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Any activity you do contributes to burning calories. Five minutes is always better than nothing. But when you exercise for at least 30 minutes, your cardio reaches a level where everything happens more quickly. You burn more calories, trigger more enzyme changes and raise your core temperature, which can dull your appetite.
While 30 minutes may feel like an eternity, it's easy to hit. Begin with 15 minutes of exercise and then add two to three minutes to each routine. Within a week or so you'll be up to 30 minutes.