Why I don’t trust anecdotal evidence and neither should you.

Has a friend ever told you to avoid a certain food because it will negatively impact your health in some way?  Chances are that this friend has also eliminated this food from their diet (or knows someone else that has) and they’ve seen amazing results.  I’ll be honest, I encounter this sort of thing all the time.  By definition this is what we call anecdotal evidence (or personal accounts) and here’s why I generally don’t trust it.

No Controls

The first reason why I don’t put a lot of faith in anecdotal evidence is that rarely are there any controls in place to ensure that the effect (let’s say weight loss) can be attributed to the treatment (a food or dietary practice).  You may have heard the phrase before: Correlation does not imply causation.  Even if it were true that weight loss followed a change in dietary practice, without controls in place there is no way to determine if the weight loss was a result of that change in dietary practice.

For example, there are people today who argue that gluten causes obesity.  They point to themselves (or others) who have eliminated gluten from their diet and subsequently lost weight.  The problem with this logic is that without being able to control for other variables, such as concurrent dietary changes, other lifestyle changes, total calories consumed, etc., there is no way to determine whether the weight loss experienced is a result of removing gluten from their diet or some other factor.  In fact, researchers have been unable to find a link between gluten and obesity, but you wouldn’t know this if you’ve been putting your trust in anecdotal evidence.

Conflicts of Interest

People have a wide range of motivations for communicating health information to you.  Even though your friends or family may genuinely just want what’s best for you, perhaps they like playing the health “expert”.  Others may stand to profit more obviously (monetarily or otherwise).  When anecdotal evidence in the form of personal experiences is used as a reason to justify a type of behaviour change, it’s important to ask yourself what the person telling this information stands to gain.  Often the incentive for communicating accurate information is overshadowed by their personal stake in you believing their claims.

The Placebo Effect

The placebo effect is a phenomenon that occurs where a placebo, or a fake treatment, has the ability to improve a patient’s condition, for no other reason than the patient believes it will be helpful.  In a person’s account of a dietary change there is no way to determine whether the effect they experienced is due to the treatment or a result of placebo.  In contrast, scientifically designed studies can determine whether the treatment has a greater effect than placebo alone, which is ultimately what you should be looking for.


So, the next time someone comes to you with nutrition advice, I encourage you to ask them: “Where is the evidence coming from?”  If it sounds anecdotal to you, be skeptical of the claims.  Chances are there’s a lot more to the story than what is being presented.  Remember the old adage is holds – if something sounds too good to be true, it probably is.


*originally published on on May 30, 2015

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