High or Low Intensity Exercise – Which is Better for Fat Loss?

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Along with female clients often telling me that they don’t want to do weight training because they don’t want to get big muscles, another common question I get asked on a regular basis is what type of exercise should I be doing in order to lose weight?

There could be a few different answers to this question depending on the individual asking it. However; I will try to answer it in the context of low or high intensity exercise. So which is best for losing weight? During the course of this article I hope to answer this question while dispelling a lot of common misconceptions about training at a low intensity level.

High intensity exercise generally means regularly providing more effort during a movement or moving at a faster pace, either results in burning a great amount of energy. Typical examples would be sprinting, weight training rugby or basketball. Low-intensity exercise generally means working out with less effort, but keeping up a consistent pace. Some examples would be walking, speed walking, cycling, aerobics classes and light jogging (for those with average and higher fitness levels).

To lose weight, any exercise will play its role, as all will burn a greater amount of calories than doing nothing. Although it comes as a surprise to many, the majority of research shows that aerobic exercise in the so-called “fat burning zone” is not a very effective way to lose fat.

Your body burns either fat or carbohydrates depending on the intensity of your activity. But when it comes to losing weight, calories are calories. You burn fat even when you’re in couch-potato mode. Yet, a lot of misunderstanding prevails. Get ready to break down some of the myths people have about burning fat!

The body completely shuts off one fuel source when it turns on the other.
False, the real answer is that what has often been misunderstood by both exercisers is that the body relies on both fat and carbohydrates for energy all the time, albeit in different ratios. In fact, as you sit here reading, you may be burning about 50-60 percent fat and 50-40 percent carbohydrates. You’re not using much of either, however, because the amount of calories you need probably amounts to about one or two calories a minute. If you were to get up and start jogging in place, your body would need to supply you with some quick energy to do so, so the metabolism ratio might shift to drawing upon more carbohydrates, say 70 percent, and less fat, say 30 percent. If you were to continue jogging, then, in order to preserve the carbohydrates (which can run out since you have limited stores in the body), your body would gradually shift its metabolism ratio again to say, 60 percent fat and 40 percent carbohydrates. From an energy efficiency point of view, it pays to be fit. The endurance athlete would be able to make the shift sooner, and his fat-burning percentage might be 65-75 percent.

However, in practical terms this is purely technotalk, and these ratios don’t make a big difference when it comes to losing weight and decreasing your body fat. For the most part, athletes are often leaner not because they might rely on slightly more fat for fuel, but because they practice their sport two to three, or more, hours a day – this burns a lot of calories. To lose weight, you need to burn more calories than your body consumes and uses everyday. Exercise is one main way to burn a lot of calories. But when it comes to weight loss, what matters is how many calories you burn, not so much whether they are fat or carbohydrate calories.

Another common misconception is that: Exercise done at a low intensity, such as walking, is better at fat burning than other high intensity activities, like running or cardio activities where you push yourself very hard.

This is false and I am going to explain why. What the truth is, is that in a strict scientific sense, these claims are true because working at a lower intensity requires less quick energy and a higher percentage of fat is burned. Also in theory while working at a low intensity the fuel you use to exercise is more likely to be fat and glucose.

However you’ll also burn fewer calories than you would if, for the same amount of time, you work out at a higher intensity (running versus walking). If you’re trying to lose weight, even though a higher percentage of fat is being used, a lower total amount of fat is lost.

The most fundamental aspect of any fat loss program is to create a calorie deficit – to burn more calories than you eat.

Whether increased fat burning will result in actual weight loss is dependent upon several variables, including the total calories burned (which include both fat and carbohydrate calories) and the total fat calories burned. If you do work at a low intensity, you need to increase the time spent exercising to burn more calories. What matters most is the total number of calories burned. If you burned 250 calories every day from a short, fast jog, you’d see a bigger difference in weight and fat loss than if you walked everyday for the same amount of time. The number of fat calories you burn isn’t that important, because even if you burn a lot of carbohydrate calories, these need to be replaced both by the carbohydrate you eat in your diet and also within your body. Your fat stores will be broken down and transformed into carbohydrates when you need fuel. Even if you’re burning lots of carbohydrate calories and less fat calories through exercise, your fat still inevitably gets used.

