Wednesday, 23 April 2014

What slows us down, Part 2

In my previous post "What slows us down, Part 1" I dived into the topic of why it's hard to maintain pace when running long races, it's a big topic though, so only covered the topics:
  1. Conscious and sub-conscious, the role of Central Governor
  2. Homoeostasis
  3. Liver and Muscle glycogen depletion
In this post I'll cover just Fat vs Glycogen/Glucose metabolism

4. Fat vs Glycogen/Glucose metabolism

Our muscles and various organs primarily use fat and glucose/glycogen to power the function. When glucose is stored in muscles and the liver it's stored in a slightly altered form called Glycogen.  Our muscles will use the local glycogen preferentially while stocks are still high, but as glycogen stores diminish our muscles have to turn increasingly to metabolising fat or source glucose from blood glucose.

When our muscles work hard they almost exclusively use glycogen as fuel and at the highest intensities burn the glycogen anaerobically (without oxygen) which is great for a quick burst of power but very only uses 1/18th of the energy available in glucose, one of the bi-products of this crude but quick process is lactate which contains the remainder of chemical energy and hydrogen irons that lowers our muscle Ph making it more stressful environment for metabolism and fibre integrate.  A small amount of anaerobic metabolism generates a great deal of lactate so even when running at marathon pace where about 1% of power comes from anaerobic metabolism we could envisage nearly 20% of available fuel at marathon pace being in form of lactate.

Our muscles also can use lactate as fuel through a process called the Cori cycle, this process isn't 100% efficient though, and lactate can only be metabolized using aerobic pathways, so it's only our slow twitch or aerobic fast twitch fibres that will be able to utilize it.  For an ultra runner we should be running more aerobically than a marathoner so the fuel mix will see much less lactate, it will still be there, especially if you push on too hard up a hill.  If you feel the burn going uphill during an ultra it's a strong sign that you are working too hard and should ease off immeditately.

Our brains and gut have a strong preference for using glucose so when glycogen stores are getting low our bodies increasingly reserve the remaining glycogen stocks in our liver for use by the brain. The body uses the hormone Cortisol to signal for the liver to release glycogen and at the same time signal for the muscles to take up less blood sugar.  This is very clever adaptation for survival but it does mean that the longer we run, the more our liver gets depleted the higher cortisol will be, so that our muscles take second dibs on any sugar we consume during the race.   Therefore avoiding high cortisol levels may be crucial factor in making sure that we fuel both our brains, gut and muscles with the food we eat.

A big part in avoiding depletion of glycogen stores in our liver and muscles is how efficiently our muscles can metabolize fat.  Our diet, training and genetic make up all have a role to play in just how much we metabolize fat at different intensities, the following two charts plot how much fat and carbs (glucose/glycogen) that our complete bodies use a different intensities (HR along the horizontal axis, Calories per/hour vertical axis).  The first chart is from data collected when Scottish Ultra runner Caroline McKay was tested in here run up to the Highland Fling in 2013, the second is for a publish paper that looked at well trained athletes.

Fat/Cab metabolism for Ultra runner Caroline McKay
Far/Carb metabolism for well trained athletes.
The differences are startling - Caroline is able to use 70% fat, 30% carbs at intensities we run Ultramarathons at, while the conventionally training athletes came it at around 30% fat, 70% fats.  One of the likely keys to this difference is that Caroline was following a broadly low carb Paelo diet at the time of the tests.  Here ultra marathon focused training will also be a factor, but likely less significant than the diet.

Genetics will also play a role - we all have different mixes of slow and fast twitch fibres, those with more slow twitch have a higher capacity for aerobic metabolism and as fat can only be processed aerobically it puts a strong bias towards in favour of slow twitch dominated athletes.   Also on average females have more slow twitch fibres than males.

Results for ketogenic (very low carb diet) athletes have shown even higher utilization of fats, however, ketogenic athletes have yet to show benefits in races performances, while there a numerous cases of ultra-runners adopting lower carb diets and seeing good improvements to their ultra-running.  Very little research has been done in this area so it's not yet possible to say what level of carb/fat/protien mix be idea, and it's likely to be different from individual to individual.  I suspect through the 80's, 90's and 00's we were consuming too much carbs to be healthy let alone perform to our best in ultra marathons so going back to times before heavily processed food and large amounts of high GI carbs will likely help most runners.

I count myself as just one example of a ultra-runner that has seen improvements with going from a typical western high carb diet (I'd guess roughly 60% carb, 25% fat, 15% protien) to lower carb diet (~35% carb, 50% fat, 15% protein).

