Podcast episode review:

Science of Ultra podcast hosted by Shawn Bearden, PhD.

Episode #145, Burn, with Dr. Herman Pontzer, PhD First published on March 4, 2021.

Written by: Lachlan Mitchell

The Science of Ultra Podcast is directed at ultra-endurance athletes looking to do more than improve race performance. It interviews guests from a range of backgrounds, including scientists and athletes, providing listeners with insights into how they can improve as an ultra-endurance athlete—or any athlete, for that matter.
The guest for Episode #145 was Dr. Herman Pontzer, PhD, an evolutionary biologist/ anthropologist from Duke University whose research into human physiology helps us understand how ecology, lifestyle, diet, and evolution influence metabolism and health. Dr Pontzer is the author of Burn, with key messages from his research regarding energy metabolism. What he has to say offers insights into Relative Energy Deficiency in Sport (RED-S) and what we say to athletes with Low Energy Availablity (LEA)

The constrained energy theory
With respect to energy expenditure in humans, the traditional thinking is that the human body is akin to an engine: the more work performed, the more energy is expended, while less work means less energy. However, measures of energy expenditure from the Pontzer laboratory (and other labs) do not appear to fit this model. Instead, some form of constraint exists around energy expenditure, where the body tries to keep total expenditure within a narrow range.
The constrained energy theory explains these observations. In cases where an individual is more active than usual for an extended period of time, the body responds to this high activity level to keep average energy expenditure within a narrow window. For example, an individual starts running about 6-7 miles every day, equating to 40-50 miles per week. If there is no adjustment to energy intake, the individual loses weight. After 3 months, weight loss stops. After 5 months, the individual burns the same amount of energy per day now as they did before commencing the running program, despite the 6-7 miles of running each day.

Two caveats are given for this hypothetical example.
• First, we don’t know what limit can be conserved. That is, is the body capable of conserving (constraining) the energy-equivalent of 40-50 miles per week of running?
• Second, the timing of this adaptation is unknown. That is, would this adaptation/conservation of energy occur at the 3-month mark, the 5-month mark, or some other time point?

Observational evidence has suggested an answer to the first caveat. Pontzer and his colleagues undertook a very unique observational study of runners completing the Race Across America, a running event beginning on the Pacific coast of the United States and finishing on the Atlantic coast. The runners completed a marathon (42 km) a day, six days per week. The researchers measured the energy expenditure of these runners during the first week and the final week of the run. During the first week, energy expenditure rose dramatically, as anticipated. When they measured the energy expenditure during the final week of the race, the energy expenditure was still elevated relative to pre-race, but the total expenditure was approximately 600 kcal less per day than what was expected. Based on this observation, the researchers suggested that the amount of energy that can be conserved by the body should be 600 kcal per day.

The constrained energy theory and limits to total energy expenditure
Dr. Ponzer uses the constrained energy theory to explain observations of extreme energy expenditure, including the Tour de France, Arctic expeditions, ironman events, and pregnancy. He suggests the upper limit of long-term energy expenditure and intake is 2.5 times the basal metabolic rate (BMR).

The constrained energy theory and low energy availability/relative energy deficiency in sport
The final topic of conversation in the episode was relating the constrained energy theory to observations of low energy availability. Researchers studying low energy availability have suggested there is no true overtraining, only underfueling. This comes off the back of the reasonably well-established 30 kcal/kg fat-free mass threshold for low energy availability. This can be viewed as essentially an imbalance between energy intake and energy expenditure during exercise.
Prolonged periods of low energy availability have well-documented physiological outcomes. Contrary to this viewpoint, Dr Pontzer asserts, in studies of low energy availability where the athlete substantially increases energy intake, the signs and symptoms of low energy availability remain. Ponzer suggests that in these cases, total energy expenditure has reached the 2.5 x BMR threshold. Regardless of how much additional energy is consumed, it cannot be used by the body. The only way to address the physiological symptoms of low energy availability in these cases is to reduce exercise energy expenditure. Dr Pontzer notes that further research in this field is required to ascertain what exactly is going on in these situations. That leaves us wondering: Are we giving the right messages to athletes with RED-S?

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