Re: can you digest
Artifical sweeteners have essentially zero calories... so they can't make you fat.
Unless you have a condition known as phenylketonuria (PKU), the two amino acids that make up Aspartame can be metabolized (high levels of the unmetabolized phenylalanine can cause brain damage). While it is possible to have a sensitivity to the metabolic byproducts, I've not heard of Aspartame being harmful to your kidneys. Splenda, on the other hand, can't be digested so it is excreted in your waste, largely unchanged in structure.
Although both these sugar substitutes have been FDA-approved for human consumption (early 80s and late 90s, respectively), like with most foods, I would not consume these excessively.