Editor's Note: Is 20 minutes of cardio, three times a week enough for you to shed that unwanted body fat? Tom discusses this issue in this article and provides some useful information to help you design your own effective cardio program.

Dear Tom,


I just read an article in a bodybuilding magazine that said all you need to lose fat is three days a week on a cardio machine for twenty minutes. It said that low intensity, long duration cardio workouts are not the best way to lose fat and that a high intensity twenty-minute workout is more efficient. Is this true? I don't have a lot of time to work out so it would be great if I could get my cardio done in only twenty minutes.

Yes it's true that higher intensity cardio workouts burn more calories per unit of time AND increase metabolism more after the workout than low intensity workouts. High intensity cardio, including high intensity interval training (HIIT), is very effective and time efficient, although it's not for beginners.

It's well known that low intensity exercise utilizes primarily fat as fuel and high intensity exercise utilizes more carbohydrate as fuel. In the past, this was the basis for the idea that low intensity, long duration aerobic exercise was superior for fat loss. Some people were were afraid to exercise too intensely because they thought it would take them out of the "fat burning zone" and make them them burn only "sugar" and not body fat. Today, research has proven that this belief was false.

For example, a 1995 study conducted by Grediagin, et al, and published in the Journal of the American Dietetic Association (95(6):661-5) compared fat loss in two groups over a 12 week period. One group performed exercise at 80% of VO2 max for a duration sufficient to burn 300 kcal, the other group performed exercise at 50% of VO2 max for a duration sufficient to burn 300 calories. Hydrostatic body composition testing revealed that each group lost an identical amount of fat.

The authors concluded, "This study suggests that fat loss is a function of energy expended rather than exercise intensity. Therefore, if fat loss is the goal and time is limited, persons should exercise safely at as high an intensity as tolerable to expend as much energy as possible during their allotted time."

In my opinion, that conclusion pretty much hits the nail on the head when it comes to answering the questions "how long and how hard should your cardio workouts be?" In fact, it bears repeating again:

If fat loss is the goal and time is limited, persons should exercise safely at as high an intensity as tolerable to expend as much energy as possible during their allotted time."

Another study published by Ballard, et al in the same journal (51(2):142-6, 1990) showed identical findings. High (80-90% VO2max) versus low (40-50% VO2max) intensity rates were compared in two groups with duration carefully controlled to ensure each group burned the same number of calories. The high intensity group exercised for only 25 minutes and the low intensity group for 50 minutes, yet both groups lost the same amount of body fat!

Keep in mind BOTH approaches worked, but the high intensity group got it done in half the time! Regardless of whether your cardio sessions are 15 minutes, 30 minutes, 45 minutes, or whatever, the higher the intensity during that time period, the more calories you will burn. Although many factors are involved in exercise-induced fat loss, the most important factor is the total number of calories burned, NOT whether the calories burned are fat or carbohydrate.

When discussing the effects of exercise intensity on fat loss, it's also important to consider energy expenditure after the workout, not just the calories burned during the workout. Higher intensities not only burn more calories per unit of time, but they also elevate your metabolism more at rest after the workout is over. This post workout increase in metabolic rate is known as "excess post exercise oxygen consumption" or EPOC for short.

It has been proposed, based on the results of several studies comparing the amount of calories burned at rest after low intensity versus high intensity exercise, that HIIT is a superior method of fat loss due to its effect on post workout metabolic rate.

Clearly, HIIT is the logical protocol of choice if you are already fit and you have little time to work out. However, it's also logical that time permitting, more frequent and longer duration exercise might cause even greater overall fat loss if intensity is sufficient, simply because more total calories can be burned over the course of a week.

For example, if you do 20-25 minutes of very intense cardio, you might burn about 400 calories. That's a lot of calories for such a brief workout. But it only adds up to 1200 total calories in one week if your frequency is only three days per week. If you gradually built up your frequency to six days per week, you double your caloric expenditure to 2400 calories per week.

If you also increase your duration, your intensity will decrease so you'll burn fewer calories per minute, but the calorie expenditure for the entire workout is higher, which increases your total weekly calorie burn even further.

