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    What comes first: fat burn or muscle loss?

    I'm just curious to know if the body will go after fat first, or muscle, when it realizes it's under a severely restricted caloric intake.

    Personally, I go for the weight loss over time approach, like a pound or so a week or every two weeks. I hear of multiple pounds lost per week on some diets, though, and thought of this question.

    I wonder if the body finds it easier to burn fat in these instances, or muscle.

    Anybody know?

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    Mercrudgeon Bikedud's Avatar
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    Fat is a normal source of fuel when your body is exercising for long periods at a moderate level of intensity.

    http://www.cruciblefitness.com/nutrition/etips/Fat.htm

    http://www.jmtrainingsystems.com/art...dyfatloss.html

    Dieting is the same (I pretty sure) in that if carbohydrates are not available your body will burn fat for fuel. There are possible issues with burning fat for fuel over a period of time when blood sugar falls too low.

    http://wwhttp://www.ketosis-ketoacid...ifference.com/

    http://www.survivediabetes.com/ketosis.htm


    If you are consuming muscle as a fuel source you are in serious need of nutrition and at risk for possible health issues.

    Your correct, the best way to lose weight is too modify your behavior and correct poor eating habits over time.
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    Senior Member Richard Cranium's Avatar
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    Good question, but most likely no hard and fast answer.

    At least some of the factors would have to do with the "rate" of the constant calorie deficit. Another would be the type of caloric "load", are you just walking around or are you freezing on a mountain top?

    The other side of fat-burning not often discussed but actually more important, "Why do certain individual's "lipid profiles" change when in calorie deficit while others stay the same??

    In other words, diet helps reduce cholesterol for some people but not others......

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    All I know is that I'm at 210 now, from 215 a few weeks ago.

    The big, high-tech diet I'm on? NOTHING. Just watching portions. Eating fruits/veggies before the main course, etc. No big deal.

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    Just ride. roadbuzz's Avatar
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    Originally posted by Andy Dreisch
    The big, high-tech diet I'm on? NOTHING. Just watching portions.
    Give it up! That can't possibly work!

    It's just a guess, but I'm thinkin' it probably mainly burns fat for calories, and muscle when protein is insufficient.

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    Bike Happy DanFromDetroit's Avatar
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    Originally posted by Andy Dreisch
    ...
    Eating fruits/veggies before the main course...
    What a wonderfully simple and practical idea! I am astounded that it never occured to me.

    Dan
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    Pat
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    Originally posted by Andy Dreisch
    I'm just curious to know if the body will go after fat first, or muscle, when it realizes it's under a severely restricted caloric intake.

    Personally, I go for the weight loss over time approach, like a pound or so a week or every two weeks. I hear of multiple pounds lost per week on some diets, though, and thought of this question.

    I wonder if the body finds it easier to burn fat in these instances, or muscle.

    Anybody know?
    The thing is that "that depends".

    The body burns 2 types of fuel. It burns carbohydrate or fat. Carbohydrate is great for high levels of exertion. Burning carbohydrate gives twice the energy yield per O2 molecule and power output is generally oxygen limited. However, you only have a limited amount of carbohydrate (about 2000 calories).

    Why is carbohydrate so limited if it is such a great fuel? Well it weighs more then fat - twice as much for the energy content. So you body has a nearly endless supply of fat which is great at long sustained moderate exercise.

    Now the body HAS TO HAVE some carbohydrate. The nervous system can only burn carbohydrate. So if you are not eating it, it has to come from somewhere. Where? Well you body takes muscles, breaks them down into proteins, and converts the proteins to carbohydrates.

    So if your diet is overly carbohydrate restricted, you will lose muscle mass. In facts, in many if not most diets, people lose 1 lb of muscle for every lb of fat lost. People who exercise quite a bit and who eat enough carbohydrate can actually gain muscle mass whilst losing fat. However, on most fast weight loss diets, I bet that nearly all the weight lost is muscle. Fat is 3500 calories per pound and it takes a sedentary person days to burn off a lb of fat so losing weight by losing fat is just not feasible quickly unless you are exercising quite a bit.

