Aspartame breaks down in the digestive track and doesn't enter circulation. It breaks down into three well known compounds that are found in much greater amounts in our natural diets, phenylalanine, aspartic acid, and methanol.
The amounts of these digestion products are much lower than those obtained from many other natural dietary sources.3,25 For example, the amount of methanol in tomato juice is 6 times greater than that derived from aspartame in diet cola.25 The amino acids aspartate (ie, anion of aspartic acid) and phenylalanine are very common in the diet, found in foods such as lean protein, beans, and dairy, with 100 g of chicken providing an almost 40 times greater intake of aspartate and a 12.5 greater intake of phenylalanine than a diet soda.25 In the body, the 3 digestion products follow their normal metabolic pathways, being broken down further, taken up by tissues in the body, or excreted. Thus, due to the rapid digestion of aspartame in the gastrointestinal lumen and small intestinal mucosal cells before reaching the bloodstream, the intact aspartame molecule is never present in internal tissues in the body or breast milk.3,25,28 The absence of aspartame in the breast milk of lactating women consuming aspartame was recently confirmed.21
https://academic.oup.com/nutritionreviews/article/74/11/670/...