Marathon Calorie Guide: How to Fuel Your Weekly Training Load

Energy deficiency in high-mileage training is not an edge case. Research suggests that the majority of recreational marathon runners — even those not attempting to lose weight — underestimate calorie expenditure by 15–30%, particularly during peak training weeks.

The consequences range from mildly annoying (persistent fatigue, poor adaptation) to clinically serious: Relative Energy Deficiency in Sport (RED-S) impairs bone density, hormonal function, immune competence, and training adaptation. It affects both female and male athletes.

Accurate calorie estimation is the foundation of intelligent marathon nutrition.

How Calories Are Burned During Running

The primary model for estimating exercise energy expenditure uses Metabolic Equivalent of Task (MET) values — a ratio expressing the metabolic rate of an activity relative to resting metabolic rate (approximately 3.5 mL O₂/kg/min).

The general formula:

$E = \text{MET} \times M \times t$

Where:

• $E$ = energy expenditure in kilocalories
• $M$ = body mass in kilograms
• $t$ = duration in hours

MET values by running pace:

A 10K easy run at 8:00/km pace (80 minutes total) burns approximately:

$E = 8.0 \times 70 \times \frac{80}{60} = 747 \text{ kcal}$

Weekly Training Load Estimation

A typical intermediate marathon build for a 70 kg runner:

Note that this is exercise-only expenditure. Total Daily Energy Expenditure (TDEE) adds:

$\text{TDEE} = \text{BMR} + \text{NEAT} + \text{Exercise expenditure}$

Where BMR (Basal Metabolic Rate) for a 70 kg, 35-year-old male of average height is roughly 1,750 kcal/day and NEAT (Non-Exercise Activity Thermogenesis) adds 400–800 kcal/day depending on occupation.

Peak marathon training week TDEE for this runner: approximately 3,200–3,400 kcal/day.

Carbohydrate Periodisation

Not every day requires the same carbohydrate intake. Carbohydrate periodisation — matching carbohydrate availability to training demand — optimises adaptation and manages total energy intake without underfuelling.

General framework:

For a 70 kg runner on a long run day: $8 \times 70 = 560$ g of carbohydrate. That is a large amount — roughly 2,240 kcal from carbohydrates alone. Planning is required to hit these targets without relying on processed foods.

Protein: The Often-Neglected Variable

Endurance athletes historically under-prioritised protein. The evidence now strongly supports 1.6–2.2 g of protein per kg body weight per day for athletes undergoing high training loads, primarily to support muscle protein synthesis and tendon/bone connective tissue remodelling.

For a 70 kg runner: 112–154 g of protein per day. This requires deliberate planning — roughly:

• 2 eggs + protein from mixed day foods (breakfast)
• 150 g chicken or 200 g Greek yoghurt (lunch)
• 200 g salmon or lean beef (dinner)
• Post-run recovery shake or 300 g cottage cheese

Protein does not cause "bulkiness" in endurance athletes. At high training volumes, the primary risk is muscle catabolism from undereating, not over-adapting from adequate protein.

Signs of Underfuelling

Running on insufficient calories is surprisingly common and often misread as overtraining:

The pattern to watch for: a runner logging the same mileage as a month ago who is getting slower, not faster, and feeling worse, not better. Fitness adaptation requires both training stress and caloric sufficiency.

Practical Strategies

1. Track for two weeks, not forever. Two weeks of calorie and macronutrient tracking during a representative training block establishes baseline understanding. Ongoing tracking is not required and can increase anxiety around food for many athletes.

2. Use training-day vs. rest-day eating patterns rather than uniform daily targets. Eating substantially more on long run days and moderately less on rest days is a practical and sustainable implementation of carbohydrate periodisation.

3. Pre-run meals for sessions > 75 minutes. Consume 1–2 g/kg of carbohydrate 2–3 hours before any long run. Fasted long runs have a role in early-season low-intensity base building but should be abandoned once training intensity rises.

4. Recovery window. The 30–60 minute post-run window is physiologically privileged for muscle glycogen resynthesis. A 3:1 or 4:1 carbohydrate-to-protein meal or drink (0.8 g/kg carbs + 0.2–0.4 g/kg protein) in this window accelerates recovery compared to delayed eating.

Summary Targets (70 kg Runner, Peak Marathon Training)

Scale all of the above linearly to your actual body weight. A 55 kg runner needs substantially less absolute intake; a 90 kg runner substantially more. The per-kilogram targets remain constant.

Nutrition in marathon training is not complicated at the macro level. The single most common error — chronic underfuelling during high-mileage training — is solved almost entirely by eating more on the days when you train hardest.