Breakfast: Correlation is Not Causality

We've been saying for years that breakfast is the most important meal of the day. We've reported on research with a variety of results, including:

1. Protein at breakfast will help you feel fuller through the morning (10/13/10);
2. Those who ate a higher-fiber breakfast ate less at lunch (10/17/07);
3. Eating two servings of high-fiber cereal per week led to a reduction in risk of heart failure ("What is a healthy breakfast?"); and
4. Those who ate cereals or quickbreads for breakfast had, in one study, a lower Body Mass Index than those who ate meat and eggs ("What is a healthy breakfast?")

A common pitfall in interpreting research is mistaking a relationship between two facts (correlation) with one of the facts directly causing the other (causality). A good illustration of mistaking correlation with causality is as follows: "It's hot during the summer. There are more murders during the summer. Hot weather causes murders." While the facts might be true when taken at face value, given the number of other factors that are involved, there's still no evidence that hot weather directly causes homicide.

Back to breakfast. Statements 1 and 2 above, due to the good quality of the design of those studies, are likely causal. Statements 3 and 4, however, may or may not indicate cause. Researchers in the United Kingdom surveyed a cross-section of British adults (Appetite 2013;60:51-57) to find out whether they ate breakfast, how often, what they ate when they did eat breakfast, and what effect they believed that eating breakfast might have on their lives and health. They also asked about physical activity; height, weight, and waist circumference; and gathered demographic information.

Because this study was not a long-term study, but only a snapshot in time, the researchers did not focus on health issues but rather looked at links between eating breakfast and other behaviors and attitudes. They found that those who ate breakfast were far more likely to believe that eating breakfast helped with weight control or weight loss, and those who ate breakfast were also slightly more likely to participate in vigorous exercise. That said, there was no significant difference in Body Mass Index between those who ate breakfast and those who did not - although not all respondents provided height, weight, or waist circumference measures.

What this means for you

The researchers in this study concluded that the single fact of eating breakfast should be considered an indicator of a lifestyle that is likely to be healthier on the whole. In other words, if you eat breakfast you are likelier to lead a healthier lifestyle: there is a correlation between breakfast eating and health. Does the simple fact of eating breakfast cause lower Body Mass Index or protect you from heart disease? No.

Just because the link is not causal, however, does not mean that eating breakfast is not a good idea. The fact that eating a certain kind of breakfast will help you feel fuller throughout the morning and may help you eat less at lunch are both great reasons to make sure you start your day right.

First posted: December 19, 2012