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Burning Fat/Burning Glycogen

Old 09-08-05, 12:54 PM
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Burning Fat/Burning Glycogen

I'm just full of questions today.

From what I can gleen, exercising at a lower HR burns fat, while exercising at a higher HR burns glycogen stored in the muscles and the liver. When we run out of carbs and glycogen during a training ride, event, or race, we run out of energy. We simply can't go anymore. This is called as bonking... unless you're in Australia and bonking is something else much more enjoyable altogether.

We can replace glycogen by eating carbs (complex are better) during exercise.

We also have a window of time after exercising to restore glycogen to the muscles and the liver.

Do I have this right so far?

Does the body turn stored fat into glycogen? Are fat and glycogen linked together in a cycle, or are they two separate energy sources?
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Old 09-08-05, 01:02 PM
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You have summarized my own confusion very nicely.
I am looking forward to the answers along with you!

Ciao for now.
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Old 09-08-05, 05:52 PM
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There is always a combination of energy pathways metabolizing various energy "substrates" at varying rates. The intensity of exercise, dictates to some extent the proportions or the "rate of metabolism" of some of these energy substrates.

Typically, people who casually toss around terms relating to metabolic processes seldom understand the "big picture" and tend to incorrectly give inappropriate attention to a single process regarding some given energy source being utilized at some given intensity.

Much like your remarks.
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Old 09-09-05, 02:04 AM
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Originally Posted by rjkresse
From what I can gleen, exercising at a lower HR burns fat, while exercising at a higher HR burns glycogen stored in the muscles and the liver.
It's not quite so simplistic. There's a ratio of fat-to-carb calories that's burnt. People often confuse the percentage of fat-burning with total-calories of fat burned. In general, the higher the intensity up to your LT, the higher the total calories burned from both fat and carbs. Fat-calories also goes up with intensity, just not as much as glycogen. It really depends upon your level of fitness and condition. Let's compare a beginning-rider vs. an elite athlete:

BEGINNER RIDER
50% max-HR = 500 Cal/hr = 75/25% fat/carbs = 375 fat-calories/hour
65% max-HR = 650 Cal/hr = 60/40% fat/carbs = 390 fat-calories/hour
80% max-HR = 800 Cal/hr = 25/75% fat/carbs = 160 fat-calories/hour

ELITE FIT ATHLETE
50% max-HR = 750 Cal/hr = 75/25% fat/carbs = 560 fat-calories/hour
65% max-HR = 1000 Cal/hr = 70/30% fat/carbs = 700 fat-calories/hour
80% max-HR = 1250 Cal/hr = 60/40% fat/carbs = 750 fat-calories/hour
90% max-HR = 1400 Cal/hr = 25/75% fat/carbs = 350 fat-calories/hour

Basically, the more fit you are, the more fat/hour you can burn at a higher pace. Your energy and cardiovascular systen becomes more efficient at converting fat to ATP for burning, without resorting to anaerobic fermentation and phospho-creatine at high-effort levels. So the more fit you become, the more total fat-calories you burn at a higher-intensity, even though the percentage is lower compared to a lower effort.

The latest studies appear to show that total-weight loss is more related to total-calories burnt in the workout regardless of whether it's fat or carbs. If you burn off 3000 calories in a workout and eat only 2500 calories a day, you'll lose about 1-lb fat/week regardless of the intensity. You can ride easy for 6-hours a day or ride really hard for 2-hours. The weight-loss will end up being the same. What happens is that after the ride, your body will convert fats into glycogen to replenish your energy stores. So it doesn't matter if you burned up that fat during the ride, or if it's converted later to be used on the next ride, it's still gonna be used up at one point or another to make up the calorie deficit.


Originally Posted by rjkresse
When we run out of carbs and glycogen during a training ride, event, or race, we run out of energy. We simply can't go anymore. This is called as bonking... unless you're in Australia and bonking is something else much more enjoyable altogether.

We can replace glycogen by eating carbs (complex are better) during exercise.

We also have a window of time after exercising to restore glycogen to the muscles and the liver.

