Should I try carb depletion for a sub-3 marathon?

At some point in your sub-3 journey, you’ll probably hear about Ron Hill’s approach to carb depletion. One of Britain’s most iconic marathoners, Hill helped popularise a strategy that many runners still toy with today: run your glycogen stores low in the days before a race, then reload with carbs to trigger so-called “supercompensation”. The idea is simple: by first depleting your body’s carbohydrate reserves, you can potentially coax your muscles into storing more glycogen than usual once you start eating carbs again.

Modern running science, including Tim Noakes’ Lore of Running, tends to view this theory with scepticism. There’s little solid evidence that depletion significantly boosts glycogen capacity — and none that it consistently improves performance. But that doesn’t mean it’s without value. I’ve tried it. And while I wouldn’t claim it made me faster directly, it did help with a very real taper-week challenge: managing weight, appetite, and routine.

Why I Tried It

I gave carb depletion a go in the week leading up to the 2025 London Marathon. It wasn’t about chasing marginal gains — it was about feeling more in control during taper week. In peak training, I’m often running close to - or over - 100 miles a week. That volume demands fuelling, and plenty of it. But once mileage drops, my appetite doesn't always adjust. Unless I’m very careful, I start gaining weight — usually through water retention linked to continued high-carb eating.

Carb depletion helped me reset. I kept protein high for recovery but reduced carbs for three days in advance of a Sunday marathon - Monday through to Thursday morning. It gave structure to an awkward phase of training, helped stabilise my appetite, and kept my weight in check without feeling like I was starving myself.

What It Feels Like

It’s not especially pleasant. Your brain thrives on glucose, and when it’s in short supply, your mood often dips. I felt slightly irritable and mentally flat — not dramatically, but noticeably. That was compounded by taper restlessness: lower mileage, more unstructured time and the usual whispering of maranoia in the background.

Training runs during depletion felt heavy and sluggish. There was less spring in my stride. But I reminded myself that this didn’t reflect my fitness — it was just the cost of fuelling differently. For me, it was manageable. But I wouldn’t push through those signals blindly: if you feel lightheaded, faint, or off, it’s a clear sign to eat. Health and readiness always come first.

The Carb Rebound

Once I began reintroducing carbs on the Thursday, I felt noticeably better. That shift brought a genuine lift — physically and psychologically. I didn’t feel bloated in the way I sometimes do with carb-loading, and the transition felt smooth rather than forced. It’s hard to say whether that was due to the depletion phase or just the structured taper, but the effect was positive.

London went well. I ran 2:45:01 — twelve minutes faster than the year before. That came down to many factors: better training, smarter pacing, improved fuelling. But the taper strategy helped. I felt leaner, calmer, and more in control.

That said, it’s important not to overdo it. Your muscles can only hold so much glycogen. Carb-loading isn’t about eating until you feel stuffed — it’s about fuelling smartly in the final days. If you eat far more than your body needs, you’ll likely just retain excess water or store fat. A couple of days of clean, simple carbs, paired with a solid breakfast and fuelling plan on race day, is usually enough.

Would I Recommend It?

Not for everyone — and definitely not for first-timers. If you're chasing sub-3 for the first time, there are more important things to get right. Pacing. Fuel. Confidence. Introducing a new carb strategy that hasn’t been tested in training can easily backfire or become an unnecessary stressor.

But for experienced runners who’ve already found consistency in their fuelling and taper routine, it might be worth experimenting with — provided you do so with care. If it gives you structure, helps you manage your appetite, and lets you feel lean without sacrificing energy, then it can serve a purpose. But if it leaves you feeling drained or anxious, abandon it. Nothing is more important in marathon week than being well-fuelled, well-rested, and in the right headspace.

Five Things to Know About Carb Depletion for Sub-3 Runners

1. It’s not magic, but it can serve a purpose.
The physiological benefits are highly debated, but the structure and focus it brings to taper week can help.

2. Think structure, not speed.
For many, the value lies in appetite control and feeling less bloated — not in performance enhancement.

3. Expect sluggishness — but monitor it.
Some fatigue is normal. But if you feel too lightheaded, nauseous, or off, refuel. Fast and without guilt.

4. Not for first-timers.
Stick to what you know unless you’ve tested it before. Simplicity beats stress on your first sub-3 attempt.

5. Psychological gains are still gains.
Even if it’s mostly in your head, feeling sharper and lighter on race day can give you the edge when it counts.