I recently sat down with Dr. José Areta, Associate Professor in Exercise Metabolism and Nutrition at Liverpool John Moores University, for what turned into one of the most thought-provoking conversations I have had on athlete fueling in recent times. José has spent years studying what happens to the human body during periods of energy deficit, and his latest research is genuinely challenging some of the most deeply held beliefs in sports nutrition. If you have ever worried about low energy availability, or been told you need to stay above 30 kcal per kg of fat-free mass at all costs, this one is for you.
The Study That Questions Energy Deficit Thinking
In 2024, José published a landmark paper titled Endocrine, Metabolic, and Skeletal Muscle Proteomic Responses During Energy Deficit with Concomitant Aerobic Exercise in Humans. It was the first ever protein-level investigation of what short-term energy deficit actually does to human muscle, and it did not deliver the results most people expected.
Ten healthy, physically active males were put through a tightly controlled 15-day protocol. The first five days were an observation period. The second five days were spent in energy balance. The final five days dropped energy availability from 45 kcal per kg of fat-free mass all the way down to 10, while keeping exercise consistent at 60% VO2 max for 90 minutes, three times per week. That is a reduction of around 78%. Extreme by design, and the point was to create a measurable physiological signal.
What they found was remarkable. Using a cutting-edge technique called dynamic proteomic profiling, José and his team could look at individual protein changes inside the muscle rather than just overall muscle protein synthesis. Two things stood out. First, the mitochondrial fraction of muscle actually increased. Rather than energy deficit downregulating the muscle, the machinery responsible for aerobic energy production was upregulated. Second, the extracellular matrix proteins, the structural scaffolding of skeletal muscle including collagen-related proteins, showed signs of being reduced. Interesting, and nuanced. Not a simple good news or bad news story.
Fat Oxidation, Ketones, and the Carbohydrate Myth
Here is something that will challenge a lot of athletes: these participants were eating approximately 2.4 grams of carbohydrate per kilogram per day during the energy deficit period. That is not a ketogenic diet by any traditional definition (<50g per day). Yet fat oxidation went through the roof, free fatty acid availability climbed, and beta-hydroxybutyrate rose enough to indicate ketosis.
This matters because the narrative pushed in certain circles is that you need to cut carbohydrates aggressively to increase fat oxidation. What this study suggests is that it is the energy deficit itself, not the carbohydrate restriction, that drives the metabolic shift toward fat burning. You do not need to slash carbs to zero. As José put it, weight loss diets are, functionally, ketogenic diets. When you go into a meaningful energy deficit, your body responds accordingly, even if you are still eating a significant amount of carbohydrate.
The practical takeaway for athletes is clear: you can pursue fat adaptation and body composition goals without gutting the fuel source your muscles rely on for quality training. Given it wqs during a significant calorie sdeficit (78% reduction), it was still fascinating to see the driver being the deficit as opposed to the carb amount. How big that deficit has to be in order to drive improvements in fat oxidation might be the next question to be answered in future resaerch from this group. The premise behind periodized nutrition is this. Fuel for what you are doing in terms of duration, intensity and purpose (i.e. weight loss/maintain/gain). This it is exactly what we build into Fuelin.
The Real World: A World-Class Cyclist at the Tour de France Femmes
To ground all of this in something tangible, José shared a remarkable case study. A high-performing female cyclist he had worked with agreed to be monitored throughout the Tour de France Femmes in 2023. Using double-labelled water, the gold standard for measuring energy expenditure in free-living athletes, the data revealed that she was burning nearly 7,500 calories per day on average, a physical activity level of around 4.2 times her resting metabolic rate. Her energy deficit across the race was approximately 2,500 calories per day, resulting in a weight loss of 2.2 kilograms over eight stages.
Yet her performance did not fall apart. Stage seven, a climbing stage, produced the highest mean maximal power output for both 30 and 60 minutes of the entire race. She finished the event and went on to place tenth in the World Time Trial Championships only weeks later.
This is the part that makes coaches and practitioners think harder. She was eating around 14 grams of carbohydrate per kilogram per day. She was fueling aggressively. And she still could not close the gap on that deficit. It raises the honest question: was there a way to reduce it, perhaps by increasing calorie-dense foods like fats and concentrated carbohydrate products between stages? Possibly. It was unclear from the study why that did not happen.
But the bigger lesson is that energy deficit, even significant and sustained deficit under race conditions, does not automatically destroy performance. At least in the short term. The issue that is sure to be raised, is yes, short term no impact on performance, long term, what is the impact on this athletes or similar athletes health AND performance. We know from this case study that the athletes menstrual cycle was negatively impacted and so too, T3 levels. Bone health was not assessed in the short or chronic phases and that is an area where the impact of LEA deserves renewed attention to understand just how big an impact these short term perdiods of energy can have on the body.
