CO2 blood levels

[left coaster]

This may be too esoteric for the board, but thought I would ask if anyone has experimented with this or knows of any relevant research.

It is my understanding that CO2 in the blood during exercise is a byproduct muscle metabolism. I also understand that the more CO2 the more pressure on the lungs to remove it from the body, the more acidic the blood gets over time etc.

I've read that the lowest CO2 production during exercise occurs when we are using fat for energy, rather than carbohydrates. I realize there may be a confound in terms of immediately available glucose for muscle function, but here are my questions.

If someone were adjusted to a ketogenic diet, would they tend towards slower respiration during exercise due to lower CO2 levels in the blood?

Following that, do we reach a stage with endurance exercise where we are actually 'fat burning' like the media tends to reference? If so, is there a transition point in endurance exercise where we begin to produce less CO2 and our respiration becomes more efficient? Would this be measurable, even if not subjectively noticeable?

This leads me to other questions, but they're dependent on a positive response to the ones above...

[hjs]

It has to do with the muscle fibers, the slow ones can work on fat or on lactate and te fast ones only produce lactate. With the hybrid ones who can be developed as both.
The reason long aerobic work is done below 2 mmol lactate is to only get the aerobic system training and not involve the anaerobic.
I do believe getting in ketoses shifts the energyuse to fat burning, but if you get above the point your slow muscle fiber can get the job done, you have to use the aerobic one and start using carbs. During ketoses the top end is limited due to the lack of glycogen there is to use.

[ergo sum]

If so, is there a transition point in endurance exercise where we begin to produce less CO2 and our respiration becomes more efficient?

At rest there is a lowering of CO2 produced:O2 consumed when eating a low carb diet (besides fat, protein also requires less CO2 to burn) and the source of what you are burning can be measured reasonably, with a mask on your breath, looking at their concentrations. But as you exercise and need your aerobic energy (see Henry's comment above), excess CO2 will be produced and if it became too much for diffusion to handle in the lungs via blood-gas exchange, the body being a dynamic system picks up the slack with the kidneys to remove excess CO2 (and lactate), another good reason to drink lots of water when you exercise which prevents the blood pH from accumulating CO2 causing pH too low that would make you sick. It's part of a dynamic system though, so what happens at rest on a ketogenic diet isn't the same problem as what happens during exercising. Small changes like those at rest from diet are easily compensated for with blood pH and the lungs and can be measured. When exercising the emphasis seems to be on conditioning an individual's lactate threshold?

[brianh]

do we reach a stage with endurance exercise where we are actually 'fat burning' like the media tends to reference? If so, is there a transition point in endurance exercise where we begin to produce less CO2 and our respiration becomes more efficient?

Yes...ish, but not from diet, and this might be mixing up cause and effect. Your fitness relative to the intensity of your activity is what drives your CO2 production and the efficiency of your respiration.

Training at the lactate threshold (aka anaerobic threshold) in particular (time trial pace anywhere between 20 minutes and an hour) should yield the most benefits for this. This is expected to train the lactate system, which scrubs lactic acid from the system and actually uses it as an additional fuel source. It's also expected to increase mitochondrial enzyme levels; this directly helps the muscles use fat instead of oxygen & carbs. That said, there are training benefits to every intensity level, and you'll get the best results if you train at all intensities.

The "intensity" that gets labelled fat burning is also referred to by a lot of sources as "active recovery" or "rest." It is SLOW, and it's most useful as a rest between higher intensity intervals or if you're taking a rest day but still want to move to work the kinds out. It's called fat burning because your muscles are being fueled directly by lipids in your blood as opposed to carbs & oxygen--it is NOT called fat burning because it's the ideal intensity for losing weight. It will take a long, long time to burn through any appreciable amount of body fat, and it offers basically no other fitness benefits. You're far better off working out at a higher intensity; even if you are not engaging the fat burning system much while doing so, afterwards your body will be burning through extra fat to restore muscle glycogen.

