I was surprised to find out that, indeed, Aspartame does decompose
into three primary byproducts either during metabolism, or during
heating. Two are amino acids, aspartate and phenylalanine. The other
product is methanol. Methanol is commonly known as wood alcohol. It
is a toxin in humans.
That said, it is important to remember that mainly compounds that are
toxins may not be toxic at the level which we encounter them in our
environment. Rather than a "anything causes cancer/death in a high
enough dose", I would instead view that statement as simply a note
that we should be concerned when we encounter compounds at or near
toxic levels. The only time this may not be true is when the compound
has a cumulative effect, that is to say, it builds up in our bodies
(such as formaldehyde is proposed to do in some of the comments
above).
I searched on Medline for "aspartame AND methanol" and reviewed the
abstracts of a number of scientific articles and one personal letter.
The personal letter was of particular interest as it critiques the
statements made on a number of the pages linked in the first three
comments.
The reference is:
T.R. Tephly. "Comments on the purported generation of formaldehyde and
adduct formation from the sweetener aspartame." Pharmacology Letters.
65(13):PL157-60, 1999.
In the article, Dr. Tephly suggests that many of the studies that have
been carried out are not accurate in their measurement of methanol
measurement due to the differences in methanol metabolism between
higher order primates (us included) and lower order lab animals such
as mice/rats.
Dr. Tephly goes on to discuss some studies they have done on
metabolism of aspartame and methanol in monkeys in an attempt to study
the idea that methanol is formed at high levels and the idea that
formation of methanol would lead to formaldehyde bound up in the body.
"These direct studies of methanol in monkeys at large doses cast
considerable doubt on the proposal that formaldehyde adducts are being
formed from extremely low doses of methanol or aspartame."
His conclusion is fairly blunt:
"Based on the experimental data presented, aspartame appears not to
constitute a hazard."
While I cannot vouch for Dr. Tephly's nonaffiliation with an aspartame
manufacturer/user, he is faculty at a University rather than simply an
employee of Coke/Pepsi/etc.
The summary findings of many of the articles say that they found no
increase in methanol levels in the blood after consumption of large
quantities (e.g. 25x the 90% percentile consumption), nor did they
find elevated levels of a formaldehyde precursor. They did find
elevated levels of a formaldehyde precursor in the urine.
Many of the articles go on to note that indeed, more methanol is
ingested during the consumption of fruits and alcoholic beverages
derived from fruits (brandy). I did read a series of the links above
that state that the methanol ingested in fruit is different due to
factors found within the fruit. I would request more information
about this claim.
In summary, the bulk of the scientific literature does not state that
the quantity of methanol produced after consumption of even very large
quantities of aspartame poses a health hazard (very well support) nor
is the formation of formaldehyde adducts within the body a concern (I
found few articles on this topic). I found only one exception which
was mentioned several times in the links above and is the topic of the
first half of my answer: Trocho C. Pardo R. Rafecas I. Virgili J.
Remesar X. Fernandez-Lopez JA. Alemany M. Formaldehyde derived from
dietary aspartame binds to tissue components in vivo. Life Sciences.
63(5):337-49, 1998.)
Other abstracts I thought you might find interesting are included
below:
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"Safety of long-term administration of 75 mg/kg of aspartame per day
was evaluated with the use of a randomized, double-blind,
placebo-controlled, parallel-group design in 108 male and female
volunteers aged 18 to 62 years. Subjects received either aspartame or
placebo in capsule form three times daily for 24 weeks. No persistent
changes over time were noted in either group in vital signs; body
weight; results of standard laboratory tests; fasting blood levels of
aspartame's constituent amino acids (aspartic acid and phenylalanine),
other amino acids, and methanol; or blood formate levels and 24-hour
urinary excretion of formate. There also were no statistically
significant differences between groups in the number of subjects
experiencing symptoms or in the number of symptoms per subject. These
results further document the safety of the long-term consumption of
aspartame at doses equivalent to the amount of aspartame in
approximately 10 L of beverage per day."
Leon AS. Hunninghake DB. Bell C. Rassin DK. Tephly TR. "
Safety of long-term large doses of aspartame." Archives of Internal
Medicine. 149(10):2318-24, 1989 Oct.
------------
Aspartame (APM) is a widely used dipeptide sweetener
(L-aspartyl-L-phenylalanine methyl ester). It has been suggested that
excessive use of APM might elevate plasma aspartate, phenylalanine,
and/or methanol concentrations to levels that are potentially harmful.
Six normal young adults ingested eight successive servings of
unsweetened and APM-sweetened beverage at one-hour intervals in a
balanced crossover design. In one part, the beverage was not
sweetened. In the other, each serving of beverage provided 600 mg APM,
a dose equivalent to the amount provided by 36 oz of APM-sweetened
diet beverage. Plasma aspartate concentration was not significantly
increased after ingestion of unsweetened or APM-sweetened beverage.
Similarly, ingestion of the unsweetened beverage had no significant
effect on plasma phenylalanine concentration. However, ingestion of
APM-sweetened beverage significantly increased plasma phenylalanine
levels 1.41 to 2.35 mumol/dL above baseline 30 minutes after
ingestion. Plasma phenylalanine values reached a steady state after
administration of four to five servings and did not exceed normal
postprandial values at any time. Blood methanol and formate
concentrations remained within normal limits. The data indicate ready
metabolism of APM when administered at levels that may be ingested by
normal individuals who are heavy users of diet beverages.
Stegink LD. Filer LJ Jr. Bell EF. Ziegler EE. Tephly TR. "Effect of
repeated ingestion of aspartame-sweetened beverage on plasma amino
acid, blood methanol, and blood formate concentrations in normal
adults." Metabolism: Clinical & Experimental. 18(4):357-63, 1989 Apr.
--------------
Toxicology is based on the premise that all compounds are toxic at
some dose. Thus, it is not surprising that very large doses of
aspartame (or its components--aspartate, phenylalanine, and methanol)
produce deleterious effects in sensitive animal species. The critical
question is whether aspartame ingestion is potentially harmful to
humans at normal use and potential abuse levels. This paper reviews
clinical studies testing the effects of various doses of aspartame
upon blood levels of aspartate, phenylalanine, and methanol. These
studies demonstrate that blood levels of these compounds are well
below levels associated with adverse effects in sensitive animal
species.
Stegink LD. The aspartame story: a model for the clinical testing of a
food additive. [Review] American Journal of Clinical Nutrition. 46(1
Suppl):204-15, 1987 Jul.
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