Monday, March 7, 2011

Fasted Cardio part 1

Update: Due to time constraints and the finding of new material to review this will be a 2 parter

Finally spring appears to be right around the corner. With the emergence of the the sun occuring before noon (ok 7 am), a recent bodybuilding event, and a few recent articles I've come across I wanted to a post on morning exercise, or fasted exercise in general. Personally when the sun is up and it's not freezing outside, I find that I am more inclined to want to get in some morning exercise as opposed to when its dark and cold in the morning. So that is what piqued my interest to look into the science behind fasted cardio this year, along with the bodybuilding event I mentioned.
 This past weekend, the Arnold Classic took place. The show is essentially the 2nd biggest show in pro bodybuilding, next to the Mr. Olympia. I read some of the contestants columns in Muscular Development magazine, and you can be sure each of them used some form of fasted cardio to get leaner for the show. In fact alot of serious fitness enthusiasts incorporate fasted conditioning into their exercise regimens in an effort to "maximize fat loss" since they are exercisng on an empty stomach. I realize several will argue that one shouldn't base decisions off of anecdotes for several reasons including confounders, personal bias, subjective outcomce meausures, chance outcomes, etc. Nonetheless,  I still believe in anecdotes, but I decided to look into a bit more more. By chance, the first "scientific" article I came across was from the most recent Strength and Conditioning Journal which was waiting for me back at my mom's house this weekend. Anyways I was none too impressed with the article. Here's why...

First off the author Brad Schoenfield, acknowledges the popularity and rationale behind fasted cardio, but concludes science does not back its efficacly. Here I will have to disagree. He goes on to write that, "First and foremost, it is shortsighted to look solely at how much fat is burned during an exercise session. The human body is very dynamic and continually adjusts its use of fat for fuel. Substrate utilization is governed by a host of factors (i.e., hormonal secretions, enzyme activity, transcription factors, etc), and these factors can change by the moment ." when considering the rationale behind one burning more fat during fasted cardio than blood glucose. However this statement in contradictory.

Despite acknowledging substrate utilization (fat or carbohytdrate) is affected by hormone secretion and gene expression, the article does not take this into account throughout its entirity, later writing, " fat burning must be considered over the course of days-not on an hour-to-hour basis-to get a meaningful perspective on its impact on body composition".  I am under the impression, Schoenfield assumes that exercising with food in the stomach will allow one to exercise longer or harder and thus burn more calories. One of the references Schoenfield uses is a classic cycling study out of Time Noakes lab showing that a preexercise meal enhances time to exhaustion in a bout of cycling compared to a fasted state (1). The key finding of this study was subjects that consumed breakfast rode for  136 minutes (+/-14 min) compared with 109 minutes (+/-12 min) when they cycled in the fasted state (109+/-12 min). These findings are not at all relevant to the average person looking to shed some fat.While going harder or longer is ideal for athletes, the general population is looking for fat loss and riding a bike to exhaustion for 2.5 hours is not quite neccessary for fat loss.  Calories burned is not as important as the hormonal response and subsequent gene expression. I have explained this before when discussing how intervals are superior to steady state cardio for fatloss despite burning far fewer calories in a session (see here).

Interval training increases stress hormones such as epinephrine, norepinephrine, and growth hormone(2, 3). Increasing these hormones during the exercise bouts chronically increases enzymes that are key to fat metabolism such as Citrate Synthase (4) and fat transporting proteins such as FAT/CD36 (5) . So without sounding all fanatic, like many fitness websites out there, interval training essentially increases the bodie's ability to burn fat, whether during exercise or at rest.

Now here is where things get real interesting in the Schoenfield article, the author acknowleges, " multiple studies show that consumption of carbohydrate before low-intensity aerobic exercise in untrained subjects reduces the entry of long-chain fatty acids in the mitochondria, thereby blunting fat oxidation. This is attributed to an insulin-mediated attenuation of adipose tissue lipolysis... and a decreased expression of genes involved in fatty acid transport and oxidation"  Basically this equates to eat he recognizes eating before exercise causes an increase in insulin, and insulin stops the breakdown of fat (adipose tissue lipolysis) and decreases the actions of genes that express for enzymes important in fat metabolism. One of the referenced studies is a study infact showing that eating before exercise decreases fat burning or lipolysis (6) but then the author states,  "both training status and aerobic exercise intensity have been shown to mitigate the effects of a pre-exercise meal on fat oxidation." That is not neccessarily the case. Its true that intense exercise will lower insulin levels and increase the bodies ability to "burn" fat, but for the average person who has an active life or other fitness goals such as body building, power lifting, etc. or maybe even an athlete just trying to supplement their current training, high intensity work is not always practicial. Moderate intensity does not suppress inuslin or increase the stress hormone response to exercise to the same extent as high intensity exercise. This can be read in any exercise physiology text. The following figure will show whats going on.

(BTW this is figure 5.10 from Physiology of Sport & Exercise by Jack Wilmore, Dave Costill, and W. Larry Kenney)

Basically exercise increases  glucose rate of appearance from the liver to be used as fuel. There is a subsequent decrease in insulin caused by the stress hormones I mentioned earlier in the post. One might ask how we can use that extra glucose if we have a decrease in insulin? During exercise we experience a shift in a protein that allows glucose into the cell, called GLUT4, to the cell membrane (7). This allows for the decrease in insulin while still allowing our cells to handle more glucose than would be encountered at rest. In fat as one adopts an exercise plan, they increase GLUT4 content, and the ability to translocate these proteins to the cell membrane. This is one of the key mechanisms that results in the exercising population being more insulin "sensitive" (8). Unfortunately once you stop exercising this increase in insulin sensitivity can diminish in as little as a week according to some studies. I am not so sure about this personally, but let me stop going off on a tangent.

Here is a visual of the GLUT4 transporter at the cell membrane to get a better picture of what is going on...

With more GLUT4 translocation occuring without it, less insulin is needed resulting in increased insulin sensitivity.

So hopefully what you have been able to gather is that a bout of exercise is about much more than burning calories. Particularly you are looking for a hormonal and general physiological response that will improve the bodie's ability to burn fat fat in the long run. This includes increasing enzymes and different proteins that help facilitate this process as well as decreasing circulating insulin. Fasted cardio seems to be advantageous in this department. First off after an overnight fast, you start off with lower insulin. Subsequently, the stress hormone associated with exercise drive it down further than would be experienced after a meal (breakfast) thus inhibiting insulin's fat storing effects and enhancing the lipotytic effects of the stress hormones. In regards to the GLUT4 adaptation and resting insulin levels, I am not sure at this point in time. In part 2 of this post, I plan to follow up on this by reviewing a couple of randomized experimental trials that indeed show fasting cardio is superior over the long term to exercising at the same intensity while fed.

I also just want to mention that I have no bones with Brad Schoenfield. I have read alot of his previous material and he seems like an intelligent guy. I know he has helped alot of people reach their fitness goals. However, I feel in his most recent article discussing fasted exercise, he over contextualized. It's true some people need a little food in their belly before an intense bout of exercise whether it be conditioning, sports, or weight training. With that said, there is nothing wrong with those looking to lose fat exercisng in the fasted state to facilitae metabolic adaptations that will encourage their body to metabolize fat.

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