    Now, I did manage to do this once. I spend 2 weeks on a road bike tour which featured over 70 miles per day and over 4,000' of climbing per day. I avoided fats in my diet and consumed ample carbohydrates to replenish my glycogen stores. I also exercised at a reasonably moderate pace. I mean I was faced with climbs in excess of 7 miles and I did not think I could hammer up a 7 mile long hill without my legs giving out before I ran out of hill. I managed to lose 10 lbs in 2 weeks. Not too shabby. It can be done, but that was a rather special situation.

    As a rule of thumb, losing a lb per week is a more realistic goal.

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    Thanks, Pat ... excellent info.

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    Senior Member Cipher's Avatar
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    Originally posted by Andy Dreisch
    Thanks, Pat ... excellent info.
    2nd that! (To all who contributed).
    Speed Kills...It kills those that don't have it!

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    Just ride. roadbuzz's Avatar
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    Originally posted by Pat
    Where? Well you body takes muscles, breaks them down into proteins, and converts the proteins to carbohydrates.
    That sounds a little dubious to me. Are you saying that the body doesn't break down fat and burn it?

    I mean, your body doesn't burn fat either... it has to be broken down to simple sugars to be utilized. It fails the common sense test, too. Why would animals develop a very efficient mechanism for storing energy for hard times (known as fat), and be so bad at storing protein (known as muscle) if it were a good source of energy for hard times. I've heard it said that "fat burns in the flames of carbohydrates," the message being that you need to maintain some level of carb intake in your diet for efficient fat burning.

    I don't buy it... but am willing to be educated. Can you clarify or point to some online (or offline) research that supports the above statement?

  11. #11
    Pat
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    Originally posted by roadbuzz
    That sounds a little dubious to me. Are you saying that the body doesn't break down fat and burn it?

    I mean, your body doesn't burn fat either... it has to be broken down to simple sugars to be utilized. It fails the common sense test, too. Why would animals develop a very efficient mechanism for storing energy for hard times (known as fat), and be so bad at storing protein (known as muscle) if it were a good source of energy for hard times. I've heard it said that "fat burns in the flames of carbohydrates," the message being that you need to maintain some level of carb intake in your diet for efficient fat burning.

    I don't buy it... but am willing to be educated. Can you clarify or point to some online (or offline) research that supports the above statement?
    No I did not say that the body does not burn fat. It certainly does burn fat.

    And you are wrong, fat cannot be broken down into sugars. Fat is a fat not a polysaccharide. Fat is burned by beta oxidation while carbos are burnt via the KREBS cycle. Taking advanced graduate biochemistry did teach me something.

    By the way, you don't need any carbos to burn fat. Under most conditions, atheletes burn a mix of fat and carbo and that might be where the saying come from. But one can deplete one's glycogen stores and still keep going by burning fats only - you just don't go real fast because burning fat takes twice the oxygen per energy unit released as burning carbos. But you can do it.

    What I was saying is that under certain conditions the body prefers burning Carbos. And neural tissues can ONLY burn carbos and not fat (which is where you might have gotten the mistaken notion that I said we can't burn fat - we have lots of non neural tissues which can burn fat). And the body can break up proteins to make carbos if it has to. That is why many people lose muscle mass whilst dieting. Their body is burning up their muscle mass.

    I sort of suspect that the human body LOVES fat. I think our bodies horde fat for the famine that is coming up. Of course, famine never comes for most of us.

    As to the references, you can find nearly all of this information in almost any introductory Biochemistry text. Of course, it would take you a long time to make any sense out of it.

    And if you think the introductory texts are hard to make sense of, you ought to try reading the original research papers which generally assume that you are an expert.

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    Originally posted by Pat
    ... And the body can break up proteins to make carbos if it has to. That is why many people lose muscle mass whilst dieting. Their body is burning up their muscle mass.
    This was the reason for my post in the first place.

    Thanks, again, Pat, for the great info.

