[quote]Bill Roberts wrote:
It isn’t fructose the muscles are using in your example, it’s glucose. So there was nothing wrong with my statement when read as intended. There was no intention of an implication that the muscles can’t use what was once ever fructose someplace else in the body but is glucose now when reaching the muscle.[/quote]
Ok, so what I’ve been wondering was…once fructose is converted to glucose the body should use it to replenish muscle glycogen stores. Sure it won’t be as fast as straight complex carbs like dextrose which goes to the bloodstream right away, and sure that may give an edge to a PWO shake, but even so, why do people say something towards the line of “oh no that’s too much fructose, it won’t be used to replenish muscle glycogen, it will be turned to fat.” And isn’t that what your statement here implies as you said:
“Your suggestion that if liver glycogen is already fully restored then still-further glucose would still get converted to glycogen and simultaneously other glycogen would be hydrolyzed to yield glucose would be a nice mechanism fot the liver to effectively convert fructose to glucose supplied to the bloodstream on the fly, but at least so far as I was taught in biochemistry and anything I’ve ever read on it since, this is not what happens, but instead the liver uses the fructose to produce fat.”
But yet you also said “There was no intention of an implication that the muscles can’t use what was once ever fructose someplace else in the body but is glucose now when reaching the muscle.” It sounds like a contradiction but maybe i’m not understanding right, would you please clarify? thanks.
It sounds like that your point is that fructose to glucose conversion is not done “on the fly,” and by “on the fly” do you mean at an accelerated rate like how dextrose/glucose goes straight to the bloodstream? And sure it has to go through an extra step in the liver for conversion, but how much longer will it take anyway? Ultimately, it gets converted to glucose. And as far as that “golden window” of opportunity for post workout feeding, an hour is ideal yeah, but i’ve read other things that said the body is in that “golden window” state for a lot, lot longer. And what about the bodybuilders of the past who mainly would eat solid food post workout (like pasta, steak, etc) which has to go through longer digestion/processing as compared to the “on the fly” liquid glucose carbs such as dextrose. Yet those were what helped sculpt the Olympian pros. So in regards to fructose not being glucose-available “on the fly,” why does it have a bad rap as “not useable for the muscles - it will convert to fat.” Again, I’m still not clear why the energy from fructose would turn to fat unless there is no need for glucose i.e. excess, but the body will have a need because muscle glycogen is depleted, on top of that you have an accelerated metabolism due to exercise, so fructose should be acceptable PostWorkout and replenish muscle glycogen without worry of getting converted to fat it seems to me… Your thoughts?
I did a little bit more research, and I read that fructose gets converted to glucose in the liver. The glucose is then released to the bloodstream for use by the body. And any excess glucose gets converted to fat (no big surprise there).
So this seems to suggest that once in the bloodstream the glucose should be able to refill muscle glycogen…because the body has a need for it.[/quote]
You’re assuming that this is happening promptly enough on the fly to support the PWO usage, which is what was being discussed. If you were right then wouldn’t blood glucose rise and insulin rise just as much from fructose intake as from glucose intake?
But that is proven to be not remotely the case. Blood glucose does not rise with fructose intake anywhere near like what it does with glucose intake.[/quote]
Does it have to happen “on the fly” (per my definition/understanding that i stated above) VS. fructose having to be converted to glucose in the liver… well, after conversion, again, it seems we both agree that the body should still be able to use it.
It sounds you are bringing up an issue of blood glucose rise, and from what I understand this is determined by the GI of foods, right? You said that Blood glucose rise is not anywhere like what glucose would give, but what about the higher fructose foods, like carrots? Ofcourse we have to take out the fiber in the picture here, so “carrot juice” is what I’m thinking. This is a high GI food over a hundred+. And maybe you can explain the whole issue of GI in relation to this. How I picture GI, in layman’s terms, is simply like this: you can picture a bathtub, and the pressure/speed of the water running is the “GI,” water being blood glucose, and the size of the bathtub is in relation to the body’s needs for the glucose/energy, so when your exercising the bathtub is bigger and it could hold more water, and so high GI foods are OK in that instance, and if your body doesn’t need the energy, then the bathtub is smaller, and if you overflow this bathtub, well then there are problems… obviously, excess energy being converted to fat is one of those problems. Ok, my point is this: sure the food in question may be a lower GI/blood glucose response i.e. the faucet is running slower, but the point is…the faucet is still running and the bathtub will still get filled up. So, again, I still don’t get why people don’t see fructose as being able to be a food source that will replenish muscle glycogen since fructose gets converted to glucose and glucose to liver and muscle glycogen.
