Just as your car burns gasoline to move forward, your body requires energy to keep your metabolism humming. That energy comes primarily from glucose, which is stored in the body as glycogen, or from fat. Because glycogen is easier for your body to use as energy, it's used before fat, so if your glycogen stores are full, your body doesn't burn fat. To start burning fat, you need to diminish your glycogen stores so your body has no other choice than to use stored fat for energy. If you're having a hard time losing weight, consult your doctor or a dietitian to help you design a fat-burning program that suits your particular needs.
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Glucose serves as the primary source of energy for your body (ref 4 para 1). When you eat bread, fruit, beans and other carb-containing foods, your body breaks it down into glucose. Some of the glucose is used for energy right away and the rest is converted into glycogen and stored in your liver and muscles (ref 1 para 1,3). Between meals, your body draws on these stores to keep blood glucose at a steady level so you stay energized (ref 1 para 2). Glycogen is especially important for your brain because glucose is its preferred source of fuel (ref 1 para 2).
Glycogen Stores Versus Fat for Energy
Your body can store about 600 grams of glycogen -- 100 grams in the liver and 500 grams in your muscles. When you're not exercising, the glycogen in your liver is broken down to maintain blood glucose levels and feed your brain. However, when you're exercising, the glycogen in your muscles fuels your workout. Glycogen is converted into energy more easily than fat, which is why it's used first. After about 30 to 40 minutes of moderate-intensity aerobic exercise, such as a brisk walk, your body depletes its glycogen stores and turns to fat to fuel the rest of your workout.
Your body also uses fat for energy during periods of starvation, but not right away. At first, if you stop eating by fasting or from starvation, your metabolism breaks down muscle and other proteins, turning them into glucose. After several days without food, though, your body starts to burn your fat for energy.
Eat Less to Burn Fat
While starvation can help you burn through your fat stores, you don't need to go through such extremes to lose unwanted weight. Eating fewer calories than your body needs to maintain its current weight encourages your body to burn fat as a fuel source. Don't drop your calories too low though. Women shouldn't eat fewer than 1,200 calories and men, no fewer than 1,800 calories.
One pound of fat contains 3,500 calories, so reducing your calorie needs by 250 a day should help you lose a 1/2 pound over a week's time. To determine your calorie needs for weight loss, use an online calorie calculator to figure your maintenance needs; then subtract 250 calories from that number. For example, if you need 2,000 calories a day to maintain your weight, dropping down to 1,750 calories a day should help you burn some of your fat.
Exercise to Burn Fat
Increasing the amount of time you exercise is essential if your goal is to burn more fat. Only after you've been working out for 30 minutes does your body even start to burn fat. To get the most fat-burning benefits, you should aim for more than 40 minutes of aerobic exercise, according to the University of Michigan Medical School. While walking is a good way to exercise and burn fat, you can also do some gardening, clean the house, dance or go on a bike ride, as long as it's continuous and moderate in intensity. To get the most fat-burning benefits, try to get this amount of exercise five days a week.
- Biochemistry: Glycogen Metabolism
- University of Pennsylvania: Making and Storing Fat and Retrieving it to Supply Energy
- University of Michigan: Timing is Everything: Why the Duration and Order of Your Exercise Matters
- Georgia State University: Sugars
- Frontiers in Physiology: The Role of Skeletal Muscle Glycogen Breakdown for Regulation of Insulin Sensitivity by Exercise
- McKinley Health Center: Breaking Down Your Metabolism
- World Health Organization: What is Moderate-Intensity and Vigorous-Intensity Physical Activity?
- Centers for Disease Control and Prevention: How Much Physical Activity Do Adults Need?
- Baylor College of Medicine: Adult Energy Needs and BMI Calculator