Copyright: Christopher Elwell

Is your diet cola blinding?

Pivot Points is a monthly column by EuroScientist writer David Bradley.

The artificial sweetener aspartame is one of the darling molecules of the scaremongering tabloids and blame-seeking activists, there’s even a Facebook page aimed at banning it. I’ve written about its purported toxicity several times over the years and am yet to be convinced by any of the horror stories; the chemical evidence against it simply doesn’t stack up.

Aspartame comprises two amino acids, phenylalanine and aspartic acid hooked together by a methylene (CH2) bridge. Break aspartame down in vivo and the amino acids are released together with a small amount of methanol. Fundamentally, this is what happens when you ingest the compound and the three chemicals released are then handled by the body as if they were ingested separately.

The most recent toxicity study that has come to my attention discusses dosing rats in the laboratory with 500 mg or 1000 mg of aspartame in water every day for six months and apparently shows detrimental effects on liver function in rats.

Let’s take a look at those dosages. A single can of diet soda might contain 180 mg of aspartame. The molecular weight of the molecule is about 294, methanol has a weight of 32. A quick calculation would suggests that there is approximately 20 milligrams of methanol per can. If it is all on ingestion and ignoring the chemistry and absorption (given that digestive systems are not 100% efficient in extracting nutrients from food and drink, then what happens to the methanol, the purported toxic culprit in aspartame-containing products?

Methanol is certainly toxic. Just 10 ml can cause permanent blindness. The fatal dose can be as low as 25 ml but is usually about 100-125 ml. Methanol is about 800 mg per ml density. So the 20 mg I mentioned equates to 0.025 ml. Conversely, 10 ml of methanol is 8000 mg, or 8 grams. Forty cans would equate to 1 ml exposure, theoretically.

However, there is an additional factor to take into consideration. The Materials Safety Data Sheet (MSDS) for methanol suggest that it is a cumulative poison, which is why people who drink methanol-contaminated hooch gradually succumb to its blinding toxicity. However, if the methanol is fully released from aspartame and readily absorbed from the gut, peak serum concentration will occur an hour. The methanol is then only slowly metabolized in the liver to aldehyde (and then quickly to formic acid, which is the agent of blinding harm). Within 90 minutes, half of the methanol has been broken down and excreted. So, could it accumulate quickly enough if you’re a regular diet drinker?

Drink a single can and you would, as mentioned, ingest 0.025 ml. If you were to drink a six pack in 90 min that would give you about 0.15 ml. About 1.5% of the minimal harmful dose. After 90 min, half of that would be gone. So, you would be down to 0.075 ml. If you drink another six-pack of diet soda during the next hour and a half you would ingest another 0.15ml, bringing your total up to 0.26 ml. That figure would itself half in 90 minutes. Then you drink another six-pack bringing it up to 0.28 ml. If you repeat the process all day the cumulative dose will tend towards 0.3 ml. That is merely 3% of the minimal dose known to cause problems with the retina.

To reach a cumulative level of the hazardous quantity, 10 ml, you would have to drink several hundred cans of diet soda every day.

Much of the scaremongering lingers on this breakdown of aspartame and claims that diet drinks exposed to high temperatures are somehow more dangerous; they have even been blamed for Gulf War Syndrome. But, the three components are released into your system physiologically so thermal degradation is irrelevant. Of course, people with the rare genetic condition phenylketonuria must not ingest phenylalanine from any source including proteins in their diet or aspartame. It is possible that these two amino acids, and others, are excitotoxins that at high dose can interfere with normal neuronal activity and this has been implicated as a causative effect in the elusive etiology of the disorder known as fibromyalgia. But, any evidence regarding the purported health effects of aspartame hinge on very high exposure.

Indeed, the rats in the experiment I mentioned earlier were drinking 1000 mg daily, the equivalent of an adult drinking 400 12oz cans of artificially sweetened drink every day for six months. Now, even the most ardent cola fan on a “diet” could not physically achieve that and if they were trying in earnest to do so, then one would assume that they had far worse health problems and psychological issues than any that aspartame itself might cause.

Featured image credit: Christopher Elwell via Shutterstock

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David Bradley

David is a freelance science writer with more than thirty years in science communication. His best-selling book, Deceived Wisdom is available now.
David Bradley

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