What Is Metabolism?
Metabolism encompasses all of the chemical processes that occur within the cells of a living organism, in order to sustain life. There are many factors that influence metabolism – some that we can control, most that we cannot.
Metabolism can be split in to 2 categories – anabolism and catabolism.
Catabolism: This is the breakdown of organic substrates, for example, carbohydrate is broken down to glucose, protein breaks in to amino acids and triglycerides are broken down to fatty acids and glycerol. This process produces energy.
Anabolism: Anabolism has an opposite effect to catabolism whereby it synthesises new organic molecules for functions such as structural maintenance or repair, support of growth or to store nutrient reserves.
Your daily total energy expenditure can be split in to three components; these influence your overall metabolic rate. The diagram below outlines the activities that our bodies use energy for and how significantly each activity contributes to our daily energy output. The three components are then discussed in more detail.
1. Basal metabolic rate/ Resting metabolic rate
Basal Metabolic Rate (BMR) is the amount of energy required for your body to maintain homeostasis (The ability to maintain internal equilibrium). In other words, it is the amount of energy your body uses at complete physical and mental rest in a thermoneutral environment (to avoid heat-generating processes such as shivering which burns more energy). If a state of complete rest is not maintained, the energy expenditure is referred to as resting metabolic rate (RMR) which expends more energy than the BMR.
The amount of energy you burn at rest can be increased by increasing your lean body mass i.e. muscle mass. This is because lean mass uses more energy.
BMR and RMR contribute largely towards total daily energy expenditure (on average 60-75%). By increasing your lean mass, the amount of energy your body expends in a day can be markedly increased. This is particularly useful for individuals who are trying to lose weight. Increasing your muscle mass as well reduces the risk of injury in training.
However, there is a difference between being lean and being “skinny”. Being skinny is not always an indicator of good health or of being lean. Someone who is very thin can have minimal muscle mass and can still have visceral fat hidden away – the most dangerous form of fat. So it is possible to be thin but still have a higher percentage of body fat than is healthy.
Leanness is rather an indicator of greater muscle mass and less fat mass.
What influences BMR/RMR?
The largest influencing factor is body size and composition. Other factors that affect BMR/RMR are age, gender and hormonal status; discussed below.
- Body size – the rule here is that the larger the body surface area, the higher the metabolic rate will be. So those who are taller will have a higher RMR than those who are shorter. However, an individual who is short yet wide will have a lower RMR than someone of the same weight/ surface area who is taller and lean. This is because fat cells burn significantly fewer kilojoules than other tissues, such as muscle – this is where body composition comes in to it.
- Body composition – As previously discussed, fat free mass is the metabolically active tissue in the body. Therefore the more you have of it, the higher your RMR will be and the more energy you will burn at rest.
- Age – Metabolic rate is higher during periods of rapid growth e.g. the first years of life. During growth, much of the energy received from food is dedicated to new tissue.
Resting metabolic rate decreases with age. This is likely due to the tendency for muscle tissue to be lost and fat to be gained as activity levels decrease. This can be avoided by maintaining muscle mass through exercise.
- Gender – There is commonly a difference in metabolic rate between men and women. This is largely due to size and body composition. Due to differences in anatomy, women are naturally predisposed to store more essential fat than men. For example, women store more fat around their hips and chest than do men. As a result, a male of the same weight and height will generally have a higher RMR as their percentage of muscle mass to fat mass will often be higher – for the sake of comparison, this is assuming all other factors like muscle mass and age are also the same.
- Hormones – Your hormones can affect your metabolism too. In hormonal disorders such as hyper or hypothyroidism, your energy expenditure is either increased or decreased. Additionally, stimulation of the sympathetic nervous system can increase cellular activity and hence metabolism e.g. during periods of stress or excitement. RMR as well gradually increases during gestation.
- Genetics – Genetic metabolic disorders will as well affect metabolism in different ways. This includes disorders such a phenylketonuria (PKU).
- Other – Other factors than influence RMR include nicotine, caffeine, alcohol, fever, infection and extremes in environmental temperature. These factors all increase the metabolic rate as it attempts to restore homeostasis.
2. Thermic effect of food (TEF)
This is the energy your body uses to eat, digest, absorb and metabolise food. Factors that affect the TEF are as follows:
- Composition of food – The TEF is higher after the consumption of protein and carbohydrate foods opposed to high fat foods. This is because fat is metabolised very efficiently where as some carbohydrate and protein is wasted during metabolism.
- Routine – Individuals who follow a routine with their diet have a higher TEF response than those who do not have a regular eating pattern.
- Extreme dieting, skipping meals or fasting – By consistently eating well under your energy requirements or skipping meals, your body is tricked in to a starvation response and the metabolism is consequently slowed down in order to conserve energy. This decreases BMR and can also deteriorate your muscle mass which further slows the rate at which your BMR burns energy.
- Type of food – Spicy foods increase the TEF and also prolong it. For example, meals with chilli and Mustard may increase the metabolic rate by up to 33% opposed to meals with no spice! This effect can last for over 3 hours.
3. Thermic effect of physical activity
This includes the amount of energy that is burned during voluntary physical activity such as sport and incidental activity, referred to as Non-Exercise Activity Thermogenesis (NEAT). Incidental activity, or NEAT, includes activity of everyday living e.g. shopping, chores and fidgeting.
Factors affecting activity thermogenesis (AT) include:
- Age – there is a correlation between age and a decline in AT. This is attributed to the decrease in lean body mass and increase in fat mass that is generally seen with increasing age.
- Excess post exercise oxygen consumption (EPOC) – This is the continued elevation in metabolic rate once exercising has finished. It is influenced by the duration and intensity of physical activity.
Metabolism can in part be determined by your genetics; however, if someone is lucky enough to have a “fast metabolism”, it is likely due to a combination of the above factors.
By altering your dietary habits and physical activity level, weight loss and good health can be achieved much easier.
Some of the influences on an increased or decreased BMR are shown in the images below.
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This article was written by our dietitian Belinda Elwin who is a Dietitians Association of Australia member and Accredited Practising Dietitian and Nutritionist.
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