Duration and intensity are inversely related, so the longer the workout, the lower the intensity. But that doesnt mean a 30 or 45 minute workout necessarily has to be "low" in intensity, with little effect on post workout metabolism. A 30 or 45 minute steady state workout can be "moderate" or "moderately-high" in intensity.

The combination of the highest intensity you can muster with a 30-45 minute duration can create an enormous calorie burn. Some of that calorie burn will occur after the workout as well, because studies have shown that EPOC is influenced not just by intensity, but also by duration.

In order to achieve optimal rates of fat loss, it's important to consider the total number of calories you burn both during and after exercise over the course of each week. You must also consider the interplay between different combinations of intensity, frequency and duration because total calorie expenditure is not just a function of intensity, but of intensity times duration and frequency.

Although infrequent and very brief (15-20 minutes or less) HIIT workouts have recently gained great popularity (and deservedly so), I would like to see more research done to identify the effects of various combinations of intensity, duration and frequency on body composition for various populations of various fitness levels with various goals.

Organizations such as The American College of Sports Medicine (ACSM) still recommend longer daily and cumulative weekly exercise duration when an individual's goal is fat loss. The ACSM position stand titled, "The recommended quantity and quality of exercise" states, "A threshold level for total body mass and fat mass loss generally would require at least 30-45 min of exercise per session for a person of average fitness.

If the primary purpose of the training program is for weight loss, then regimens of greater frequency and duration of training and moderate intensity are recommended. Shorter duration, higher intensity programs may be recommended for healthy individuals at low risk for cardiovascular disease and orthopedic injury."

To avoid overtraining, injury or aerobic adaptation, which become risks with higher frequency, duration and intensity, it's important to build up slowly and to get your physician's clearance before attempting high intensity cardio. If you want to double your calorie expenditure to lose fat faster, it's not wise to double your training volume overnight, but rather to increase gradually.

You should also alter your volume and intensity in a "seasonal" fashion based on your goals at the moment, a practice known as "periodization" in the sports conditioning world. Staying on high volume cardio all year round is counterproductive and may lead to aerobic adaptation and a plateau in fat loss.

If your current goal is to maintain your level of body fat and stay healthy, I'd recommend at least 20 minutes of aerobic activity 3 days per week. If your goal is maximum fat loss, then time permitting, I would recommend higher frequency and duration, sometimes building up to much as 30-60 minutes 5-6 days per week, if necessary, based on weekly results.

You can also consider the option of alternating long, moderate intensity sessions with brief, high intensity sessions. Once you reach your desired percentage of body fat, then you can gradually shift back into a "maintenance" program of lesser frequency, duration and intensity.

Genetics also play a role in the ideal volume of cardio for fat loss. If you're one of the few people who are genetically blessed with the fast metabolism and physical attributes to burn fat easily, then three days a week for twenty minutes often provides sufficient stimulus for results. In fact, I know a few people with hyperactive metabolisms who stay ripped all year round without doing any cardio at all. Not many people are that fortunate.

I've seen very few people who can lose fat at the maximum rate with a cardio frequency of three days per week. On the other hand, I have never seen anyone do six days a week of cardio for 30-45 minutes or more per session and NOT lose a lot of body fat (assuming their nutrition was satisfactory).

The bottom line is that a single prescription, such as "three days a week for 20 minutes" will not work for everyone and high intensity training is also not for everyone. Exercise programs must be prescribed on an individual basis and they are not static - frequency, duration and intensity need to be adjusted based on results. Three twenty-minute cardio sessions may or may not be sufficient, depending on your goals, your current level of fitness and your actual results, but longer and/or more frequent cardio sessions are sometimes a necessary evil.

Read other articles by Tom Venuto

About the Author

Tom Venuto is an NSCA-certified personal trainer, lifetime natural bodybuilder, certified strength & conditioning specialist (CSCS), and author of the #1 best selling diet e-book, "Burn The Fat, Feed The Muscle." Tom has written hundreds of articles and has been featured in IRONMAN, Natural Bodybuilding, Muscular Development, Exercise for Men and Men's Exercise. To contact Tom or get information on his e-book, visit www.BurnTheFat.com