Do I have this right so far? Does the body turn stored fat into glycogen?
Looks good so far. You have about a 10-15 minute window to eat carbs right after ride. About 1.5gm/kg body-weight is about optimum. If you wait longer than this, the body will disassemble perfectly good muscle to convert into glycogen to replenish your energy stores. This is the prefered source rather than fats. So you cannot starve yourself to make your body burn fat, it'll take apart muscle instead. Fat-catabolism is initiated at the same time and is triggered by low blood-sugar. Glucagon is then secreted to signal the body to convert fats into glycogen (through glycerol->glucose) But this process is much slower than converting muscle & protein, so you have to eat carbs right after a ride.


Originally Posted by rjkresse
Are fat and glycogen linked together in a cycle, or are they two separate energy sources?
They are linked somewhat. Here's a summary of the processes:



The bottom-line is that the citric acid cycle (Krebs cycle) is what generates the ATP for muscle contractions. It is fed acetyl-CoA from either carb or lipid metabolism. The carb pathway is much quicker and can allow the kreb cycle to generate more ATP/second than the lipid pathway.* So that's why as intensity picks up, a larger percentage of energy comes from burning carbs. But the amount generated by lipid metabolism also increases as well, just not as quickly.

Here's how they are linked (a closer look at the fatty-acid spiral):


Notice that the fatty-acid spiral also generates acetyl-CoA for the kreb's cycle, but it initially requires the input of ATP. Where does this ATP come from? It comes from carb metabolism! So you cannot burn fats without having carbs to burn as well! This is a double-whammy when you bonk, because once you go through your carbs, you have nothing left to ignite the lipid pathway either. Your body shuts down carb & lipid metabolism and resorts to burning proteins and muscles for energy.

So that's why it's so important to replenish your carbs during long rides, once you bonk, that's it, you pretty much gotta stop unless you want to tear your muscles apart. About 250-300 cal/hour is the maximum your body can absorb, so you gotta start eating early if you're doing a long ride at high intensity (to burn as many calories/hour per workout).

Another catch is that in the intestines, in order absorb a single glucose molecule, you have to have an exchange of a sodium ion. You sweat away about 1000mg of sodium per hour, so you have to make sure you get electrolytes as well on the ride. You can end up with a triple-whammy when you bonk if you run out of carbs and electrolytes. You've got a stockpile of fats you can't burn because there's no carbs to initiate the process. Then you can't absorb carb scraps you've been picking off the ground because you've got no sodium... heh, heh... no wonder cyclists who bonk look like zombies!

-----------------------------
* Slight error in this summary chart. Fructose is not entered into glycolysis direclty. It has to go through the liver and get converted from a 5-carbon fructose into a 6-carbon glucose first. Then that glucose is fed into glycolysis.

Last edited by DannoXYZ; 04-24-06 at 04:24 AM.
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Old 09-09-05, 05:53 AM
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Old 09-09-05, 07:42 AM
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DannoXYZ -

Thank you. This was exactly what I was looking for.
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Old 09-09-05, 07:56 AM
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Originally Posted by DannoXYZ
Then you can't absorb carb scraps you've been picking off the ground because you've got no sodium... heh, heh... no wonder cyclists who bonk look like zombies!
Very good post, as usual. Another bonking note: the central nervous system _only_ burns carbs, and (IIRC) can't store them - zombie!
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Old 09-09-05, 08:58 AM
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Originally Posted by danch
Very good post, as usual. Another bonking note: the central nervous system _only_ burns carbs, and (IIRC) can't store them - zombie!
"Carbs" would be the wrong term. Carbohydrate is what you eat. It's a macronutrient. Upon consumption it is stored in the muscles and liver as glycogen. When it is required for fuel, it is used as glucose.

.
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Old 09-09-05, 09:52 AM
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Originally Posted by Richard Cranium
There is always a combination of energy pathways metabolizing various energy "substrates" at varying rates. The intensity of exercise, dictates to some extent the proportions or the "rate of metabolism" of some of these energy substrates.

Typically, people who casually toss around terms relating to metabolic processes seldom understand the "big picture" and tend to incorrectly give inappropriate attention to a single process regarding some given energy source being utilized at some given intensity.

Much like your remarks.
Sometimes people like to respond to posts that may contain incorrect statements with an air of condescension. However, they offer little or no help in rectifying the OP's mistake, making their comment largely useless.

Much like your remarks.

PS Thanks Danno for providing some real information.
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Old 09-09-05, 01:16 PM
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Originally Posted by Cornbread
Sometimes people like to respond to posts that may contain incorrect statements with an air of condescension. However, they offer little or no help in rectifying the OP's mistake, making their comment largely useless.

