Sunday, October 17, 2010

What do fasted training and low carb athletes have in common?

I saw this excellent blog post by Martin Berkhan over at Lean Gains and my thought-track went on a tangent which led me to writing this post.  I've been meaning to write a series of posts on our fuel systems and our carbohydrate needs for various activity levels and goals for quite some time.  Unfortunately, a perfectionist mindset held me back from tackling such a large [and somewhat ambiguous] beast.  It has to start somewhere ...

Fasted Training For Superior Insulin Sensitivity And Nutrient Partitioning -- Martin Berkhan, Lean Gains
A few highlights:
  • "When exercising at higher intensities than 65% VO2Max, fat oxidation is progressively reduced and becomes almost non-existent at 82-87% VO2Max.
  • C [fed/carbohydrate group] saw a greater increase in VO2Max ...  C improved VO2Max more as they could train harder due to providing the proper substrates for fueling the activity. On the other hand, F [fasted training group] became progressively more efficient at oxidizing fat at higher levels of intensity as evidenced by the increase in FATmax. This is, in turn, could be explained by the substantial increase in the fat burning enzymes FAT/CD36 and CPT1.
  • The fasted training group saw significant improvements in all parameters relevant to improving body composition and health [glucose tolerance and insulin sensitivity, GLUT4/AMPK increased, markers for fat metabolism, body composition], where as the fed training group saw comparatively lackluster results here."
My notes: There is a lot of neat stuff in that post and in the study's linked.  Read between the lines: fasted training appears to improve markers related to insulin sensitivity, health, and body composition.  However, the fed athletes appeared to be capable of a higher work output (while they also gained a little wieght).
Should I eat those potatoes?

I'd like to point a few things out:

  • At rest or low intensities, we can use fat as our primary energy source.
  • Ketogenic diets and physical performance recounts a few examples of groups and individuals who survived for a year or more on a diet mostly or entirely devoid of carbohydrates--as well as presenting a study where cyclists improved their performance after adapting to a strict protein and fat only diet.
  • On the other hand, fat is metabolized to ATP slower relative to carbohydrate and cannot be burned anaerobically.  At high intensities, we are using primarily carbohydrate for fuel.  Period.
  • With clever diet modifications and time to adapt, our body can begin to burn greater amounts of fat at various exercise intensities (which spares glycogen), however, we will still be burning some glycogen/glucose. 

So where does the glucose come from?  From the blood.  We have some glucose floating in our bloodstream (about a teaspoon).  Our body also has a capability to store glucose as glycogen in the muscles (~400 grams) and liver (~100 grams)--these are generally filled by carbohydrates that we've eaten.  We can also make some glucose  in the liver via gluconeogensis using amino acids and the glycerol backbone of fat (this is how people have survived with no DIETARY carbohydrate).  So as we draw glucose from the blood it can be replenished via our glycogen fuel stores (muscle and liver) or from something we eat before or during exercise.

Where I like to draw attention to it this:
How much fuel do you have in your stores and how much fuel are you going to burn during your exercise?  If your muscle and liver glycogen is full to the brim, then the token athletic male could fuel in the realm of 15-20 miles of running on just what he has in his tank.  Hence, it's widely recognized that carbohydrate containing sports drinks do not improve performance in efforts less than 60 minutes at high intensity.

So what about Fran?

With a quick, extremely intense workout like Fran, an athlete is no doubt burning almost exclusively carbohydrate for fuel.  However ... he may be done in 2-5 minutes.  So Fran is not all that taxing on your fuel reserves (what it does to your muscles, CNS, and soul is another story).
Whether your "Fran-fuel" (glucose) comes from glycogen storage from something you ate a few days ago, if it's boosted by something that you ate before your workout, or if your liver made it from amino acids and fat backbones ala gluconeogenesis may or may not matter as much as the amount that is available vs. the amount you are going to use.

An athlete could skate by (and probably do well) on a low carb diet if their training regime consisted of some low-moderate intensity aerobics, some weight training with sufficient rest between sets, and/or VERY SHORT, INFREQUENT high intensity exercise sessions.  But! forget about hitting 20 minute metcons 4-5x/week with firebreather intensity.

SO, Should you train fasted (or do a very low carb diet)?

Well, what are your goals?  Do you want to have shredded body composition, are you firing all cylinders chasing performance,  desiring to live in to the 100's, some combination of all three? (remember that generalizing means not being optimal in everything ... Mario didn't jump as high as Luigi, but he was an all around pretty good guy)

Or maybe you're looking for something else?

Your goals (and comparing where you are now vs. where you want to be) will be the primary driver of how often and what type of training you'll need.  And how often and what type of training you're doing will be a big driver of what/how you need to eat.  Conversely, your goals will drive what/how you should eat.  And what/how you eat will impact what level and frequency you are capable of training.  

Within that goal-diet-training matrix, you'll have some options to play around with:  a static diet with a daily pattern, a dynamic pattern of re-fueling according to that days training demands, a weekly cyclical diet with days of higher consumption and days of lower, a daily cycle with mostly veg/protein/fat throughout the day and tossing bulk of carbs post-workout, or maybe something else?  Each has pros and cons and what works for some may not work for others.  Tinker,  find something that works for you, adapt, and proceed.

Hope to hit this topic more in the future.  Will certainly be hitting the different fuel systems/energy pathways, where they fit in to our modes of exercise, and how they affect our fueling.


  1. informative post, but the conclusion is the usual, lazy "find what works for you" prompt. What works for each person is not very definitive. Who knows what combo of training and diet leads to living beyond 100 etc. There is no evidence or model to go by, so certainly setting out to find out for oneself is not enough.

  2. You're right. Problem is, there just isn't enough research looking further into this subject. I try to stick to writing what I can share with confidence-- what will work/be applicable to a large audience that I wont have to retract in a year or two. [not saying I wont take something back if proven otherwise]. Still, until there's further exploration on this topic, I'm afraid this is about as far as the train goes. If you're needing more specific advice consider a personal consultation with a nutrition/fitness professional.


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