Training & Nutrition - Inconsistency re: fat-burning zones?

I've recently gotten a HR monitor and have started to use it in my training and have been reading up on determing all of the salient HR numbers and what they mean. The heartzones.com website has been recommended several times here and I found a lot of useful info there, but there's one thing that seems confusing to me -- there seems to be inconsistent information there re: the fat-burning zone. For instance, if you go to the HR calculator link you'll find info on the 5 zones and what they mean, and on that page you'll see Zone 2 (60% - 70% of MHR) listed as the fat-burning zone. The wording in that section seems to imply that this is the best zone for burning fat, but if you check out other documents found on the site here (http://www.heartzone.com/whitepaper.pdf) and here (http://www.heartzone.com/warning.pdf) you'll see the optimal fat-burning zone as being just below the anaerobic threshold, with the latter document seeming to me to most directly contradict the info found on the HR calculator page. What I get from these documents is that while it's true that you burn a larger percentage of fat for fuel in the lower zone, you burn up a lower number of calories thus a lower absolute amount of fat and would be better off closer to but not over your anaerobic threshold. This seems a lot more intuitive than the notion that you can putt along at a relatively easy pace and burn more fat than at higher levels. So, am I missing something or is there a contradiction?

I'm not a doctor, etc. etc., but from what I've read, it's something like this...

You burn more fat and glycogen (per unit time) at the higher heart rate. At the lower heart rate you can exercise almost exclusively burning fat, but your body will still preferentially burn glycogen, until you've depleted your stores. In either case, your body will burn a higher percentage of fat at the lower HR.

The point that may be missing is that it takes a bit more time on the bike to burn the fat at the lower HR. Another good tip on this that I have been using is to ride 45-60 minutes at the lower HR just after getting up and not eating anything except the required (at least for me) cup of coffee. Seems the glycogen store in ones blood is at its lowest level when we wake and we will burn fat very quickly in this state. But one has to be careful not to over do it. 60% HR and no more than an hour. Working well for me as I am down 30+ pounds since starting same and restricting my food intake to something more reasonable.

Originally posted by Grendel
What I get from these documents is that while it's true that you burn a larger percentage of fat for fuel in the lower zone, you burn up a lower number of calories thus a lower absolute amount of fat and would be better off closer to but not over your anaerobic threshold.

Yup - that's it!

That first article did a good job of explaining various factors involved and as ParamountScrapin said time/distance is a factor as well.An important point is as your fitness improves your ability to utilize fat at higher intensities improves (in fact what is low/high intensity keeps shifting up).Another factor mentioned but not focused on is how the food timing and composition affect all this.Again as Paramount said if your blood sugar is depleted you will begin burning fat sooner also if you continually fuel the workout with simple sugars you are training your body to utilize sugar not fat as fuel(also many of those increased calories burned are offset by the calories taken in.).
IMO you would do best to vary the training taking advantage of the strengths of both approach (ie work to increase both distance and intensity)

The notion is that you burn more fat at lower heart rates then at high heart rates.

The biochemistry shows you that it takes twice as much oxygen to produce the same amount of energy burning fat as it does carbohydrate. So if you want more energy (power) production, you should burn carbohydrate. Since when you heart is really pounding, you want power, well I would think that near your anaerobic threshold, you would burn nearly 100% carbohydrate. Of course, people might vary on that one. Sometimes the body does not always behave in a sensible fashion and individuals vary.

However, carbohydrate stores in the body are limited (2,000-2,500 cal) and fat stores are functionally limitless (at 3,500 cal per lb and most of us have plenty of lbs).

The reasoning is that if your heart is beating fast, then you will be burning nearly 100% carbo. At lower heart rates, your body should "save" the carbo and burn fat. Of course, the body seems to HORDE fat. Our bodies seem to want as much fat as possible for that famine that is just around the corner (bodies are slow to catch up with current events). I suspect that only people who are trained at longer rides, burn much fat at lower heart rates. Of course, if you deplete your carbohydrate, you will burn virtually 100% fat. But riding that way does not feel good.

