The 10 km Runner Dieting and Fasting at the Same Time
A 41-year-old man, 82.4 kg, 178 cm, office worker, enters a 10-week programme to run 10 km under 60 minutes. He currently runs three times a week (5 km easy, 6 x 400 m, 8 km long) and has set a parallel target of losing 5 kg in the same 10 weeks. He trains at 06:30 before breakfast because 'fat burns better on an empty stomach', drinks nothing during the run, and reports dizziness at the end of two of the last five sessions, held in 28 degrees C and 70% humidity. His food record shows about 1,500 kcal per day with 55 g of protein. Yesterday he weighed 82.4 kg immediately before a 60-minute run and 81.0 kg immediately after, having drunk 300 mL during the run. His resting heart rate is 7 beats above his usual and he describes himself as 'flat' on easy runs.
1. Calculate his sweat rate from the weigh-in data and build a hydration plan for the 60-minute training run and for a 60-minute 10 km race in the same conditions. Show the arithmetic.
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Arithmetic. Mass change 82.4 - 81.0 = 1.4 kg, i.e. 1.4 L of sweat. Add the 300 mL ingested: 1.4 + 0.3 = 1.7 L lost in 1.0 h, so sweat rate = 1.7 L/h. That is at the upper end of the normal 0.5-2.0 L/h range and is plausible for 28 degrees C and 70% humidity.
Replacement target. Aim to replace 70-80% during exercise: about 1.2-1.4 L/h, not 1.7, because full replacement blurs the thirst signal and causes gut discomfort. He currently drinks nothing, so build up: 150-200 mL every 15 minutes, roughly 600-800 mL/h, and only increase if he tolerates it.
Practical plan.
- 2 h before: 400-500 mL water; 15 min before: 200 mL.
- During: a bottle on a loop or a belt, 150-200 mL per 15 min; sip, do not gulp.
- Sodium: 1.7 L x ~800 mg/L is roughly 1,300 mg/h. Use a drink with 400-700 mg/L, more if he is a salty sweater (white streaks on kit, stinging eyes).
- After: 1.25-1.5 L per kg of body mass lost.
- Race day: start with pale-yellow urine; do not exceed ~800 mL/h on course. Drinking more than ~1.5 L/h of plain water in a 60-minute race is a hyponatraemia risk, not a safety margin.
2. Evaluate whether he is in low energy availability. Estimate his energy needs and set an intake that still delivers 5 kg in 10 weeks without wrecking the training.
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Estimate. Resting metabolic rate by Mifflin-St Jeor: (10 x 82) + (6.25 x 178) - (5 x 41) + 5, is about 1,730 kcal. With a desk job and three runs a week, total daily expenditure is roughly 1.45-1.5 x RMR, so ~2,500-2,600 kcal/day. A 60-minute run costs about 600 kcal at his mass.
Energy availability. Fat-free mass at roughly 18% body fat is about 67 kg. EA = (intake - exercise energy) / fat-free mass = (1,500 - 600) / 67, which is about 13 kcal/kg fat-free mass, far below the 30 kcal/kg threshold. That is low energy availability, and it fits the +7 bpm resting heart rate, the flatness, the dizziness and the 55 g protein (0.67 g/kg, well under the 1.6 g/kg he needs).
Target. 5 kg in 10 weeks is 0.5 kg/week, a deficit of about 500 kcal/day. That means roughly 2,000-2,100 kcal on training days and 1,800 on rest days, with protein at 1.6 g/kg (~130 g) and carbohydrate at 3-5 g/kg (~250-400 g). Weight loss at 0.5-0.75% of body mass per week is compatible with the sub-60-minute goal; 1,500 kcal is not, because the training quality that produces the time is the first thing lost.
Flag in review: persistent dizziness, a metallic taste and a raised resting heart rate are not purely nutritional. Check hydration, iron status if fatigue persists, and sleep.
3. What should change about the 06:30 fasted runs, and how would you fuel the 48 hours before the target race?
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Fasted running. Fasted low-intensity running is not harmful in itself when total daily energy is adequate - 'fat burning' is a substrate shift, not a performance advantage for a 10 km. The problem here is the combination: fasting plus 1,500 kcal/day plus 600 kcal of running produces the EA of about 13 kcal/kg computed above. Two changes:
- 30-40 g of carbohydrate before the 06:30 run (banana, toast, sports drink). This is enough to blunt the dizziness and does not compromise a fat-loss goal at the margin.
- Keep the fasted run explicitly low intensity. Do not put the 6 x 400 m session on an empty stomach in the heat.
Race week.
- Two days out: raise carbohydrate to 5-7 g/kg (~400-550 g), lower fibre and fat slightly, keep protein constant.
- Day before: familiar foods, no new supplements, 500 mL fluid with the evening meal.
- Race morning: 2-3 h before, 1-2 g/kg carbohydrate (~100-150 g); 15 minutes before, 200 mL fluid.
- In-race: 30-60 g carbohydrate per hour for a 60-minute effort is optional but helpful; a mouth rinse or a small gel at 30 minutes is enough for most at this duration.
Rehearse all of this in training. Race-day nutrition that has never been tested is a plan, not a preparation.