Much like your remarks.

PS Thanks Danno for providing some real information.
+1

OP was just asking a question, and was showing how he understood the issue.

Some people are just....pshhh.
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Old 09-09-05, 01:21 PM
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he said bonking
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Old 09-09-05, 01:39 PM
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Danno, although I zombied out my first attempt at college and thus trashed my biochem potential, from a layman's standpoint, even though I did not totally understand the fine details, your presentation here provided the glue to cement my understanding of these processes that I have been studying regarding cycling and metabolism. This was an outstanding presentation. My hat's off to you.
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Old 09-09-05, 02:09 PM
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Originally Posted by bigfatdud
OP was just asking a question, and was showing how he understood the issue.
Yep. I was just presenting the issue as I understood it and asking questions. DannoXYZ, your reply was brilliant, comprehensive, understandable, and should be made a sticky. And, paraphrasing Rodney Crater, I didn't understand all of it, but I understand what goes on in a way that will help me a lot. Again, thank you.

Mr. Cranium... It was not my intent to pass along misinformation. I was presenting my misunderstanding and asking for clarification, "big picture" and all. Curiosity is a good thing.
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Old 09-09-05, 02:33 PM
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Originally Posted by NoRacer
"Carbs" would be the wrong term. Carbohydrate is what you eat. It's a macronutrient. Upon consumption it is stored in the muscles and liver as glycogen. When it is required for fuel, it is used as glucose.

.
thanks for the correction
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Old 09-09-05, 02:43 PM
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Danno - thanks for the explanation. That answered so many questions that I've had, but I'm at the stage where I don't know exactly how to ask them.
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Old 09-09-05, 03:50 PM
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Originally Posted by Cornbread
Sometimes people like to respond to posts that may contain incorrect statements with an air of condescension. However, they offer little or no help in rectifying the OP's mistake, making their comment largely useless.

Much like your remarks.

PS Thanks Danno for providing some real information.
My thoughts exactly.

Thanks, Danno. That was very helpful.
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Old 09-09-05, 09:49 PM
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Originally Posted by danch
Another bonking note: the central nervous system _only_ burns carbs, and (IIRC) can't store them - zombie!
Hehehehehehe
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Old 09-10-05, 12:07 AM
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Danno, that post was the ****. You're the man.
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Old 09-11-05, 10:56 PM
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Yeah...what he said.

I understand it...but only because I studied it for my AFAA exam. I couldn't have added graphics like that though!!
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Old 09-12-05, 09:08 AM
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Danno, that was one of the all-time best posts I've read on any forum. I'm a bit lost in the graphs, but I get the gist of it. I do have 1 remaining question though.

How was the recomendation of 1.5g of carbohydrates/kg of body weight established. At 173 pounds this means I should be eating 105 grams of carbohydrates within 15 minutes of a ride. Shouldn't this recomendation be adjusted for the intensity of the workout? 105g seems rather high. Do the carbohydrates I consume immediately prior to my morning commute count at all?

Am I right to consume these carbohydrates in the form of fresh fruit and vegetables? I try to have a banana/apple/orange, when I get in to work in the morning (12 miles at about 15mph, 60-80% max HR). Before the morning ride I have a bowl of whole grain-based cereal with a banana/berries or if I'm running late, a Kashi GoLean breakfast bar.
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Old 09-12-05, 12:22 PM
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DAnno's post was great from a fat burning perspective but from a cycling improvement perspective here's my question.

I commute to work monday to friday (zone 1) and spin three times (zone 3) during the week.

Weekends I have 2 long hard rides and my friend is suggesting that I should turn one into a long recovery ride as it will help build my base up, making my body burn fat more efficiently at the lower zones? Is this the best way to go about it or should I maximize my time by going to the longest, hardest rides I can do?
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Old 09-15-05, 03:59 PM
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Thanks for the props folks and you're all most welcome. A couple more questions:

Originally Posted by DerekU2
Am I right to consume these carbohydrates in the form of fresh fruit and vegetables? I try to have a banana/apple/orange, when I get in to work in the morning (12 miles at about 15mph, 60-80% max HR). Before the morning ride I have a bowl of whole grain-based cereal with a banana/berries or if I'm running late, a Kashi GoLean breakfast bar.
Yup, any kind of low-fat source of carbs is fine along with lean protein. On such a short ride, that breakfast alone should be fine. Recent UK study showed kids getting protein (beans) along with their toast in the morning did better on exams, not sure on the exact mechanism yet.


