Burning fat for endurance running

[Zelenius]

So let's say someone is a casual runner and they want to run longer distances or train for a marathon. With training, does their body start to adapt to burning fat better to help them fuel their long runs? And if so, how does this happen at the biochemical level? Please note, I am not interested in weight loss, I am a very slim individual.

To my understanding, glucose is the body's preferred fuel when we are active. It is the easiest fuel source to use and metabolize. While we are also burning fat much of the time(I believe we burn mostly fat when at rest or while walking slowly), the faster we run, the more the body will rely on stored carbohydrate, in the form of glycogen(stored glucose) for fuel. A sprinting person is using almost nothing but glycogen for fuel. When it comes to distance running, I think while the body shifts to burning more glucose at first, it shifts to burning more fat after about 20 to 30 minutes, right?

Unfortunately, the body can only store so much glycogen in the liver and in the muscles, enough to very roughly power a 20 mile run. When a runner uses up all his glycogen, he/she starts to feel fatigued(unless he is well-trained), and this phenomenon is called "hitting the wall". Because all his glycogen is used up, his body has to switch to burning fat for energy, and fat doesn't burn as efficiently or cleanly. This is my understanding of how it works, correct me if I am wrong.

Now does just running very long distances(20 + miles) on a regular basis force the body to adapt so it will burn fat more efficiently? Do well-trained endurance runners still "hit the wall" at 15 to 20 miles, or does regular training mostly or completely do away with it? I would really love to spare my glycogen by using more fat for long runs. Also, does consuming more fat and less carbs force the body to adapt faster?

I've been looking over the science, though I am not sure how to interpret some of it:

Effects of short-term fat adaptation on metabolism and performance of prolonged exercise.

Despite the brevity of the adaptation period, ingestion of a high-fat diet by endurance-trained athletes results in substantially higher rates of fat oxidation and concomitant muscle glycogen sparing during submaximal exercise compared with an isoenergetic high-CHO diet. Higher rates of fat oxidation during exercise persist even under conditions in which CHO availability is increased, either by having athletes consume a high-CHO meal before exercise and/or ingest glucose solutions during exercise. Yet, despite marked changes in the patterns of fuel utilization that favor fat oxidation, fat-adaptation/CHO restoration strategies do not provide clear benefits to the performance of prolonged endurance exercise.

Fat adaptation followed by carbohydrate restoration increases AMPK activity in skeletal muscle from trained humans.

Strategies to enhance fat utilisation during exercise.

A perspective on fat intake in athletes.

Although this is one of the worst places to ask such questions, this is the best of the worst places. The misinformation, extreme stupidity, and lack of skepticism on most fitness forums is appalling and potentially dangerous.

 

[MuDPhuD]

I would really love to spare my glycogen by using more fat for long runs. Also, does consuming more fat and less carbs force the body to adapt faster?

I've been looking over the science, though I am not sure how to interpret some of it:

Effects of short-term fat adaptation on metabolism and performance of prolonged exercise.



Fat adaptation followed by carbohydrate restoration increases AMPK activity in skeletal muscle from trained humans.

Strategies to enhance fat utilisation during exercise.

A perspective on fat intake in athletes.

Although this is one of the worst places to ask such questions, this is the best of the worst places. The misinformation, extreme stupidity, and lack of skepticism on most fitness forums is appalling and potentially dangerous.

Don't have time to check your links, but AFAIK you cannot "spare your glycogen". The body will use glycogen first as its preferred fuel. When glycogen stores are depleted from liver and muscle cells, ( typically after about 20 minutes of hard exercise like running), then the metabolism switches to burning fatty acids. Regular training will increase the amount of glycogen the body stores, therefore will increase the amount exercise before the switch over occurs.

Restricting carbohydrate intake will not prevent glycogen storage, as fatty acids can be converted to glucose. The body will store glycogen no matter what you want it to do. Glucose is essential to brain function.

I am not aware of controlled studies which confirm the supposition that the "runners wall" is actually coincident with the depletion of glycogen stores, but then again I haven't spent any time looking for them.

 

[Minoosh]

One of the abstracts brought up something I had always heard, which is that caffeine will enhance fat burning.

Since you are already equipped to screen out bogus arguments, see what others have written and if it's backed by a study go to the study and see if it's confirmed.

Intuitively I wonder if: doing strength training before a long low-intensity run would deplete glycogen and shift you to burning fat.

Of, if you eat a couple of hours before, the energy from that meal might never go into storage so you use those calories to fuel the run, and the next meal is needed to fuel baseline activity so it doesn't turn to fat.

