Why carbohydrate periodization only works if you periodize protein, too.
For years, endurance athletes have heard one simple rule:
Fuel every session.
For race day? Absolutely. For key interval workouts? Without question. BUT - what if fueling every workout the exact same way is also why adaptation eventually slows?
That's the idea behind training low - a strategy that deliberately reduces carbohydrate availability during selected sessions to create a stronger training signal. The goal isn't to make workouts harder.
It's to make your body adapt differently.
What does "train low" actually mean?
Training low simply means beginning a workout with lower glycogen availability than usual.
Athletes commonly achieve this by:
- Sleeping low after an evening glycogen-depleting session
- Completing an easy morning workout before breakfast
- Performing two workouts close together without fully restoring glycogen
- Delaying carbohydrate intake after selected aerobic sessions
Notice the theme?
None of these strategies are designed for high-performance workouts.
They're designed to increase the metabolic stress during carefully chosen low-intensity sessions.
Why athletes do it
When muscle glycogen falls below a certain threshold, your body begins activating different cellular pathways than it does during fully fueled training.
Researchers call this the glycogen threshold hypothesis.
Instead of simply repeating the same metabolic response every workout, lower glycogen availability can increase signals associated with:
- mitochondrial biogenesis
- fat oxidation
- metabolic flexibility
- long-term endurance adaptations
In other words:
Same workout. Different signal.
Sometimes that's exactly what's needed to move beyond a plateau.
The catch
Metabolism doesn't create free adaptations.
If carbohydrate availability drops too low, your body still needs energy.
One place it finds it? (drumroll, please)
Amino acids....
...which often come from skeletal muscle. That's exactly what researchers at the University of Toronto investigated.
What the study found
Eight trained endurance runners completed a genuine sleep-low protocol before spending eight hours in a metabolic research facility while scientists measured protein metabolism using stable isotope tracer techniques.
The results were striking.
Compared with normal glycogen availability:
- Amino acid oxidation increased by 11%
- net protein balance decreased by 12%
- estimated daily protein requirements increased by roughly 0.12 g/kg/day
The adaptation signal became stronger, but so did the cost.
The solution isn't more carbs
Interestingly, the answer isn't abandoning training low altogether, but matching protein intake to the increased demand.
Current evidence suggests that athletes incorporating carbohydrate periodization should aim for approximately:
1.8–2.0 g of protein per kilogram of body weight per day
And on low-glycogen training days?
Consider increasing the size of your immediate post-workout protein feeding. That helps support muscle protein synthesis while preserving the very tissue responsible for producing endurance performance.
Fuel for the work required
Modern sports nutrition has largely moved beyond "always fuel everything" and "always train fasted."
Instead, researchers increasingly recommend a framework known as Fuel for the Work Required.
The idea is simple: high-intensity sessions deserve high carbohydrate availability, while easy aerobic sessions can occasionally be performed with lower glycogen to stimulate different adaptations.
Recovery nutrition changes accordingly, and carbohydrates become periodized.
Protein should be, too.
The takeaway
Training low isn't about seeing how long you can survive without carbohydrates - it's about strategically exposing your body to a different metabolic environment.
Used sparingly (typically one or two lower-intensity sessions per week) it can be a valuable tool for improving endurance adaptations.
Just don't ignore the protein bill!!
Because the goal isn't simply to train low.
It's to adapt high.
References
Gillen JB, West DWD, Williamson EP, Fung HJW, Moore DR. Medicine & Science in Sports & Exercise. 2019.
Impey SG, et al. Sports Medicine. 2018.
