What is your ideal weight?
We know that there are disadvantages in being overweight, including strong correlations between obesity and various forms of heart disease and cancer. But there are also good reasons to avoid being too thin. Low body weight and low body fat will make you ill. The link is especially strong in women, for whom low body fat is associated with irregular periods, brittle bones, and reduced fertility (see Chapter 4). Reducing your food intake to levels which make you underweight will deprive you of essential nutrients. If you have too little body fat, your body will break down muscle and other essential tissues, which is unhealthy as well as damaging to your running performance. So the trick is to find an equilibrium weight which keeps you trim and healthy, and without making you too thin.
There are two basic ways to estimate whether you need to lose weight. The simplest is to see whether you weigh too much for your height. The more complicated approach is to estimate how much of your total weight is accounted for by body fat. These approaches are explained below. But everybody (and every body) is different, so there are no hard and fast rules.
Body mass index (BMI)
The most common way to think about whether you are overweight is to look at your body mass index (BMI). You can calculate your Body Mass Index here. It is calculated by comparing your weight with your height, using the following formula:
BMI = weight in kilograms / (height in metres) 2
Example: I weigh 74kg and my height is 1.87m, so my BMI is
74 / (1.87) 2 = 21.2.
The following table sets out the average assessments of your BMI measurement:
Table 6.9 Body Mass Index assessment
|18.5 or less||Underweight|
|18.5 – 24.9||Normal|
|25.0 – 29.9||Overweight|
|30.0 – 34.9||Obese|
|35.0 – 39.9||Very Obese|
|40 or greater||Extremely Obese|
However the BMI scale, and other similar methods based on comparing weight and height, are fairly crude and should be treated with caution. You may weigh a lot for your height because you have excess fat (which might be a problem). But you may have a high BMI because you are have a stocky build, or because you work out in the gym and have more than average lean muscle tissue. Also, this BMI scale is only suitable for adults aged 20-65 and it may not be appropriate for all ethnic groups.
Percentage body fat
In addition to comparing your weight with your height, you can also look at your percentage body fat. To measure this accurately requires full submersion in a tank of liquid, which isn’t practical for most of us. However, your body fat can be approximated in a number of other ways, including measuring the ratio of your waist to your hips, using callipers to measure your near-surface body fat, or using equipment which estimates your fat by measuring your electrical resistance.
For example, you can buy bathroom scales which estimate your body fat by measuring your electrical resistance. These work best for tracking changes in body fat if you measure yourself at the same time of day. However, this form of measurement can be significantly distorted by your level of hydration.
Once you have got an estimate of your percentage body fat, what does it mean? According to the American Council on Exercise, body fat levels of greater than 25% for men, and 31% for women indicate clinical obesity.
Table 6.9 Implications of percentage body fat
|Minimum level of fat||10-12%||2-4%|
|Obese||32% plus||25% plus|
Lots of us start running because we want to “lose weight”. Some competitive runners want to lose weight in order to increase their performance. This section is about the principles of weight loss for runners.
Obesity is a growing problem in Britain, with around half of women and two thirds of men currently overweight or obese. This can lead to poor health, including heart disease and diabetes and reduced life expectancy. It also reduces quality of life, both physically and psychologically.
Many of us find that running is a good way to escape this disease. Runners benefit from a virtuous circle of weight loss, increased self-esteem, improved performance and commitment to a healthier lifestyle.
Running is an ideal way of losing fat and improving your appearance. It increases your energy consumption, allowing you to continue to eat satisfying amounts of food while reducing your levels of body fat. Conversely, weight loss is an effective way to improve your running. For competitive athletes, reaching the correct body weight is an important component in improving performance. However, if you reduce your weight and body fat too much, you can make yourself ill, and your performance will deteriorate.