  13. #13
    One day at a time chaztrip's Avatar
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    Hmmm good Info here

    Thanks

  14. #14
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    Originally posted by Pat
    No I did not say that the body does not burn fat. It certainly does burn fat.

    And you are wrong, fat cannot be broken down into sugars. Fat is a fat not a polysaccharide. Fat is burned by beta oxidation while carbos are burnt via the KREBS cycle. Taking advanced graduate biochemistry did teach me something.
    All right- actually fat is a lipid. Specifically, fat is a triglyceride, composed of three fatty acids and a glycerol molecule. Triglycerides play an important role in metabolism as energy sources. They contain twice as much energy (8000 kcal /Calorie /kg) as carbohydrates. In the intestines, triglycerides are split into glycerol and fatty acids and then move into blood vessels. In fat catabolism , triglycerides are hydrolyzed (uses water to split the triglyceride bond) into fatty acids and glycerol . The glycerol is then converted into glyceraldehyde-3-phosphate, and enters glycolysis (process of breaking down glucose to ATP thatís used for energy) and eventually the ctric acid cycle. Fatty acids are broken down through a process known as beta oxidation which results in acetyl-CoA to be used in the citric acid cycle. (See step 1 of the Krebs Cycle/Citric Acid Cycle)
    By the way, you don't need any carbos to burn fat. Under most conditions, atheletes burn a mix of fat and carbo and that might be where the saying come from. But one can deplete one's glycogen stores and still keep going by burning fats only - you just don't go real fast because burning fat takes twice the oxygen per energy unit released as burning carbos. But you can do it.
    Expanding on this, under most ideal conditions everyone burns a combination of fats and carbohydrates. Whether you burn your calories from fat or carbohydrates depends in part on how high your anaerobic threshold is, at what point your body uses carbohydrates predominantly for fuel from exercising, and if oxygen is present during exercise (or rather, if youíve gone into oxygen debt because the demands you place on the body from exercise exceeds the amount of oxygen you intake). You will always burn some fats and some carbohydrates, even when youíre just sitting still- the body needs energy to perform the tasks youíre doing, after all!

    During exercise, there are two states of exercise- aerobic and anaerobic exercise. Aerobic exercise is exercise done in the presence of oxygen- that is, exercise done where the demands do not exceed the amount of oxygen you intake. During this time, fat is the primary fuel utilized as energy. Fat is only burned in the presence of oxygen. Do not become confused here by this next statement- you will burn a higher percentage of fat in the aerobic state, but the higher your exercise exertion level, the more overall calories you burn, and the lower the percentage of fat that will be used to burn those calories. At higher intensities, the body will go into oxygen debt, resulting in the heavier breathing, out of breath feeling you have during intense exercise. (Drop me a PM if this throws you off). When the body goes into the anaerobic phase, not enough oxygen is present to sufficiently burn fat, so the body converts to anaerobic pathways instead- the process where carbohydrates are used instead as its energy source. In this case, the carbohydrates are broken down into its simplest components- glucose. This sugar (glucose) then is metabolized into pyruvate. When oxygen is not present, the pyruvate is then broken down into lactic acid (sounds familiar? Lactic acid buildup in the muscles???) and CO2, plus 2 measley ATP molecules. Had oxygen been present, that CO2 would have been converted to the Acetyl-CoA, and entered the Krebs Cycle at step 1. (See step 1 of that diagram). When oxygen is not present, however, the lactic acid builds up and the amount of ATP produced is insufficient to continue the higher intensities at longer periods. So the body slows down, and as the breathing slows, oxygen returns into the body, which leads to the Krebs cycle being re-activated, and fats (and some carbohydrates) are again used to create more ATP so you have the energy to continue working out. Add that to the lactic acid thatís already in the muscles- it is also converted back to pyruvate with the addition of oxygen, and that pyruvate can be used in the Krebs cycle.

