Not content to leave methanol without drawing some theoretical conclusions, here’s the last I have to say on the topic.
- Blood’s buffering capacity is around 38.5 mEq/L/pH at physiological pH levels, and is more capable of buffering in an acidic than an alkaline direction.
- 1 mEq of formic acid is 46 milligrams.
- Acidosis is clinically defined as a drop in blood pH below 7.35 (Edit: I forgot to mention, blood’s usual pH is 7.4). By those numbers above, I figure it would take about 88 milligrams of formic acid to reduce the pH of a litre of blood by this amount – remember, you’ve got around 5 litres of blood. You’d have to have over 20 times as much formic acid as is produced by the consumption of one can of artificially sweetened soft-drink. That’s a pretty big margin of safety right there. I mean, as that professor in the document I linked below says, toxicity is all about the dose.
“The adage among toxicologists is “the dose makes the poison”–vitamin
A, iron, and selenium, to name a few, are required by the body but are
toxic at too-high levels.”
So – unless someone can show how methanol harms humans in some way unrelated to the production of formic acid, I’m pretty satisfied this particular metabolic byproduct of aspartame is safe. This is consistent with empirical studies. Next, the other two! Well later, anyway. Feel free to tear down my logic or chemistry in the meantime. It’s been a loooong time since I’ve tried to figure out some of this stuff.
Blood is a pH buffered solution, and the average adult has around 5 liters of it. A standard can of diet coke has approximately 200mg of aspartame, 10% of which is metabolised to methanol. That 20mg of methanol is metabolised rapidly by alcohol dehydrogenase into formaldehyde. This formaldehyde is then rapidly metabolised into formic acid via formaldehyde dehydrogenase – alcohol dehydrogenase is the rate limiting step in this process. By and large, the pathological effects of methanol are related to metabolic acidosis (a drop in blood pH) from build-up of formic acid.
I’ve been messing around with the chemistry for an hour or so, but am way too out of practice to write anything sensible about it. So for now, I’m going to go with the following bits and pieces I’ve found out:
- The lowest recorded fatal dose of methanol is 6 grams. That’s 300 times the amount of methanol produced by the metabolism of one can’s worth of aspartame.
- Methanol is metabolised with a half-life of around 1.5 to 2 hours in humans at levels found in artifically sweetened drinks, fruit juices and red wine.
- A glass of tomato juice contains enough pectin to produce approximately 85mg of methanol during metabolism – as compared to apple juice (21mg) and diet coke (20mg).
Draw whatever conclusions you like from that. Perhaps when I’ve got a bit more free time I might have another stab at calculating the ability of blood to buffer the formic acid. If you feel like having a stab, here’s a journal article outlining the buffering capacity of human blood.
Oh and check out this article too, on aspartame metabolism relating directly to methanol production.