Originally Posted by DerekU2
How was the recomendation of 1.5g of carbohydrates/kg of body weight established. At 173 pounds this means I should be eating 105 grams of carbohydrates within 15 minutes of a ride. Shouldn't this recomendation be adjusted for the intensity of the workout? 105g seems rather high. Do the carbohydrates I consume immediately prior to my morning commute count at all?
The recovery amount came from a lot of studies which tested day-after efforts based upon ingesting various amounts of carbs after a ride. The most optimum amounts is then what would allow you to make full-recovery and do your maximum-distances rides day after day, such as 3-4 hour endurance events. Blood-glucose and insulin levels are also monitored to check absorption rates. Amounts ingested above the optimum quantity doesn't appear to aid recovery-rates. So the best sequence actually looks something like this:

TIME____AMOUNT
0-20min = 0.6gm/kg body-weight
every 30min = 0.6gm/kg body-weight
after 4hrs = 4.0gm/kg body-weight TOTAL (add up all previous steps)

It gets difficult trying to time eating every 30-minutes, so I just take in 1.25-1.50gm/kg right after a ride to ward off muscle-catabolism, then do a full meal within 4-hours. The size of that meal does take into account the amount burned off during the ride. The sequence of calories-in vs. calories-out may look something like this for a 3-hour endurance workout:

+300 cal pre-ride meal
+400 cal on-ride snacks
-2500 cal burnt on ride (700 ingested, 1300 glycogen, 500 fat)
+300 cal post-ride snack
+400 stored-fat conversion to glycogen
+600 cal post-ride meal within 4-hours
------------
-500 cal deficit TOTAL

The physiological events happening in the body after a ride is quite a whirlwind. The lowered blood-sugar stimulates secretion of glucagon and cortisol along with remaining adrenaline and epinephrine from the workout all combine to convert stored fats in adipose tissue as well as protein into glucose to restore the depleted glycogen stores in the muscles. The purpose of the immediate meal is to quickly raise blood-sugar to reduce the glucagon/cortisol levels. It also raises insulin levels which triggers absorption of glucose from the bloodstream into the muscle-cells. This limits the rate of muscle-breakdown. Conversion of fats & lipids to glucose happens at the same time at a steady pace, but it can only occur so fast. Your body's gonna try to stock up a full supply of glycogen no matter what. You want to keep the supply of glucose from ingested carbs high until the glycogen stores are full to prevent the body from disassembling muscles. That very last post-ride meal is then adjusted for the length of the ride.

The trick is balancing all the processes at their maximum rates. Fat conversion is the slowest and about the fastest you can do is about 1000cal/day without tearing apart muscle at the same time. When blood-sugar is low, fat-conversion cannot occur fast enough to replenish the depleted glycogen stores, so you have to add carbs to keep up to the glycogen accumulation rate (glycogenesis). This can take 8-24hours depending upon depleted you got.


Originally Posted by jslopez
Weekends I have 2 long hard rides and my friend is suggesting that I should turn one into a long recovery ride as it will help build my base up, making my body burn fat more efficiently at the lower zones? Is this the best way to go about it or should I maximize my time by going to the longest, hardest rides I can do?
The purpose of endurance rides are to tax your energy-delivery system. Being able to converted stored glycogen to ATP, being able to convert fats to glucose and absorbing glucose into the muscle-cells. The kind of ride that does that is one that burns up as much energy as fast as possible, say... 2500-5000 calories.

The trick is to balance it with your fitness level. If you can only hold a 1000cal/hr pace for only 1-hour, it won't tax your energy system as much as going at a slower 700/cal/hr pace for 3-hours. Because you'd be able to burn off twice as much. I got your weekend-schedule on email and yes, you probably want to do a longer distance event on one day so that you can burn off more. You really do want to do a 3-4 hour ride at a fast enough pace so that you burn off lots of calories/hour, but slow enough so that you can keep it up for 3-4 hours. The idea is to burn off as many calories as possible per ride and as you get more and more fit, you can go faster and burn off even more calories in that time and tax your energy system even more.