    My hypothesis at this point is that it takes less energy to use carbohydrates rather than fats is simple- it takes many more steps to break the bonds of the fat molecule to the fatty acid and glycerol, then bind the fatty acid to make the acetyl coA than to simply use a water moleule to break down carbs to pyruvate, then pyruvate to carbon dioxide, forming acetyl CoA for the krebs cycle. There are also more carbons and hydrogens in a fatty acid than in glucose, so it takes a shorter amount of time to break down a carbohydrate than a lipid (fat). It is certainly less work during the anaerobic process when the body metabolizes carbs to pyruvate, which then converts to lactic acid and carbon dioxide.

    What I was saying is that under certain conditions the body prefers burning Carbos. And neural tissues can ONLY burn carbos and not fat (which is where you might have gotten the mistaken notion that I said we can't burn fat - we have lots of non neural tissues which can burn fat). And the body can break up proteins to make carbos if it has to. That is why many people lose muscle mass whilst dieting. Their body is burning up their muscle mass.b
    I am unsure as to what neural tissues are, as Iíve never heard of those terms before, and a quick scan of my biochemistry books make no mention of neural tissue. ďNeuralĒ refers to the nervous system, and the nervous system is NOT involved directly with fat, carbohydrate and protein catabolism.

    It is possible for the body to metabolize proteins as an energy source. Fat is an important energy source for the body- it is used to break down some vitamins, as well as to produce large amounts of ATP, maintain all of your cells- the wall structures and integrity, etc. But if youíre not intaking enough fat for producing ATP, your body can use protein instead- it is a last resort, but it can be done. In this case, the protein is catabolized to produce amino acids, and these amino acids can be used in the Krebs Cycle to produce the ATP needed to keep your energy levels up. Itís more than just the body using your muscle to break down protein (which it does)- itís also the protein you intake daily- instead of the body using it to provide protein to build and maintain muscle, it is used instead to provide energy for the bodyÖ you end up losing more muscle than fat over the long run, which is how you can end up with that saggy look when dieting and you restrict your calories (or your fat and carbohydrate intake) too much.

    I sort of suspect that the human body LOVES fat. I think our bodies horde fat for the famine that is coming up. Of course, famine never comes for most of us.

    As to the references, you can find nearly all of this information in almost any introductory Biochemistry text. Of course, it would take you a long time to make any sense out of it.

    And if you think the introductory texts are hard to make sense of, you ought to try reading the original research papers which generally assume that you are an expert.
    You got it! The human body loves fat- fat is used to protect the organs, provide heat, provide structure to the cells, dissolve the fat soluble vitamins we need for the body, provide energy needed for the body to exercise and perform daily tasks, etc. The body is efficient- itís all about self-preservation. So if you restrict your food intake or exercise to the point where you are losing too much fat, or you are not getting enough fat, then the body will look to other ways to provide energy- breakdown of carbohydrates and protein.

    So the answer to the question the first person had- what does the body burn first- fat or muscle? It burns fat first, BUT if you are not taking in enough fat and calories to maintain your activities, and the carbohydrates begin to run low, then the body will turn to protein (muscle and the protein you ingest in your food) to maintain itís activities. At what point that happens, I could not tell you- itís going to depend on the individual and what youíre needing to maintain your activities. If you really wanted to know, you could check into a performance lab or a local university with an exercise physiology or kinesiology department for tests, and they could perform some expensive tests on you, but youíd at least have your answerÖ right?

    If you have questions, PM me, I did a post that explains the Krebs Cycle a bit more, and it would be good to read the link, then re-read the post I did and see if it makes any sense, and also check out the picture diagram I am including in this post where you can view how the Krebs Cycle works. When I find that link, Iíll edit the post and add it in for clarification.


    Sorry so long....