During the same amount of time you don’t use more calories at lower exercise intensities. If you’re trying to lose weight and you have only 30 minutes to work out, you would burn fewer calories walking at a moderate pace compared to walking at a fast pace. Working out at higher intensities may cause you to burn a lower percentage of fat, but since you burn more total calories, you still use more fat calories.

Low to moderate intensity exercise can burn a significant number of calories over a period of time. If you aren’t fit enough to push yourself to work at a high intensity, or you have a physical weakness that prevents you from doing so, you can still burn a lot of calories by doing low-intensity workouts for a longer period of time.

Running, cycling, or other cardio activities are more fat burning once you’ve been doing them for more than 15 or 20 minutes.

The above statement is another common misconception. What the truth is that technically, once you’ve been exercising for 15 or 20 minutes, your body has made the shift to using a higher percentage of fat for fuel. But again, if you’re trying to lose weight, it’s about the total number of calories burned, not necessarily the fuel source.

For example, say that at rest you burn up to 60 percent fat. When you enter the initial phases of intense exercise, the ratio changes. When you exercise at a low intensity the ration is about 50% carbohydrate and 50% fat as a fuel source. You may now burn only 30 percent fat because your body is using quick-energy carbohydrates so the ratio is 70% carbohydrate and 30% fat.

Once the exercise is sustained, the body switches back to using a higher percentage of fat to fuel the movement (up to 75 percent fat). In this aerobic phase of exercise, a higher percentage of fat is being used for energy. But if you aren’t working out for a very long period, you may still burn more total calories and, therefore, more fat calories working out harder. Put another way, if burning as many calories as you can is the best way to lose weight, even a dummy can figure out which activity of the following is going to give the best results (answer: jogging and sprinting), even though their fat burning quota is on the low end of the ratio.

I once read somewhere that working out for 20 minutes at a high intensity, doing something like Interval or hill running is more beneficial that going for a long steady jog for 60 minutes. At first I dismissed it but upon more investigation the statement began to make sense and here is why:

Excess post-exercise oxygen consumption (EPOC), which is the name given to the increase in calorie expenditure following a workout, is more likely to occur after high intensity exercise. Moderate-intensity aerobic exercise (such as walking or jogging) has very little effect.

What this means is that your body is still burning calories and expending energy long after the workout is finished. Which is why a lot of us have a high core body temperature a few hours after a big session in the gym.

One pound of fat contains the equivalent of roughly 3,500 calories. So, assuming your calorie intake remained static and your weight was stable, you’d need to burn an extra 500 calories per day to drop just one pound of fat over the course of a week. To lose fat at a decent rate (around two pounds per week) you’d need to burn 1000 extra calories per day. And the type of workout that burns 1000 calories, in terms of both time and effort, is not a realistic goal for most people.

I use the measurement of +500 for weight gain (lean muscle) and -500 for weight loss. Realistically an aim of 1 pound of fat a week is a target that is achievable for the average Joe. Using this theory you can apply it to your training.

For aerobic exercise to be effective, you need to do a lot of it. Scientists from Canada, for example, report that a walking program was enough to cut body fat levels by an average of 13 pounds over three months, or just over one pound per week [6]. Although this is a lot more than some of the other studies I’ve looked at it, the people taking part in this trial trained for more than one hour, every single day. And not everyone has that much spare time to devote to exercise.

And that brings us to another problem. Most modern exercise machines have digital readouts telling you how many calories you’ve burned. The most reliable way to assess energy expenditure during exercise is to measure oxygen consumption. Each liter of oxygen that you consume generates approximately five calories of energy. For example, if you exercise for 30 minutes and consume 30 liters of oxygen, you’ll have burned 150 calories. But without directly measuring oxygen consumption, it’s difficult to get an accurate estimate of how many calories you’ve really burned. But in most cases, 30 to 40 minutes of moderate intensity cardio three or four times a week isn’t going to deliver the results you want. A full body training program that includes both cardiovascular and resistance exercise, combined with a proper diet, is a far more effective way to drop the pounds.

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I'm a certified personal trainer specializing in circuit training, nutrition and weight management, training in different environments, sports conditioning along with lifestyle and fitness assessments. I'm also a group studio cycling instructor. See my profile page for more information!

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