The Problem With How We Define and Diagnose LEA
José was direct on one of the most contested issues in sports nutrition: the 30 kcal per kg of fat-free mass threshold as a diagnostic marker of low energy availability. His position, shared with co-authors Asker Jeukendrup, Craig Sale, Charles Pedlar, Lara Van Genechten, Neil Walsh, and Carl Langan-Evans in their critical review of the IOC consensus statement on Relative Energy Deficiency in Sport (REDs), is that the evidence underpinning that threshold is fragile.
The threshold was derived from short-duration, laboratory-based studies in predominantly sedentary females doing a small amount of exercise. Extrapolating it to real-world assessments of field-based athletes introduces enormous error. José noted he has measured energy availability at minus 80 in athletes, which is clearly well beyond any margin of error. But if someone is sitting at 29 versus 30? That difference is essentially noise.
More importantly, the hormonal and physiological markers used to flag REDs, including T3, leptin, IGF-1, and bone formation markers, are all influenced by factors potentially beyond energy intake. Mental stress, sleep disruption, competition pressure, and the kind of type-A psychological profile common in endurance athletes all affect the same endocrine pathways. As José put it, energy is not the only nail, even if nutrition is the only hammer most sports dietitian's are holding. The counter argument I had to this would be that if nutrition is identified as a potential source of causing few or several of these issues, then address it first as it is "low hanging" fruit. If the results is positive then it does not matter whetehr or not nutrition was directly or indirectly attributed to the signs & symptoms. What is important is that the athlete is feeling better. My biggest fear with the article proposed by these authors is that it is used by coaches or athletes to undermine the recent advances in awareness around fueling that has been made and benefitted so many
What Athletes Should Actually Take From This
None of this is an argument for reckless weight loss or deliberate under-fueling. José was emphatic on that point, and so am I. But there are a few things worth internalising.
Short-term, controlled energy deficit can produce real physiological adaptations, including improved fat oxidation and mitochondrial up regulation, without necessarily compromising performance or causing catastrophic muscle loss. The key words are short-term and controlled. The moment high-intensity training is layered on top of a significant calorie deficit, the picture changes quickly. Glycogen stores deplete, performance drops, and recovery suffers. Strategic deficit works. Chronic, unintentional or intentional deficit does not. At least, it can work yet the consequences will catch up wit those doing it sooner rather than later.
The athletes I worry about most are not the ones deliberately managing their body composition under supervision. It is the ones who genuinely do not know they are under-fueling, often because no one has sat down with them and done the work. At Fuelin, around 94% of users come in with a weight loss goal. The most important thing we can do is help them pursue that goal intelligently, keeping carbohydrate intake aligned with training load, protecting recovery, and making sure they understand what the body is actually asking for on any given day. This is why we will recommend a "calorie floor" as well - the point to which we refuse to drop calories further based on the training schedule and training volume coupled with the intensity and athlete biometrics. Our view is that we are not prepared to be responsible for drastic weight loss as a result of extreme caloric restriction that is not tightly controlled in a lab. That is our company position and one we are happy to stand by. If an athlete wants to do extreme dieting they can, just not using Fuelin.
Energy availability matters. However, we acknowledge it is one variable in a complex system that includes training stress, psychological load, sleep, and individual physiology. Where energy availability fits into the picture of chicken or egg, that is still being understood. Some may argue about the name and whether or not the "low energy availability" really is the central cause. For me personally, I do believe that nutrition is a simple, first port of call to address for many athletes suffering from the myriad of issues described under the LEA/REDs paradigm.Athletes deserve to understand the full picture, not just a number they cannot accurately measure anyway.
Cheers,
Scott
References:
Nishimura, Yusuke et al. “Endocrine, Metabolic, and Skeletal Muscle Proteomic Responses During Energy Deficit With Concomitant Aerobic Exercise in Humans.” FASEB journal : official publication of the Federation of American Societies for Experimental Biology vol. 39,21 (2025): e71163. doi:10.1096/fj.202502384RR
Areta, Jose L et al. “Energetics of a World-Tour Female Road Cyclist During a Multistage Race (Tour de France Femmes).” International journal of sport nutrition and exercise metabolism vol. 34,5 253-257. 25 May. 2024, doi:10.1123/ijsnem.2023-0275
Jeukendrup, Asker E et al. “Does Relative Energy Deficiency in Sport (REDs) Syndrome Exist?.” Sports medicine (Auckland, N.Z.) vol. 54,11 (2024): 2793-2816. doi:10.1007/s40279-024-02108-y
Speakman, John R. “Why does caloric restriction increase life and healthspan? The 'clean cupboards' hypothesis.” National science review vol. 7,7 (2020): 1153-1156. doi:10.1093/nsr/nwaa078
Areta, José L. “Physical performance during energy deficiency in humans: An evolutionary perspective.” Comparative biochemistry and physiology. Part A, Molecular & integrative physiology vol. 284 (2023): 111473. doi:10.1016/j.cbpa.2023.111473