    Koffee

  15. #15
    One day at a time chaztrip's Avatar
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    Koffee you must type 100 words a min... I might have to print this out and read it on my next flight

    Thanks for the Info

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    Whoops, forgot the pic of the Krebs Cycle:

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    Originally posted by Koffee Brown
    [Referring to burning protein and, possibly, muscle in the absence of carbs and fat in the diet]Ö you end up losing more muscle than fat over the long run, which is how you can end up with that saggy look when dieting and you restrict your calories (or your fat and carbohydrate intake) too much.
    Koffee, I have read your earlier post re; KREBS cycle, etc. Very interesting stuff although I must say I'm on my fourth reading of this stuff and probably need another 3-4 readings to fully understand it. Biology ain't my strong suit but I find it very fascinating. Thanks for the insights.

    Now, to your quoted statement above ... I suspect the "saggy" look you refer to is a possible outcome of the Atkins diet. Not to start a diet war here, but I wonder if you think I'm right in this leap of logic.

    Thanks, again.

  18. #18
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    Nope, you're wrong.

    I was talking about people who do the fasting diets, people who plain just don't eat enough fats and carbs in their diet, people who do too much cardio and no strength training, people who have poor nutrition habits, etc.

    Atkin's wasn't even on my mind when I did the post. I do have issues with Atkins, but that's a whole nuther thread...

    Koffee

  19. #19
    Jungle lady cbhungry's Avatar
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    Originally posted by Pat
    What I was saying is that under certain conditions the body prefers burning Carbos. And neural tissues can ONLY burn carbos and not fat (which is where you might have gotten the mistaken notion that I said we can't burn fat - we have lots of non neural tissues which can burn fat). And the body can break up proteins to make carbos if it has to. That is why many people lose muscle mass whilst dieting. Their body is burning up their muscle mass.b


    Originally posted by Koffee Brown

    I am unsure as to what neural tissues are, as Iíve never heard of those terms before, and a quick scan of my biochemistry books make no mention of neural tissue. ďNeuralĒ refers to the nervous system, and the nervous system is NOT involved directly with fat, carbohydrate and protein catabolism[/i]
    I think neural tissues refers to the central nervous system or brain. Although glucose is the primary energy substrate of the brain, it's absence (hypoglycemia or lack of insulin which prevents gucose from entering the brain cells ) does cause the brain to resort to the short chain metabolites of the free fatty acids, acetoacetic and B-hydroxybutyric acids or ketone bodies. Here, the brain efficiently protects itself from damage by resorting to utilising by- products of fatty acids nder conditions of hypoglycemia.

    And Pat is right about using protein to make carbs. The upshot of the Kreb's cycle is that the precursors for hepatic glucose synthesis (important source of the fuel burned during most athletic activities) are lactate/pyruvate and amino acids, primarily the amino acid ,alanine, derived from muscle and the glycerol comes from fat. Amino acids constitute the primary substrate for gluconeogenesis! Most of the lactate is recycled from preformed glucose (Cori cycle), the only net contribution coming from the breakdown of muscle glycogen. I probably oversimplified things a bit, so forgive me Koffee.
    Last edited by cbhungry; 09-28-03 at 04:27 PM.
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  20. #20
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    Originally posted by cbhungry
    I think neural tissues refers to the central nervous system or brain. Although glucose is the primary energy substrate of the brain, it's absence (hypoglycemia or lack of insulin which prevents gucose from entering the brain cells ) does cause the brain to resort to the short chain metabolites of the free fatty acids, acetoacetic and B-hydroxybutyric acids or ketone bodies. Here, the brain efficiently protects itself from damage by resorting to utilising by- products of fatty acids nder conditions of hypoglycemia.


    Ok, I can give him that- it wasn't obvious to me he was talking about the brain, since the discussion had to do with muscle and exercise. At the same time, CB, I know you are 100% correct, and you are the queen...

    And Pat is right about using protein to make carbs. The upshot of the Kreb's cycle is that the precursors for hepatic glucose synthesis (important source of the fuel burned during most athletic activities) are lactate/pyruvate and amino acids, primarily the amino acid ,alanine, derived from muscle and the glycerol comes from fat. Amino acids constitute the primary substrate for gluconeogenesis! Most of the lactate is recycled from preformed glucose (Cori cycle), the only net contribution coming from the breakdown of muscle glycogen. I probably oversimplified things a bit, so forgive me Koffee.
    Dang, also totally true- I hadn't even thought about that part of Kreb's... well, just a few secs, but overlooked it in the end. If there's anything I said out of turn in my explanation or that needs correction, feel free to expand on.... thanks, CB!

