Humans have a natural tendency to store fat — it’s a survival mechanism to protect us against the possibility of famine, and some fat is essential for our general health. The trouble is that today many people have access to an abundance of food, especially energy-dense fatty and sugary foods, yet they undertake little energy-burning physical activity. This means that many people have an imbalance between their energy intake (food and drink) and their energy expenditure (general metabolism and physical activity).
The net result is an energy surplus, which is efficiently stored as body fat by a physiology that developed in times when famine was a likely and life-threatening risk. The storage of this excess fat causes many people in the developed world to become overweight or, to be more accurate, ‘over-fat’, which in more extreme cases is classified as obesity. There is an associated health risk with being over-fat and even more so with obesity, as it increases our risk of heart disease, diabetes, and some forms of cancer.
Maintaining a healthy level of body fat is a matter of balancing calorie intake (food) with expenditure (our basal metabolism and exercise/physical activity). To reduce body fat, you have to expend more calories than you take in, and this can be achieved either by eating fewer calories and/or by expending more energy through greater levels of physical activity.
Fat and carbohydrate (mainly glucose and related molecules) are the two main types of fuel used to support your body’s metabolism. They come from food and drink, and following the process of digestion are absorbed into your blood stream to either be transported to sites for immediate use or storage for later use. Fat is stored as adipose tissue around your body, including under your skin, in muscles, and around vital organs. Carbohydrate is stored as glycogen (formed by joining many glucose molecules together) in your liver and muscle cells.
Many activities of the body that take place when you are at rest, for example, brain activity, the pumping of your heart and the functions of your internal organs, use glucose as a readily available source of energy. Likewise, you will use small amounts of fat in your basic everyday activities. So, small amounts of fat and carbohydrate are continually used to maintain your basic daily metabolism. However, muscle contractions that occur during exercise require a considerable amount of additional energy, and vigorous exercise can increase your energy use by 10-fold. This additional energy expenditure will therefore increase your use of fat and carbohydrate, some of which will come from your body’s stores.
‘Burning fat’ or ‘fat-burning’ means using stored fat as a fuel to support body function. Reducing total body fat (which is what most people desire when they say that they want to ‘lose weight’) involves burning more calories each day (whether from stored fat or stored glucose) than are replaced by calories consumed as food. In a large part, weight loss is achieved by meeting this goal, often with the assistance of exercises that burn fat and exercises that build muscle (because muscle cells burn more calories at rest than do fat cells).
To use energy and substantially increase the use of fat and carbohydrate, you need to undertake exercise that uses the large muscles of your arms and legs, and to perform it for a prolonged duration. For example, walking, jogging, swimming, cycling or doing aerobics for 30 minutes or more. The biggest factor determining how many calories (that is, the amount of energy) you use during exercise is how much you do. Walking 3 km uses almost the same amount of energy as running 3 km. But of course, walking the 3 km will take longer than running it.
A second factor that influences the amount of energy you use is the exercise intensity. Jogging for 30 minutes will use more energy than walking for 30 minutes, the primary reason being because you will travel a greater distance in 30 minutes of jogging than in 30 minutes of walking.
Exercise intensity also influences the proportion of energy that you get from fat and carbohydrate. Low intensity exercise, such as walking, predominantly uses fat with some carbohydrate. The harder you exercise, the more energy you use per minute, and the greater the proportion derived from carbohydrate. Very vigorous exercise predominantly uses carbohydrate (muscle glycogen), but this does not mean that it is not effective when trying to reduce your fat stores. By depleting your glycogen stores, some of the carbohydrate you eat will be used to replenish the glycogen, and you are less likely to store the excess as fat.
So you need to find a balance between exercising as hard as you can and doing it for a reasonable duration (30 to 60 minutes per day). For example, it would be better to walk for one hour than to jog for 5 minutes. But it would also be more effective to walk briskly (or jog) than walk slowly.
The most important factor is for you to choose an exercise (or a variety of exercises) that you will do on a regular basis — ideally at least 5 times a week, and preferably everyday. You should then perform it at an intensity that you can sustain for at least 30 to 60 minutes a day. And for fat loss, a target of one hour per day is a desirable objective.
