6 The metabolic System
What is Metabolism?
6.1 Metabolism is the process of transforming foodstuffs into tissue elements and into energy (movement and heat) for use in growth, repair and general functions of the body, and to running a marathon. Metabolism happens in your muscles and organs and the result of it is what we commonly refer to as “burning calories”. Metabolism is essentially the speed at which your machine body’s motor is running. It is the metabolic rate that influences your energy level, mood and exactly how quickly you gain or lose weight. Most people are born with normal metabolism but their lifestyle influences it for better or worse.
6.2 Basically, our body burns calories to sustain three main functions: –
(i) Basal (Resting) Metabolic Rate (BMR) – This is the amount of energy or caloric expenditure required by the body when it is at complete rest – even when we are lying down doing nothing – to maintain body functions such as breathing, heartbeat, muscle tone, maintaining body temperature etc. BMR accounts for 60-70% of the daily calories burnt for an average person. The other 30-40% helps maintain body tissues and muscle mass. To roughly calculate, BMR multiply the body weight in kilos by 22.
(ii) Activity – This is the energy used doing movement and exertion – from lifting our arm to operate the remote control to doing housework, walking and exercising. The calories burnt, depends upon how active or passive lifestyle we have. The activity level calories roughly are 30% of BMR for inactive persons, 50% of BMR for average activity and 75% of BMR for strenuous activity.
(iii) Dietary Thermogenesis – This is food induced heat production – the digestive calories burnt in the process of eating, digesting, absorbing and assimilating food in the body. Digestive calories will be about 10% of both BMR and activity level calories. As an example, drinking 2 litres of iced water a day will burn 100 calories just heating the water to normal body temperature.
6.3 Example – For a person weighing 60 kg with average level of activity, the guideline average daily calories will be: –
1320 BMR calories (60 x 22) + 660 average activity level calories (50% of BMR calories) + 198 dietary thermogenesis calories (10% of both BMR and average activity level calories) = 2,178 calories.