By René Borg, Running Coach Ireland
www.runningcoach.ie | @running_coach_ireland
The carb vs fat debate exploded again recently after that roundtable with Tim Noakes, Phil Maffetone, and Paul Laursen made waves in the endurance world. We discussed it on our Trail Running Ireland podcast where we suggested we may be asking the wrong question entirely.
I've watched runners swing between extremes for years. Someone reads about fat adaptation and suddenly they're afraid of fruit. Another hears about carb loading and starts downing pasta like it's going out of fashion. Meanwhile, they're both missing what actually determines whether their fueling works.
Your body isn't a car engine - It's a Battery
Most runners think about fuel like filling up a car. Carbs = petrol, fats = diesel. But your body operates more like a rechargeable battery, and this isn't just a metaphor.
At the cellular level, your body literally runs on electron flow. Mitochondria transfer electrons across membranes to produce ATP, the energy currency your body uses for everything from muscle contraction to nerve transmission. A body with better mitochondrial function can generate the same power output with less fuel input than a poorly functioning system.
Research shows that mitochondrial efficiency varies significantly between individuals, and those with higher ATP production per unit of oxygen consumed literally need less substrate to produce the same energy output. This explains why two runners can follow identical fueling strategies and get completely different results.
Sleep, training, stress, nutrition, even your emotional state - they all charge or drain this biological battery. And the state of your battery determines how you'll respond to different fuels.
Research support
John Kiely's work on periodization shows something crucial: adaptation isn't a simple input-output equation. It's an emergent property of a dynamic system that passes through multiple human filters - your beliefs, expectations, emotional state, and life situation - before producing any physiological effect.
Studies show that what you believe about your training and nutrition shapes your biochemical responses. Your stress hormone profile changes based on whether you expect a stressor to be harmful or beneficial. Placebo responses trigger real neurochemical releases like dopamine and endorphins. Even believing a milkshake is "indulgent" vs "healthy" changes your hunger hormone response to identical food.
How you feel about your fueling strategy isn't just mental - it directly influences how your body processes that fuel.
What this looks like in practice
Here are three real examples from athletes I coach (names changed):
Sinead tried low-carb after reading about fat adaptation. Felt amazing for six weeks: better focus, stable energy, dropping body fat. Then she got moody, recovery slowed, and she picked up a nagging injury. Adding back strategic carbs after hard sessions while keeping easy days fat-focused reversed this. Her "battery" could handle the metabolic flexibility training, but only to a point.
Liam constantly bonked in races despite perfect carb loading protocols. His high-stress job was already maxing out his cortisol response and draining his system. We shifted focus to better recovery and stress management first, then adjusted his race fueling. His battery was too depleted to effectively use the fuel he was giving it.
Linda swore by her high-carb approach through years of successful racing. Then menopause hit, and suddenly the same foods that energized her before left her feeling sluggish and inflamed. We adapted her strategy to match her changing physiology. Same person, different battery state.
What variables really matter?
Instead of micro-managing macros, pay attention to:
Your current battery state: Are you sleeping well? Stressed at work? A runner juggling three kids and a demanding job has a more depleted system that needs different fuel support than someone with flexible schedule and solid recovery.
Your training phase: Base building requires different fueling than race prep. Your body's asking for different things at different times, and your battery's needs change accordingly.
Your metabolic machinery: Some people have naturally efficient fat-burning enzymes. Others have better glucose handling. Most of us can adapt either way with the right approach, but your starting point matters based on your mitochondrial genetics and ancestral roots.
Your actual goals: A 5K runner and an ultra runner shouldn't fuel the same way. This seems obvious, but you'd be surprised how many people follow generic advice that doesn't match their event demands.
Your Practical Action Plan
Stop asking "Should I go low-carb or high-carb?" Instead, ask yourself:
- How's my energy throughout the day? Steady energy suggests your current approach is working. Crashes and cravings suggest your battery isn't getting what it needs.
- How do I feel during and after training? Good sessions that leave you energized are the goal. Struggling through workouts or feeling demolished afterward means something needs adjusting.
- How's my recovery? Are you sleeping well? Mood stable? Getting stronger? These are better indicators of your battery state than any macro calculator.
What's my life context right now? High stress periods might need more carb support. Easier phases might be perfect for fat adaptation experiments.
The Bottom Line
The fat vs. carb debate often overlooks a more useful perspective: nutrition isn't one-size-fits-all. Your fueling strategy should reflect your current reality - your battery state, life context, training phase, and individual physiology.
The best athletes I know aren't dogmatic about any particular approach. They're adaptable. They pay attention to what their system needs in the moment and adjust based on how their body is responding now, not on what worked last season or what works for someone else.
The goal isn't to win arguments but to optimise your own biological battery for the specific demands you're placing on it.
Want to dive deeper into this approach? Listen to our full discussion on Trail Running Ireland podcast, Episode 115, where we also chat with Jared Martin about preparing for the World Mountain and Trail Running Championships.
www.runningcoach.ie | @running_coach_ireland
Key Research Supporting the Body Battery Approach:
- Mitochondrial efficiency (ATP/O ratio) varies between individuals and directly affects fuel requirements (Salin et al., 2015)
- Higher mitochondrial efficiency correlates with lower fuel consumption for equivalent work output (Salin et al., 2015)
- Individual differences in substrate utilization patterns during exercise are well-documented (Brooks & Mercier, 1994)
- Beliefs and expectations directly influence stress hormone profiles and performance outcomes (Keller et al., 2012)
- Placebo responses trigger measurable neurochemical releases including dopamine and endorphins (Benedetti, 2005)
- Food beliefs alter hunger hormone responses even when food content is identical (Crum et al., 2011)
- Mitochondrial haplotype and fuel preferences
