You've already figured out that the path to getting a flatter stomach lies not in doing loads of situps, but in cranking out the cardio -- so you're already well ahead of the pack when it comes to your approach to fitness. Since there's no such thing as "spot reduction," there are really no shortcuts to getting a slimmer body, including a smaller waist. Still, there's no simple way to say how much cardio it's going to take to get there, since it all depends on how much excess fat you have to lose.
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To lose weight, you have to create a calorie deficit, meaning you're burning more calories than you're taking in. To be more precise, you need to create a 3,500-calorie deficit in order to lose 1 pound of fat. That's certainly going to involve doing cardio at least four to six days a week, for as long as you have to spare, or as long as you can stand to work out on any given day. If you're doing a workout that burns 1,000 calories every day, for example, it will take you seven days to burn 1 pound of total body fat. If you only had a little belly to begin with, repeating that cycle every day for three weeks might be enough to get the flat stomach you want.
Big Calorie Burners
If you have all the time in the world -- and all the desire and willpower -- you could do nearly any type of cardio to help you create that calorie deficit. However, doing exercises such as walking or cycling are not as efficient as other types. Burning more calories requires more intensity as well as more time. Some of the biggest calorie burners out there include running, which will burn about 906 calories per hour for a 160-pound person. Cross-country skiing will burn about 600 calories per hour for that 160-pound person, and swimming will burn about 618 calories. Compare that to walking, which burns about 522 calories per hour, and you may begin to see how choosing the more intense exercise can help you create that calorie deficit faster.
A safe amount of weight to lose per week is about 2 pounds, which equates to a deficit of about 7,000 calories. Sure, you could simply eat the same amount as you've always eaten and just burn an additional 1,000 calories, but the other way to go about it is to reduce the number of calories you're consuming every day. By cutting about 250 calories each day -- roughly the number of calories in a bottle of soda -- you'll end up with a 1,750-calorie deficit in a week's time. If you're burning roughly 500 calories working out five days a week, you'll be creating a deficit that can help you lose more than 1 pound each week.
If working out for an hour at a time seems impossible or is just not feasible because of your schedule, you're not alone. Fortunately, though, there's another option for the days when time is limited. High intensity interval training, or HIIT, is a workout you can do in as little as 12 minutes, and according to the American College of Sports Medicine, it can increase your metabolism for the entire day following the workout as well as helping you burn subcutaneous abdominal fat. Warm up by walking or jogging for five minutes, and then sprint at about 90 percent of your maximum for one minute. Go back to walking or jogging for another minute, and then repeat the sprint-recover cycle a total of eight times. Try HIIT workouts on the days you don't have time for a longer workout, or about two days a week.