Metabolic Efficiency: Defined and Put in its Place

by Ilana Katz MS, RD, CSSD

As a sports dietitian, as well as an endurance athlete (triathlon and marathons) 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 that struggle with the same dynamic. I thus 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 taught me that just to maintain my current body weight (without gaining), or dropping even a minimal amount, would require less energy (calories) than my already restrictive daily intake even with an above average of training hours per week.

So what then is metabolic efficiency and does it exist? Metabolic efficiency seems to have a positive ring to it, but it can be a very frustrating state, for those that seek weight loss as a benefit of endurance training.

Metabolic efficiency is not a new concept. In fact, the potential for energy efficiency among athletes was highlighted in the early 80’s when many competitive athletes reported what would seem to be inadequate energy consumption to meet the demands of their excessive training regimens.  (Drinkwater, Deutser, Dahlstrom, Beidelmann). 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, there was evidence that triathletes, gymnasts, marathoners and distance swimmers maintained their body mass over extended periods despite low energy intakes (less than 35 calories per kilogram of body weight). Thompson et al, 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 1500 calorie per day intake between the two groups). Thompson’s results definitely lean towards metabolic efficiency existing, because regardless of the difference in energy intake, both groups’ body composition (body fat and muscle percentages) had remained stable for over two years.

One could easily argue that what has been reported as intake, may be under-exaggerated. This in fact, is a common phenomenon, particularly in athletes of aesthetic sports, where it is not uncommon for body image distortion to be rampant. Wilmore and Schultz measured energy expenditure in female runners in a respiratory chamber. Their experimental group reported significantly lower energy intakes that their training would require but the measured energy expenditure showed no evidence of efficiency metabolically. In these studies, metabolic efficiency has a more objective, physiological definition: a lowered resting metabolic rate and an increased energy expenditure during intense activity. Similar studies were done by Beidelmann in 1995, who like Wilmore and Schulz, 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 under exaggerating energy intake.

Thompson et al, counter-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 rather than extended time and also measured oxygen consumption in a respiratory chamber.  Surprisingly, lower energy-intake athletes (low again by almost 1500 calories of estimated energy requirements for the experimental activity) had physical energy expenditure, resting metabolic rate, and sleep energy expenditure lower than the adequate-intake athletes. Furthermore, what was defined as adequate intake, was controlled, to prevent over or under-reporting in the short duration.    

My initial perception of metabolic efficiency (very slow metabolism) was now totally upended by this research. The physiologically objective amount of oxygen consumption as a measure made me realize that many athletes, do not see a drop in weight potentially because many physically active people become more sedentary in the non-exercising portions of the day, therefore, expending less energy, than is being reported, on average.  This would explain that lower intakes of energy prevent body mass from dropping, because on average there is a balanced of daily sedentary energy expenditure. 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 where they are not doing much training other than very low intensity and shorter durations, as compared to there short (probably 2 or 3 months at the most) peak season.  

In most of these studies, metabolic efficiency was determined by a resting metabolic rate, energy expenditure during exercise, and an average of a daily energy expenditure.  Obviously having an objective measure is important and moreover, it provides an explanation of the existence of metabolic efficiency. Metabolic efficiency can be positive for many, especially athletes in aesthetic sports who can thus justify eating less yet still perform effectively. Even for endurance athletes, as for them, it may take longer to “hit the wall” as they burn less 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. Based on experience with my recreational athlete clients, many often think that their training will now allow them to eat more. The research above has helped carve out a good summary for these athletes: unless metabolism is raised simultaneous to training, weight gain is more likely to result versus weight loss.  

A higher metabolism means a higher consumption of oxygen, the rate at which calories are burned. This includes at rest and during exercise. Duration, intensity of workouts, as well as body composition and food consumption, not only determine calories burned during workouts, but also calories that continue to burn post workouts. Exercise itself is the highest contributor to increasing one's metabolism and many of wanting to reverse our efficient metabolisms may need to take a closer look at how we work out. Our diet does contribute to overall metabolism (calories burned based on the composition of macronutrients/ the thermic effect of food) but to a lesser extent than exercise.  Whether it is a high-intensity aerobic workout or a lower intensity longer duration work out, or even a muscle building weight lifting session in the gym, all of these contribute in some way to raising one’s metabolism. The rate at which calories burn at rest is affected by workouts too, a concept referred to as “excess post-exercise oxygen consumption” or EPOC. For me the answer was consulting with an exercise physiologist, who designed a plan that maximized EPOC, increase the optimal burn at rest, based on the work out strategy.

References:

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