Population-level data, when combined with aspartame consumption, is enough to give us an approximate upper limit on how toxic aspartame is. The largest issue w

Population-level data, when combined with aspartame consumption, is enough to give us an approximate upper limit on how toxic aspartame is. The largest issue with estimating the upper level of toxicity would be if another unaccounted for population change decreased the risk of cancer at the same time aspartame increased the risk. Then, it's unfair to assign aspartame the entirety of that increased risk. There's probably hundreds of things introduced in modern life that have some impact on health but aren't fully accounted for in population level data. All of this is to say, if aspartame caused cancer at the same rate as smoking, then we'd probably know about it by studying population-level data. There is a level of health impact that we should be concerned about but could get lost in all the other things happening to the population.