I have come across this problem lately. As I am finally (FINALLY!) able to ride for an hour or so at a decent aerobic load, I have found that after just 30 mins or so, I start feeling light headed and nauseous. I have never NEVER had this type of issue ever in my life. I knew immediately it was to do with fuelling. I possibly suspected the amputation may have had an effect too, but I completely underestimated the scale of the issue and the factors influencing this.
Reasons why carbs might run out faster than they used to for an athlete
1. Reduced aerobic fitness (very common)
If you’ve had time off cycling, illness, injury, or a big lifestyle change, your body:
- Burns more carbohydrate at the same pace than it used to
- Is less efficient at using fat for fuel
- Depletes liver and muscle glycogen faster
Result: what used to be “easy” now quietly becomes glycogen-draining, and 30 minutes is enough to tip you into feeling shaky, nauseous, or weak.
👉 This can happen even if you don’t feel unfit.
2. You’re riding at a higher relative intensity
Even if your speed or power looks similar:
- Your threshold may be lower
- So you’re actually riding closer to your limit
That shifts fuel use toward carbs and away from fat.
Same ride on paper ≠ same ride metabolically.
3. Low baseline glycogen (under-fueling day to day)
If you’re starting rides with partially empty stores, you’ll crash sooner. Common causes:
- Skipping or delaying meals
- Low-carb dieting (even unintentionally)
- Long gaps between meals
- Training later in the day without refueling
Cycling taps liver glycogen early, so once that’s gone, nausea and “I feel ill” symptoms can show up fast.
4. Hormonal changes (especially relevant if this is new and persistent)
Hormones strongly affect:
- Blood sugar regulation
- How fast glycogen is used
- How well fat is mobilised
Changes related to:
- Perimenopause / menopause
- Thyroid function
- Chronic stress (elevated cortisol)
can all reduce tolerance for fasted or low-fuel exercise.
This often shows up exactly as you describe:
“I used to be fine for an hour. Now I’m not.”
5. Illness, iron status, or overreaching
If carbs suddenly feel non-optional, consider:
Recent viral illness (even weeks later) Low iron or ferritin Accumulated fatigue / poor recovery Poor sleep quality
These all reduce aerobic efficiency and increase carbohydrate dependence.
I know that this is not a sign of me getting soft in the head, or weak in the constitution. I needed to find out what exactly was going on so I can keep going for longer as I build up my fitness (properly this time).
My other factors
1. Amputation permanently changes energy cost
After a leg amputation, cycling (and walking) costs more energy at the same external workload and I must stop overlooking this as a trivial fact because it is not. The reasons:
- I’m using more stabilising muscle
- There’s increased neuromuscular demand
- Asymmetries increase overall metabolic load
- My cardiovascular system works harder for the same output
Even years later, this means:
I burn carbs faster at any given pace than I did pre-amputation.
So an hour unfueled before is not comparable to an hour now — even if the ride feels similar.
This isn’t deconditioning alone. It’s a new baseline.
2. Long-term fitness loss changes fuel pathways
Extended periods of reduced activity (which are unavoidable after an amputation) lead to:
- Reduced mitochondrial density
- Lower fat oxidation capacity
- Earlier reliance on glycogen
I can rebuild some of this, but not all of it — and it takes longer and more fuel than before.
Trying to train “like you used to” often backfires metabolically.
3. Menopause shifts the rules again
Menopause adds a second, powerful layer:
- Estrogen normally helps spare glycogen
- Loss of estrogen → greater carb dependence
- Blood glucose becomes less stable during exercise
- Fasted or under-fueled training tolerance drops sharply
This is why many menopausal athletes report:
- Sudden nausea
- Shakiness
- Feeling ill rather than “just tired”
It’s not subtle. And it’s not optional.
4. Why the “30-minute wall” shows up
What’s likely happening:
- Liver glycogen is partially depleted at the start
- Exercise demand exceeds fat oxidation capacity
- Blood glucose drops
- Nervous system reacts → nausea, weakness, “I feel awful”
Your body is saying:
“I cannot safely buffer this anymore without external carbs.”
That signal is protective, not dramatic.
5. The most important reframe (this matters)
You are not failing at endurance. I am not failing at endurance.
Our physiology has:
- Higher energy cost
- Less hormonal buffering
- Less margin for error
So fueling early is not “cheating” or “giving in” — it’s matching input to reality.
TAKE AWAYS FOR ME
- I must fuel correctly to prevent these symptoms
- Small frequent carbs
- Fuelling before, after AND during , Protein before, carbs during.
- Electrolytes are even more important, as menopause increases sodium loss.
Final, straight talk
Your fueling needs are not excessive.
They are appropriate for:
- Increased metabolic cost
- Reduced hormonal buffering
- A body that deserves support, not testing
Fueling early is what will let your endurance improve.
Fine-tune this to your usual ride length Build a gut-training progression Simplify this into a one-page checklist
I have some better fuelling strategies to research. Be back in a few weeks to report progress. Meanwhile I am keen to hear from anyone who is walking at least one of these dramas too!
