Exercising to Burn Fat: how intense is your workout?

An informative argument for adults seeking to burn fat through exercise

[Authors]

Fall 2010

A paper for Women’s Health and Men’s Health Magazine

Are you one of the millions of Americans that will be hopping on that treadmill to burn some of those extra pounds (of non-muscle) gained from the Thanksgiving Day feast? And when it’s in between the holiday eating seasons, do you exercise to burn off some of that excess fat? Even if you don’t currently exercise but have been thinking about trimming some fat there is an important question to consider before lacing up the sneakers or taking the bike in for a tune-up: does a more intense workout burn more fat?

Consider exercise intensity, on the surface it seems obvious—exercise harder, be more “winded” and you’ll be burning off more fat; however this may not be the case. You probably understand the concept of calories-in vs. calories-out—If you use more calories than you eat then you’ll lose weight because your body will need to burn other sources of fuel to function. Fat happens to be a great source of fuel for your body. But what if you burn those calories running vs. walking? Furthermore what if you burn the same amount of calories running vs. walking? Will it result in different amounts of fat to be burned? Well a variety of studies over multiple decades have sought to answer exactly that.

This question isn’t just limited to walking vs. running; it could be the difference between swimming intensities or biking intensities. Whatever type of exercise you favor when trying to burn fat, it may be the case that exercising at a higher intensity vs. a lower one does not have an effect on how much fat you burn. This could mean that as long as you burn the same amount of calories, a long walk could be just as effective as a short run.

The truth is, with all of the research being done in the field of exercise and human physiology scientists still have not come to a definitive conclusion on the effects of exercise intensity on fat burning. This is evident when looking at the conflicting data arrived at by various studies. See figure 1 & 2. However, by analyzing the current wealth of knowledge we can determine whether or not you should be out-of-breath in order to burn off more fat. Currently some researchers believe that higher intensity workouts do burn more fat, while others are convinced that the level of intensity has no effect—so long as the right amount of calories are burned.

Figure 1: Depending on the study some yellow bars are longer than blue bars, while some blue bars are longer than yellow. These differing results suggest the issue remains unresolved.

* Statistical significance between intensity groups (p<0.05)

Figure 2a: Depending on the study some yellow bars are longer than blue bars, while some blue bars are longer than yellow, suggesting different outcomes for different studies.

Figure 2b: shows minimal difference between intensity groups

Note: figure 2b, Grediagin et al. 1995 did not provide pre or post test values to allow for % calculation

* Statistical significance between intensity groups (p<0.05)

There is no “ideal” study that would allow us to “close the book” on this topic; instead we must analyze numerous imperfect studies that share the pursuit of determining how exercise intensity affects fat burning. These studies utilize different exercise methods and populations; however they are unified by the observation of varying intensities on fat burning. The imperfections and variations of each study must be taken into consideration and will be addressed in greater detail in the following sections.

The conclusion drawn in this article from analyzing these studies can be applied to the general adult population because they consisted of men and women with all different body types; the populations varied between healthy, overweight, and obese adults ages 18-70. The population within a given study was a bit tighter than 18-70; most studies used an age range of about 15 years. But either way chances are my conclusion applies to you.

Despite the population variation seen in each study, there were a few factors that unified the various populations. All populations consisted of sedentary subjects; this is a malleable term determined by the researchers, they may define “sedentary” as a person who gets less than 2 hours of exercise a week or someone who participates in structured physical activities no more than twice a week, so you wont find marathon runners or champion weightlifters in a sedentary group.

Another thing worth noting is that the specific research question being asked in each study was to some degree different from one another and because of this the variables observed in each study were different; some studies included variables like fitness (maximum oxygen intake), weight loss and cholesterol changes. Some studies also analyzed changes in different types of fat; certain studies looked at overall fat composition, while others analyzed changes in subcutaneous or visceral fat.

It should be stated that not all fat is the same. Subcutaneous fat is the fat located just below the skin; it’s the fat you can pinch. Visceral fat is the fat located inside the abdominal cavity; it’s the deeper fat that hangs around the organs. Visceral fat is harder for researchers to analyze due to its position in the body; so a greater number of studies tested for changes in subcutaneous fat because it can be measured much more easily by simply pinching different regions around the body with calipers. (See image a & b below)

Images: a) Location of visceral fat compared to subcutaneous fat.

b) Calipers used to measure subject’s subcutaneous fat.

a) http://lowerbloodpressurecheap.com/wp-content/uploads/2009/10/visceral-fat-279x300.jpg

b) http://www.chiefdietician.com/wp-content/uploads/2009/02/skinfoldcaliper.jpg

Due to the strong relationship that calories-in vs. calories-out have on fat burning, maintaining close control over calories is very important when designing a good study. In a study sense it means having subjects burn a controlled amount of calories, which is known as keeping the study ‘isocaloric’; this takes care of the calories-out aspect of a study. A way to control calories-in, if feasible, is to have subjects consume a specific amount of calories. Controlling subjects’ diets is very difficult, but two studies managed to accomplish this level of calorie control; other studies utilized diet logs to monitor caloric intake. Taking these measures are necessary in order to remove calories (overall energy expenditure) as a factor that could skew the data collected. Imagine for a moment that you have one group of subjects running and the other group walking; if each group spends a different amount of energy exercising, then how can you be certain that the fat burned is the result of running vs. walking (intensity) when it could have been due to the amount of energy spent. So to avoid this confusion most of the studies kept their design isocaloric—they had both exercise groups burn the same amount of calories. The runners ran for a shorter duration, and the walkers walked for a longer one, but all in all each person burned the same amount of calories.

