What's Best for Fat Loss - High Intensity or Low Intensity Workouts?

Editor's Note: This article jumped out at me when I first read it because I often wondered if I should be spending more time on the treadmill doing low intensity workouts or less time doing high intensity workouts (I was hoping for the latter). One of Tom's clients emailed him a question related to this and Tom provides an excellent answer in this article. Let's just say the answer depends on what your weight loss goals are and how much time you have to exercise daily.

Dear Tom,

I just read an article in a bodybuilding magazine that said all you need to lose fat is three days a week of cardio 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.

ANSWER:

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 or those with certain health problems).

It's common sense if you think about it - work harder, burn more calories, right?

Here's where the confusion has come from:

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 hard 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 in exercising at a low intensity to stay in the "fat burning zone" was false. At lower intensities, you burn more calories from fat, but you burn fewer total calories.

For example, a 1995 study conducted by Grediagin, et al, 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 (took a lot longer, of course). 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?"

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...


***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 20 minutes, 30 minutes, 45 minutes, or whatever, the higher the intensity during that time period, the more TOTAL calories you will burn. The more TOTAL calories you burn, the more fat you 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.

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 healthy, 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.

Remember, it's all about the intensity and the calories burned, not necessarily whether the workout is performed with intervals or in a steady state.

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 four, five, then even six days per week, you could 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 doesn't mean a 30 or 45 minute workout necessarily has to be "low" in intensity.

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.

Although infrequent and very brief (15-20 minutes or even less) HIIT workouts have recently gained great popularity (and deservedly so), that doesn't mean you should never do steady state cardio, nor does it mean that certain individuals aren't better off with longer, less intense cardio.

Respected organizations such as The American College of Sports Medicine (ACSM) still recommend longer daily and cumulative weekly exercise duration when the 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 intensity, frequency, and duration, it's important to build up slowly and ALWAYS get your physician's clearance before attempting high intensity cardio.

Naturally, of course, it's not wise to dramatically increase your training volume or intensity suddenly, but rather to increase gradually.

If your current goal is to maintain your level of body fat and stay healthy, I'd recommend starting with at least 20 minutes of cardio 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 5-7 days per week, if necessary, based on your weekly results.

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. This is a form of "cardio periodization," similar in nature to the periodization of weight training used by elite athletes. Staying on high volume cardio all year round is counterproductive and may lead to overtraining, aerobic adaptation and a plateau in fat loss.

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 (I hate those people, don't you?)

The bottom line is that a single cardio workout prescription, such as "three days a week for 20 minutes" will not work for everyone. Exercise programs must be developed on an individual basis and they are not static. The frequency, duration AND intensity all need to be adjusted based on your results.

If the intensity is high enough, three twenty-minute cardio sessions may be sufficient for you, 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."

You can learn much more about calories, metabolism and fat burning foods by visiting www.BurnTheFat.com

Editor's Note: Christian Finn, another certified personal trainer, wrote an article titled, A Better Way to Burn Fat Faster - Or Is It? In the article he discusses the "high intensity vs. low intensity" debate as well.

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