Going back to the start...If you had a 30 minute best of 7500m (2:00/500m, 202.5 watts), you might be able to rely solely on fat as your fuel source up to about 110 watts (2:27/500). At that level of fitness, doing 165 watts (2:08.5/500) would probably be mid-to-low aerobic; you might be able to do a half marathon at that pace, but it wouldn't be easy. If you built your fitness up to be able to do a 1:45/500 split for 30 minutes (302.3 watts), your fat burn pace might go up to 165 watts, and you'd be doing it with pretty the same relative effort and easy breathing of the original 110, while burning 50% more energy and therefore 50% more fat per unit of time.

So where does a ketogenic diet fit into this? In my non-medical, completely unqualified opinion, nowhere. I'd imagine that without carbs in your diet, you wouldn't be able to do as much high quality work at and above lactate threshold, which would limit your rate of fitness improvement and therefore reduce the amount of fat burn you could get out of workouts at any intensity.

[left coaster]

Very informative replies, thanks!

I'm doing parallel research (doctoral student, late in life -- I know!) on how the brain utilizes energy sources (glucose, lactate, ketone) during endurance exercise. My background is in psychology though, not physiology, and how the muscular system adapts is new information for me.

Brain, your comments about training the lactate system make a lot of sense, as do the parts about burn rate of fat/calories, thanks.

I have no interest in a ketogenic diet, but I remain very interested in what happens to our body and brain when we run out of glycogen/carbs during prolonged exercise or during periods of food shortage. There are times when I'm carb depleted and seem to do OK and others when I'm depleted and just start to sweat and hate life almost immediately when I exercise. The times when I'm carb depleted and manage to exercise hard I tend to feel very spaced out and 'decorticated', something referred to in the literature as 'hypofrontality'. I don't think the explanations currently available for hypofrontality during exercise are adequate though. All very interesting!

[Shawn Baker]

General topic of interest for me as well- Volek/Phinney have done a fair bit of research with Keto-adapted athletes looking at respiratory quotients and other metrics that seem to be favorable in especially ultra endurance athletes.

My personal experience as an "aging" athlete that has to consider longevity and health as part of the equation is that low carb/Keto stuff is helpful for weight control and probably confers a great deal of protection against chronic disease- I find hi intensity workouts generally require some degree of glycogen repletion to maximize performance- so when I do a particularly hard lifting session or rowing sprint time trial I often will add some carbs back in (still experimenting as to when and how much)

[Edward4492]

I have zero science to back this up. But having done numerous 15k's on the erg, HM's and one horrid FM. And several dozen brutally hard RR's on the bike (60-80 mile affairs).There have been many times when I had what I can only describe as the "Low Carb Buzz" when energy levels started to plummet and I would get a generally fuzzy feeling. Pretty vague, I know. But I also learned that if I could just survive and get through it, some sort of second wind would kick in. And I always assumed that this was the switch over point when the carbs were gone and we were switching to fat burning metabolism. And if I made it that far, I was generally able to finish in relative comfort. Nothing specific here....but I've had these sensations on many occasions.

[brianh]

I remain very interested in what happens to our body and brain when we run out of glycogen/carbs during prolonged exercise or during periods of food shortage. There are times when I'm carb depleted and seem to do OK and others when I'm depleted and just start to sweat and hate life almost immediately when I exercise.

This might be a useful, then. Glycogen is stored in the muscles and in the liver. Muscles can use glycogen in either location, and use muscle-stored glycogen first, but only glycogen in the liver can be made accessible to other organs such as the brain. If you start a workout with muscle glycogen low but liver glycogen ok, you'll be ok for a while (and then hit a wall once you deplete what's in your liver). Start with both depleted, and it's going to be awful.

In terms of training in general, working out in tempo/UT1 should have the most effect for convincing the body to increase muscle glycogen storage. As usual with this stuff, that's because that's the intensity that relies most heavily on muscle glycogen. Couple that with lactate threshold training so that the intensity at which you can rely on fat burning, and with proper carb intake while exercising, and you can stave off depletion for a good amount of time.

If you're going into a single workout and feel that second form of depletion, your best bets are probably to drop the intensity to practically nothing and just stick to fat burn to still get your body moving, or just call a rest day.

