The human body can't break it down, or use it in any way. And as it turns out, modern wastewater treatment methods don't break down Splenda either.
Smitha Ramakrishna, a finalist in the 2009 Intel Science Talent Search, found that the sweetener can accumulate in the water supply after people excrete it. This could potentially cause harm to fish and other living creatures.
Scientific American reports:
"She tried to start doing research at Arizona State University, though since she was the first high schooler her lab had ever had ... [E]ventually she was allowed to subject sucralose to various treatments, like bacterial digestion, typically used in wastewater treatment plants.
She found that sucralose resisted most of these treatments ... that means almost all the sucralose people eat or drink winds up in the ecosystem."