Popular but reputable magazines (Prevention?) might bridge the gap between super-specialized studies and random Web sites. I briefly bought bodybuilding magazines. Bodybuilding isn't fitness, but there's an overlap since those folks are horrified by body fat and want to optimize muscle growth.

 

[blutoski]

So let's say someone is a casual runner and they want to run longer distances or train for a marathon. With training, does their body start to adapt to burning fat better to help them fuel their long runs? And if so, how does this happen at the biochemical level? Please note, I am not interested in weight loss, I am a very slim individual.

To my understanding, glucose is the body's preferred fuel when we are active. It is the easiest fuel source to use and metabolize. While we are also burning fat much of the time(I believe we burn mostly fat when at rest or while walking slowly), the faster we run, the more the body will rely on stored carbohydrate, in the form of glycogen(stored glucose) for fuel. A sprinting person is using almost nothing but glycogen for fuel. When it comes to distance running, I think while the body shifts to burning more glucose at first, it shifts to burning more fat after about 20 to 30 minutes, right?

Unfortunately, the body can only store so much glycogen in the liver and in the muscles, enough to very roughly power a 20 mile run. When a runner uses up all his glycogen, he/she starts to feel fatigued(unless he is well-trained), and this phenomenon is called "hitting the wall". Because all his glycogen is used up, his body has to switch to burning fat for energy, and fat doesn't burn as efficiently or cleanly. This is my understanding of how it works, correct me if I am wrong.

Now does just running very long distances(20 + miles) on a regular basis force the body to adapt so it will burn fat more efficiently? Do well-trained endurance runners still "hit the wall" at 15 to 20 miles, or does regular training mostly or completely do away with it? I would really love to spare my glycogen by using more fat for long runs. Also, does consuming more fat and less carbs force the body to adapt faster?

I've been looking over the science, though I am not sure how to interpret some of it:

Effects of short-term fat adaptation on metabolism and performance of prolonged exercise.



Fat adaptation followed by carbohydrate restoration increases AMPK activity in skeletal muscle from trained humans.

Strategies to enhance fat utilisation during exercise.

A perspective on fat intake in athletes.

Although this is one of the worst places to ask such questions, this is the best of the worst places. The misinformation, extreme stupidity, and lack of skepticism on most fitness forums is appalling and potentially dangerous.

Endurance athletes' performance isn't really about fat 'burning' efficiency. The physiological changes that improve endurance are more about increasing glycogen and fat storage in the muscle tissue itself (larger granules closer to point of use), muscular mitochondrial density, long/short muscle fiber ratio, reducing excess weight so there is less work being performed, larger stroke volume of the heart, and biochemical production particularly oxidative enzymes.

I'm not sure what you meant by 'spare glycogen' - fat burning is started when glycogen is depleted.
Glycogen will be restored during recovery. Diet does not affect this.

 

[GlennB]

I would really love to spare my glycogen by using more fat for long runs.

I'm intrigued as to why you care about sparing your glycogen. It will be replenished pdq.

 

[Zelenius]

Endurance athletes' performance isn't really about fat 'burning' efficiency. The physiological changes that improve endurance are more about increasing glycogen and fat storage in the muscle tissue itself (larger granules closer to point of use), muscular mitochondrial density, long/short muscle fiber ratio, reducing excess weight so there is less work being performed, larger stroke volume of the heart, and biochemical production particularly oxidative enzymes.

I'm not sure what you meant by 'spare glycogen' - fat burning is started when glycogen is depleted.
Glycogen will be restored during recovery. Diet does not affect this.

That would explain why running 20 miles is getting easier for me.

What I mean by "sparing glycogen" is using a higher percentage of fat to fuel a long run so I can delay "hitting the wall" for as long as possible, or simply not depleting all my glycogen too soon. I realize glycogen is the body's preferred fuel, but it would be great if that vast reservoir of fat could be utilized more.

In the study I posted in the OP, A perspective on fat intake in athletes, it says:

Studies have shown that fatigue is associated with reduced muscle glycogen and that increasing muscle glycogen or blood glucose prolongs performance while increasing fat and decreasing CHO decreases performance. This has led to an emphasis on CHO intake in athletes in endurance sports, which quite often leads to low caloric intake. It is well known that trained subjects have higher levels of fat oxidative capacity, which spares glycogen during endurance sports. Data from recent studies in trained athletes, who were fed iso-caloric high-fat diets (42% to 55%) that maintained adequate CHO levels, have shown an increase in endurance in both men and women when compared to diets composed of low fat intake (10% to 15%). The magnitude of the effect on endurance was significant at high percentages of maximal aerobic power and increased as the percentage of maximal aerobic power decreased. Based on this review, a baseline diet comprising 20% protein, 30% CHO and 30% fat, with the remaining 20% of the calories distributed between CHO and fat based on the intensity and duration of the sport, is recommended for discussion and future research.

