Don't Spike Your Insulin Levels
There are several ways you can ramp up how quickly your body is burning fat, say nutrition and exercise experts. Nutrition-wise, you want your body to access stored body fat for energy, says nationally known nutritionist and famed author Ann Louise Gittleman, Ph.D.
To make this happen, you need to have a sufficient amount of protein in your diet, she says. You also need to make sure your carbohydrate intake is moderate, not high. That means getting no more than 50 percent of your calories from carbs. High levels of carbs lead to high insulin levels.
"Your body cannot access stored body fat with high insulin levels," Gittleman says.
Converting Fat to Energy
Your adipose, or fat, tissue is a storage depot, says Gregory J. Morton, research assistant professor of medicine at the University of Washington Diabetes and Obesity Center of Excellence, Division of Metabolism, Endocrinology and Nutrition.
When you eat foods, your body absorbs energy. Some is used right away, some is converted to glycogen and stored in the liver and muscles, and the rest is stored as fat. Some foods, such as simple sugars, are rapidly converted into energy and readily packaged and stored as fat. Other foods, such as vegetables, are digested over a slower period of time and, as a result, aren't packaged into fat as quickly.
Your stored glycogen is used relatively quickly when your body is in an energy deprivation state, which means your body must mobilize your fat stores for the energy it needs.
What Happens When You Fast
The most obvious time for your body to turn to its fat stores is during a period of fasting, such as when you are asleep and not consuming food.
One key player in this process is the hormone insulin, made by beta cells in the pancreas when you eat. Your body needs insulin to use and store energy from food. When insulin is abundant lipogenic, or fat-storing, enzymes are stimulated and enzymes that break down fat are inhibited.
"When there's no insulin around, the opposite situation happens," Morton says, so when insulin levels are low, key enzymes in your fat tissue activate to break down fat and use it for energy.
Exercise Brings on Fat Burn
Morton says exercise also creates a state of negative energy balance. Your body needs to mobilize fuel to compensate for the energy you burn up. The body turns first to its stored glycogen. Then, it starts to mobilize fat from its adipose tissue depot.
Turbo Charge the Fat Burn Process
Your body turbo charges this process when you do interval training, says David K. Spierer, assistant professor of sports sciences at Long Island University, Brooklyn Campus. Interval training involves the repetition of high and low intensity bouts of exercise.
"This causes the igniting of carbs over and over and over again, so fat calorie burn from interval training is higher than from continuous aerobic exercise," Spierer says.
Your body goes through three processes over and over when you use interval training. The first process happens within 6 to 10 seconds of activity. In this process, cell-powering adenosine triphosphate, or ATP, is produced anaerobically, he says. ATP is your body's energy currency.
The second system breaks down glucose, the storage form of sugar, to produce needed energy and also produces lactic acid. The glycolytic system takes 10 seconds to 2 minutes, he says, and also is powerful. In fact, a muscle fiber's energy use can be 200 times greater during exercise than when at rest, according to "Physiology of Sport and Exercise," Jack H. Wilmore, January, 2008.
The first two systems are not capable of supplying the energy your body needs beyond this point. That's when your body turns to the oxidative system, Spierer says, in which the body uses oxygen to generate energy. In this aerobic process, the body utilizes fats more than glucose, he says. This can take about 20 minutes to achieve.
Body Continues to Burn Fat
Your increased fat burn from higher intensity exercise continues after your workout, Spierer says, because your body is still burning heat, or calories, due to your workout 9 to 10 hours later and working to replenish everything you used during your exercise session. Bouts of high-intensity exercise also lead to a higher accumulation of lactic acid. Your body needs work to clear this acidic environment in its blood. Your body also works to bring down your body temperature and regenerate energy from carbohydrate.
Duration also has an impact on your afterburn. "There's lots of repair going on, and all of this uses heat, or calories. That's why we burn more calories when we do intense activity instead of moderate," Spierer says. "The harder you work out, the more bang for the buck you get once you're done."
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