With the amount of conflicting data seen from various studies (figures 1 & 2) it is difficult to arrive at a definitive answer on whether or not exercise intensity matters when trying to burn fat. Many studies did not observed any significant correlation while other studies arrived at the conclusion that higher-intensity workouts are significantly more effective when compared to low or moderate intensity workouts when it comes to burning off fat. So which side got it right?

I analyzed different features relevant to all the studies in order to compare the quality of each study against one another. This was done in an attempt to resolve the issue of whether or not higher-intensity exercises burn more fat. I will argue that, for sedentary adults, a high-intensity workout is no more effective at burning fat than a low to moderate intensity workout. Some people argue that a high-intensity workout is more effective at burning fat. After analyzing the studies it seems that the stronger, more valid studies have arrived at the conclusion that there is no correlation between higher intensity exercises burning more fat. Note: individual study results can be seen in figures 1 & 2.

It appears that one thing researchers can agree on is the importance of energy expenditure (calories) for a given workout—only two of the seven laboratory-based studies failed to employ some sort of method to make sure both high and low intensity groups were burning the same amount of calories during a given workout session. One study, which failed to utilize a strict isocaloric design, was Tremblay et al. 1994. This study also happened to see the strongest correlation in regards to high-intensity exercises being more effective at burning fat than low-intensity exercises. Bryner et al. (1997) also failed to utilize an isocaloric design; they assigned the same amount of exercise duration (40-45 min.) to each intensity group and had the subjects exercise at different heart rates. This study also concluded that high intensity exercise was more effective at burning fat. The rest of studies utilized some sort of isocaloric method, most commonly done by adjusting the duration of the exercise, adjusted to each subject. It appears that two of the three lab-based studies, which observed a correlation between high intensity exercises and more effective fat burning, failed to utilize a good isocaloric method. Based on the two studies lack of a strict isocaloric design, energy expenditure may have affected the results of their studies, compromising the data and validity of the studies as a whole. Additionally, this lends support to the notion that fat burning is not affected by exercise intensity.

Another calorie method, which increased the individual credibility of some studies, was the researchers’ attempt at dietary control or monitoring. Most studies, to some degree, had their subjects keep track of their dietary habits. I say to some degree because certain studies were more thorough than others. In fact, two studies actually controlled their subjects’ entire diets by providing them with snacks and three meals daily for the entire intervention period (and no, these subjects weren’t eating for free) but this method did allow for greater calorie control through the study period. It also happens that the two studies that provided controlled diets to their subjects saw no correlation between exercise intensity and fat burning. Only one study, Tremblay et al. 1994, failed to utilize any sort of diet record or control; this was the study that also saw the greatest benefits from a high intensity workout when compared to a low intensity workout. The rest of the studies utilized some sort of diet log. Based on the importance of calorie expenditure vs. calorie intake, better diet and calorie control gives rise to a better study. The breakdown of diet control can be seen in table 1.

Table 1: Individual Study Design: degree of dietary control

Correlation / No Correlation
Use of Controlled Diet / Nicklas et al. (2009)
Ballor et al. (1990)
Use of Diet Log / Irving et al. (2008)
Bryner et al. (1997) / Slentz et al. (2005)
Grediagin et al. (1995)
No Dietary Observance / Tremblay et al. (1994)

The side that observed no correlation utilized better dietary control (caloric intake).

In the field of exercise research, a larger sample size (more subjects) means more data; and when you conduct research in an attempt to establish or disprove certain correlations with the goal of applying the results to a larger, general population, it helps to have more data. As noted before, an isocaloric design is very important to the validity of a study seeking to determine how exercise intensity affects fat burning. In order to keep a study isocaloric researchers had to conduct exercise treatments in a lab, under the supervision of trained personnel. Having to conduct research in a supervised lab means small sample populations for researchers to work with, but either way, more subjects mean more data and that’s a good thing. Of the four studies that observed a correlation, three studies managed to test 69 subjects within the laboratory. The fourth study, Tremblay et al. (1990) surveyed 2623 men and women. Studies, which observed no correlation, managed to test 309 subjects within a laboratory setting. Testing more subjects means receiving more data, and in exercise science more data means better founded conclusions. Studies which observed no correlation tested 230 more subjects within a laboratory setting; this means that these studies had 230 more subjects to base their conclusion on. Individual study population sizes can be seen in Table 2.

Table 2: Individual Study Design: Population Size

Correlation / No correlation
Population of Subjects Tested in a Laboratory Setting / 27- Irving et al. (2008)
15-Bryner et al. (1997)
27-Tremblay et al. (1994)
2623(survey)-Tremblay et al. (1990) / 95- Nicklas et al. (2009)
175- Slentz et al. (2005)
12- Grediagin et al. (1995)
27- Ballor et al. (1990)
Population of Subjects Surveyed / 2623
Total Number of Participants Tested in a Laboratory setting / 69 / 309

More subjects were tested in a laboratory setting by the studies that observed no correlation.

A large sample size is not the only thing that contributes to the gathering of more data; study duration also contributes to the amount of data collected by researchers—a longer study results in more data—and more data affords better conclusions. When comparing the overall duration of studies that observed a correlation vs. studies that did not, the break down is 46 weeks vs. 64 weeks respectively. Studies that observed no correlation between exercise intensity and fat burning had a combined duration of 18 additional weeks when compared to the studies, which support a correlation. This translates into 18 additional weeks of data collection to base a conclusion around. Individual study durations can be seen in Table 3