As to the fat burning zones. There is a way to determine them experimentally. Fat and carbohydrate produce different levels of CO2 for the amount of O2 consumed. So in a lab setting, by monitoring C02 production and 02 consumption, you could experimentally determine fat burning zones. I don't think that anyone has ever done this. I do know that quite a few sources give 220-age as maximum heart rate and that one apparantly has absolutely no experimental basis, someone seems to have just made it up. Look, maximum heart rate is pretty easy to determine. If a popular figure is bogus on that, don't you think the fat burning zones are bogus also?

One of the problems I'm having is that I don't yet know my exact MHR, since I haven't been tested for it. The old 200-something minus age formulas (and a few derivatives I've found) put me as a 38 year old somewhere around 182 to 185 MHR, but I know this can't be right because I can get up to 187 - 189 on a short, hard sprint and I know I'm not going at my absolute, nothing left in the tank max effort. I begin to go anaerobic at around 165, so if you figure the anaerobic threshold to be 80% that puts my MHR at around 205. I think that might be a bit high and my anaerobic threshold is probably a little higher than 80%, so for the sake of figuring I'm pegging my MHR at 195. Given that, a zone that would put me at 60% - 70% MHR would be 117 - 137 which I easily surpass on a normal ride and would have to work at staying that low. My 'just cruising around' rate is between 140 - 150, and my monitor alarm is set for a range of 143 - 163 and I can stay within that pretty easily for an hour or more. I guess what I'm saying is that the lower zone just doesn't seem right to me based on what I've observed so far, and a calculated MHR in the 180's would make those numbers even lower. I think I'm in pretty decent cardio shape with a RHR of about 47, so it may be that I've already moved the zones upward?

Yep, your AT is probably higher than 80% MHR. But, for the purposes of fat burning, you're right. If you've pushed your AT up, the other bands move up as well, affecting the amount of oxygen available for burning fat. In general terms, the harder you're breathing, the less fat (percentage-wise) you're burning.

Another salient point... the expression goes "fat burns in the fire of carbohydrates." On long rides, keep eating...

From what you have stated I can tell you are in pretty good shape. Do you want to lose weight? You would be better off keeping up with your current training and burn more calories.

The fat burning range you have stated is what I would consider a recovery range. If you trained in this range only you would never get faster and not burn as many calories unless you rode alot longer.

What are your goals, weight loss or performance?

My goal is a little of both, I guess. I want to get faster and add about 3 - 4 mph to my average so I can stay with the faster group on the weekend rides, but I also want to drop about 20 pounds to get to my target weight of 175. I guess I was hoping that one would take care of the other, that if I worked hard enough to get my speed up that at least some of the weight loss would take care of itself, but the reading I've been doing lately leads me to believe that maybe it doesn't work that way.

I read an article recently about the old 220-age maximum heart rate. There are actually quite a few variations on this formula. However, the experimental data shows a significant variation in the maximum heart rate for any age. A given individual can vary from any of the formulas by 10-20 bpm.

What it all means is that the old standby formula is as good as any for use as a general guideline. It is a poor predictor for any individual as will be any formula that is based solely on age.

Regarding fat burning rates at various heart rates. As my faulty memory recalls, low exertion levels burn primarily fat and little glycogen. That's why you can walk all day yet still be able to sprint in the evening. As your exertion increases, the body begins using glycogen to supplement the fat energy. Since you have limited glycogen stored in the muscles and it takes much longer to replace glycogen than to burn it, a long bike ride often results in the 'bonk'. This is when you have depleted your glycogen stores and now must rely almost exclusively on fat burning for energy. Since fat burning produces less energy than the combination of fat and glycogen, you slow down.

