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 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.
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.