Metabolism refers to the chemical (metabolic) processes that take place as your body converts foods and drinks into energy. It’s a complex process that combines calories and oxygen to create and release energy. Your basal metabolic rate (BMR) refers to the minimum number of calories your body needs to function while you’re resting. This amount varies from person to person. Your BMR fulfills 60% to 70% of the energy your body uses. Rapid weight loss and aggressive calorie restriction decrease your BMR — this is one reason why weight loss is usually not linear and may stall at some point. Someone with a fast metabolism or fast BMR burns a lot of calories even while at rest. If you have a slow metabolism or slow BMR, your body needs fewer calories to keep it going.
A fast metabolism does not necessarily lead to thinness. In fact, studies show that people with overweight/obesity often have fast metabolisms. Their bodies need more energy to keep basic body functions going. Your metabolism plays a critical role in keeping your body functioning. Certain factors like age, muscle mass and physical activity can affect how your metabolism uses calories for energy. Having a fast or slow metabolism isn’t really a factor in weight gain or loss. Your weight has more to do with calories in versus calories out. Drastically cutting back on calories can have a negative effect on how your metabolism works. Talk to your healthcare provider about other ways to achieve a healthy weight. The BMR refers to the amount of energy your body needs to maintain homeostasis. Your BMR is largely determined by your total lean mass, especially muscle mass, because lean mass requires a lot of energy to maintain. Anything that reduces lean mass will reduce your BMR. As your BMR accounts for so much of your total energy consumption, it is important to preserve or even increase your lean muscle mass through exercise when trying to lose weight. This means combining exercise (particularly weight-bearing and resistance exercises to boost muscle mass) with changes towards healthier eating patterns rather than dietary changes alone as eating too few kilojoules encourages the body to slow the metabolism to conserve energy. Maintaining lean muscle mass also helps reduce the chance of injury when training, and exercise increases your daily energy expenditure. An average man has a BMR of around 7,100 kJ per day, while an average woman has a BMR of around 5,900 kJ per day. Energy expenditure is continuous, but the rate varies throughout the day. The rate of energy expenditure is usually lowest in the early morning. During strenuous or vigorous physical activity, our muscles may burn through as much as 3,000 kJ per hour. The energy expenditure of the muscles makes up only 20 per cent or so of total energy expenditure at rest, but during strenuous exercise, it may increase 50-fold or more. Energy used during exercise is the only form of energy expenditure that we have any control over.
However, estimating the energy spent during exercise is difficult, as the true value for each person will vary based on factors such as their weight, age, health and the intensity with which each activity is performed.
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