Against Glycogen Repletion...

[NU_nutrition_TS]

Have a look through this thread:
http://forum.myprotein.co.uk/diet-nu...carb-diet.html

[anab0lic]

Hey NU, could you explain without getting too technical how fats are used to fuel a weight workout or something like sprints/HIIT... What I have read seems to say that dietry fat cant be converted fast enough to ATP to fuel intense excercise....I understand intramuscular triglycerides can be though? but not much of this is stored??? I believe this is why 'experts' say that you need to restore glycogen (which is then used as a rapid fuel source during training) with a weekly carb up...

[NU_nutrition_TS]

In short, fats cannot be used to fuel high intensity exercise to any great degree. In fact, at very high intensities, the predominant energy pathway is anaerobic glycolysis which uses glucose exclusively.

However, there is always some input from fatty acid oxidation in most physical activity and the more that this can be optimised the more glucose/glycogen sparing it will be. The biggest input from fatty acid oxidation is during recovery when the speed of ATP synthesis is not important but fatty acids will yield a much higher net ATP synthesis than glucose ready for future exertion.

In the case of fatty acid oxidation, which exclusively uses the oxidative pathway, it is not the amount of available fatty acids that is the limiting factor but merely the turnover to produce ATP (as you say, it is much slower than anaerobic glycolysis of glucose) as well as insufficient oxygen availability. There are triglycerides stored within the muscle (intramuscular triglycerides or IMTG) just as there are glycogen stores but they are usually much smaller. Again, a higher fat, lower carb diet will tend to optimise the IMTG storage capacity of muscle.

If you are not training at high intensities any more frequently than every other day then muscle glycogen can be maintained even on a lower carb diet without carb loading. Glucose can be synthesised endogenously (gluconeogenesis) and this process is fuelled primarily by fatty acids, which is why you can shed body fat during rest and recovery from high intensity exercise if you limit the carb load at this time. Just ensure sufficient protein is consumed to provide raw materials for muscle protein synthesis and to provide substrates for gluconeogenesis. Immediately after high intensity exercise there is plenty of lactate floating around which will be shuttled to the liver where it also can be converted to new glucose.

[anab0lic]

Thanks, making alot more sense now!

You seem to really know your stuff... have you ever thought about putting all your knowledge in this area into a book? I'd certainly buy it.

[NU_nutrition_TS]

Thanks for the compliment but there are already books out there by authors who not only know their stuff but helped me to know mine!

[James]

You should make a thread:- Nu's recommended reading list

[NU_nutrition_TS]

I tend to stick to basic biology/physiology textbooks - rather than read any of the 'fad' routine books. An exception is Doug McGuff and John Little's book Body By Science - because it is based on solid biology/physiology/biochemistry. But, as with anything, you shouldn't take what you may read as 'gospel' without cross-referencing and trying things out for yourself in the real world.

[NU_nutrition_TS]

As there has been a lot of discussion in other threads about muscle glycogen (with attendant water) and how that impacts on weight loss on various diets with specific reference to low carb diets, I have decided to post this here.

Here's the deal. The percentage of body mass that is skeletal muscle, in average males, is 42% and somewhat less in average females. Studies have shown that, even with supra-physiological levels of glucose and insulin administration, the most glycogen that can be stored in muscle is 4g per 100g of muscle wet weight. The usual amount varies between 1 and 3g per 100g of muscle w/w. The upper limit being attained by elite athletes. The amount of water that is stored with glycogen has been quoted as anywhere between 2.4 and 4g per gram of glycogen.

So, in an average male weighing 75kg, he will have around 32kg of skeletal muscle storing anywhere between 320g and 960g of total muscle glycogen holding between 768g and, at most, 3.84kg of water.

Even seasoned strength athletes only use around a quarter of their muscle glycogen stores during a training session with sets taken to failure. This is only from the worked muscles as muscle glycogen cannot migrate out of the muscle. So, if someone uses a split routine, where different muscle groups on different days, they will not be depleting a quarter of their total muscle glycogen but merely a quarter of the glycogen stored in the trained muscle(s). Even if a whole-body routine is employed, the most glycogen that will be used, in our average male, would be between 80g and 240g. The water associated with this loss of glycogen would be between 192g and 960g.