    Koff

  21. #21
    Pat
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    Well, I threw out the neural tissues thing and I guess that was not well advised. It just confused things.

    The central nervous system can not burn fat. It requires carbohydrate. So in a starvation diet, one will completely deplete one's carbohydrates. Of course, going brain dead is not an option. So where does the body get the carbohyrdates? It rips up muscles!

    Now every now and then, you hear of someone dying from a rather extreme diet. I believe these cases are caused by heart failure. The body rips up muscles and eventually gets to the heart....

    Now, I can give you my opinions as to when fat gets burned and when muscles get ripped up and when carbohydrates get burned.

    One can experimentally determine the balance of carbohydrate vs fat burning in a person. But it is not a particularly convenient thing to do and I can not think of anyone who has done it on a large number of subjects under various levels of exercise.

    In the typical weight loss diet many people restrict carbohydrates and they do not exercise. Losing weight by dieting is REALLY tough because many people's bodies respond by lowering the metabolic rate! Also, since carbohydrates are very low, the body often needs more and it rips up muscles. So a dieter often loses as much muscle by weight as fat.

    If one exercises and diets modestly, the results are different. The body can not scale back metabolic rate because the person is active. The body has to give a priority to muscles because it uses them. Of course, this only works if the person is careful to keep carbohydrate consumption adequate to allow exercise without the body needing to rip up muscles.

    It is my impression, that totally unfit people burn very little fat when they exercise. That is because virtually any exercise is intense if you are out of shape. I believe that as one gets closer to your maximum effort, you burn proportionally more carbohydrate and less fat. I have ridden numerous centuries and have discovered that if I ride really really hard I will deplete my glycogen about 50-80 miles into the ride, called the bonk. If I slow down a bit, I can do the century without glycogen depletion. However, I have never ridden really far without bonking say 150 miles+ but come to think of it every time I went for really long distances I was WIRED and rode fast initially.

    Locally, we have some riders who ride FLAT out for say 00 miles out of 40. I have found that if I do those rides, that my appetite is incredible when I get home and I really want something anything sweet - carbohydrate craving. I suspect that this is the result of the large expenditures of carbohydrate at high intensities. I have found that even on longer rides of say 70 miles, if I do not ride at near anaerobic threshold, I don't try the huge craving for sweets when I get home. So I take this as an indication that I burn relatively more fat if I back off a bit then if I really hammer.

    Hammering is good too because it increases fitness. And with an increased fitness, I can cruise at a higher speed in a "fat burning zone".

    Of course, this stuff about the fast rides, has to be taken with a grain of salt. It is merely my interpretation of my subjective experience and it is not clinically verified.

    I do know that individuals do have their own responses to diet and exercise. A friend of mine who cycles developed diabetes. He has been able to manage the situation with diet, exercise and some drugs. Interestingly enough, he had to learn exactly what foods spiked his blood sugar levels. This apparantly varies with different people. Different people do respond differently to what they eat.

    Also different people have different responses to exercise. I am a terrible sprinter. I know lots of people who can outsprint me. I suspect that I don't have a fast twitch muscle fiber in my body. But I do have a whole bunch of aerobic power. I can wear down a sprinter by forcing them to sprint repeatedly without letting them recover. Now this is just an example.

    But as you can see we each respond to what we eat in our various ways and we each have our own atheletic strengths and weaknesses. So we will not respond to training in the same ways.

    And then, we all have our different mental quirks. Some people, for example, are extremely competitive and they live to drop other people.

    The basic biochemistry is the same. But we do have enough individual differences to make our personal experiences a fair bit different. The trick is to know enough about the science to use it in a beneficial way to aid your own goals. Also learning your own quirks is a good thing too. What works for your buddy, might work for you, but then again it might not.

