Approximately 10% of aspartame (by mass) is broken down into methanol in the small intestine. Most of the methanol is absorbed and quickly converted into formaldehyde. Some experts/scientists believe that the metabolism of aspartame does not damage the body because: (a) the quantity of methanol produced is too small to disrupt normal physiological processes; (b) methanol and formaldehyde are natural by-products of human metabolism and are safely processed by various enzymes; and (c) there is more methanol in some natural fruit juices and alcoholic beverages than is derived from aspartame ingestion.^[16][17]

Other experts/scientists believe that (a) fruit juices and alcoholic beverages contain protective chemicals such as ethanol that block conversion of methanol into formaldehyde, while beverages with aspartame contain no "protective factors"; (b) exposure to very low levels of methanol and formaldehyde have been proven to cause chronic toxicity in humans; and (c) the low levels of methanol and formaldehyde in natural human metabolism are tightly-controlled and small increases above these levels can contribute to chronic poisoning.^[18][19]

In 1998, a team of scientists in Spain conducted an experiment on rodents to indirectly measure the levels of formaldehyde adducts in the organs after ingestion of aspartame. They did this by radiolabeling the methanol portion of aspartame. The scientists concluded that formaldehyde bound to protein and DNA accumulated in the brain, liver, kidneys and other tissues after ingestion of either 20 mg/kg or 200 mg/kg of aspartame.^[20] However, representatives of the manufacturer of aspartame have argued that these scientists were not directly measuring formaldehyde, but simply measuring levels of some by-product of the methanol from aspartame.^[21] Tephly thinks that the by-product was not formaldehyde. The researchers have stated that the data in the experiment proves it was formaldehyde.^[22]

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