The Endurance Engine Isn’t Just in Your Legs, by Hillary Allen

Professional ultrarunner, scientist and coach Hillary Allen (left) brings a rare dual perspective to performance nutrition: She understands what matters in the lab, and what actually works when you’re deep in the pain cave.

In this review, she digs into the latest CurraNZ research through both lenses, unpacking its relevance where physical and mental fatigue collide.

Hillary writes:
There’s a moment in every race when it stops being about your lungs. It becomes about your brain.

I’ve felt it at the top of a vertical kilometer, when my heart rate is maxed out and the only thing keeping me moving is rhythm. I’ve felt it in a marathon, when pacing discipline starts slipping and the mind begins negotiating. I’ve felt it deep into an ultra, when technical terrain demands sharp decisions long after the legs are tired.

That moment, when focus begins to fade, is where performance lives or dies. And recently, I read an exploratory study that made me rethink how much of that fatigue is muscular… and how much might be something else entirely.

A 2025 paper (1) published in Muscles explored whether the performance effects of New Zealand blackcurrant extract (CurraNZ) depended on muscle fibre type. Instead of asking the generic question  “Does it work?”  the researchers asked a better one: “Who does it work best for?”

Participants were categorized based on fatigue patterns that indicated whether they were more Type I dominant (endurance-oriented) or Type II dominant (power-oriented). After seven days of supplementation, the Type I–dominant athletes increased their total running distance by 17% and their high-intensity running distance by 15%, without additional cardiovascular or metabolic strain.

That’s not trivial. It suggests that the “endurance engine,” the slow-twitch fibres responsible for sustained effort, may respond differently to anthocyanin-rich blackcurrant extract than fast-twitch fibres do.

Type I fibres are built for durability. They rely heavily on oxidative metabolism, store glycogen, and utilize intramuscular triglycerides. Previous work has shown CurraNZ may enhance fatigue resistance (2) and fuel handling in these fibres (3). So what we may be seeing isn’t just antioxidant support, it could be improved efficiency in the fibres that carry us through long efforts and repeated surges.

For someone like me, who races everything from vertical kilometers to 50Ks to 100 mile races, that matters. Even in shorter events, endurance fibres anchor performance. They clear metabolites, stabilize rhythm, and help you recover between hard efforts. In ultras, they determine whether you can respond to a move at mile forty or whether you simply survive.

But here’s where this becomes more personal. I’m not just an athlete. I’m also a coach and currently in graduate school.

Which means I live in two fatigue worlds at once: physical fatigue from training, and cognitive fatigue from reading research papers at night and writing before dawn. And what we know from broader performance science is that mental fatigue alone can reduce time to exhaustion, even when muscles are capable. The brain can slow you down before your body has to.

That’s why this research catches my attention beyond the muscular story. Anthocyanin-rich blackcurrant juice has been associated in previous literature with vascular and cerebral blood flow responses (4), which are discussed mechanistically in relation to fatigue resistance. Racing, especially on technical terrain, is cognitively demanding. So is academia. Decision-making, pacing recalibration, emotional regulation, they all require bandwidth.

If your endurance fibres resist fatigue longer and your cognitive clarity stays sharper, your performance ceiling shifts. As a coach, this excites me even more.

We’re moving toward a future where performance nutrition isn’t generic. Not everyone responds the same way to the same stimulus. Some athletes are durability machines. Some are explosive and powerful. Some fade late. Some surge late. Understanding fibre tendencies, even indirectly through fatigue patterns, allows for more intelligent conversations about training and support strategies.

It’s not about finding a magic edge. It’s about aligning physiology with preparation. What I appreciate most about this study (1) is that it doesn’t overpromise. It acknowledges variability. It suggests direction. It invites personalization, and that’s how high performance should work, because in the end, endurance isn’t just about the legs. It’s about sustaining output, muscularly and mentally, when it would be easier to fade.

For me, balancing professional racing with graduate-level study, durability is everything. I don’t just need to finish races strong. I need to show up the next day in class, in coaching sessions, in life. The endurance engine isn’t just in the muscle fibres. It’s in the systems that keep them, and the brain that directs them, working together. That’s the edge worth paying attention to.

For me, CurraNZ isn’t about chasing marginal gains for the sake of it, it’s about supporting the systems I rely on every day. The endurance fibres that carry me through long races, the cognitive clarity that helps me make decisions under pressure, and the mental stamina required to balance elite sport with graduate study. Training will always be the foundation. But when something aligns with both the physiology and the lived experience of fatigue, it earns a place in my routine.

References

1. Willems, M.E.T.; Blacker, S.D.; Perkins, I.C. Effects of Blackcurrant Extract During High-Intensity Intermittent Running: An Exploratory Study of Possible Muscle Fibre-Type Dependence. Muscles 2025, 4, 56. https://doi.org/10.3390/muscles4040056

2. 1. Mark E. T. Willems * , Megan Bradley, Sam D. Blacker and Ian C. Perkins Effect of New Zealand Blackcurrant Extract on Isometric Contraction-Induced Fatigue and Recovery: Potential Muscle-Fiber Specific Effects, Sports 2020, 8, 135; doi:10.3390/sports8100135

3. L. A. Jones, L. Leung,  J. S. Barrett,  H. O. Fortis,  J. A. Strauss, S. O. Shepherd, New Zealand blackcurrant extract augments muscle glycogen storage at rest and enhances intramuscular triglyceride degradation during prolonged exercise, 2025 European Journal of Applied Physiology https://doi.org/10.1007/s00421-025-05995-9

4. Watson AW, Scheepens A, Kennedy DO, Cooney JM, Trower TM, Haskell-Ramsay CF. The pharmacodynamic profile of "Blackadder" blackcurrant juice effects upon the monoamine axis in humans: A randomised controlled trial. Nutr Neurosci. 2020 Jul;23(7):516-525. doi: 10.1080/1028415X.2018.1525950. Epub 2018 Oct 5. PMID: 30289026.