Why running is a good way to lose weight
In my view, diets are not a good way to lose weight because:
- many people on diets are perpetually hungry; and although they may reduce their food intake for a time, it is unlikely that they will be able to maintain their lower weight;
- the human body reacts to low food intake by reducing its metabolic rate (your body goes into “starvation mode” to conserve energy); this defeats the point of the diet since it means that your body slows down your calorie consumption;
- constraining your food intake restricts your intake of key nutrients (e.g. vitamins and minerals), the absence of which can eventually make you ill;
- psychologically, dieting reinforces a personal sense of self-disgust and dissatisfaction with your own body.
By contrast, losing fat by exercising increases your energy consumption, so that you can continue to eat normally, your body increases its metabolic rate (even when you are not running), you get plenty of key nutrients, and you feel good about yourself. For me it is a no-brainer: if you want to lose weight, take up running.
Do you really want to lose weight?
People who say they want to “lose weight” may be using this as a short-hand for a number of different implicit and explicit goals. These might include:
- reducing visible body fat;
- improving muscle tone;
- increasing muscle size;
- reducing the appearance of cellulite;
- increasing life expectancy and reducing health risks such as heart disease and diabetes;
Very few of us, except perhaps professional athletes and horse jockeys, are really concerned about what we weigh. We are using body weight as a measure of our progress (or lack of it) towards underlying goals, which we are sometimes embarrassed to express openly.
Achieving lower levels of body fat and improving your appearance does not necessarily mean reducing your weight. Because muscle is made up of protein, which is heavier than fat, many people who begin to exercise find that they actually increase their weight, at least at first, as they build up their muscles, even though they are losing body fat. So despite increasing their weight, they are still achieving their goals of improving their appearance and reducing their body fat.
It is therefore important to be clear about what you are really trying to achieve. You may make good progress towards your goals without seeing any evidence in your weight or changes in your body mass index. The danger is that you become too focused on “losing weight” and so make changes in your diet and lifestyle which makes it harder, not easier, to achieve your real goals.
How to reduce your body fat
The principles for losing fat are simple. Your body uses fat to store energy that is surplus to its requirements. So if you take in (in other words, eat or drink) more energy than you use up, your body will store the excess calories as fat. If you use up more energy than you take in, then you will burn stored fat to provide the extra energy.
From the point of view of reducing your body fat, it doesn’t matter where the calories come from. A surplus calorie will be stored as body fat, whether it was originally from protein, carbohydrate or fat. So the composition of your food intake is much less important for losing fat than the total amount of calories you consume and the amount of energy you use.
In other words, to reduce your body fat you must eat fewer calories and/or burn more energy. This straightforward principle has some important implications.
Eating and drinking fewer calories
Most diets are aimed at getting you to eat fewer calories. This usually means either restricting the total volume of food you eat; or eating foods which contain fewer calories per mouthful, so that you eat as much food but it contains less energy. (Some diets are intended to get you to burn more energy by increasing your metabolic rate – see below).
It doesn’t matter to your overall weight loss where the calories come from; but if you are going to restrict your calorie intake, it is generally a good idea to cut back on alcohol, harmful fats and refined sugars.
Alcohol is high in calories (about 90 kcal for a glass of wine or 170 kcal for a pint of beer). It is also diuretic (which means it makes you pee), contributing to dehydration. While alcohol is not necessarily unhealthy in moderation, it does not do you much good, and cutting back on calories from this source will make you less hungry than cutting back on your basic food intake.
The reasons for cutting back on fats are:
- fat has more than twice as many calories per gram than carbohydrates or protein; so you won’t feel as hungry if you cut back on fat as if you cut out the same number of calories of carbohydrate or protein;
- some fats are positively harmful and so you should aim to reduce these in your diet;
- most of us eat more fat than we should in the first place; so cutting back on fat often brings us back towards a more desirable, balanced diet.
The reasons for cutting back on refined sugars are that they lead to large swings in blood sugar levels. In children, it is thought by some people that refined sugar is associated with hyperactivity, though there is little compelling evidence for this.
This is not primarily a book about dieting. My view is that the best way to lose weight is to exercise more. But if you decide to cut back on calories, it seems that the most effective ways to do this are ones that don’t leave you feeling perpetually hungry. This means that you need to eat foods that have high volume for each calorie. Hence low fat, low sugar, high fibre diets are likely to be the most sustainable ways to reduce calorie intake.
