A Rant on Correlational Studies

This is something that has been on my mind often lately. I’ve been reading a book called ‘The China Study.’ I’ve been quite annoyed at much of what I have read so far. The main reason it vexes me is that the author relies far too much on correlations from uncontrolled/observational research.
Let me first say, that uncontrolled/observational research isn’t useless. It’s just that in many cases, it has fooled individuals into making choices about food that are flat our wrong. If you look at many of the topics I write about that mention nutrition, you may notice I dispel a lot of myths. Many, but not all these fitness myths were created from individuals who misinterpret epidemiological research.
Let’s go over a few examples. You may have heard that artificial sweeteners are associated with higher BMI’s. Here is study that found that tea and coffee drinkers who consume artificial sweeteners (AS) have larger BMI’s than tea and coffee drinkers who stay away from AS. BMI’s in the AS vs the non-AS group was 28.2 vs. 27.1 kg/m(2) in men, 28.4 vs. 27.1 kg/m(2) in women.
So essentially what this study found, is that individuals who consume a cup of coffee with AS are fatter than people who don’t consume AS. AS contains trace calories (if any) and yet it makes us fatter? If this doesn’t make any sense to you check this out to understand energy balance. If you are consuming a packet or two of AS, maybe you are adding 2-4 kcals per day in your diet. How is that small amount of calories going to make you fat? It won’t!
So what was the reason that these individuals had a higher BMI? There really are endless possibilities. Could it be that people who had AS in their beverages also had a donut/cookie/croissant with their morning caffeine fix? Was their caloric intake higher than the non-AS group? Without knowing the answer to these (and many other) questions, you can’t say that AS causes people to have a higher BMI. You need to keep in mind that the results of this study are correlations. Correlation doesn’t equal causation!
A very smart man by the name of Alan Aragon just published a new research review. He goes over this subject a little and he talks about different kinds of studies. He states that randomized control trials (RCT’s) are the gold standard of evidence. What is so special about RCT you may be thinking. Well RCT’s find an cause-effect relationship.
An RCT could be used to find out how an artificial sweetener such as aspartame effects the BMI of individuals. You’d have two groups, a control groups who would consume aspartame and a placebo group consuming well, a placebo! The researchers would look at how the groups differ after a period of time passes in terms of body mass index (BMI) and waist circumference (WC) for example. This study would show how an individual substance can cause an effect.
All the questions that you answer in an epidemiological study are erased. You are left with a variable, so if the results between the AS and non-AS group are different, you know that the variable is the reason for this difference. This is key!
I could go on and on with examples. I don’t want to waste your time though, when you could do a quick search on this here sight and find a ton of myths that I have revealed on my site. If you want to see more myths, just look!
I guess I can understand how some people come to believe what they are told by magazines, media etc. Not many people spend time on pubmed searching out if what they hear is the truth. If you really care about how to find out about what is true or false in the fitness industry keep coming back here. You can also check this out if you like a more hands on approach: How To Read Fitness Research by a great coach named Mark Young (I don’t get anything if you buy, it’s just a great resource).

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