Could sugary and diet soft drinks cause depression?

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In a recent blog post I wrote about research which linked consumption of artificial sweeteners and aspartame in particular with certain cancers. This sort of ‘epidemiological’ evidence cannot prove that aspartame or other artificial sweeteners cause cancer. However, as I pointed out in the blog post, we have research which shows that giving aspartame to animals in not-excessive doses has the ability to provoke similar cancers. We also know that one element of aspartame is methanol, which can metabolise in the body into formaldehyde – a chemical that recently made its way onto the official ‘cancer-causing’ (carcinogen) list. Put this all together, and it’s my belief that there’s more than enough evidence for us to avoid consuming aspartame.

Some other epidemiological evidence just announced might also give us pause for thought. The research (you can read about it here), due to be presented at the American Academy of Neurology’s 65th Annual Meeting in San Diego in March, has linked diet drink consumption with an increased risk of depression. Sugar-sweetened drinks also had some links here, but these were weaker than those for diet-drinks. Coffee, on the other hands, was associated with a reduced risk of depression.

Again, epidemiological studies only reveal links between things, and do not tell us if one thing is causing another. However, are there any plausible mechanisms that might explain these associations?
Well, one prominent theory is that at least some cases of depression are caused by inflammation in the brain. Inflammation is what is behind the redness, swelling and pain that may come after bashing our thumb with a hammer. However, as far as depression goes, what we’re really talking about here is lower-grade inflammation that permeates the body and brain. Spikes in blood sugar (as a result, perhaps, of glugging down cans of sugary soda) are known to be inflammatory. So, here we have at least a potential explanation of the link between sugary soft drinks and depression.

What about diet drinks? Well, other than methanol, aspartame contains aspartic acid and phenylalanine. Aspartic acid is sometimes referred to as an ‘excitotoxin’, which means it has the potential to stimulate and damage nerve cells. And phenylalanine has the ability to interfere with the functioning of neurotransmitters that can impact on mood. These effects might possibly, therefore, provoke psychological symptoms including depression. Perhaps we should not be too surprised when we find a well-conducted study showed giving aspartame to individuals prone to depression worsened their symptoms compared to placebo [1].

And what of coffee? Coffee is rich in substances known as ‘polyphenols’ which have ‘antioxidant’ (disease-protective) function. Some polyphenols have the ability to quell inflammation, and there’s the possibility therefore that coffee might genuinely reduce the risk of depression through this mechanism.

In the final analysis, we simply don’t what impact sugared or artificially sweetened drinks have on mood, but as with the cancer research, I think there’s enough evidence and basic chemical knowledge to be wary of them. I also see nothing here which persuades me I should ditch my morning coffee.


1. Walton RG, et al. Adverse reactions to aspartame: double-blind challenge in patients from a vulnerable population. Biol Psychiatry. 1993;34(1-2):13-17

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