Half-Marathon in 12 Weeks: Setting Macros and Fueling When the Client Wants to Lose 5 kg
Dev, 29, is a 78 kg, 1.78 m recreational runner who strength trains three days a week. He has entered a half marathon in 12 weeks and wants to drop from 78 kg to 73 kg (about 6.4% of body weight) before race day. He currently runs 30–35 km/week and is about to build to 50–55 km/week. His recent 10 km time is 48 minutes, and he has no injury history. He works a desk job, walks roughly 6,000 steps a day, and estimates intake at 2,900 kcal/day. He has no consistent pre- or post-run fueling, does his long run at 7 a.m. on an empty stomach, and has "bonked" twice on runs longer than 90 minutes. He asks for calorie and macro targets plus "whatever I need to take during the long runs."
1. Estimate Dev's total daily energy expenditure and set an initial calorie and macronutrient prescription with a stated rate of loss.
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Start with a resting estimate using Mifflin–St Jeor for a male: (10 × 78) + (6.25 × 178) − (5 × 29) + 5 = 780 + 1,112.5 − 145 + 5 ≈ 1,753 kcal.
Activity multiplier is judgement, not arithmetic. Desk job plus 6,000 steps plus 30–55 km/week plus three lifts is a moderate-to-high factor; I would use ~1.55, giving a TDEE of roughly 2,700–2,800 kcal. His self-reported 2,900 kcal is consistent with maintenance, which is a useful sanity check.
Prescription:
- Deficit: 300–500 kcal/day → 2,250–2,450 kcal.
- Protein: 1.6–2.0 g/kg = 125–156 g (say 140 g, 560 kcal) - protects lean mass in a deficit and supports adaptation.
- Fat: 0.8–1.0 g/kg = 62–78 g (say 70 g, 630 kcal) - hormonal and energy-density floor.
- Carbohydrate: remainder. At 2,350 kcal: 2,350 − 560 − 630 = 1,160 kcal ÷ 4 = ~290 g/day, about 3.7 g/kg - appropriately high for this training load.
Expected rate: a 300–500 kcal deficit yields roughly 0.3–0.5 kg/week, i.e. 0.4–0.6% of body weight per week, in the commonly cited 0.5–1% band.
Note the assumption stack: the multiplier is an estimate, so week one is a calibration period. Hold intake steady for 14 days, average daily weights, and adjust by 100–200 kcal rather than reacting to a single reading.
2. How would you periodize carbohydrate across the week and fix his long-run fueling?
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Match carbohydrate to the demand of the day rather than holding it flat.
Weekly structure:
- Key session days (long run, intervals/tempo): highest carbohydrate, 4–5 g/kg, with most of it in the 24 hours before and the meal after.
- Easy run and strength days: 3–3.5 g/kg.
- Rest day: lowest, ~2.5–3 g/kg, with protein and fat making up the difference.
In practice, he eats the same total across the week but shifts 60–120 g of carbohydrate into the days that need it. Protein stays constant every day.
Long-run fueling - the part he is actually failing at:
- Pre-run: 1–4 g/kg carbohydrate 1–4 hours before, depending on tolerance. For a 7 a.m. run, a 60–80 g carbohydrate snack (banana plus oatmeal, or a sports drink) 60–90 minutes out is realistic and testable.
- During: sessions over 60–75 minutes get 30–60 g carbohydrate per hour; beyond 2.5–3 hours he can push toward 90 g/hour using multiple transportable carbohydrates (glucose plus fructose). Start at the low end.
- Fluid and sodium: estimate sweat rate with pre/post weighing; replace roughly 125–150% of the loss afterward, and include sodium for sessions over 90 minutes.
- Post-run: 20–40 g protein plus 1–1.2 g/kg carbohydrate within a few hours to restore glycogen.
Every product gets rehearsed in training, never first used on race day. His two bonks are almost certainly a fasted long run compounded by low habitual carbohydrate, not a need for a supplement.
3. Is 5 kg in 12 weeks the right goal? What would you renegotiate, and what signs would make you pause the deficit?
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The math works out to about 0.42 kg/week, roughly 0.54% of body weight per week - technically inside the recommended band. The problem is timing, not arithmetic. He is simultaneously building from 30–35 to 50–55 km/week. Adding training load while running a deficit is the classic setup for low energy availability: adaptation stalls, sleep and mood degrade, and injury risk climbs, usually at the worst possible moment.
What I would renegotiate:
- Reframe the goal from a scale number to a performance and composition goal: arrive at race day at 74–75 kg, healthy, having run the mileage. A 3 kg loss over 12 weeks is 0.25 kg/week and far more defensible during peak mileage.
- Front-load the deficit. Take the larger deficit in weeks 1–6, then reduce it to near-maintenance for the final three weeks including any taper. Race week is not the time to be in a deficit.
- Protect the key sessions. If a long run or interval session is compromised, that day becomes a higher-intake day. Performance is the feedback signal.
Pause the deficit and refer if I see:
- Persistent fatigue, unusually high resting heart rate, poor sleep, irritability, or declining pace at the same effort.
- Recurrent illness or nagging soft-tissue pain.
- Any sign of disordered eating, or a client who will not entertain a lower rate of loss.
He has a 48-minute 10 km - he is not a beginner. Treat him like an athlete whose fueling is the limiter, because that is what the bonks and the fasted long runs are telling you.