  22. #22
    Senior Member miamijim's Avatar
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    The other side of fat-burning not often discussed but actually more important, "Why do certain individual's "lipid profiles" change when in calorie deficit while others stay the same??
    Genetic pre-disposition??

    Two hours before I had my cholesterol checked I ate two grilled cheese sandwiches. My bad cholesterol was normal and my good was low. My primary care provider just rolled her eyes and shook her head.

    Today in surgery we had a patient with a pre-disposition for high lipids. This particular patient had nothing to eat for at least 12 hours prior to surgery. Upon closing the chest you could see the lipids glistening in the pooling blood. Very impressive and very educational for my perfusion students. If I ever get a pic of this I'll post it.
    WWW.CYCLESPEUGEOT.COM 2005 Pinarello Dogma; 1991 Paramount PDG 70 Mtb; 1976? AD Vent Noir; 1989 LeMond Maillot Juane F&F; 1993? Basso GAP F&F; 1989 Terry Symmetry; 2003 Trek 4700 Mtb; 1983 Vitus 979

  23. #23
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    Whoo! Bring on da pics, dood!



    Koffee

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    Protein

    Koffee, you have some outstanding knowledge to share to the public in tearms we understand. For that I am thankful.
    I have a couple of questions I would like to ask you if you would be kind enough to help me out. Would you mind if I emailed you a PM?? can you email me at greg.cato@gmail.com

    Quote Originally Posted by Guest View Post
    All right- actually fat is a lipid. Specifically, fat is a triglyceride, composed of three fatty acids and a glycerol molecule. Triglycerides play an important role in metabolism as energy sources. They contain twice as much energy (8000 kcal /Calorie /kg) as carbohydrates. In the intestines, triglycerides are split into glycerol and fatty acids and then move into blood vessels. In fat catabolism , triglycerides are hydrolyzed (uses water to split the triglyceride bond) into fatty acids and glycerol . The glycerol is then converted into glyceraldehyde-3-phosphate, and enters glycolysis (process of breaking down glucose to ATP thatís used for energy) and eventually the ctric acid cycle. Fatty acids are broken down through a process known as beta oxidation which results in acetyl-CoA to be used in the citric acid cycle. (See step 1 of the Krebs Cycle/Citric Acid Cycle)


    Expanding on this, under most ideal conditions everyone burns a combination of fats and carbohydrates. Whether you burn your calories from fat or carbohydrates depends in part on how high your anaerobic threshold is, at what point your body uses carbohydrates predominantly for fuel from exercising, and if oxygen is present during exercise (or rather, if youíve gone into oxygen debt because the demands you place on the body from exercise exceeds the amount of oxygen you intake). You will always burn some fats and some carbohydrates, even when youíre just sitting still- the body needs energy to perform the tasks youíre doing, after all!

    During exercise, there are two states of exercise- aerobic and anaerobic exercise. Aerobic exercise is exercise done in the presence of oxygen- that is, exercise done where the demands do not exceed the amount of oxygen you intake. During this time, fat is the primary fuel utilized as energy. Fat is only burned in the presence of oxygen. Do not become confused here by this next statement- you will burn a higher percentage of fat in the aerobic state, but the higher your exercise exertion level, the more overall calories you burn, and the lower the percentage of fat that will be used to burn those calories. At higher intensities, the body will go into oxygen debt, resulting in the heavier breathing, out of breath feeling you have during intense exercise. (Drop me a PM if this throws you off). When the body goes into the anaerobic phase, not enough oxygen is present to sufficiently burn fat, so the body converts to anaerobic pathways instead- the process where carbohydrates are used instead as its energy source. In this case, the carbohydrates are broken down into its simplest components- glucose. This sugar (glucose) then is metabolized into pyruvate. When oxygen is not present, the pyruvate is then broken down into lactic acid (sounds familiar? Lactic acid buildup in the muscles???) and CO2, plus 2 measley ATP molecules. Had oxygen been present, that CO2 would have been converted to the Acetyl-CoA, and entered the Krebs Cycle at step 1. (See step 1 of that diagram). When oxygen is not present, however, the lactic acid builds up and the amount of ATP produced is insufficient to continue the higher intensities at longer periods. So the body slows down, and as the breathing slows, oxygen returns into the body, which leads to the Krebs cycle being re-activated, and fats (and some carbohydrates) are again used to create more ATP so you have the energy to continue working out. Add that to the lactic acid thatís already in the muscles- it is also converted back to pyruvate with the addition of oxygen, and that pyruvate can be used in the Krebs cycle.

