Understanding the difference between correlation and causation is essential in being able to interpret nutrition information presented in the media. New studies related to nutrition come out virtually every day, and many of them end up in the headlines of your favorite newspaper, magazine, or website. Sometimes these studies can provide useful information, but often their results are presented in a misleading way.
As a result, we end up forming beliefs that may not be reflected by the actual science. A great example of this is the connection between breakfast and weight loss.
Every child hears the phrase “breakfast is the most important meal of the day.” Actually, we hear it as adults as well. It’s unclear where that belief started, but it may have been based on research. There are lots of studies, like this one, that connect eating breakfast with maintaining a healthy body weight. Some headlines even suggest that the larger the breakfast is, the better it is for you.
So, based on this information, it is reasonable to say that there is a connection (correlation) between eating breakfast and lower rates of obesity.
This does not, however, mean that simply eating breakfast when you haven’t already been doing so will lead to weight loss. Simply eating breakfast does not cause weight loss or a lower body weight by itself.
The best way to determine this scientifically is via randomized, controlled trials (RCT’s). This type of study isolates a variable (in this case, eating breakfast vs. not eating breakfast) and randomly assigns it amongst a large, random group of people. This can determine whether that variable is causing the change. This is different than an observational study, which can help identify factors that are connected (or correlated) in populations that are already exhibiting a certain behavior.
Observational studies often show that people who eat breakfast are less likely to be overweight or obese. RCT’s like this one and this one, however, show that randomly assigning people to eat or not eat breakfast does not affect likelihood of weight loss. The second linked study also demonstrates that eating breakfast does not positively impact resting metabolic rate during the rest of the day, as many in the nutrition field have long believed.
What does this mean? It simply means that adding in breakfast without making other changes is not a useful weight-loss intervention. It’s likely that people who consistently eat breakfast also have other consistent habits that do help with weight management. Perhaps they are also more likely to get quality sleep, exercise regularly, or eat more vegetables. Or maybe they’re just generally more consistent people, or more health-conscious. Maybe they eat breakfast because they’re health-conscious, and they’ve heard that eating breakfast is good for them. Which, all things considered, it probably still is.
I happen to believe that, for most people, including a quality breakfast with a protein source and a whole grain or fruit is generally beneficial for health and overall eating patterns. The lack of a causal relationship between eating breakfast and weight loss doesn’t change that. There’s a lot more to health than body weight. But if you don’t eat breakfast and think that you will lose weight simply by eating something when you wake up, it’s not quite that simple.