra kilojoules are used primarily to increase the amount of important body tissues such as bone, muscle, blood and body organs. Some of this extra may also be stored as body fat, which can be used later as a source of energy.
A negative energy balance will not allow for proper growth and development, limiting the normal weight gains associated with children and adolescents. See your doctor if you suspect you have a metabolic disorder. Some genetic disorders of metabolism include:. This page has been produced in consultation with and approved by:.
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Summary Read the full fact sheet. On this page. Two processes of metabolism Metabolic rate Metabolism and age-related weight gain Hormonal disorders of metabolism Genetic disorders of metabolism Where to get help.
Two processes of metabolism Our metabolism is complex — put simply it has two parts, which are carefully regulated by the body to make sure they remain in balance. They are: Catabolism — the breakdown of food components such as carbohydrates , proteins and dietary fats into their simpler forms, which can then be used to provide energy and the basic building blocks needed for growth and repair.
Anabolism — the part of metabolism in which our body is built or repaired. Anabolism requires energy that ultimately comes from our food. When we eat more than we need for daily anabolism, the excess nutrients are typically stored in our body as fat. Thermic effect of food also known as thermogenesis — your body uses energy to digest the foods and drinks you consume and also absorbs, transports and stores their nutrients.
Thermogenesis accounts for about 5—10 per cent of your energy use. Energy used during physical activity — this is the energy used by physical movement and it varies the most depending on how much energy you use each day. Physical activity includes planned exercise like going for a run or playing sport but also includes all incidental activity such as hanging out the washing, playing with the dog or even fidgeting! Maintaining lean muscle mass also helps reduce the chance of injury when training, and exercise increases your daily energy expenditure.
Amount of lean muscle tissue — muscle burns kilojoules rapidly. Crash dieting, starving or fasting — eating too few kilojoules encourages the body to slow the metabolism to conserve energy.
Age — metabolism slows with age due to loss of muscle tissue, but also due to hormonal and neurological changes. Growth — infants and children have higher energy demands per unit of body weight due to the energy demands of growth and the extra energy needed to maintain their body temperature. Gender — generally, men have faster metabolisms because they tend to be larger.
Genetic predisposition — your metabolic rate may be partly decided by your genes. Hormonal and nervous controls — BMR is controlled by the nervous and hormonal systems. Hormonal imbalances can influence how quickly or slowly the body burns kilojoules. Environmental temperature — if temperature is very low or very high, the body has to work harder to maintain its normal body temperature, which increases the BMR.
Infection or illness — BMR increases because the body has to work harder to build new tissues and to create an immune response. Amount of physical activity — hard-working muscles need plenty of energy to burn. Regular exercise increases muscle mass and teaches the body to burn kilojoules at a faster rate, even when at rest. Drugs — like caffeine or nicotine, can increase the BMR.
Dietary deficiencies — for example, a diet low in iodine reduces thyroid function and slows the metabolism. We can measure the energy people use during various activities by measuring their oxygen use. See Figure 2. Approximately 20 kJ of energy are produced for each liter of oxygen consumed, independent of the type of food.
Table 2 shows energy and oxygen consumption rates power expended for a variety of activities. Work done by a person is sometimes called useful work , which is work done on the outside world , such as lifting weights. Useful work requires a force exerted through a distance on the outside world, and so it excludes internal work, such as that done by the heart when pumping blood.
Useful work does include that done in climbing stairs or accelerating to a full run, because these are accomplished by exerting forces on the outside world. If a person needs more energy than they consume, such as when doing vigorous work, the body must draw upon the chemical energy stored in fat. So exercise can be helpful in losing fat. However, the amount of exercise needed to produce a loss in fat, or to burn off extra calories consumed that day, can be large, as Example 1 illustrates.
If a person who normally requires an average of 12, kJ kcal of food energy per day consumes 13, kJ per day, he will steadily gain weight. How much bicycling per day is required to work off this extra kJ? Table 2 states that W are used when cycling at a moderate speed. The time required to work off kJ at this rate is then. If the person uses 13, kJ but consumes only 12, kJ, then the amount of fat loss will be.
Figure 2. A pulse oxymeter is an apparatus that measures the amount of oxygen in blood. Such measurements can indicate the level of athletic conditioning as well as certain medical problems. Figure 3. This fMRI scan shows an increased level of energy consumption in the vision center of the brain.
Here, the patient was being asked to recognize faces. All bodily functions, from thinking to lifting weights, require energy. See Figure 3.
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