It seems to suggest that glycogen sparing does happen in trained athletes and percentage fat intake plays a role, though maybe it is more total calories that are important here.

 

[Bikewer]

Most of what I know of this is from reading about pro cyclling... I know riders in a big stage race will ingest huge amounts of food.... Often over 8000 calories per day.
There is the "carbo loading" and the "glycogen window".... The glycogen window is a period of a couple of hours after hard exercise where the body maximizes it's uptake of carbohydrate to replenish glycogen stores.

But, these guys are working pretty hard...6-8 hours in the saddle putting out an enormous amount of watts...Especially on the mountain stages.
I imagine the typical pro cyclist could not have more than a few percentage points of body fat.

 

[casebro]

Yeah, there is a leanness to cyclers.

But is bonking 'running out of muscle fuel' ?

I'd sure like to know the best way to recover. I've got a mitochondrial problem and am severely limited in endurance. Ragged red fibers and actual stored lipids within my cells means I don't burn fats well. Do 'we' know what bonking is? Obviously running out of something, I do wonder about 'neurological fatigue', it takes energy to send the message to muscle to pump the pedals.

 

[alfaniner]

The simplest guideline I used was, you use 100 stored (glycogen) calories per mile. You can store about 700-800 calories. If you don't have some fat storage you start burning muscle protein, unless you replenish you glycogen.

Taking one of those energy packets or Sports Beans every five miles or so worked for me. (six Marathons completed.)

 

[aggle-rithm]

People who swim the English Channel (and other absurd feats) have to gain about 20 pounds of extra fat before they attempt it. By the end of the swim, they're 20 pounds lighter.

 

[GlennB]

The simplest guideline I used was, you use 100 stored (glycogen) calories per mile. You can store about 700-800 calories. If you don't have some fat storage you start burning muscle protein, unless you replenish you glycogen.

Is that even possible in a healthy person, even an extremely lean one?

My understanding was that burning muscle protein for energy is an extreme starvation response.

 

[casebro]

Is that even possible in a healthy person, even an extremely lean one?

My understanding was that burning muscle protein for energy is an extreme starvation response.

Exactly Glen.

But even a lean person has 100,000 calories stored as fat. A marathon only burns 5,000.

The problem is mobilizing the fat, which I suspect takes some other component that we DO run out of, and it takes a while for our bodies to build it from it's component parts. Some enzyme or protein?

Anybody ever tried vinegar? It's a fatty acid of only ONE link, it ought to be immediate lipid fuel, like glucose is immediate carb.

eta: but the body can't handle the pH? That is probaly why we transport fats as 'triglycerides", three fats and a (?glycogen? glucose?) so the body first has to link the vinegar into a triglcyeride, then break the tgl up again in the cell? so not fast?

 

[jt512]

My understanding was that burning muscle protein for energy is an extreme starvation response.

The body is always breaking down muscle protein and burning it for energy. The rate of breakdown is increased during exercise, and increases with exercise intensity.

 

[jt512]

The problem is mobilizing the fat, which I suspect takes some other component that we DO run out of, and it takes a while for our bodies to build it from it's component parts. Some enzyme or protein?

What I learned long ago is that when glycogen becomes depleted, Krebs cycle intermediates, which are required for the complete oxidation of fat, become depleted.

 

[blutoski]

That would explain why running 20 miles is getting easier for me.

What I mean by "sparing glycogen" is using a higher percentage of fat to fuel a long run so I can delay "hitting the wall" for as long as possible, or simply not depleting all my glycogen too soon. I realize glycogen is the body's preferred fuel, but it would be great if that vast reservoir of fat could be utilized more.

In the study I posted in the OP, A perspective on fat intake in athletes, it says:



It seems to suggest that glycogen sparing does happen in trained athletes and percentage fat intake plays a role, though maybe it is more total calories that are important here.

Interesting... that's contrary to what I've read in the literature to date, but it could be a new finding. Or perhaps they're talking about extending the overlap period (the transition from glycogen to fat is gradual as one winds down and the other ramps up) I'll read the study before commenting further.