[left coaster]

I remain very interested in what happens to our body and brain when we run out of glycogen/carbs during prolonged exercise or during periods of food shortage. There are times when I'm carb depleted and seem to do OK and others when I'm depleted and just start to sweat and hate life almost immediately when I exercise.

This might be a useful, then. Glycogen is stored in the muscles and in the liver. Muscles can use glycogen in either location, and use muscle-stored glycogen first, but only glycogen in the liver can be made accessible to other organs such as the brain. If you start a workout with muscle glycogen low but liver glycogen ok, you'll be ok for a while (and then hit a wall once you deplete what's in your liver). Start with both depleted, and it's going to be awful.

In terms of training in general, working out in tempo/UT1 should have the most effect for convincing the body to increase muscle glycogen storage. As usual with this stuff, that's because that's the intensity that relies most heavily on muscle glycogen. Couple that with lactate threshold training so that the intensity at which you can rely on fat burning, and with proper carb intake while exercising, and you can stave off depletion for a good amount of time.

If you're going into a single workout and feel that second form of depletion, your best bets are probably to drop the intensity to practically nothing and just stick to fat burn to still get your body moving, or just call a rest day.

Thanks again for all the input.

You all may find it interesting that lactate flows across the blood brain barrier in an uncontrolled manner that is concentration gradient dependent. When we're sitting still and using our brain, lactate flows out, when we're exercising hard, lactate flows in. Like the muscles, newer discoveries show that lactate is also used as a fuel in the brain but the verdict seems to still be out in terms of exactly how it is used and if it is preferred to glucose.

Another interesting tidbit is that at about 40% aerobic threshold cognition is facilitated across all domains, reaction time, attention, working memory, executive function, with the only exception looking to be inhibitory control. However, once you cross above about 65% AT, higher order executive type functions start to show impairment while reaction time and sensory motor skills remain facilitated. This makes a lot of sense from an evolutionary perspective i.e. when it's time to fight or flight, thinking about the future is a waste of resources -- it's all about surviving now. It also dispels the idea that testing cognitive function during exercise places excessive demands on the attention system, otherwise the divided attention remains would also emerge at lower (40%) aerobic threshold levels.

I'm coming to believe that the higher order cognitive impairment at higher intensities has to do with the amount of lactate entering the brain and that it might be competing with glucose initially and later with ketones as our body becomes glycogen depleted. Still in development though...

[Galeere]

I have zero science to back this up. But having done numerous 15k's on the erg, HM's and one horrid FM. And several dozen brutally hard RR's on the bike (60-80 mile affairs).There have been many times when I had what I can only describe as the "Low Carb Buzz" when energy levels started to plummet and I would get a generally fuzzy feeling. Pretty vague, I know. But I also learned that if I could just survive and get through it, some sort of second wind would kick in. And I always assumed that this was the switch over point when the carbs were gone and we were switching to fat burning metabolism. And if I made it that far, I was generally able to finish in relative comfort. Nothing specific here....but I've had these sensations on many occasions.

IMO you want to be careful on experementing on fuel for the muscles. When glycogen is gone at high intensities the fat burning metabolism might takt over all right but you will not be able to perform on the same level. And IMO it´s easier to focus on the fat burning metabolism by going slower @ 60-70% of MHRR. Put in other words: I am not going to diet while on training. Quite on the contrary I am eating more than before with a focus on long-chain carbohydrate rich foods (as opposed to industry sugars).

[bonefixer]

Personally I think most of this is over-analysis.

Firstly, don't worry about CO2. It's not possible to produce too much CO2 for your lungs to blow off. You are not going to get acidotic from CO2 excess if you're otherwise healthy.

Secondly, all this stuff about 'UT2 burns fat' etc may well be right, but afterwards everything equilibrates such that within 24 hours (or less, probably) muscle and liver glycogen and blood sugars will be back to baseline levels, and any excess calories burned during exercise over those consumed in food will be lost as fat.

Am I wrong?

[Galeere]

Personally I think most of this is over-analysis.

Firstly, don't worry about CO2. It's not possible to produce too much CO2 for your lungs to blow off. You are not going to get acidotic from CO2 excess if you're otherwise healthy.

