Heart Rate Training (HRT) – Part 10

Part 1 of this series.

Dr Noakes to the Rescue?

I think we are trying to fit big feet into small shoes by trying to make Low Carb fit into exercise modes which are by design not compatible with being fat fueled.

I have great respect for Dr Noakes, but in my opinion the following video illustrates how we are missing the point when it comes to athletic performance and Low Carbohydrate diets.

Dr Noakes (@20:00) makes a startling comment in the video (@20:04).

There must be an exercise duration for which fat adaptation is the preferred state for optimal performance and I don’t know if we have found it yet…

Dr Noakes is humble in this video and admits that his own studies which showed advantages may have been biased – that was helpful for those of us who bought the hype that we can perform any sport on LC and do it well.

However, this video completely misses the point in a weird sort of way. People do low carb and get great benefits but find it doesn’t mix well with their particular athletic modality. In fact, they may find it doesn’t mix well with any competitive athletic modality. And, all athletics is artificial in some sense so to conclude that it is a bad diet based on some artificial measurement is missing the mark. Low Carb has found a home with triathletes and ultra-marathon runners but those are fairly elite choices.

What Dr Noakes is missing is that the real issue isn’t one of duration. The issue is one of fuel source. Certainly intensity and duration are inversely related. Sprints are intense and short. Ultra-marathons are low intensity but very long in duration. Few people want to do an ultra marathon.

We low carb athletic proponents are fond of touting the advantages of fat adaptation. It is true that the low carb athlete is better at accessing fuel but only out of necessity since the body spares carbohydrate stores on the low carb diet. The body doesn’t want to spend carbs on exercise since those stores are limited. But if you push your intensity up high enough it will pull from carb stores and these are limited in both depth and speed that can be accessed on a low carb athlete. This necessarily reduces exercise intensity to levels that are not competitive.

This is why folks who do Crossfit or take up wrestling find that they need to supplement carbohydrates. They chose to alter their diet to fuel their exercise rather than picking an exercise mode which matches their diet. Personally, I view this as a fundamental mistake.

What is the Answer?

The right answer is to engage in athletic activities which utilize the fat as fuel and don’t rely on carbohydrates as fuel. When a person is completely fat fueled they have a very deep tank of storage for energy. The use of fat as fuel necessarily translates to a reduced intensity which is problematic on any medium duration sports.

What intensity level is this done at? Here’s where some data can illustrate the point. Ben Greenfield published his own VO2max testing and I’ve plotted his VO2max numbers vs his RER numbers. (Remember that RER is a measure of fuel mixture. An RER of 0.7 indicates that the person is 100% fat fueled and an RER of 1.0 indicates a person is 100% carbohydrate fueled. An RER of 0.85 would be 50-50 mix of fat/carbs.)

Ben’s data is pretty jumpy so the red line is a 3rd order polynomial which is fitted to the data (the R^2 was 0.85). It shows a dip (more fat burning) at around 35% of VO2max. The data also shows a fairly flat line across the entire range from 25% to 50%. The actual data showing the dip is:

This is a very specific point in time where Ben Greenfield’s data showed him burning 97% of his energy from fat – clearly the sweet spot for a person who is fat fueled since almost none of the energy he was using came from carbs. Ben never got an RER below 0.7 so this is the best he did during that time. Unfortunately, that point is a pretty low point level of intensity. Well, sorta. Ben’s VO2max is 61.1 and that point was at a VO2 rate of 27.2 so the point where Ben was being as fat fueled as he possibly could be was at 45% of his VO2max.

This correlates well with other published data:

This shows a peak fat oxidation rate at 65% of VO2max but the issue isn’t one of maximum fat oxidation since at 65% the fat is mixed with carbohydrate oxidation. That does produce the most efficient exercise but only in a non-fat fueled athlete. The attractiveness of the data is that it clearly shows a fairly wide curve. The whole graph is relatively flat from 50% to 75%. But what is different between these two points is the fuel source in the body.

Fine Tuning Fat Burning

The spot is the highest intensity which can be reached where 100% of energy comes from fat and 0% from carbohydrates. I don’t know if there’s a term for this but let’s call it the Maximum Glucose Sparing/Fat Burning the MGSFB point.

I happen to know my MGSFB from my VO2max data. Here, I’ve added a bulls eye at the MGSFB point. At this point my RER was 0.7 and rising.

[Note for people trying to lose fat: I have no interest  at all in fat burning at this point in time but the implications of the above should be obvious for those who are interested in fat burning. You can get the most bang for the buck in fat burning at a level which is much less than your max heart rate. If you are beating yourself up in the gym that may be good news.]

My MGSFB was at 65% of my VO2max and it was at a HR. This matches the point at the top of the curve shown of fat oxidation rates vs VO2max. In fact, that could well be the very definition of fat adapted – ie, the ability to most efficiently burn fat at the highest VO2max point.

This was at a HR of 117 bpm. This correlates well to the center of the Maffetone HR value of 112-122 bpm. In fact, it might suggest that I should drop my range to 107-117 max since that guarantees I stay in the fat burning range for the entire activity

So how do you figure this out for yourself?

Here’s the approach I essentially took to get to this number:

  • Get fat adapted by adopting a Very Low Carb diet (< 30g of carbohydrates)
  • Do this for the TBD (days/weeks/months) adaptation period
  • Get your VO2max tested
    • If your VO2max doesn’t show you with an RER of 0.7 for a large portion of the test time you are not yet fat adapted
  • Plot the curve or just look at the data and find the highest point where your RER is still 0.7 or less
    • The point at which it goes over 0.7 you are no longer burning fat exclusively
  • Find the corresponding heart rate
    • That is your max heart rate for exercising

A Cheaper/Easier/Faster/Close-Enough Way

Just use the MAF calculation. It’s close enough. I like it enough that I wrote an MAF calculator and put it on-line here.


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