When you reach your anerobic threshold (sprinting), your muscles switch over to anerobic conversion of glycogen. This gives you tremendous energy but at the cost of rapid consumption of glycogen and a buildup of lactic acid. Once the lactic acid raises the pH of the muscle cell above a threshold, energy production shuts down.

The bottom line is that lower exertion levels result in a higher percentage of fat burning, but less fat burning overall than from higher levels of exertion (below the anerobic threshold). Ideally, if you exercise at just the right rate, you would deplete your glycogen right at the finish of your workout. This results in the maximum total fat burn without bonking.

Originally posted by Grendel
My goal is a little of both, I guess.
Well, I'd say it's possible to lose the weight without stressing too much over what HR is best for burning fat and what not. My weight has dropped lower than it's been in 20 years this year, and I wasn't even trying... just focused on getting in my mileage (usually between 120-150 miles / week), and quality work-outs (in my case, that means lots of AT threshold and anaerobic work, with recovery ride days between). All I've done, really, is try to avoid out-and-out empty calories. My diet and ride nutrition has focused on performance and recovery. Stepped on the scales for the first time in months a couple of weeks ago and discovered I now weigh 160, about 10 lbs less than I thought I weighed!

I went to a lab and had my AT tested. I am 45, a female, 5'5" and weigh 170 pounds. (size 10) I had a body composition test that showed 31% body fat. For fat burning I am supposed to stay between 122-134. That means if I walk and swing my arms at only 3.5 mph on the treadmill, I am at 130. I regularly rollerskate near and above AT which is 171. The few times I have worn the heart rate monitor while cycling shows an average around 155-165. I do not know what my max heart rate is. So... rather than slow down on cycling and rollerskating--which I use for cardio and enjoyment, I added a walk 2-3 times per week for fat burning. I didn't want to take the excitement and drive out of my favorite sports.

grendel- we are in the same boat and have similar numbers. i am also 38 and don't get the numbers. my HRM only serves to remind me to go easy on my easy days. i am used to going balls out for as long as i can and have stagnated, so i am trying to back off a bit. i will save me the physical and mental wear and tear.

if it's worth anything, i started making gains in my speed as i backed off (probably as a result of a better recovery effort) i don't spend all that much on recovery products(proper protein/carb ratio, etc)

an increase of 3-4 mph is a lofty goal. i've been told i have the speed, but the endurance needs work. unfortunately, with my life i can only get in 1 maybe 2 rides of > 2 hrs per week and none > 4 hrs.

Rather than focus on what the heart zones website says about fatburning, the best thing you can do is get the books that explain the theory behind the explanations given on the websites. Those explanations Grendel highlighted with the webpages are excellent sources of explaining the difference between working in your fat burning ranges vs. looking at which zones burn more fat vs. burning glucose by your body as it's primary fuel sources are not talking about the same thing.

I recommend Sally Edwards' books: "The Heart Rate Monitor Book" as a great starter that explains everything down to its basic level. Afterwards, pick up her book called "The Heart Rate Monitor Book for Outdoor and Indoor Cyclists". In that book, she'll explain the zones more clearly, plus go into the discussion on why working out around anaerobic threshold is good for maximum fat loss, and you'll get the workouts you can do both indoor and outdoors, plus an idea of how to formulate a training schedule so you can achieve these results.

Just keep in mind, those two articles are not talking about the same thing, and you won't get confused or think the articles are inconsistent. The more you know, the more you realize that the articles actually complement each other.

Koff

OK, stop all the jazz, the only important thing is for you to find more "games to play" regarding your workouts and HR monitor.

You mentioned trying to lose 20 lbs. if this is true, you have a long road ahead and the big picture is finding interesting ways of measuring your progress.

The "real reason" stuff like "fat burning zones" are popular is they provide a guide to performing workouts at such a low intensity that people DO NOT become discouraged and quit.

My experience suggests, you might focus on keeping your workout schedule. Whenver possible, making it longer as long as it does't cause you to start skipping workouts.....there's time enough for intensity AFTER the weight comes off.. Good Luck.