Interesting question, technically a pure fat bar would be the lightest at 9 cals per gram but i doubt that any good for racing. Its more the water content of regular riding food that will affect their weight but because you need water anyway if you carry it in your back pocket or in your bottle cage you still have to carry it. After racing a 24 solo I can confidentally say carry what every you can eat because once the food supply stops the legs will soon follow.