In [1], Hill and Belsito report on the case of a 60-year old Caucasian woman with a 6-month history of eyelid dermatitis. Despite application of a corticosteroid-containing ointment and discontinuation of eyelid cosmetics and nail polishes for 2 months, her rash did not clear. Several years back, in response to a facial dermatitis, she had been patch tested and found to be allergic to formaldehyde, quaternium-15 and fragrances. As expected, her facial dermatitis was resolved by swtiching to formaldehyde-, quaternium-15- and fragrance-free facial and nail cosmetics.
For this current condition, she was again patch tested. Positive reactions occured to formaldehyde (++), quaternium-15 (++), diazolidinyl urea (+), DMDM hydantoin (+) and imidazolidinyl urea (++), with her hair care products and cleansers containing multple sources of these. She was instructed on ways of avoiding such allergens, with particular emphasis on formaldehyde and formaldehyde releasers. Despite doing so for three weeks, her condition improved only slightly. A local pharmacist suggested avoiding aspartame, which she had started using as an artificial sweetner 5 months prior to the onset of her condition (approximately 80 mg per day). Within a week of discontinuing use, her eyelid dermatitis was resolved completely and has yet to recur.
The resolution of the case, of course, naturally suggests that the aspartame was the cause of her eyelid dermatitis. Indeed, following consumption, aspartame is hydrolyzed in the intestine to phenylalanine, aspartic acid, and aspartic acid methyl ester. The methyl ester is converted into methyl alcohol (methanol) and carried by the portal vien to the liver. Here it is oxidized into formaldehyde and then converted into formic acid by alcohol dehydrogenase, aldehyde dehydrogenase and the microsomal oxidase pathway. This process occurs not just in the liver but in other parts of the body containing these enzymes, including the eye.
To be fair, previous studies have shown that does of aspartame beyond 100 mg/kg are required to raise methanol blood levels, hence formaldehyde and formic acid, above contol. This correponds to drinking 35 cans of diet beverage in one sitting for a 70 kg person. Nevertheless, it may well be the case that blood and urine are not the right places to look for formaldehyde.
References
[1] A.M. Hill and D.V. Belsito, "Systemic contact dermatitis of the eyelids caused by formaldehyde derived from aspartame?" Contact Dermatitis, vol. 49, pp. 258-259, Nov. 2003.