Lean muscle mass burns more calories, that is why trainers try to build up their customers who are trying to lose weight and stay in shape. If you look at endurance athletes, and other people who participate in endurance activities, they are lean and not overly muscular. Fitness will aid tremendously in a survival situation, and putting on a few extra pounds prior to an anticipated drop in caloric intake is not a bad idea, if you are physically fit already. Fitness is good, but body builders will not last long without food.