To lose weight using a treadmill, the best speed is a mixture of speeds.
Many treadmills show a graph that depicts your "fat burning zone" at a low heart rate. As exercise intensifies, you move into a "cardio zone," then an "aerobic" zone. There is a myth that very low intensity training (what your treadmill calls the "fat burning zone") is the best way to burn fat with exercise. This false belief is a misinterpretation of exercise's relationship with weight loss.
Debunking the Fat-Burning Myth
When you exercise on a treadmill, your body gets energy from burning both fat and carbohydrate. You are always burning some of each, but the proportions of carbohydrate and fat fluctuate with intensity. Since the amount of carbohydrate your body can store is limited, it prefers to burn fat for fuel whenever possible. Carbohydrates deliver energy faster than fat, so as intensity increases, you use more carbohydrate to keep up with energy demand.
In the "fat-burning zone" (when energy demand is low), it's true that most calories come from fat (about 60 percent). Fat only provides only about 40 percent of the energy at higher intensities, but the total number of fat calories burned is still higher than at low intensity. Say you burn 5 calories walking on the treadmill at 3 mph for a minute: 3 calories from fat and 2 from carbohydrate. Running 6 mph you burn about 10 calories per minute: 6 from carbohydrates and 4 from fat. So in the "fat burning zone" (walking), you burn 1 fewer calorie from fat, and 5 fewer calories overall per minute. Multiply that for a 45-minute workout, and you could burn 45 extra calories from fat, and 225 more calories overall. Since weight loss is determined by total caloric deficit (no matter what kind of calories), faster treadmill speeds are more efficient for weight loss.
Post-Exercise Calorie Burn
A study done at Colorado State University proved that intense exercise increased post-exercise calorie burn. Subjects exercised either at a low intensity (50 percent effort) or high intensity (75 percent effort) until they burned 500 calories. They found that in the 3 hours after exercise, the high-intensity group burned about 50 percent more calories than the low-intensity group. Despite equal expenditure during the workout, intense exercise burned more overall calories.
Intervals for Maximum Weight Loss
Some gym-goers can't maintain an intense effort for an entire workout. Luckily, you can pack tons of intensity into a short time by doing intervals of hard work followed by a low-intensity rest period. Intervals maintain a higher average pace than steady efforts and maximize post-exercise calorie burn even for experienced runners.
Warm up for 10 minutes, then increase from a walk to a jog, or a jog to a run. Find a pace challenging enough that you couldn't comfortably hold a conversation, and hold it for a minute. When the minute is up, walk or jog easily for another minute. Start by doing five intervals and add one repetition each week. Once you can do ten intervals, increase the interval time, decrease the rest time, or increase the interval speed or incline to continue to maximize calorie burn.
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