Changing to a low carb diet is more likely to deplete liver glycogen before (and at a faster rate) than muscle glycogen if the 'dieters' are not engaged in regular exercise (whether short duration, high intensity or long duration, low intensity/endurance) outside of normal daily activities. At least in the short term. This is because, until the metabolic 'machinery' required to optimally use fatty acids and ketones is up-regulated, the body is still going to be dependant on a greater input from glucose and glycogenolysis of liver glycogen is the primary endogenous source when exogenous sources are restricted.

Once adaptation is complete (typically 2 weeks) glucose is spared by increased use of fatty acids and ketones and the gluconeogenic pathway of endogenous glucose production will be increased enough to supply the lesser demand for glucose, including the replenishment of glycogen stores.

So most of the water loss associated with dieting will occur within the first two weeks and will be primarily from the losses of glycogen. However, even in fasting, the glycogen is not lost at a greater rate than it can be produced by gluconeogenesis. In a study (quoted elsewhere) subjects were fasted and the endogenous production of glucose was measured by subtracting the rate of glycogenolysis from the rate of glucose appearance in serum. As glucose can only come from three sources - exogenous intake, breakdown of liver glycogen or endogenous production - and the first of those was eliminated by the fast, it was found that (after 18 hours of fasting) 96% of the glucose was coming from gluconeogenesis and only 4% from the breakdown of liver glycogen.

After full adaptation to a low carb diet, there is no reason to suppose that gluconeogenesis cannot provide enough glucose to keep glycogen levels up (even if at a slightly reduced absolute level). So any intital water losses associated with this initial drop in glycogen can be made back up.

So even if you were to fully deplete all available glycogen in the body - which is highly improbable - the most water you could lose is 3.84kg and most of that would be made back up in the subsequent weeks after embarking on the diet. Again, for our average dieter, the amount of water loss due to total glycogen depletion (unlikely as that is) would more likely be 2.3kg. As we don't deplete all of the glycogen in our bodies (until we are dead!) it will be less than this. The reason why some studies (where the constituents of weight loss are actually quantified) shows significantly more water loss is that all tissue lost will comprise some water, not just the glycogen. Lean muscle tissue has 75% water, fat tissue is 10% and even bone is 22% water.

[elmo322]

so your saying that glycogen levels never get fully depleted in the muscles even between transition?

[ATZ]

To play devils advocate as per usual here:

Quote

Alas, we come to a critical point of contention. One of the biggest misconceptions is that bodybuilding training is not glycogen-depleting. Okay, sounds convenient, but let's look at the research. Roy & Tarnopolsky studied the effects of resistance training on healthy young men and observed an average muscle glycogen decrease of 36% [J Appl Physiol. 1998 Mar;84(3):890-6]. The striking aspects here are this was after 9 total sets of 10 reps, and taking sets to failure was not specifically prescribed. Perhaps most importantly, the subjects were NOT fasted leading into the trial. In fact, they each consumed 3 mixed meals approximately 3 hrs apart leading into the trial, which was 3 hours after the last meal. The subjects' daily intake was just over 3000 kcals, so they were far from starving.

Taking the above data into account, the following scenarios stand a good chance of significantly depleting glycogen: Many bodybuilders do much more volume per muscle than what was done in that trial (9 sets total for quads), and in some cases follow up the training bout with cardio. Many bodybuilders train first thing in the morning, and neglect pre, during, and/or post-training carb intake for a variety of unfounded reasons. Many bodybuilders will group up bodyparts in such a way that pushing muscles & pulling muscles are trained together on separate days, increasing the overlap of muscular work especially in the bi's & tri's. Often times lower-extremity cardio (cycling, walking, jogging, stair stepping, etc, etc) will be done in close proximity if not back to back with leg training, and overlap occurs there as well. The magnitude of overlap only increases in pre-contest training where cardio volume and general rep volume tends to increase, all this during a reduction of carb intake.

Now, one can always play devil's advocate and say that their plan has absolutely no overlap of muscles within any given 24-48hr period, and thus glycogen can be replenished by the time the next training bout hits. Well, think of it this way - those who replenish muscle (& liver) glycogen to capacity quicker will spend greater durations with fully hydrated/swollen cells. Thus, they will remain anabolic for longer periods before the next training bout than those who barely approach local glycogen replenishment by the time the next workout arrives. Just some food for thought to chew on

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