Metabolic efficiency: More smoke and mirrors coaching

ESP Tipcast 89: Clearing the smoke and breaking the mirrors on Metabolic Efficiency

This article first appeared July 1, 2016. The article is republished here with a brief addition near the end (BLUE TEXT)

The fitness and nutrition market is driven by sexy new terms and training methods. Few have garnered more interest than Metabolic Efficiency training. The idea has been sold hard by fitness and nutrition guru Bob Seebohar, who may or may not have coined the term. However, much like aerobic decoupling, metabolic efficiency is actually just a repackaging of an old idea to fabricate a half-truth. The purpose of this article is to cut through the smoke and cover the mirrors to reveal the real face of metabolic efficiency, namely the cross-over concept.

Metabolic Efficiency: rebranding doesn’t change bioenergetics

It is becoming increasingly more common for coaches to use an existing concept to sell their cool-aid. Metabolic efficiency is itself less of a concept, and more a description of the cross-over concept elucidated by one of the pre-eminent exercise physiologists of our time, George Brooks. As Brooks defined it, cross-over is a model to:

…integrate the divergent effects of exercise intensity, nutritional status, gender, age and prior endurance training on the balance of carbohydrate (CHO) and lipid metabolism during sustained exercise…as exercise intensity increases, a shift in substrate use toward carbohydrates occurs, even in the trained state.

The cross-over from fat to carb burning is largely dictated your exercise intensity, as well as your fitness level. Not surprisingly, the longer you exercise, the lower your intensity needs to be, thus increasing your reliance on fat.

The cross-over from fat to carb burning is largely dictated your exercise intensity, as well as your fitness level. Not surprisingly, the longer you exercise, the lower your intensity will be, thus increasing your reliance on fat.

The cross-over concept, depicted above, can be further summarized as follows:

    • Endurance training results in muscular biochemical adaptations that enhance lipid oxidation as well as decrease the sympathetic nervous system responses to given submaximal exercise stresses. (i.e., promotes FAT BURNING during mild- to moderate-intensity exercise)
      • Increases in exercise intensity are conceived to increase contraction-induced muscle glycogenolysis, alter the pattern of fiber type recruitment, and increase sympathetic nervous system (SNS) activity. (i.e., high intensity exercise increases SNS activity, fast twitch fiber use, and hence glucose use)
    • The pattern of substrate utilization depends on the interaction between exercise intensity-induced responses (which increase carb use) and endurance training-induced responses (which promote fat burning).

Carb and fat use ultimately depend on how well-trained you are and how hard you are exercising. In other words, no matter how fit you are, your exercise intensity ultimately determines fat and carb use.

Metabolic Efficiency Point: A new “lactate threshold”?

To recap, the cross-over concept helps to explain the key factors involved in substrate (e.g., fat, carbs, protein) use during exercise. The harder you exercise, the more carbs you need, but also that the more fit you are, the more fat you will burn. According to Bob Seebohar, the metabolic efficiency point (MEP) is the point where you are using 50% carbs and 50% fat; an example of a MEP is depicted below. Correct me if I’m wrong, but MEP looks like it occurs at EXACTLY the same point as the cross-over, which is simply a marker, not some magical point of metabolism. Incidentally, MEP sounds a lot like the bioenergetic equivalent of lactate threshold, and my thoughts on LT training are no secret. However, Bob Seebohar has been very successful selling this idea without offering any substantive evidence to support it.

It doesn't take an exercise physiologist to figure out that the metabolic efficiency point is the same as the cross-over point. Like lactate threshold, nothing magical happens there.

It doesn’t take an exercise physiologist to figure out that the metabolic efficiency point is the same as the cross-over point. Like lactate threshold, nothing magical happens there.

The argument behind MEP is too broad and frankly, too full of holes, to fully review here. The gist of it can be summarized by these points:

  • Poor utilization of fat stores and increased reliance on endogenous CHO stores
  • Increased need for supplemental CHO and higher risk of GI distress
  • Higher body weight and fat
  • Increased risk of disease

Incidentally, the low carb and Paleo people make the same arguments for their diets. By utilizing the MEP training approach Seebohar claims that athletes will be able to increase fat burning and carbohydrate use as well (a twofer!) while improving blood sugar control, reducing GI distress (because you won’t be eating all that sugar), and you will lose body fat. How will all this magic happen?

