3.6 The Energy of Food: Calories and Kilojoules

Transcript

Whilst there is much much much more to losing weight than simply how many calories you eat, it is still an important part of the equation, so let’s explain this in more detail.

The cells that make up our bodies need energy in order to work, just like a car needs fuel to go.  This energy in food is represented as either calories or kilojoules, some countries use calories and other kilojoules.  As a general rule 4 kilojoules equals roughly 1 calorie, for example 100 calories are the same as 400 kjs, so you can convert them easily yourself.

So that is energy going into our bodies via calories or kilojoules.  But when we go about our lives we are also burning energy and we do this in a variety of ways.

Most of the energy we use is simply from our bodies just doing its usual processes, like pumping blood around, using our brains and breathing, and this is called our basal metabolic rate or BMR.  This BMR accounts for 60-75% of the daily energy we use. The rest of the energy we use is from the extra movement or bodily processes we do, like running or studying.

So that is energy going out.

So now we have a simple equation.  We have the energy we put in and the energy we use, and then the difference between the two can either be we have taken in more energy than we need, or less, or the same amount.  If we consistently take in more energy, like food calories, and don’t burn that energy off, we have an excess of energy and we store that as fat. It is important to understand that fat is simply the bodies way of storing energy for later use.  Basically the body is reserving that energy as fat so that if there comes a time you don’t get any food, it will use the fat as an energy source and you can survive longer.

So for example let’s say a couple of potatoes has 140 calories, and we walk 3km we burn 150 calories, then we are neutral.  But if we have that same number of potatoes and sit down all day, we have 150 extra calories which get stored as fat, because fat is just the bodies way of storing energy.

And if you use more energy than you take in, by either increasing your activity levels or reducing your food intake then you are going to have a deficit of calories and you will lose weight.

The total amount of energy a person should have a day depends on your gender, body build and how active you are.  This can range from 1800 calories or 7600 kilojoules a day for inactive women, to 3000 calories or 12700 kilojoules a day for an active man  You can calculate this easily for yourself by googling BMR calculator.

Obviously, if you want to lose weight you need to be in a calorie deficit which means you are using more than you have put in.  If we want to lose 2kg a month, which would be 24kg after a year, that generally means we need to be in a calorie deficit of 500 fewer calories a day or 3500 fewer calories a week. So that might mean eating 500 calories fewer a day or burning 500 more calories a day or a combination of both.  Or we could live normally on a few days of the week and have 1000 calories less on select days, meaning 3500 fewer calories a week. There are lots of ways to achieve this, but overall you have to be in a calorie deficit.