Losing weight while training for a race or HYROX
Dieting and training hard compete for the same energy. The rate of loss, and where the deficit sits in the training cycle, decide whether an athlete gets lighter and faster or lighter and slower.
An athlete can lose fat during a training cycle without losing performance if the rate stays around 0.5 to 1% of body weight per week, protein stays high, strength work continues and the deficit is moved away from the hardest weeks. Faster rates cost lean mass and session quality. The best-controlled evidence for the rate comes from a trial with elite athletes.
The rate: 0.7% per week, not 1.4%
Garthe and colleagues (2011) randomised 24 elite athletes to a slow or a fast weight-loss target while all continued four strength sessions a week. The slow group aimed for 0.7% of body weight per week and reduced energy intake by about 19%. The fast group aimed for 1.4% and cut intake by about 30%. Both lost around 5.5% of body weight. The slow group took about 8.5 weeks and increased lean body mass by 2.1%. The fast group took about 5.3 weeks and kept lean mass unchanged. The authors recommend the slower rate for athletes who want to keep or gain lean mass and strength.
For an 80 kg (176 lb) athlete, 0.7% a week is about 0.55 kg (1.2 lb). Because the 3,500 kcal per pound rule overstates long-run loss (Hall, 2008), a starting deficit of 300 to 500 kcal a day, adjusted after two to three weeks of trend data, is a more reliable way to land in that range than a calculated target.
Where the deficit goes in a training cycle
| Phase | Energy approach | Why |
|---|---|---|
| Off-season and base | Main deficit phase, 0.5 to 1% per week | Training intensity is lower, sessions are easier to fuel |
| Build | Smaller deficit, maintenance on key-session days | Quality sessions need glycogen |
| Peak and race-specific | Maintenance | Recovery and session quality outweigh scale progress |
| Taper and race week | Maintenance, carbohydrate up before the race | Glycogen and the water with it are race fuel |
Inside a week the same logic applies. Most of the deficit belongs on rest and easy days. Long sessions, threshold work and HYROX simulations are fuelled. Dieting hardest in the heaviest training week is a common route to illness and injury.
Protein and strength work
Protein intake of about 1.6 to 2.2 g per kg (0.7 to 1.0 g per lb) of body weight per day and continued resistance training are the two levers that protect lean mass in a deficit. For hybrid athletes the strength work is already in the plan. The cost of concurrent training is covered in the interference effect, and that cost rises when energy is short.
Body recomposition: when it works
Losing fat and gaining muscle at the same time is realistic for athletes new to strength training, athletes returning after a break or an injury, and those carrying more body fat. Lean, well-trained athletes recompose slowly. For them, a defined deficit phase followed by a maintenance or small-surplus phase usually gets further than an open-ended recomposition. This is an editorial judgment from the training literature, not a single study’s result.
Measuring progress without being fooled by water
Training athletes swing 0.5 to 1.5 kg (1 to 3 lb) between mornings from glycogen, sodium and gut content alone (see water weight for athletes). Weighing fasted under fixed conditions and judging a 7 to 10 day trend is the minimum. Predicting the next morning from the trend and the previous day’s food and training, then checking the scale against the prediction, separates fuel from fat even faster. Stalls that last under two weeks are usually water (see plateau or water?).
Warning signs: REDs
The 2023 IOC consensus statement on Relative Energy Deficiency in Sport (Mountjoy et al.) describes the health and performance effects of low energy availability. Signs that a diet has gone too far include stalling or falling performance, repeated illness or injury, poor sleep, low mood or libido, and menstrual changes in women. When these appear, the deficit stops, whatever the scale says.
FAQ
- How fast can an athlete lose weight without losing performance?
- About 0.5 to 1% of body weight per week. In a trial with elite athletes, a target of 0.7% per week preserved and even increased lean mass, while a faster target of 1.4% per week did not (Garthe et al., 2011). For a 80 kg (176 lb) athlete, 0.7% is about 0.55 kg (1.2 lb) a week.
- Can you lose fat and build muscle at the same time?
- Yes, under specific conditions: newer lifters, athletes returning after a break, people with more body fat, and anyone eating high protein with progressive strength training in a modest deficit. Well-trained, lean athletes recompose slowly and usually do better with separate phases.
- Should you diet during marathon or HYROX training?
- A modest deficit in base and early build phases works for most athletes. In the final 4 to 6 weeks before a key race, and in peak training weeks, eating at maintenance protects session quality and recovery. Race weight is set in the months before the taper, not during it.
- What is RED-S and why does it matter when dieting?
- Relative Energy Deficiency in Sport (REDs) is the set of health and performance problems caused by too little energy left over after training. Warning signs include falling performance, frequent illness or injury, disturbed sleep, low libido and, in women, menstrual changes. A diet that produces these is too aggressive regardless of the scale.