I happened to stumble upon this infographic about Diet Coke this morning – it’s trending on Facebook and has been repeated, unquestioned, in The Metro, The Mirror, and some other reputable news sources.

Here is the picture (click for a larger version):

The graphic, and its accompanying article, falls short on a lot of the things you might expect from a piece of scientific writing. I’m going to give some advice to the “Renegade Pharmacist” author, so that his articles can be better in the future.

You need a consistent list of references.

I understand that it is difficult to add references to such a small graphic, but I would highly recommend doing so. You don’t have to give the URL, just some information to let people find the paper, like:

Swithers SE. Trends Endocrinol Metab. 2013, 24(9):431-41.

This tells you the author’s name, the journal name, the issue number, and the page. At scientific conferences, information is often shared in the form of posters, and it is expected that posters will have a reference list.

In the longer article, you have some explanatory sections beneath each point, but then the references are all in a jumble at the bottom. Some of them have URLs, some of them have article titles, and some of them have author names. You can list them in the order that you use them, or in alphabetical order, but it’s important that they be consistent. There are several different referencing formats you could use. If you collect your sources together using a reference manager like Mendeley, it will even generate the reference list for you.

Every claim must be backed up with a reference.

Your graphic makes eight main claims:

1. Phosphoric acid is bad for your teeth.
2. Aspartame tricks your body into thinking you have eaten sugar.
3. Within 20 minutes,  drinking Diet Coke causes insulin release.
4. Within 20 minutes, Diet Coke increases the risk of diabetes and metabolic syndrome.
5. Within 40 minutes, the combination of caffeine and aspartame causes addiction.
6. After 60 minutes, drinking Diet Coke makes you want sugar.
7. After 60 minutes, Diet Coke depletes your body of essential minerals.
8. After 60 minutes, Diet Coke dehydrates you.

After you make each statement, you must immediately cite the source it came from. You can do this by putting a little number next to the statement that corresponds to the numbering in your reference list (just like Wikipedia does), or you can give the author’s name and the date of the paper (e.g. Nettleton et al. 2009) afterwards in brackets. I’m sure many of those claims are backed up by items in your reference list, but it takes effort to go through and find that out, whereas what you want is for people to be able to quickly assess the evidence you’re basing your argument on.

References must be from reputable sources.

Papers that are published in scientific journals are first scrutinised by experts in the field. Lots of papers don’t get through this reviewing process, as it involves going through the paper with a fine-toothed comb looking for problems in the science.  This peer review process is not perfect, and not every paper that is published turns out to be correct, but peer reviewed journals are still the best possible source of scientific information.

Some of your citations are from scientific journals, but many are from news articles or blogs. It’s important that your evidence is from the primary scientific literature.

You mustn’t extrapolate from unrelated studies.

One of your links to support the claim that Diet Coke causes addiction was to this paper from 2013, which looked at the effects of diet soda on rat brains. The final paragraph of its abstract reads:

“These results suggest that diet soda has adverse effect on the cerebellum of adult female albino Wistar rats.”

The scientists here have chosen their words carefully so that they do not overstate the implications of their work. This is a study that was performed on this very specific subset of rats, and gives information about this very specific subset of rats.

Generalising from animal studies to human studies is a really important area of biology, but it can’t be done on the basis of one study. The absolute best source of information is a study that looks at lots and lots of other studies, across a broad range of animals and in humans, like this one from 2007. These are called meta-analyses, and it is always worth trying to find one when you’re investigating a topic. For health information, the Cochrane library is very useful.

The credentials of the person you are referencing aren’t as important as the fact that the information is from a paper in a peer-reviewed journal.

There were a few sentences about the sources of your information that gave me pause.

“Susan E. Swithers, a professor of psychological sciences and a behavioural neuroscientist.”

“Marisa has over 20+ years as a weight loss therapist with her method proven to be the only one to work by the famous UK TV series Super Size Super Skinny”

On subjects like this, the professional qualifications of the person giving the information aren’t as important as the strength of the evidence that they’re basing their arguments on. This is why it’s important to look for peer-reviewed papers, because they (hopefully) demonstrate good evidence.

You depend too heavily on one source of information.

The paper “Artificial sweeteners produce the counterintuitive effect of inducing metabolic derangements” forms the crux of many of your arguments – in some cases, you quote the paper and its accompanying article verbatim. If you were merely providing a summary of this particular paper, this would be fine, but your article draws in evidence from lots of places, meaning that it needs to be more balanced than this.

You did some things correctly.

It is a very good idea to take a company’s claims about their product with a pinch of salt, and you rightly point out that these arguments apply to lots of different drinks, including Diet Pepsi, and off-brand versions of diet sodas.

You’ll note that I haven’t actually commented on the science put forward in the infographic. This is for a simple reason: I haven’t investigated the topic, and as such wouldn’t dream of holding forth on it.