Burning more calories
Burning more calories is generally a more sustainable and positive way to lose body fat than trying to eat fewer calories. The main ways to burn more calories are to exercise, and to change what and when you eat.
Exercising burns more calories in three ways:
- first, you use energy during the exercise itself; for example, running or walking uses up about 100 calories per mile – and this is not greatly affected by how fast you go; (clearly, you will burn more calories per hour if you run faster, because you will cover more miles per hour, but the number of calories per mile will stay pretty much the same); table 6.8 shows the calories burned for other common exercises;
- second, regular exercise increases your metabolic rate even while you are not exercising; essentially, if you exercise often your body adapts to a generally higher level of energy consumption;
- third, exercise increases the amount of lean muscle tissue in your body; and this in turn increases your metabolic rate;
Other ways to increase your metabolic rate (and hence energy consumption) are:
- altering when you eat; in general, a large breakfast seems to kick‑start your metabolism so that you burn up more calories during the day;[i] in addition, there is evidence that eating little and often (e.g. 4-5 moderate sized meals and snacks a day, rather than 2-3 larger meals) maintains a higher metabolic rate through the day;
- changing the time you exercise; exercising in the morning before you have breakfast may increase your metabolic rate during the day by more than exercise in the evening.
There may also be a marginal impact on your metabolism from changing what you eat. For example, your body seems to respond to carbohydrates by increasing your metabolism more when you eat fats. However, the size of this effect is probably small enough to ignore. Diets that restrict you to particular kinds of food really work by reducing the amount you eat (since there is only so much pineapple that a person wants to eat).
How fast to reduce your body fat
If you do decide you want to reduce your body fat, you should not attempt to lose weight too rapidly. In particular, you should not aim to lose more than ½kg a week; nor should you try to consume less than 85% of your daily equilibrium calorie needs.
The best approach to estimating how fast you should be losing weight is:
- decide how much exercise you realistically intend to do;
- estimate your equilibrium daily calorie requirement (using the calculation described earlier in this chapter, which you can do automatically here) at that level of exercise, given your current bodyweight;
- work out how many calories a day you need to eat to lose weight without consuming less than 85% of your daily needs or losing more than 500g a week.
So, for example, if your equilibrium daily calorie requirement is 3000 kcal a day, you should not eat less than 2550 kcal (which is 85% of 3000). If you cut back to 2550 kcal a day, you will lose 3,150 kcal a week – which means you’ll lose about 350g of fat each week (since fat weighs in at 9 calories per gram). This is the maximum realistic rate of weight loss for you.
Should you run more slowly to lose more fat?
You may have heard the claim that you should run more slowly to burn more fat. My local gym has signs on the aerobic equipment which identifies a “fat burning zone”. The implicit message is that if you are exercising to lose weight, you are better off staying within this zone.
This is an excellent example of Alexander Pope’s dictum that “a little learning is a dangerous thing.” As we shall see in the next chapter, it is true that the proportion of energy that comes directly from fat is higher at low rates of exercise intensity. But this doesn’t mean that you are going to have less body fat if you exercise at lower intensities, for two reasons:
- first, it doesn’t matter in the slightest for your overall level of body fat where the fuel comes from while you are exercising; your body will replenish and rebalance your energy stores when you are recovering. In the end, a calorie surplus or deficit will always end up affecting your level of body fat by the same amount, irrespective of which particular fuel was burnt during exercise;
- second, what matters is the number of calories that you burn. A higher proportion of the calories may come from fat if you run more slowly, but you are also burning fewer calories in total. We can all devote only a limited number of hours to exercise, and the way to maximise the number of calories used in that time is to run as far and as fast as we can in that time. We will not burn more calories by going deliberately slowly.
As we shall see in Chapters 9 and 10, there are very good reasons for running slowly some of the time: but increased weight loss is not one of them. Whatever you may have been told to the contrary, it is not true that you will lose more fat, or more weight, by running more slowly.
[i] British Nutrition Foundation, see http://www.nutrition.org.uk/