Come race day you can't massively alter how much fat your can metabolize, but what you eat and when can alter things a little.  Eating a high Glycemic Index (GI), high carb meals in the days and hours prior to race will push up blood insulin and shift the body to storing blood sugar as glycogen and fat rather and switch off burning of fat.  To avoid this insulin spike you need to take care when carbo loading prior to race, make sure it's of low GI type, and consumed with protein and fat to slow digestion and lower the GI.

During a race our muscle and liver become more sensitive to insulin so that little is required for our bodies to take up and manage blood sugar without risking a big insulin spike and shutting off fat metabolism.  You should be able to eat higher GI foods when doing your ultras, but you'll still need to take care because of gastric stress, big intakes of sugary liquids and solid foods are still likely to cause problem just of different types.  I'll touch on this topic in a later item.  In general I'd suggest consuming a modest amount of easy to digest carbs continuously through a race, consuming as little as 100 carb calories/hour will probably work fine for most runners, especially those able to burn fats efficiently.  

I find it useful to think about fuelling the liver and brain to keep them happy, rather than attempting to fuel the muscles on sugars as you simply can't keep up with their needs - muscles should burn primarily fats during ultras and we should do everything we can to help them do this.

 Another big factor we have control of during the race is our pacing, if you look at the graphs the amount of fats metabolized drops off very rapidly as intensity increases.  The rate of drop off is also much more severe for athletes that can't metabolize fats well so it's a double whammy.  For these sugar dependant athletes it's become far more critical to avoid spikes in intensity, while for fat burners they will likely be able to accommodate an occasional pacing error with less detrimental effect.

For the sugar dependent types pacing as evenly as possible is critical, for this I'd recommend considering racing by heart rate (HR) as I discussed in my recent blog entry  "How to pace a perfect Marathon or Ultramarathon"  Racing by HR will also help fat burners but they can probably be bit less strict on just how even they pace.

Another approach with pacing is to use the splits percentages that the winners of ultra races use rather than the average splits from all runners in a race as is typically done.  From my analysis of Devil O'Highland and Highland Fling Ultra the winners almost always demonstrate the most evenly paced race in the whole field.  Following what the best do on their best days is likely to be much more efficient than pacing to what the masses on average do.  A few of the "masses" run perfectly paced races like the winners do, but the vast majority go out way too fast and suffer in the later stages. Glycogen depletion associated with these fast starts is key factor why they suffer.

I have a post "Good day, Bad day splits" in the works for the Fling, so I'll wrap up now and try to get on to posting this tomorrow - only three sleeps till race day now :-)

Thanks for reading.


  1. It is clearly highly desirable for an ultra- runner to develop the ability to metabolise fat, but I would be cautious about placing too much emphasis on a high fat diet to achieve this. A large body of evidence indicates that high fat diet promotes to chronic inflammation, and in the long term, might increase risk of injury, heart disease and host of other chronic conditions. Conversely a diet rich in high GI carbs also promotes chronic inflammation. Thus, I think that the dietary strategy must be a little more subtle than either high fat/ low carb or high carb/low fat . A growing body of evidence indicates that a Mediterranean diet high in fruits, vegetables, nuts, and moderately high proportion of fish, aiming to achieve a good balance between omega-3 fats and other fats, is the optimum for heart health and general health. I therefore think that an athlete engaging in rigorous training is likely to enjoy long term health and freedom from injury by attending to the nature of both the fats and carbohydrates, as much as to the simple proportions. The evidence so far suggests the Mediterranean diet might be the optimum.

    1. Hi Canute,

      I don't believe the evidence for the Mediterranean diets is especially compelling for endurance athletes - the studies into the diet I have seen haven't looked at athletic performance. I would generally agree that it can be a healthy diet and good health should be the basis on which one trains. I believe the Mediterranean diet is also far better than the typical western diet that has sadly become the norm in the last thirty years. I would view the Mediterranean diet is a good diet, but little reason to believe that it would be the optimum for the an endurance athlete, starting with an Mediterranean diet and then adjusting it for the needs of the type of training and event would probably be a good strategy.

      With the Mediterranean region the diets the actually eat is rather varied, some eat lots of carbs, some eat much less, some goes with other parts of the diet. Given this variability within what people of the Mediterranean eat, which one is optimal? Pick one ignore the rest? Or perhaps just choose the variants that have been so far studied? Or pick one that is towards the ends of what you want your body to metabolize when you training and race?

      As for low carb diets promoting chronic inflammation, I agree this could be a risk, particularly if the diet isn't well balanced in itself. For instance eating a low carb high protien diet will cause ill health, while a low carb, high omega-6 diet is also very likely to lead to ill heath. There are lots of potential pitfalls with adopting a very low carb diet that one has to be very careful about managing it. From my own experience one doesn't have to go to a very low carb diet to see benefits in general health, training and racing. Simply going from a high carb to a modest carb diet you can see most of the benefits without the risks associated with going very low carb.