How Many Minutes of Cardio to Start Burning Fat?

The duration of your cardiovascular workout, along with the intensity, helps determine how much fat your body burns during exercise -- and longer, slower workouts generally burn a higher percentage of fat. In terms of weight loss, however, immediate fat burning is much less important than your total calorie expenditure. If you're new to exercise, see your doctor before beginning a workout program.

When you perform cardio exercise or any other physical activity, your body needs to burn calories for energy. These calories may come from a variety of sources, including glycogen -- a form of sugar stored in your muscle tissues -- body fat or even muscle protein. When you begin your workout, your body primarily uses energy from glycogen stores. After about 20 minutes, it starts to use more stored fat than glycogen. Therefore, you'll typically burn the most fat during cardio sessions that last longer than 20 minutes.

You may see a "fat-burning" setting on treadmills or other cardio equipment, which can be misleading. While your body does burn a higher percentage of fat during low-intensity cardio such as slow walking, you burn more total calories during intense activity. Therefore, the actual number of fat calories burned may be higher with more vigorous exercise. That said, you need adequate oxygen levels to burn fat, so intense cardio that leaves you breathless can hinder the process. Optimal fat burning typically occurs during moderate-to-vigorous activities such as casual jogging or hiking, according to Columbia Health.

You may shed a negligible amount of fat during exercise, but at the end of the day your total calorie expenditure versus calorie consumption determines whether you lose weight. To shed pounds, you need to create a deficit so your body runs out of calories from food and must burn stored fat to fuel exercise and nonexercise activities. One pound of fat is about 3,500 calories -- far more than than the several hundred you might burn in a cardio session -- so significant weight loss takes weeks.

To healthily burn more calories than you eat, reduce portion sizes and choose high-volume, low-calorie foods rich in fiber and water, such as fruits, vegetables, lentils, brown rice and whole-wheat products. In general, whole, unrefined foods are more filling and less fattening than processed foods such as white bread or doughnuts. Perform cardio most days of the week for 30 to 60 minutes, and also do strength-training activities twice weekly for all body regions. For example, lift weights or perform situps, lunges and squats. Strength training won't burn much fat, but by increasing muscle tissue it speeds up your metabolism so you constantly expend more calories.