T Nation

Getting Ripped with Cardio

I am in my 2 week cycle of getting cut up and I generally do a lot of cardio for this. Anyway, I was noticing on the treadmill at the gym that they have a chart which shows your age and target heart rate, and that the target heart rate for my age at the higher range is best for cardio, but a mid heart rate is best for fat loss. Therefore, according to this when I run and reach the higher ranges of heart rate, apparently I am not burning as much fat as when I only walk briskly and keep my heart rate in the mid range. Though I realize all the charts and computer gizmos on a treadmill are not necessarily accurate for everyone, could someone explain why walking briskly and keeping my heart rate in the mid range will burn more fat that running and getting my heart rate up to the higher levels. If I can burn more fat while only walking, that is certainly beneficial to running my tail off to exhaustion. Thanks for any help you can give me.

This is really a misconception. Here’s how aerobic exercise works: You burn fat at a farily steady rate. As your exercise intensity increases (and thus your heart rate), you can only burn so much fat per minute, so your body increasingly burns more glycogen (sugars). The amount of fat burned at high or low intensities is essentially the same (per minute). The idea with lower intensities utilizing more fat is only on a percentage basis of total calories burned. The important thing for fat loss is: total calories burned greater than total calories taken in. By exercising at higher intensities, you can burn more calories in a shorter amount of time, while increasing the amount of time for your body to return to its resting metabolism, thereby burning more calories.

This is a pretty common misconception…While it’s true that you might burn a higher % of calories from fat in the “fat-burning zone”, you will burn far more calories at higher intensity (for a given period of time) and that is probably the more important factor to consider.

Dan,
The target heart rate for “fat burning” is a misnomer [It is B#$%SHIT]. My understanding is that at this lower heart rate a higher percentage of your energy expanded comes from fat. However at higher heart rates, you burn higher amounts of total calories per minute and therefore more total calories.
BOTTOM LINE - Run/jog at the highest level you can maintain for 15 minutes [approximately 85% HR]& gradually increase your time at this HR.

These charts are always a bit misleading. Also, the concept of absolute calories from fat vs. percentage of calories from fat during a given exercise can be confusing as well.

It is true that at say 60 % of max heart rate, you are burning a higher percentage of calories from fat than at say 75% max heart rate. You may still be burning the same absolute amount of calories from fat.

Look at it this way. Let’s say you burn 300 cal per hour when you walk at 60% max heart rate (actually Kcal, but we won’t get picky) and you are deriving the energy 80 % from fat and 20 % from carbs. Then if you run at 75 % max heart rate you are only deriving energy 40 % from fat and 60 % from carbs. Son of a gun only half as much fat. The thing is you may burn 600 cal per hour.

So, during walk, 80 % * 300 cal = 240 cal from fat. During run 40 % * 600 cal = 240 cal from fat. So actually under both scenarios you are burning the same amount of fat. During the run at 75 % though, you are burning twice as many calories, which may ultimately result in greater fat loss depending on diet.

Where cardio vs. fat burning really becomes relevant is during higher intensity exercise. Above what is called the lactate threshold (most people would say you are anaerobic) fat burning is severely curtailed. That means when lactate accumulates, lipolysis is reduced. At this point you are burning only carbs. If you do this exercise for a prolonged period, it will reduce your carb stores (glycogen) which will either have to be restored (eat carbs) or will negatively impact workouts and initiate a catabolic state.

If you do cardio below the lactate threshold, you will avoid this problem and maximize fat loss. The best way to guage this is to simply pay attention to your breathing. If breathing becomes labored, it is likely that lactate is accumulating in the blood. It is possible that other factors may be contributing, but this is the most likely explanation. So, the old tried and true, "if you can carry on a conversation you are burning fat" typically holds true. There are more sophisticated ways to approach this, but for most individuals this will suffice.

I tend to agree with what McGregor said regarding his calculation (aside from the numbers being different depending on fasted or fed), but another point to consider with respect to intensity is the extra kcal burned post-exercise (the EPOC), which will be higher with a higher intensity. So more kcals burned during and after with higher intensity (below LT of course) and about the same kcals burned from fat during and more after, not be mention higher insulin sensitivity post-exercise.

Longer duration would also increase EPOC, but would potentially be catabolic. Romijn’s paper from '93 is the classic ref on intensity and substrate utilization. Also look up other papers from Coyle’s lab on pubmed. Good luck.

The numbers I posted were arbitrary and used simply to demonstrate the concept of substrate utilization vs. exercise intensity. There will certainly be differences based on feeding state and mode of exercise employed.

With regard to the EPOC consideration, it will be a factor, but it really is not going to make a substantial difference unless you get above the LT. At this point the catecholamines will increase greatly, and this will really impact EPOC. That is why there is a large impact on EPOC with weight lifting even though actual aerobic metabolism (and consequently, lipolysis) is minimal. Again, unfortunately above LT, you will inhibit lipolysis, and also increase cortisol levels during cardio.

This is not to say I am disagreeing with CR, just clarifying my point.

word. I just didn’t want people to take your numbers literally as they might here.

For the EPOC, that’s kind of what I was getting at. The whole bout won’t be at LT, but if most of it is close to it, and even a few minutes above it, EPOC would contribute some versus the 50% of max HR some people are still traditionally stuck on. And most of the extra calories would come from fat. What % I don’t think is known, but insulin sensitivity would be higher and conceiveably glycogen synthase:phosphorylase would be high, leading to glycogen storage rather than breakdown.

I haven’t seen any data on cortisol increasing with 20-30 min aerobic activity near LT. Have you?

Yeah, but I don’t know if it’s published yet and I’m not an author (just helped out) so I probably shouldn’t say anything. Suffice to say with higher intensity exercise, in general, cortisol goes up.

It makes sense to a certain extent, since cortisol is produced by the adrenal glands and typically moves in tandem with epinephrine. Granted there is sometimes separation with C vs E response, and of course they are produced in different parts of the adrenal, but typically they move together. ACTH will go up under periods of stress and cardio at threshold is going to be perceived as stress by the body.

Even for highly trained or elite endurance athletes I never recommend more than 8-10 min bouts at or slightly above threshold and a total of 45 min at or above threshold per workout. These are very well trained individuals in peak condition, accustomed to this type of training, but it still takes its toll. Cortisol levels will go through the roof during these peak training periods.

CR is correct to a certain extent though, E responds a lot faster than C, so if the bout goes over LT for a minute or two and the total training session is less than an hour, C should be kept in check. I guess for me though, the small increase in EPOC with an increase in E isn't worth risking the potential elevation in C and inhibition of lipolysis.

Steve

Bottom line for fat burning!!! Use the talk test. If you can walk or running and singing joddies (I don’t know, but I’ve been told, T-mag is better than gold(to yourself)) then you are in the best training zone for general fat burning. For additional information, check out Covert Bailey’s Fit or Fat book series.