Secondly, all this stuff about 'UT2 burns fat' etc may well be right, but afterwards everything equilibrates such that within 24 hours (or less, probably) muscle and liver glycogen and blood sugars will be back to baseline levels, and any excess calories burned during exercise over those consumed in food will be lost as fat.

Am I wrong?

Probably not by much. Either one gears up his/hers matabolism by erging or by any other serious sport endeveaours, loses surplus weight in the process and stays with it or stops and has to buy bigger trousers again.

[brianh]

Secondly, all this stuff about 'UT2 burns fat' etc may well be right, but afterwards everything equilibrates such that within 24 hours (or less, probably) muscle and liver glycogen and blood sugars will be back to baseline levels, and any excess calories burned during exercise over those consumed in food will be lost as fat.

Am I wrong?

Nope! That's a nice succinct way of expressing what I was trying to get at and may not have ever actually stated: just work out harder, and even though your direct fuel source isn't fat, the body will burn fat to replenish what it did use at the time. And you finish a lot sooner. And you get more health/fitness benefits. And it's not so amazingly, incredibly dull. I seriously hate going that slow.

Another interesting tidbit is that at about 40% aerobic threshold cognition is facilitated across all domains, reaction time, attention, working memory, executive function, with the only exception looking to be inhibitory control. However, once you cross above about 65% AT, higher order executive type functions start to show impairment while reaction time and sensory motor skills remain facilitated.

Interesting, makes sense, and seems about right. At 40% I'm bored out of my mind and desperately seeking some sort of entertainment. When I'm working at AT and crossing into the realm of pain, I can thoroughly distract/amuse myself by trying to mentally calculate how much time I've got left in my workout, which is suddenly amazingly difficult to figure out. By the time I can finally convince myself my answer is correct, the answer is obsolete, so I try again, and I'm done in no time.

[hjs]

Personally I think most of this is over-analysis.

Firstly, don't worry about CO2. It's not possible to produce too much CO2 for your lungs to blow off. You are not going to get acidotic from CO2 excess if you're otherwise healthy.

Secondly, all this stuff about 'UT2 burns fat' etc may well be right, but afterwards everything equilibrates such that within 24 hours (or less, probably) muscle and liver glycogen and blood sugars will be back to baseline levels, and any excess calories burned during exercise over those consumed in food will be lost as fat.

Am I wrong?

Roughly do, but there are certainly points to keep in mind. Our body prefers fat as full, apart from our brain, that prefers glucose.
When we train, we use muscle glycogen, during and right after our muscle absorb incoming carbs the most easy, we are very insuline sensitive. So to recover the fastest and get our muscle filled with glycogen again this is the best time to eat carbs, and not slow carbs, but the fast ones that are fast available. A combi with the right protein is best. Again easy absorpable preferably. Fats are now less important.
For the times of the day we are in relative rest, our body needs more fats again, but now with slow carbs and protein. If we keep on eating fast carbs we would get insuline spikes time after time. If our liver and muscle are filled our insuline sensitivety will blunten.
People who do sports not often think they can get away with poor eating, ergers are no exception, most ergers I come across are overweight. Say we erg 50k a week, say 2.00 pace to keep it simple. Thats 3 hours and a bit. This will roughly burn 3000 3500 kcal. 400/500 cal a day. 100 gram carbs, or 50 fat. Not very much...

[left coaster]

Some interesting new research also indicates that the brain 'supercompensates' for glycogen depletion following endurance exercise. In animal models, the level of compensation is related to how severely glycogen was depleted and a 85-90% muscle glycogen depletion looks to be associated with about a 60% depletion in the brain (astrocytes store glycogen and are thought to convert it to lactate for neurons to use as fuel). This 60% brain glycogen depletion in turn leads to about a 40% increase in glycogen uptake compared to when the animal was sedentary. in a regular exercise situation, i.e. most days, basal levels of glycogen look to up regulate long term.

some think this could partially account for the cognitive benefits associated with exercise. so, when sitting still, your brain has more fuel and can stay active longer. This increased metabolic activity could in turn give neurons more fuel to fire hard, and frequently, in a manner that induces synaptic and dendrite plasticity aka, new learning and skill acquisition.