There is a lot to do about aspartame. People think it increases risk of cancer, harms gut microbiome, shrinks your balls; a lot of horror stories.
Let’s look at the facts. Your body breaks up aspartame into 3 harmless substances: 50% phenylalanine, 40% aspartic acid, and 10% methanol.
If these would be very harmful to the human body, then we should also stop drinking tomato juice and eating eggs and meat: they often have more of these metabolites.
The body breaks down aspartame into these substances very quickly, so there is almost no aspartame that reaches the large bowel to have any negative effect on the gut microbiome [Magnuson 2016].
The most recent research review *again* confirms aspartame poses no harm to health [Ruiz-Ojeda 2019].
Still, we have to acknowledge that companies that also sell diet soda have funded a lot of studies on the safety of aspartame, and we should remain very critical of new evidence.
For more details on the safety of aspartame, see our article here.
In light of all the evidence, I think it’s reasonable to drink diet soda or sweetened coffee/tea if it helps you lose fat, which is in itself very good for your health [Peters 2016].