Annex for Anoraks on Weight Loss


The basic principles of weight loss are not rocket science, and it is often useful to return to basics when thinking about weight loss. Calories measure the energy content of the food you eat. You use energy – that is, burn calories – just by being alive.

If you exercise, you use more energy than if you are sedentary. If the total energy you consume in a day exceeds the energy that you use up, then the surplus is stored by your body, usually as fat.

So broadly speaking, there are two ways to reduce the amount of fat in your body:

  • eat fewer calories by eating less or differently; and/or
  • burn more calories by exercising more.

Provided you achieve either or both of these, you will reduce your body fat.

To estimate the amount of calories you need each day, first multiply your weight in kilograms by 33 to estimate your daily calorie requirement for a moderately active person who does not exercise.

So if you weigh 75kg, are moderately active but do not exercise, you need about 2475 calories a day.

Then add 100 calories for every mile you walk, jog or run. So if you weight 75kg and run 5 miles a day, you have a calorie requirement of 2975 calories a day.

If you want to lose weight, reduce your calorie intake to less than your daily calorie requirement. You should not reduce your calorie intake to less than 80% of your calorie requirement.

If your weight is broadly constant at 75kg and you take no particular exercise, then you are probably consuming about 2475 calories a day. Suppose you start running 5 miles a day. Your calories consumption will go up by 500 calories a day – or possibly a little more to take account of the increased metabolic rate. Fat weighs in at 1/9g per calorie. So this calorie deficit should reduce your fat level by 55-60g per day, or 0.4kg a week.

How weight loss programmes work

Weight loss programmes are only effective if they reduce the calories you consume or increase the energy you burn. They might work by:

  • changing the calorie content of a given volume of food, by encouraging you to eat less fat (which is 9 calories per gramme) and more carbohydrate and protein (4 calories per gramme) or water (0 calories)
  • making it difficult for you to eat so much, by forcing you to combine foods in particular ways, or by forcing you to eat unappetising or repetitive things
  • provoking your body into feeling satisfied, for example by getting you to fill up on dietary fibre, which is bulky; eating often, and so making sure you don't feel hungry; and ensuring that your blood sugar levels do not swing around too much by encouraging you not too eat foods which promote an insulin response
  • encouraging you to eat at times which increase your metabolic rate and so increase your energy consumption. For example, it might be that your metabolic rate is higher if you eat more often, or if you eat during the day rather than in the evening.

Such weight loss programmes may have value, but only if they help you to achieve the underlying basic requirement of eating fewer calories or burning more energy. Many diets are not sustainable, particularly if they work by artificially limiting what you eat.

Diets that cause weight loss by reducing your water content are certainly not going to be sustainable and may make you ill.

Do you want to lose weight or reduce your body fat?

Consider for a moment what it is you actually want to achieve. Does it really matter to you how much you weigh? Or is this a proxy for other concerns you have about yourself: for example about how your body looks, how toned your muscles are, or how fit you are? It is important to decide what you really want to achieve.

The reason this matters is that lean muscle tissue is heavier than fat. So if you exercise, you may reduce your fat stores but develop more lean muscle tissue, so that your weight may actually increase (or fall more slowly). Over time, you may look more toned and fit, and carry less fat, but weigh much the same as before.

Most people who say they want to lose weight really want to reduce their body fat. Unless there is some reason why you want to weigh less, you should focus on reducing body fat.

Exercise and dieting

It is a bad idea to increase your exercise and go on a diet at the same time. When you are a runner, you need more energy and nutrients, and you are much more likely to get ill or injured if you try to restrict what you eat at the same time.

Running helps you to lose body fat in three ways:

  • you will burn more energy while you are running – about 100 calories for every mile you jog or run
  • you will raise your basic metabolic rate, so that you burn more energy even when you are not exercising. (This effect is most pronounced if you run at least 4 times a week for at least half an hour).
  • you will increase your quantity of lean muscle tissue, which burns more calories.

Dieting will reduce your fat, muscles and water content, whereas running will reduce your fat but increase the quantity of lean muscle. In addition, running will improve the tone of your muscles, which will contribute to an improved appearance. Dieting is often difficult to sustain, because it requires you constantly to deny yourself what you want. Sometimes, the constraints imposed by diets are unhealthy, preventing you from eating a good variety of foods.

Running in the "fat burning zone"

Your body has a number of sources for energy, including glycogen (which is stored in your muscles and round your liver), protein and fat. Fat is an efficient way to store energy, but it can only be translated back into energy fairly slowly.

When you run at high levels of exertion, your body needs a lot of energy quickly, and uses glycogen, which is the most accessible source of energy. Only a modest proportion of the energy comes directly from fats. When you run more slowly, your body does not need to call on the most accessible energy stores, and a higher proportion of the energy you use comes from fat. This is why some aerobic equipment in gyms, and some books about exercise, talk about fat burning zones.

So it sounds as if you must run more slowly to burn more fat, right? Wrong.

This argument is wrong on two counts. First, you will almost certainly burn less fat overall if you run slowly, even if it represents a higher proportion of the energy you use. If you have an hour to exercise, the further you run the more fat you will burn. Second, what matters for your fat levels is the amount of energy you consume and the amount of energy you burn: not where the energy comes from when you exercise. Body fat used during exercise will soon be replaced.

So the best way to burn fat is to run as far as you can in the time available.