MAF Plus 20

Peter Defty (of OFM fame) suggests that fat adapted athletes can increase their MAF number from 180 – age (with correction factors added/subtracted) to 200 – age (same correction factors) (Primal Endurance Podcast – #90: Peter Defty Talks Optimized Fat Metabolism).

His reasoning is that the heart rate is 10-15 beats per minute faster in fat adapted athletes (from the FASTER data). He reasoned that Maffetone came up with the number based on non-fat adapted athletes and that once fat adapted the number can be shifted up.

Tempting Idea, but…

I’ve had the same thoughts before and I’d really like to accept Defty’s ideas since I’m getting tired of mostly walking. I’d like to run more. But I’ve also had no injuries in the past few months. Recovery has been so easy that I’m finding myself doing two MAF efforts a day. I’d hate to jeopardize that.

I don’t think I’m getting much faster doing MAF, but I wonder if sticking with MAF and doing intervals would improve my speed. I do feel like I am improving my leg strength at MAF and they are not a limiting factor when I’m out for more than an hour.

The limiting part of MAF is that after 4 or 5 miles I can only run a few steps until I have to start walking again.


Of course, Maffetone’s approach is that MAF is MAF. And it’s 180 – age (with correction factors).  The program is fixed and doesn’t need to be changed. The athlete who is not yet fat adapted will burn more carbohydrates at MAF and the athlete who is fat adapted will burn more fat at MAF. This shift away from carbohydrate reliance to fat adaptation is the goal of MAF when done with the recommended lower carbohydrate diet.

20 Beat Shift – VO2 Data

To see what a 20 beat increase would do, take a look at my VO2max fat/carbohydrate oxidation curve. At my MAF (122 bpm) I am currently burning nearly all fat and very little carbohydrates.

Shifting up by 20 bpm from 122 to 142 just happens to be the 50-50 crossover point of calories from fat and carbohydrates. This will cause glycogen depletion which has good and bad aspects. My current view is that staying out of that range is the smartest idea since cycling glycogen doesn’t promote lower glycogen stores since the body responds by over saturating glycogen stores.

Shifting right by 20 bpm could have the advantage of causing a further shift of the curve to the right and increasing my fat oxidation at that same heart rate. If that is the effect then it would be positive since in the end I could have a higher VO2max and improved fitness.

Critique of MAF number

One difficultly of the Maffetone MAF number is that there’s no real explanation of the basis for the number. Maffetone himself says that the number can be adjusted based on actual metabolic tests but he never exactly explains how to adjust the number nor exactly what he based the number on other than observation of a lot of his clients/patients. The number fit the tests within a few beats but Maffetone never explains the derivation of the number in enough details to explain what lab test he used and what the correlation to the tests is. Maffetone has spent a lot of energy explaining what it isn’t (lactic threshold, VO2max, percent of max heart rate, etc) but not a lot explaining what it is. Without tying it to some external metric it’s hard to judge the value of the metric.

Is MAF at the cross-over point for a non-fat adapted athlete but the point of maximum fat burning in a fat adapted athlete? It is true from my data that 122 is the sweet spot. It is literally the peak of fat oxidation (the black theoretical curve fitted line) where no carbohydrates are being burned. Ten beats lower is still in the prime fat burning zone. For me, lower numbers are even ketone burning (evidenced by the RER of less than 0.7).

Rate of Perceived Exertion (RPE)

Mowing my lawn raises my heart rate beyond the MAF range and makes me sweat. MAF makes me sweat when it’s warm outside but it’s a pretty gentle pace. I could do exercises at 142 max and it would be fine. I know because I’ve mowed the lawn (and done CrossFit) at higher rates.

I don’t think I am going to change what I am doing at the moment but I will bear it in mind for the future. I did 5 sessions last week of 5Km or longer and I’d like to keep up the volume.


Is it the Low Carb or the High Fat?

Interesting study took a look at the question of whether it is high fat or low carb (Leckey JJ, Hoffman NJ, Parr EB, Devlin BL, Trewin AJ, Stepto NK, Morton JP, Burke LM, Hawley JA. High dietary fat intake increases fat oxidation and reduces skeletal muscle mitochondrial respiration in trained humans. FASEB J. 2018 Jun;32(6):2979-2991.) (Full PDF).

High dietary fat intake increases fat oxidation and reduces skeletal muscle mitochondrial respiration in trained humans.

It’s not a surprise that your body will burn more fat when you consume less carbohydrates. The Food Quotient (Food Quotient) predicts exactly that.

Mitochondria respiration (Mitochondrial Respiration) is:

…the set of metabolic reactions and processes requiring oxygen that takes place in mitochondria to convert the energy stored in macronutrients to  adenosine triphosphate(ATP), the universal energy donor in the cell.

I don’t know enough to know whether or not reduced mitochondrial respiration is good or bad for athletic performance. It seems like reduced rates of ATP would be bad for energy but is that energy made up in other ways? Is the loss offset by the increase in BHOB (ketone bodies)?


Fasted Workouts and 24 Hour Fat Oxidation

Fasted workouts cause increased 24 hour fat oxidation (Iwayama K, Kurihara R, Nabekura Y, et al. Exercise Increases 24-h Fat Oxidation Only When It Is Performed Before Breakfast. EBioMedicine. 2015;2(12):2003-2009).

Under energy-balanced conditions, 24-h fat oxidation was increased by exercise only when performed before breakfast. Transient carbohydrate deficits, i.e., glycogen depletion, observed after morning exercise may have contributed to increased 24-h fat oxidation.

These results probably don’t hold true for low carb athletes since our glycogen stores are probably already somewhat depleted.