Metabolic Efficiency: Who Needs It?

Metabolic efficiency may sound desirable, but it’s not — that is, if it even exists.

As a sports dietitian and an endurance athlete (triathlete and marathoner) who struggles to lose weight, I often find myself frustrated with the low response my body has to my consistent training and very disciplined eating habits. I also have many athletic clients who struggle with the same dynamic.

Thus, I began to research the concept of metabolic efficiency, defined as “Energy intake based on body weight that is required to maintain current weight.” I even went as far as having a basal metabolic rate test done, only to discover mine was off the charts (low).

This demonstrated to me that just to maintain my current body weight (without gaining), or to drop even a minimal amount, I would have to consume even less energy (calories) than my already restrictive daily intake even with an above-average number of training hours per week. Am I that efficient as a machine that I do not need much fuel, even with a high level of energy expenditure through training? Maybe I am the new model of “green,” the “Prius” of athletes, at least at rest. While “metabolic efficiency” seems to have a positive ring to it, it can be an especially frustrating state for those who seek weight loss as a benefit of endurance training.

This is not a new concept. In fact, in the early 1980s, researchers highlighted the potential for energy efficiency among athletes when many competitive athletes reported what would seem to be inadequate energy consumption to meet the demands of their training regimens. Based on the energy balance equation (energy consumed equals energy expended), one would expect these athletes not only to jeopardize their performance with such low intakes, but to drop weight drastically.

However, evidence showed that triathletes, gymnasts, marathoners, and distance swimmers maintained their body mass over extended periods despite low energy intakes (fewer than 35 calories per kilogram of body weight). In the 1990s, a group of researchers studied 24 endurance athletes with the same number of hours of training in a week. Some of the athletes typically ate less than adequate energy and others had above adequate energy for their training demands (there was a difference of almost 1,500 calories per day intake between the two groups). Thompson’s results definitely provided support for the existence of metabolic efficiency, because regardless of the difference in energy intake, both groups’ body composition (body fat and muscle percentages) remained stable for over two years.

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One could easily argue that the seemingly efficient athletes underreported their true intake. This is in fact a common phenomenon, particularly among athletes in aesthetic sports, where body image distortion runs rampant. One study measured energy expenditure in female runners in a respiratory chamber. Their experimental group reported significantly lower energy intakes than their training would require but the measured energy expenditure showed no evidence of metabolic efficiency.

In these studies, metabolic efficiency has a more objective, physiological definition: a lowered resting metabolic rate and increased energy expenditure during intense activity. In similar studies published in 1995, researchers, found no significant changes in resting metabolic rate or energy expenditure (rate of oxygen consumption). With this definition secured, the only conclusion these researchers could draw was that the runners were underreporting energy intake.

My initial perception of metabolic efficiency (very slow metabolism) was now totally upended by this research. Perhaps I underestimated my food intake or overstated my levels of activity. The research found that many physically active people are more sedentary in the non-exercising portions of their day, which would foil the expectation that less energy intake would lead to a drop in body mass. To take this even further, athletes (particularly ones that are highly competitive, or involved in high-intensity endurance sports) typically have an extended recovery season when their training regimen decreases, as compared to their short (two or three months at most) peak season.

However, the 1995 study with the 24 endurance athletes mentioned above argued for metabolic efficiency again in his 1995 research. Here he set out to determine if activity energy expenditure, sleep energy expenditure, and resting heart rate in endurance athletes were similar over a 24-hour period, and he also measured oxygen consumption in a respiratory chamber. Surprisingly, lower energy-intake athletes (lower again by almost 1,500 calories of estimated energy requirements for the experimental activity) exhibited lower physical energy expenditure, resting metabolic rate, and sleep energy expenditure compared to adequate-intake athletes. Furthermore, researchers controlled the athletes’ intake, thereby preventing over- or underreporting in this short-duration observational experiment.

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In most of these studies, resting metabolic rate, energy expenditure during exercise, and average daily energy expenditure provided the measure of metabolic efficiency. These measurements of metabolic efficiency can be helpful for many, especially athletes in aesthetic sports who can justify eating less yet still perform effectively.

Endurance athletes may take longer to “hit the wall” as they burn fewer calories and spare more glycogen during their events. However, for many recreational athletes, often choosing to participate in a sport for the benefit of being able to eat more and still lose weight, metabolic efficiency may result in frustration unless they have a deeper understanding of the concept. In experience with my recreational athlete clients, many think that training will allow them to eat more. The research described above helped carve out a good summary for these athletes: unless metabolism is raised simultaneously with training, weight gain is more likely to result than weight loss.

Exercise itself is the greatest contributor to increasing one’s metabolism and those of us wanting to reverse our efficient metabolisms may need to take a closer look at how we work out. While diet does influence the metabolic rate, it does so to a lesser extent than exercise. High-intensity aerobic workouts, lower-intensity longer-duration workouts, and even muscle-building weightlifting sessions at the gym all contribute in some way to raising one’s resting metabolism. So am I really metabolically efficient, or do I need to look at the big picture: measured oxygen consumption, an exercise routine that maximizes post-exercise oxygen consumption, and using food and exercise journaling to actually calculate my energy intake and energy expenditure instead of estimating them?

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