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Food Standards Australia New Zealand established the amount of sugar substitutes you can have over a lifetime without any appreciable health risk. "To reach the Accetable Daily Intake for aspartame someone weighing 70KG would need to consume 2.8g a day - equal to about 20 cans of diet soft drink or more than 100 standard 1g packets of sweetner"
But it also says a 2004 study in the International Journal of Obesity that when we offer our bodies sweet diet drinks without the KJ's, they crave real sugar even more.