"Question. Which Spartan is best?"
"That's a ridiculous question."
"False. Master Chief."
"Well, that's debatable. There are basically two schools of thought."
"Spartans shoot Sangheili. Spartans. Sangheili.....Battle Star Galactica."
In Halo: First Strike, on page 120, there's something about this.
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What had seemed like weeks, weeks of hell, had only been but a few days. The noncom was tired. But he was a Spartan, he didn't have the time to be tired or at least to show it.
"Chief, the rest of the crew needs some rest. I'll monitor the ships systems while you all get a few minutes of shut eye." Cortana said, fully aware of the Chief's absolute requirement to shut his biological systems down. She was also fully aware that her Spartan was a proud one, and would never admit his desire to rest.
What the Chief said next surprised everyone, especially the AI.
"What I really need to do, Cortana, is to take a dump. Can we get me out of this armor - I haven't pooped in 6 days, since before Reach fell. I have a massive turtle head poking out."
"GROSS!" Said Cortana, as her olfactory sensors spiked. "DID YOU JUST FART?!"
"Hu-hu, my shields can keep stuff out but they sure can't keep anything in, can they? Hu hu." He replied.
"Woooooo-eeee! My Spartan be fartin'!" Cortana shouted!
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Back on topic, I think that in the field the Spartans take certain supplements to give them energy and keep them nourished but not to produce excess waste. Also, the average length of intestines in a grown, normal human is over 25 feet. A huge Spartan would have more, so more area to store stuff!