Humans have a natural tendency to store fat — it’s a survival mechanism to protect us against the possibility of famine. 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.
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.
Fuel for aerobic exercise
Fat represents one of the 2 main sources of fuel that support cell function in your body. The other main fuel used by your body is glucose. Fat is stored as adipose tissue around the body, and glucose is stored as glycogen in the liver and the muscle cells. Both fat and glucose are also present in the blood as products of digestion.
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. Even short bursts of high energy muscular activity lasting around a minute or so will use glucose as the main fuel.
Will my body burn fat or glucose?
Exercise intensity, exercise duration and diet are major factors affecting whether the body uses fat or glucose for fuel during exercise.
Once the activity of energy-hungry muscle cells increases beyond ‘at rest’ levels for more than about one minute, the body uses aerobic processes that combine oxygen (supplied by breathing in more air) with either glucose or fat to generate energy to sustain the increased activity. The type of active task which causes you to breathe more deeply — ‘aerobic exercise’ — is the type of exercise that has the potential to burn fat as a fuel. Examples include brisk walking, jogging, swimming, cycling, gardening, cross-country skiing and roller-blading. You’ll notice that these activities all use the large muscles of your body — those in your arms, legs and back — continuously.
‘Burning fat’ or ‘fat-burning’ means using stored fat as a fuel to support body function, whereas 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).
Low to moderate intensity aerobic exercise
Low to moderate intensity aerobic exercise tends to burn fat, whereas high intensity aerobic exercise tends to burn glucose preferentially.
People who are new to regular exercise, or who are returning to exercise after a break, can work 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.
Exercising for longer at a lower intensity is better than only managing a 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.
The role of high intensity aerobic exercise
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 which 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.)
Tips for controlling body fat
Give yourself a chance: increase your metabolic rate. 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.
Tone your muscles to burn more calories. 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 — a hand-weights circuit is ideal — in addition to your regular aerobic exercise.
To reduce total body fat (‘lose weight’), burn more energy than you consume as food, but don’t focus exclusively on calorie restriction. 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 kilo per month. Losing more than 0.5 to 1 kilo a week can indicate that you are losing 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.
To avoid re-gaining lost body fat, continue exercising regularly and keep a check on your daily energy balance. Continuing regular exercise is important in maintaining a high metabolic rate. In fact, studies have shown that athletes who suddenly stop training lose 50 per cent of their maximum exercise capacity within 10 days, and significant reductions in metabolism can also be measured at this time. So, regular exercise to control body fat is best seen as a lifelong commitment. In addition, always keep the amount of calories you eat balanced by the amount of exercise that you do. 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.
http://www.mydr.com.au/sports-fitness/exercise-to-burn-fat
myDr, 2003
© Copyright: myDr, CMPMedica Australia, 2000-2009. All rights reserved.
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