Your metabolism is the total of all the calorie-burning changes that occur in the body.


We are all governed by our metabolic rate which is exclusive to us. That's why some eating plans might work for a friend but not for you. Understanding and getting to know your metabolism can help you with weight loss and maintenance.
An athletic person who has a high ratio of muscle to fat will have a higher metabolic rate than a person of the same weight with a lower muscle-to-fat ratio. This is because it takes more calories to maintain muscle tissue than fat tissue so the high-ratio athlete will be able to eat more calories than the low-ratio person without gaining weight.
Some people are fortunate enough to be born with a metabolism that causes them little trouble but others aren't. There are times in most slimmers' lives when their metabolism becomes the enemy within. But exercise can help because it boosts the metabolism and allows you to burn fat more efficiently. This means that you are burning off calories at a faster rate than if you did no exercise at all. Studies have shown that the metabolic rate remains higher for at least fifteen hours after exercising.


Metabolic rate


Your metabolism is the total of all the calorie-burning changes that occur in the body. These changes are fuelled by the energy converted from food you eat and trigger essential physiological functions such as cell renewal, digestion, hormone release and brain activity. Laboratory tests can gauge how many calories you need to burn daily but few of us have access to such facilities.
Your resting metabolic rate (RMR) is the amount of calories you burn every day simply by being alive. The RMR is expressed as calories because it is a measure of the amount of heat liberated during the metabolic process. Exercise, body temperature, hormone changes and digestion can all increase metabolic rate. You can estimate your own RMR with a simple calculation:


  1. If you are under 31 multiply your weight by 14.7 and then add 496.
  2. If you are over 31, multiply your weight by 8.7 and then add 829.


 (Note: For this you will need to input your weight in kilograms — divide pounds by 2.2 to convert to kilograms)


Any extra calories you may need depend on your activity level so:


  • If you are sedentary multiply your RMR by 1.4.
  • If you are moderately active (mostly seated during the day but take some exercise) then multiply your RMR by 1.7.
  • If you are very active during the day and take organised exercise more than four times a week then multiply your RMR by 2.


Metabolism and exercise


An inactive woman of about 1.68m (5ft 6in) and weighing 60kg (9.5 stone) will have a resting metabolic rate of 1,400 calories whereas if she exercises regularly then she will probably burn off about 2,400 calories a day. It's virtually impossible for this woman to gain weight, indeed if she goes on doing this day after day she will lose a lot of weight. It just proves how important exercise is to weight control (see Chapter 6). If the same woman was eating 3,000 calories a day she would slowly gain weight and would therefore need to exercise more.


Metabolism and digestion


Our metabolic rate increases during digestion of food; this is called the thermal effect of food. This is why it is important to eat little and often. Even after just five hours without food the body starts to believe it is being starved and will decrease its metabolism. So after seven or eight hours of sleep and then missing breakfast, you are putting your body into fasting mode and you are more likely to store fat rather than use it.