But for those who are already slim and trim, you run the risk of burning off muscle, particularly if you are male. Don't purposely starve yourself. Eat more food, especially carbohydrates to spare protein. You've heard it said that guys are different from gals. Regarding weight loss and fuel utilization, it is certainly true.
Males are more likely to burn off muscle protein when fuel supply is limiting.
Females are more likely to burn fat. Back to losing fat. Whether it comes off the surface or from deep within, every hiker that has been on the trail more than a week has experienced the loss of abdominal flab and the hardening of the thighs and quads. Within a week off the trail, the weight is back on seeming to target around your middle. So use the energy stored in fat to build muscle while you're on the trail.