Fully fuel before, during, and after every run longer than an hour. Keep weight loss to a small 250-500 kcal daily deficit taken on rest days and light evenings. Glycogen, the stored form of carbohydrate in muscle and liver, sets marathon capacity. Draining it without refilling slows training, weakens recovery, and stalls fat loss.
Medical information disclaimer: This article is for general educational purposes only and does not provide medical advice, diagnosis, or treatment. Always consult a physician or other qualified health professional about symptoms, medications, tests, or treatment decisions.
Table of Contents
- How do you fuel long runs?
- Should you still carb-load?
- Where should the calorie cut come from?
- When does dieting hurt marathon training?
How do you fuel long runs?
Eat 1-4 g of carbohydrate per kg body weight 1-4 hours before a long run. Pick low-fibre glucose-plus-fructose foods that digest fast. A white bagel with jam plus a banana works for many runners.
The Gatorade Sports Science Institute carbohydrate review recommends about 30-90 g of carbohydrate per hour during longer exercise. At 60-90 g per hour, use glucose-fructose mixes for faster absorption and flexible gels, drinks, or chews. Rehearse that plan every long run so the gut learns to tolerate it. TrainingPeaks advises fully fueling around long runs while taking the deficit elsewhere, with practice until 30-60 g per hour feels easy and up to 90 g per hour is possible in the TrainingPeaks marathon weight-loss guide.
- Start fueling about 30 minutes into the run, not at mile 18
- Combine gel plus water with sports drink to hit your hourly target
- Use the same products, timing, and fluids you plan for race day
Should you still carb-load?
The Gatorade Sports Science Institute advises roughly 7-12 g of carbohydrate per kg daily before hard competition. For the final 36-48 hours, use 10-12 g per kg per day to maximize muscle glycogen. Pause the deficit during the load.
A 70-kg runner needs about 700-840 g of carbohydrate per day at that rate. That means large, low-fibre meals with rice, pasta, bread, juice, and sports drink. Do not try to load and diet at once. Two days at maintenance will not erase weeks of modest loss, but starting flat will ruin race day.
Where should the calorie cut come from?
The Momentum Sports weight-loss guide advises a deficit of about 250-500 kcal per day for roughly 0.5-1% body-weight loss per week. It warns against dropping below about 1,500 kcal per day because faster loss costs muscle and raises injury risk. Keep protein toward the higher end at about 1.6-2.4 g per kg daily.
That amount preserves lean mass, supports recovery, and increases fullness while dieting. A 70-kg runner needs roughly 112-168 g per day. Take the cut from rest days, light evenings, and smaller non-training meals. The Gatorade Sports Science Institute advises 1.0-1.2 g of moderate or high-glycaemic carbohydrate per kg per hour for the first 4 hours after exhaustive running, then normal meals, with glucose-fructose sources aiding liver and muscle recovery.
When does dieting hurt marathon training?
Prolonged low energy availability causes Relative Energy Deficiency in Sport, known as REDs. The IOC 2023 consensus on energy deficiency links it to impaired bone, hormones, immunity, mental health, and performance in female and male athletes. That is why aggressive dieting defeats marathon adaptation.
Missed periods, frequent illness, stress reactions, flat workouts, and low mood mean fuel is too low for the workload. These problems affect men as well as women. Protect long-run days, load days, and the recovery window first. If pace stays flat for two weeks despite training, pause the deficit and eat to full training needs.