People who are new to regular exercise, or who are returning to exercise after a break, can start with lower amounts and build up towards doing low to moderate intensity aerobic exercise for at least 20 to 30 minutes on 4 to 5 days each week. This is a practical and safe way (low risk of injury) to burn body fat. Varying the type of exercise by doing, for example, a combination of walking, cycling, swimming and going to the gym is likely to prevent boredom and ensure that you get the holistic benefits from doing different body movements.
As indicated above, exercising for longer at a lower intensity is better than only managing a very short time at a higher intensity. This approach to exercise also has significant additional health benefits such as reducing the risk of heart disease and type 2 diabetes.
If combined with healthy eating that aims for a slight energy deficit, rather than an energy surplus, low to moderate intensity, moderate duration aerobic exercise can be an effective tool in weight reduction and weight management. But you must ensure that you undertake the exercise regularly, and aim for at least 150 minutes of exercise per week. For more effective fat loss, try to attain about 300 minutes a week.
For people who already have an established level of physical fitness, a higher intensity or longer duration of aerobic exercise may be indicated in order to burn fat. However, this approach is not usually practical in people who are beginning or returning to regular aerobic exercise.
Even though high intensity exercise tends to burn the body’s stores of glucose rather than its stores of fat, in high intensity aerobic exercise that lasts say 30 minutes, the total calories burned, irrespective of the source of these calories (glucose or fat), will be higher than the calories burned in 30 minutes of moderate intensity aerobic exercise. That is, the harder you exercise in your given amount of time the more calories you will burn, and that includes after you’ve finished and when you’re recovering. So, if you have moved beyond a beginner exerciser’s level of fitness, aiming to do regular aerobic exercise at high intensity (‘as hard as you can’), may be a more useful guideline than simply continuing to exercise at moderate intensity. (Before starting high intensity aerobic exercise, seek individual advice from your doctor, and be aware of the pitfalls of over-exercising, including an increased risk of injury.)
Irrespective of dietary modification, an exercise-focussed lifestyle will increase your metabolic rate, and will inherently burn more calories than a sedentary lifestyle. In contrast, it is believed that markedly reducing the amount of calories that you eat will signal a state of potential starvation to your body. In this context, your body adjusts by slowing down your metabolism and trying to conserve fat.
Using strength training exercise to increase your percentage of muscle tissue compared to fatty tissue shifts your body composition in favour of energy-hungry muscle cells. Muscle cells consume many times more calories than do fat cells, at rest. A kilogram of muscle will burn 50 to 100 calories a day compared to 5 to 7 calories a day for a kilogram of fat. One of the best ways to increase your percentage of muscle tissue, and hence your metabolic rate, is to do a strength training routine 2 or 3 times every week, in addition to your regular aerobic exercise.
To reduce total body fat, focus on increasing your physical activity rather than drastically decreasing the energy you consume as food. As fatty foods are energy dense, selecting low-fat options is a sensible way to limit unnecessary calories in your food. Don’t cut out fat altogether: current advice recommends that you moderate total fat intake but limit saturated fats — the type of fats present in foods of animal origin such as meat and butter. As a guide, a recommended rate of weight reduction is around 0.5 to 1 kg per month. Losing more than 0.5 to 1 kg a week can indicate that you are losing fluid and muscle rather than body fat. If you are overweight or obese and are considering a restricted-calorie diet, speak to a dietitian for individual advice.
One of the major determinants of how many calories you use during exercise is the amount of exercise that you do. Exercising for an hour a day will use twice as many calories as exercising for 30 minutes. And you may like to divide the time up into 2 sessions of 30 minutes or 3 sessions of 20 minutes.
Continuing regular exercise is important in maintaining a high metabolic rate. This approach will give you the best chance of maintaining the new body you have gained through an active lifestyle that includes strength training and aerobic exercise, and calorie-wise healthy eating.
Even if your exercise programme produces only a slow reduction in weight, it doesn’t mean that it is a waste of time. Many studies have shown that regular exercise can reduce your risk of heart disease and other health problems independently of weight loss.
Last Reviewed: 31 January 2010
You are under optimum weight for your height. Being underweight can carry health risks for both men and women. Check with your doctor or dietitian to find out if your current weight is healthy for you.
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