I also did a brief google search and found this:
There has been some controversy about which type of carbohydrate is best for post exercise glycogen replenishment. Some argue that simple sugars such as dextrose are best after exercise. Others say that drinks with glucose polymers are best. Still others say that there is no need to buy fancy sports drinks and that simply eating a meal high in carbohydrates such as pasta or rice is sufficient. Studies have shown no difference between different types of carbohydrates eaten post exercise and the rate of glycogen replenishment as long as sufficient quantities of carbohydrate are consumed (Burke 1997). Even when the post exercise meal contains other macronutrients such as proteins and fats, the rate of glycogen replenishment is not hindered, given there is sufficient carbohydrate in the meal as well. These studies tell us that the rate-limiting step in glycogen replenishment after exercise is not in digestion or the glycemic index of a given source of carbohydrate. Over a 24 hour period it is the total amount of carbohydrate consumed that is important.”
No, fat cannot [convert to glucose][/quote]
I’m not too sure myself, but if you google this, the defenition for gluconeogenesis is basically the body’s ability to convert protein or fat to glucose. Maybe you can check this and see.
And, you said fructose is not necessarily converted to glucose, then what is it converted to? From what I read it is converted to glucose and then the excess glucose is converted to fat when that glucose is not needed & not fructose converted directly to fat.[/quote]
Thanks for that link. But, since you read it, if you can please summarize it’s findings in relation to my question that I asked?
The fact that blood glucose levels do not rise rapidly nor is there much insulin rise from fructose, but there is with glucose, demonstrates that your on-the-fly-conversion they’re-equivalent interpretation cannot be right.[/quote]
That is not my interpretation, nor is it my original question. I was not saying that they get converted in equal rates, obviously not. One is direct to the bloodstream, and the other is going through conversion in the liver. But, once again, I was wondering why a lot of people in the bodybuilding circle believe stuff like “No - that’s fructose, it’s not going to be used as muscle glycogen, it will turn to fat.”
I would think the reason for more stability is because insulin production is not triggered by the fructose intake, but only by glucose the fairly-steady-rate conversion process in the liver, which is not going to yield the spikiness (scientific term there) of glucose absorption from the GI tract.
Now, I would not assume that the workrate is being supported entirely or even necessarly mostly by glucose provided via conversion of the fructose. Very likely triglycerides were also produced, perhaps predominantly, and the muscles can burn fatty acids from triglycerides, especially with regard to aerobic exercise such as this.
Burning fatty acids spares muscle glycogen. So without their having measured triglycerides, I wouldn’t assume that the conversion rate to glucose was at as high a caloric rate as the workload was.[/quote]
Are you assuming maybe that the test subjects were burning body fat? Or that the fructose was converted to fat and that was available to use as energy? Because if the latter then i would ask why the body would do that since glucose is the preferred fuel compared to fat. Also, 75% is a good enough intensity to start using carbs, and from what I understand, it’s unlikely to use fat stores as the first choice of energy source after consuming carbs since the body’s preferred source of fuel is carbs not fat unless in a fasted state cardio and/or at low enough intensity…
We might also consider the rate at which glucose is released. Perhaps a more steady rate of glucose being released into the body may be the factor in the case of this study.
Also, have you read the honey study? I’ve read it before, but did a quick search and only found a summary so far:
"Nine competitive cyclists received one of three supplements in gel form per week, over a three week period: honey, glucose or a flavored, calorie-free placebo. The endurance test conducted each week was a 64 km (40 miles) time trial on each subjectï¿½??s racing bicycle, fitted to a calibrated, computerized race simulator. The cyclists received 15 grams of carbohydrate in gel form along with 250 ml of water prior to and every 16 km during the time trials.
Both the glucose and honey produced a statistically significant reduction in the time to finish, and a significant increase in the athletes’ average power. The results of the study indicate that honey is an effective and affordable alternative carbohydrate source for endurance athletes."
Basically, this to me seems to suggest that honey (which is about significantly half fructose composition) was able to sustain the energy needed for the cyclist, an in fact helped them finish significantly faster, and since the best source for fueling intense activities is glucose, it looks to be that glucose is being the primary fuel here, and here we can see that steady-rate, lower GI, and it’s actually a benefit.
[quote]The abstract seems unclear on the respiratory exhange ratio. If nothing but carbs were burned in the body, the ratio would be 1.0. If nothing but fat, 0.7.
It states plainly that in the first 5 minutes, RER was lower in the fructose group than in the glucose group, meaning that they weren’t burning as much glucose as the glucose group (no surprise) and instead were burning more fatty acids.
However it doesn’t discuss differences between the glucose and fructose groups at other time points. It may be that a difference in the average was seen but the noise (random variation) was too much to be able to report a figure. [/quote]
I guess we can’t really say if they were burning fat exactly since we don’t know the ratio. But would not the ratio somewhat follow the intensity? Meaning in the case of the endurance cycling trials, wouldn’t that be higher intensity and thus requiring a higher ratio i.e. demand for carbs as fuel.
So, perhaps Fructose isn’t as bad as people make it out to be as far as for post-workout, not to mention pre-workout as those two studies seem to suggest.