Last edited by DannoXYZ; 06-29-07 at 12:02 PM.
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Old 09-18-05, 09:18 PM
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For the slow kids in class (me), can you recommend a "for dummies" kinda book on cycling nutrition? I have read 'Serious Cycling' and didn't feel it goes as in depth as this post does. I am looking for something to help me understand all this better.

Recommended reading?
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Old 09-18-05, 11:34 PM
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this has been really great. i kind of ask questions for a living so i couldn't resist this one, although you don't have to answer it:

danno:
Fat conversion is the slowest and about the fastest you can do is about 1000cal/day without tearing apart muscle at the same time
me: ah hah. is this at all related to that recommendation of not aiming higher than 2 pounds/week weight loss, or am i being simplistic?
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Old 09-19-05, 04:18 AM
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The only reason I know this stuff is because I had an intense interest in exercise-physiology during the 10-years I was racing and emphasized that during my microbiology major. Going to med-school just didn't have the appeal of bike-racing, so I stayed with that after school.


Originally Posted by BasicJim
For the slow kids in class (me), can you recommend a "for dummies" kinda book on cycling nutrition? I have read 'Serious Cycling' and didn't feel it goes as in depth as this post does. I am looking for something to help me understand all this better.

Recommended reading?
Hmm, I'm not sure if there's a "for dummies" book on cycling physiology. I'm not even sure knowing this stuff in such detail really helps. I think it's more efficient as far as fitness-improvement vs. time ratio to just follow "do this" and "do that" guidelines when it comes to nutrition. Some good info in:

"High Tech Cycling" - Ed Burke, PhD
"Cycling Science" magazine - Chester Kyle, PhD
"Bike Tech" newsletters - Ed Burke, PhD
"Bicycling Science" - David Wilson

But if you must really know the detailed "whys" and "hows" of this stuff, you're really going to have to consult the medical journals like JAMA or NEJM. Some other studies:

Regulation of Carb & Fat Metabolism During & After Exercise
JournalAppliedPhysiology-Effect of fat adaptation and carbohydrate restoration on metabolism
JournalPhysiology-Skeletal muscle fat and carbohydrate metabolism during recovery from glycogen-depleting exercise in humans
AmericanJournalPhysiology-Postexercise fat intake repletes intramyocellular lipids but no faster in trained than in sedentary subjects
JournalAppliedPhysiology-Effect of alcohol intake on muscle glycogen storage after prolonged exercise <-- important for drunks like me

Citations in bibliographies provides further reference.

"Fat conversion is the slowest and about the fastest you can do is about 1000cal/day without tearing apart muscle at the same time

me: ah hah. is this at all related to that recommendation of not aiming higher than 2 pounds/week weight loss, or am i being simplistic?"


Yup that's correct. Conversion of stored fats to replenish energy stores actually do not go any faster in fit athletes than couch-potatoes, so you can't push this process too quickly. What happens when you try to lose weight too quickly is that you end up tearing apart perfectly good muscle; effectively negating any strengthening & fitness-increasing benefits of exercise. This occurs if you exercise a lot and do not eat afterwards. With a large calorie-deficits, you end up tearing apart muscle to make up the difference between the maximum fat-conversion calories and the insufficient amount of carbs supplied. You will may end up burning off 1000cal/day of fats, but you also take apart 250gm of muscle as well. So 1/2 of the weight loss is fat and the other 1/2 is muscle. The result is you end up getting weaker and weaker with weight loss and you end up with a skinny twig with a double-chin and body-fat percentage remains the same at 20-25%.

The other issue with large calorie deficits is that a lot of people do this through cutting calories from their intake. Malnutrition really does become a concern. You end up with mineral deficiencies leading to susceptibility to illnesses, continual fatigue and low motivation. Cutting out too many carbs also results in ketoacidosis, basically you're burning up your muscles and poisoning yourself from the inside.

The only way I've been able to get rapid weight-loss is to build muscle at the same time. So in the winter, I'll build up lots of muscle-strength through weight-training in the gym. I'll also ride lots of ong miles in early spring as well. This ends up taking apart some of the muscle I built up along with losing fat gained over the holidays.

Last edited by DannoXYZ; 09-19-05 at 12:16 PM.
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