low nutritive value food such as consuming mainly carbs, proteins, and fats...

I would say neither, but if I had to choose, I would say excretory system. Why would you say digestive system? I'm wondering. The response why I chose this is because without carbs, for instance, you wouldn't have a source of fibre. Fibre acts by changing the nature of the contents of the gastrointestinal tract, and by changing how other nutrients and chemicals are absorbed. Soluble fiber absorbs water to become a gelatinous, viscous substance and is fermented by bacteria in the digestive tract. Insoluble fiber has bulking action and is not fermented, although a major dietary insoluble fiber source, lignin, may alter the fate and metabolism of soluble fibers.