Eating to lose fat while training for an endurance event
You can lose fat while training for an endurance event if you keep the loss to roughly 0.5-0.7% of bodyweight per week. Run a smaller deficit on long and hard sessions and a larger one on rest and easy days. Keep protein high and carbs full around key workouts, and stop the deficit before your taper.
The usual mistake is a flat cut: the same 500-kcal deficit every day, including the 30 km long run. Fat loss stalls, sessions suffer, and the athlete concludes they can’t do both. The issue isn’t the goal. It’s where the deficit sits in the week.
How fast can you lose weight without hurting training?
About 0.5-0.7% of bodyweight a week is a sensible ceiling for athletes. In a randomised trial in elite athletes, Garthe et al. (2011) compared a weekly loss of 0.7% of bodyweight with 1.4%. The slower group gained lean mass, while the faster group’s lean mass stayed flat.
That study was in strength and power athletes, not marathoners. Still, the principle carries across: a slower loss leaves more energy for training and recovery. For a 78 kg runner, 0.5% a week is about 0.39 kg.
Where should the deficit go in a training week?
Put most of the deficit on rest and easy days, and run little or none on long and hard days. The fuel for the work required model (Impey et al., 2018) adjusts carbohydrate to the demands of each session rather than holding one number. A deficit can be cycled the same way. The broader argument is in eat for the training you did.
Carbs on big days should still land in the ranges from the ACSM, AND and Dietitians of Canada joint position (Thomas, Erdman and Burke, 2016):
- About an hour of moderate training a day: 5-7 g/kg.
- 1-3 hours a day of moderate-to-high intensity: 6-10 g/kg.
What does a fat-loss week look like for a marathoner?
Take a 78 kg, 178 cm, 35-year-old male runner aiming to lose 0.5% of bodyweight a week, about 0.39 kg. By the linear 7,700 kcal-per-kg rule, that is about 429 kcal a day on average, or roughly 3,000 kcal a week.
His resting metabolic rate by Mifflin-St Jeor works out as follows:
10 × 78 + 6.25 × 178 - 5 × 35 + 5 = 1,723 kcal
Cycle the deficit across a seven-day week:
| Day type | Days | Deficit per day | Weekly total |
|---|---|---|---|
| Rest | 2 | 650 kcal | 1,300 kcal |
| Easy run | 3 | 450 kcal | 1,350 kcal |
| Interval session | 1 | 300 kcal | 300 kcal |
| Long run | 1 | 150 kcal | 150 kcal |
| Week | 7 | about 443 kcal on average | 3,100 kcal |
At 3,100 ÷ 7,700, that is about 0.40 kg a week, the same weekly result as a flat cut. The difference is that the long run gets a 150-kcal deficit instead of 450.
Now check the floor. If his wearable puts a rest day at about 2,500 kcal, the rest-day target is 1,850 kcal. That stays above his 1,723 kcal RMR. If the target ever dipped below RMR, the deficit would be too aggressive for that day.
On the long-run day, his carbs should sit near 6-10 g/kg, which is 468-780 g.
How much protein do you need while cutting for an endurance event?
Hold protein at the high end of the normal range. The ISSN position stand gives 1.4-2.0 g/kg/day for people who exercise. It adds that 2.3-3.1 g/kg may be needed to hold lean mass in a deficit, a figure that comes from resistance-trained subjects.
For our 78 kg runner, the top of the ISSN range (1.8-2.0 g/kg of bodyweight) is 140-156 g a day. Helms et al. (2014) state their deficit range per kg of fat-free mass, and at 20% body fat his is 62.4 kg. The lower end, 2.3 g/kg, gives about 144 g, so both routes land near 140-155 g. Helms studied lifters, so for a runner the lower end is the more defensible one. Anchoring to lean mass rather than scale weight matters more as body fat rises (here’s why).
Why does the 7,700 kcal rule overestimate fat loss?
Because your burn adapts as you lose weight. Hall’s work on dynamic energy balance shows that the static rule overpredicts long-term loss. Trexler et al. (2014) review how metabolic rate falls during a diet.
Use the arithmetic above to set a starting point, then let the weekly weight trend tell you what is happening. Daily scale readings swing with glycogen and water, especially around long runs.
When should the cut stop?
Stop before the taper, and do not diet in race week. Race week is for filling glycogen, as covered in how to fuel a race without guessing. Plan fat loss for base training, when sessions are mostly aerobic and there is time to absorb a slower rate.
If training starts to suffer, ease the deficit. Signs include falling paces at the same effort, poor sleep or recurring niggles. If you lose periods, see a doctor. Low energy availability has health costs beyond a bad session.
How Sweatmeal handles this
Sweatmeal cycles the deficit around your training load from a connected wearable or your logged activity. It eases the deficit further on low-recovery days and never sets a target below your RMR. It also recalibrates your burn from your weight trend over time instead of trusting the 7,700 kcal rule. When a race is on your calendar, the carb floor lifts for race week. The numbers are explained on the methodology page.