By training at or below your MEP you let your body use fats and carbohydrates more efficiently by increasing the size and number of mitochondria in your cells, but if you do this aerobic exercise without a lot of available carbohydrates, your body adapts by increasing the number of mitochondria and aerobic enzymes that burn fat. While not overtly incorrect, this hypothesis has two big problems. First, it assumes that high carb burning equates to fewer mitochondria, which is unlikely the case for well-trained athletes. A bigger problem is that multiple studies have shown that high-intensity interval training actually does a better job at building new mitochondria and increasing key aerobic enzymes. In other words, this argument for the “fat burning zone” as being the best intensity to burn fat is old and tired! Dr. Asker Jeukendrup, one of the worlds leading sports nutrition experts, summed it up as follows:

“If athletes want to increase the capacity to oxidize fat, there are many different training protocols to do this, ranging from prolonged endurance exercise to high intensity interval training. There is currently no evidence that one method is more effective than another. Most of these studies are a little removed from reality anyway because who would follow a training program with only one type of training (only long and slow or only HIIT).”

MEP does not determine performance

At the end of the day, most, if not all of us want to go faster. Ultimately, our performance power/velocity is determined by several factors that were summarized in a 2008 paper by Joyner & Coyle. That model, provided below, highlights several well-known factors, like lactate threshold, maximum oxygen consumption, and efficiency. What it does not show is MEP, which again is not actually directly relevant. In fact, I know of no research that has shown that athletes who burn more carbs during exercise perform worse than those who burn more fat.

Simply reviewing all the factors that determine performance, one has to wonder where metabolic efficiency is hiding.

Simply reviewing all the factors that determine performance, one has to wonder where metabolic efficiency is hiding.

Fat burns in the flames of carbohydrate
In theory, better fat burning extends endurance, but we cannot say it directly improves performance, at least not in real world scenarios. In fact, there is very little evidence showing that low carb diets do anything more than improve fat burning; i.e., fat burning goes up without improving performance. When it comes to performance power for endurance sports (run/cycle/swim), the faster you move, the more carbs you need because fat is too damn slow! This is the critical point that the low carb crowd seems to ignore. Moreover, reliance on fat and ketones (i.e., ultra low carb diets) actually inhibits both glycolysis and reduces mitochondrial function because the unused parts of glucose breakdown are used to keep the Krebs Cycle rolling. In other words, they work fine as long as you’re exercising at low to moderate intensity.
I find it equally implausible that training at or below your MEP will do anything special. Like threshold training, nothing magical happens at these points, and this mentality is contrary to the overload training principle. It also supports the judicious use of HIT, which again, has been shown to have either a similar or superior training impact than intensities at or below lactate threshold. Interestingly, a recent training peaks article erroneously equated AEROBIC THRESHOLD (AKA, 2 mM, VT1) with cross-over. Stephen Seiler gives a great overview of these thresholds here. That 2 mM may indeed occur at cross-over, or above it, or below it. Again, it’s a marker of internal physiology, but not some magic intensity point.
I also call into question Seebohar’s testing protocol, which is far too short and may actually lead to erroneous conclusions. Exercise duration is one factor that is often overlooked, or ignored in the process of fat burning, because key hormones that result in fat breakdown and mobilization, like growth hormone, start to increase dramatically after an 1-2 hours; the more fat you release, the more you burn. Case in point, I’ve performed identical run tests on myself in the lab (data available upon request), however, some were performed after a day of rest, while others were performed as “brick” following a hard 90 min bike. The results?
My R-value, the ratio of  VCO2/VOdropped about 0.1, which actually represents a three-fold increase in fat burning! So for ultra endurance athletes competing in 4+ hour races, a 30 min test will only tell you your fat burning capacity at the beginning of exercise. I’ll also add that it is impossible to do any of this testing with just heart rate. While HR correlates very closely to oxygen use, it varies widely between persons, making it impossible to link to a specific oxygen uptake.
It should be no surprise that I think the MEP is a load of bullshit. I can be respectful of Bob Seebohar’s coaching successes, but he has failed to meet even a minimum bar of evidence on this one. There simply is nothing to support his MEP hypothesis, and frankly it’s a complete rip-off of the cross-over concept. Is it worth getting tested? By itself, MEP is a waste of money. If we go back to Coyle’s model, there are specific abilities we would find helpful, lactate threshold power, VO2 max, and economy (VO2/km). Actually, I am perplexed why coaches are not advising getting your economy tested, because we have known for at least a decade that how much oxygen you use per kilometer run (or VO2/watt) is a key determinant of performance and it is trainable! Instead, coaches are essentially stealing and rebranding old concepts that often make no difference. Bottom line? Find a good exercise physiology lab and ask them if they can test your lactate threshold and economy using 5-10 min stages.

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