The RED-s Deep Dive: Metabolism, Heart Health, and Fueling Your Potential

Written By Stevie Lyn Smith

In episode 58 of Real Fuel with SLS, I sits down with Becca McConville, a board-certified sports specialist and expert on Relative Energy Deficiency in Sport (RED-s). We break down the complexities of metabolic dysfunction, the "sneaky" ways athletes end up under-fueled, and why a low resting heart rate might not be the "fitness flex" many think it is.

From the impact of GLP-1 medications on performance to the psychology of "a la carte" movement, this conversation provides a roadmap for athletes and clinicians to move from a state of survival to true performance and more importantly— optimal health.

What Exactly is RED-S?

RED-s is more than just "not eating enough." At its core, it is a mismatch between the energy "withdrawals" going out and the "deposits" coming in. Crucially, these withdrawals aren't just limited to structured training sessions.

McConville emphasizes the impact of "a la carte" movement—unstructured activity like cleaning the house, walking to work, or chasing children—which can easily account for an additional 500 to 700 calories a day that athletes often fail to track. When the body's "bills" aren't paid, it begins to "save" energy by slowing down the metabolism, bone turnover, and reproductive health.

The "Secretariat Effect": When a Low Heart Rate is a Warning Sign

Many endurance athletes view a resting heart rate in the 30s as the ultimate badge of fitness. However, McConville warns that we must differentiate between a true "athletic heart" and a "malnourished heart".

  • The Athletic Heart: In a truly fit athlete, cardiac muscle mass increases to pump more blood volume, leading to a naturally lower resting rate.

  • The Malnourished Heart: When the body is energy-deficient, it may lose cardiac mass. The heart rate drops to conserve energy, but because the muscle is weaker, the heart rate will spike significantly (12–15+ beats per minute) just from walking down a hallway.

Monitoring your morning resting heart rate is a key indicator of recovery status. If your morning heart rate begins to creep up while your resting heart rate during the day is dropping, your nervous system is likely in a state of stress and under-recovery.

Metabolic "Red Flags" You Might Be Missing

RED-s doesn't always show up as weight loss. In fact, many athletes remain weight-stable or even gain weight while their bodies are in "starvation mode". Instead, look for these physiological shifts:

  • Thyroid Dysfunction: The brain protects the body by slowing the thyroid function (specifically T3), which can lead to low body temperature and purple-tinted hands.

  • Lipid and Blood Sugar Changes: Malnutrition can lead to elevated cholesterol because the body is increasing production and decreasing clearance of lipids for energy. Similarly, A1c levels may appear elevated because red blood cells "marinate" longer in glucose before recycling.

  • Hormone Health: A decrease in sex drive or changes in menstrual cycles (for females) and morning erections (for males) are clear signs of metabolic dysfunction.

Changes to these markers, especially A1c and cholesterol levels, can be misinterpreted and the recommendations from conventional medicine are contraindicated if the athlete is in a state of low energy availability.

The Trap of the "Protein Craze"

While protein is essential for recovery, McConville notes a growing trend of "protein-maxing" among athletes. Consuming excessive protein and fiber can be counterproductive for endurance performance:

  • False Fullness: Fiber and high protein intake can swell in the stomach, making an athlete feel full before they have consumed enough total energy (carbohydrates) to fuel their training.

  • The "Weight Loss" Mimic: Many products marketed to athletes today (protein chips, protein pop-tarts) use the same satiety-heavy formulas designed for weight loss, which can lead to accidental under-fueling.

GLP-1s and the Endurance Athlete

With the rise of GLP-1 medications (like Ozempic or Wegovy), more athletes are navigating the intersection of clinical weight loss and performance. McConville highlights significant risks:

  1. Gastric Emptying: These meds slow digestion, which directly impacts how an athlete uses carbohydrates and hydration during a workout.

  2. Glycogen Depletion: Rapid weight loss on these medications can diminish the body's ability to create glycogen, severely limiting endurance capacity.

  3. RED-s Risk: Even if weight loss is a medical goal, the energy deficit created by GLP-1s can trigger RED-S symptoms, leading to long-term health complications.

Top Takeaways

  • Redefining RED-S: Relative Energy Deficiency in Sport (RED-S) is a mismatch between energy "deposits" (nutrition) and "withdrawals." These withdrawals include structured exercise and "a la carte" movement—unstructured daily activities like household chores that can burn an additional 500–700 calories.

  • The "Athletic Heart" vs. Malnutrition: A true athletic heart has increased muscle mass to pump more blood. In contrast, a malnourished heart drops its resting rate to conserve energy; it acts like a "water gun" struggling to reach the brain, causing heart rate to spike 12–15+ beats during simple movements.

  • Hormonal and Metabolic Warning Signs: Under-fueling triggers the brain to slow the thyroid (lowering T3) to protect the body. This leads to low body temperature, digestive issues, and suppressed hormonal health, such as a loss of menstrual cycles or decreased libido.

  • The "Protein Trap": Excessive focus on protein and fiber can be counterproductive for endurance athletes. These nutrients cause early satiety and gastric swelling, which may prevent an athlete from consuming the total energy and carbohydrates required for peak performance.

  • Recovery and "Under-Recovery": Instead of "overtraining," many athletes are experiencing "under-recovery." Key markers include a rising morning heart rate combined with a falling daytime resting heart rate and a flattening of heart rate variability (HRV).

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Stevie Lyn Smith
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