    My hypothesis at this point is that it takes less energy to use carbohydrates rather than fats is simple- it takes many more steps to break the bonds of the fat molecule to the fatty acid and glycerol, then bind the fatty acid to make the acetyl coA than to simply use a water moleule to break down carbs to pyruvate, then pyruvate to carbon dioxide, forming acetyl CoA for the krebs cycle. There are also more carbons and hydrogens in a fatty acid than in glucose, so it takes a shorter amount of time to break down a carbohydrate than a lipid (fat). It is certainly less work during the anaerobic process when the body metabolizes carbs to pyruvate, which then converts to lactic acid and carbon dioxide.



    I am unsure as to what neural tissues are, as Iíve never heard of those terms before, and a quick scan of my biochemistry books make no mention of neural tissue. ďNeuralĒ refers to the nervous system, and the nervous system is NOT involved directly with fat, carbohydrate and protein catabolism.

    It is possible for the body to metabolize proteins as an energy source. Fat is an important energy source for the body- it is used to break down some vitamins, as well as to produce large amounts of ATP, maintain all of your cells- the wall structures and integrity, etc. But if youíre not intaking enough fat for producing ATP, your body can use protein instead- it is a last resort, but it can be done. In this case, the protein is catabolized to produce amino acids, and these amino acids can be used in the Krebs Cycle to produce the ATP needed to keep your energy levels up. Itís more than just the body using your muscle to break down protein (which it does)- itís also the protein you intake daily- instead of the body using it to provide protein to build and maintain muscle, it is used instead to provide energy for the bodyÖ you end up losing more muscle than fat over the long run, which is how you can end up with that saggy look when dieting and you restrict your calories (or your fat and carbohydrate intake) too much.



    You got it! The human body loves fat- fat is used to protect the organs, provide heat, provide structure to the cells, dissolve the fat soluble vitamins we need for the body, provide energy needed for the body to exercise and perform daily tasks, etc. The body is efficient- itís all about self-preservation. So if you restrict your food intake or exercise to the point where you are losing too much fat, or you are not getting enough fat, then the body will look to other ways to provide energy- breakdown of carbohydrates and protein.

    So the answer to the question the first person had- what does the body burn first- fat or muscle? It burns fat first, BUT if you are not taking in enough fat and calories to maintain your activities, and the carbohydrates begin to run low, then the body will turn to protein (muscle and the protein you ingest in your food) to maintain itís activities. At what point that happens, I could not tell you- itís going to depend on the individual and what youíre needing to maintain your activities. If you really wanted to know, you could check into a performance lab or a local university with an exercise physiology or kinesiology department for tests, and they could perform some expensive tests on you, but youíd at least have your answerÖ right?

    If you have questions, PM me, I did a post that explains the Krebs Cycle a bit more, and it would be good to read the link, then re-read the post I did and see if it makes any sense, and also check out the picture diagram I am including in this post where you can view how the Krebs Cycle works. When I find that link, Iíll edit the post and add it in for clarification.


    Sorry so long....

    Koffee

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    Quote Originally Posted by Cato82 View Post
    Koffee, you have some outstanding knowledge to share to the public in tearms we understand. For that I am thankful.
    I have a couple of questions I would like to ask you if you would be kind enough to help me out. Would you mind if I emailed you a PM?? can you email me at greg.cato@gmail.com
    this thread is 7 years old and it doesn't appear that Koffee is still a member here... good luck with that.

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