How to lose 5kg during a sub-3 marathon training block

Let’s be blunt: if you’re carrying excess body fat as a runner, you’ll almost certainly run faster if you lose it. Grab a 5kg bag of flour and hold it for a few minutes. Now imagine running a marathon carrying that weight – not in your hand, but evenly distributed around your waist, chest and thighs. It’s not subtle. It slows you down.

The performance gains from weight loss are real. A 2007 study published in Medicine & Science in Sports & Exercise estimated that each 1% reduction in body mass could improve running economy by up to 1% – which, over 42.2km, could translate into several minutes. A commonly cited (if approximate) estimate is that for every pound (0.45kg) lost, you may gain 1–2 seconds per mile. That’s 2–4 minutes over a marathon if you lose 5kg.

Of course, there’s a strict floor to this. Essential body fat levels are around 5–7% for men and 14–17% for women, with healthy ranges rising slightly with age. Drop too low and performance, immunity and mood can all suffer. But for most sub-3 hopefuls – especially those coming into running from heavier backgrounds – losing fat, not just weight, is one of the most powerful levers available.

Before London 2025, I lost 14kg. I PBed across every distance from 5K to the marathon. It made a profound difference to how I felt, how I recovered and how I raced.

So when should you lose it?

Ideally, before the block starts.
Running 100–160km per week demands energy. You need carbs to fuel training, and protein to rebuild. Trying to create a consistent caloric deficit during a serious sub-3 block is difficult – and often counterproductive. You may lose weight, but at the cost of performance, recovery and adaptation.

In fact, I often gain 1–2kg during a peak block – mostly lean mass, not fat. That’s not failure, that’s functional.

If you want to lose 5kg, the best approach is to treat the 10–12 weeks before training begins as a separate "cutting" phase – a focused fat-loss block, much like you’d treat a training cycle. Track calories. Set goals. Stick to a structure.

Studies of NYPD officers referenced in Burn: The misunderstood science of metabolism by Herman Pontzer found that the best predictor of weight loss maintenance wasn’t dieting itself – but high-intensity exercise following weight loss. This supports the idea that intense training should follow fat loss, not accompany it.

But if you must lose fat during the block…

Here are five ways to do it without sabotaging your marathon goal:

1. Use intermittent fasting strategically
On easy or recovery days, skip breakfast and delay eating until mid-morning or lunchtime. A fasted run followed by a protein shake can preserve lean mass while nudging fat loss. But never fast on hard workout or long-run days – your body needs fuel when intensity is high.

2. Maintain high protein intake
Protein protects muscle mass. Aim for at least 1.6–2.0g per kg of bodyweight per day, even during calorie restriction. This supports recovery and preserves adaptations. A scoop of whey or a high-protein dinner can help plug gaps without excess calories.

3. Time your carbs
Don’t cut carbs completely – you’ll tank your training. Instead, front-load carbs around your sessions: before, during and immediately after. This ensures glycogen replenishment and improves absorption. Keep carbs lower on rest days, but don’t eliminate them.

4. Consider berberine or glucose regulation support
Berberine – a plant-based compound – may help regulate blood sugar and improve insulin sensitivity. Some research suggests it could reduce fat storage from carbohydrate intake, though evidence is mixed. Use it cautiously and not as a shortcut.

5. Eat clean, not just less
Huge mileage isn’t a licence to eat what you like. Focus on unprocessed foods, lean meats, vegetables and fibre-rich carbs. Keep saturated fat low. Avoid takeaways and refined snacks. You don’t have to be perfect, but you do need to be consistent.

If weight loss is a goal, approach it tactically. Either start your marathon block already lighter – or accept a slower, safer rate of fat loss during training. In either case, remember: your sub‑3 is the priority. Let that guide every decision.

Getting lighter can make you faster – but only if you stay strong, fuelled and healthy along the way.