Could sugary and diet soft drinks cause depression?

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

7 Responses to Could sugary and diet soft drinks cause depression?

  1. Pippa 11 January 2013 at 8:10 pm #

    What a relief about the coffee. Best part of the day. By the way, do you think the aluminium which probably gets into the cup from using Nespresso coffee capsules could be doing my brain any harm; I have heard aluminium has been linked to Alzheimers. Is there conclusive evidence about this?

  2. Chloe 11 January 2013 at 11:52 pm #

    Coffee has it’s downsides however, it’s been shown to worsen PMT symptoms in women…

  3. Dr. Philip Domenico 12 January 2013 at 12:27 am #

    No mention of phosphoric acid, which is widely used in soft drinks and steals minerals from the body. Colas are especially loaded with this acid, whereas coffee has the beneficial chlorogenic acid, especially when not over-roasted. Could be an acid-alkaline imbalance involved.

  4. Dorothy MacVicar 12 January 2013 at 2:25 pm #

    I’ve just started drinking coffee again, one cup in the morning, after 10 years… and look forward to my morning kick!- glad it’s now officially OK… Will be showing my children your comments on fizzy drinks, not that they have them very often.

  5. Lorraine 12 January 2013 at 3:40 pm #

    Thank you Dr Briffa. Very interesting.

  6. Fiona 12 January 2013 at 11:55 pm #

    Very interesting, as always. Normal coffee makes my pulse race and my head ache, so I always drink decaff. Are any of the beneficial contents of coffee lost in decaffeination? I noticed recently that Kenco (I think) are selling ‘really decaffeinated coffee’ so presumably there’s still some caffeine left in what I’ve been buying up till now.
    in what I’ve been buying up tilll now.


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