You might have heard someone claim that because they exercise regularly, they can eat whatever they want and still be healthy. When people say this, they seem to mean that either exercise OR diet can be sufficient to foster a healthy lifestyle. However, a recent study counters this "either/or" view. Like many other studies, this one found evidence that both diet and exercise contribute to health outcomes in adults.
In this blog post, we'll explore how the study can be viewed as a factorial design (Chapter 12), and we will discuss how its main finding can map on to different factorial outcomes.
The empirical study appeared in the British Journal of Sports Medicine, which may be paywalled. I learned about the study from number of journalists who covered it, including this accessible summary in Healthline, which we'll use in this post.
Here are the opening bullet points from the Healthline story:
- A new study finds that in order to live longer, people need to eat healthily and exercise regularly.
- While a balanced diet and regular exercise independently have plenty of health benefits, the study notes the greatest reduction in mortality risk comes from combining diet and exercise together.
a) These bullet points mention the study's three main variables. One of the variables is mortality risk, or longevity (lifespan). What are the other two variables?
b) Chapter 12 concerns factorial designs, in which there are two (or more) independent variables (IVs) and one dependent variable (DV). Reflect back on the three variables you listed in a), above, and identify which are the IVs and which is the DV. In addition, identify the levels of each IV.
Here are some details about the study's method and results, as summarized by the journalist:
For the study, the researchers evaluated the health and exercise data of 346,627 individuals over the course of 11 years. They tracked how much exercise each participant completed in an average week along with how intense their physical activity was. The team also followed the participants’ eating habits. During the study window, 13,869 participants died — 2,650 from heart disease and 4,522 from adiposity-related cancers.
The research team found that any type of regular exercise was associated with a lower risk of mortality, and those who both ate well and exercised had the lowest risk of dying.
In addition, while exercise and diet are independently linked to a lower risk of mortality, high levels of exercise cannot entirely mitigate the harms of a poor diet.
I made up three patterns of data to illustrate possible outcomes of this study. You can see the graphs below, labelled Outcomes A, B, and C. Take a moment to orient yourself to the graphs, noticing how the DV is on the y-axis, one IV is on the x-axis, and the other IV is depicted using line color:
c) Which of these outcomes (A, B, or C, above) is consistent with the idea that EITHER exercise OR diet alone is sufficient to maintain a longer lifespan? Explain your reasoning.
d) The journalist wrote that "The research team found that any type of regular exercise was associated with a lower risk of mortality, and those who both ate well and exercised had the lowest risk of dying [i.e., the longest lifespan].” Two of these outcomes (A, B, or C) are consistent with this statement. Which ones?
e) In your own words, describe the difference between Outcome A and Outcome B.
f) Based on what the journalist wrote, can you tell which of these two (Outcome A or Outcome B) the study found? Specifically, which one could go with how the journalist described the study's outcome:
“In our study, those who ate a poor-quality diet and were active still had substantially reduced mortality risk than those who ate a poor-quality diet and were inactive,” Melody Ding, MPH, PhD, the lead author of the study and an associate professor of health and medicine in the Sydney School of Public Health at the University of Sydney, told Healthline. “It’s just that you had the maximum risk reduction when doing both things right.”
g) Just for more practice: Two of the outcomes above depict an interaction effect (an interaction between diet and exercise). Which 2 outcomes are they? Explain your reasoning--how do you know?
Suggested answers:
a) Besides longevity, a second variable is diet quality and a third variable is exercise habits.
b) Longevity is the DV. Diet quality is an IV (levels could be good vs. poor diet). Exercise habits are also an IV (levels good be regular exercise vs. no exercise)..
c) Outcome C. Outcome C shows that you have the same lifespan if you exercise or eat a good diet, and that doing both does not increase your lifespan any more than doing one or the other (all three conditions--exercise alone, diet alone, and both--show the same lifespan). Note that this is not actually the result that the study found.
d) Both Outcome A and Outcome B are consistent with the journalist's statement.
e) The difference between Outcomes A and B can be summarized with the terms "additive" versus "interactive."
Outcome A shows an additive effect--A good diet adds (in this example) 3 years to your life; exercising adds 3 years, and if you do both, it adds 6 years. One is good, both is better.
Outcome B shows an interactive, or multiplicative effect. It shows that if you do both diet AND exercise, you have an especially long lifespan. That is, in this example, a good diet adds 3 years to your life; exercising adds 3 years, and if you do both, it adds 11 years. One is good, both is amazing (kaboom!). In Chapter 12 you'll learn that this is an interaction--the combination of the two IVs results in something even bigger than simply adding the two together.
f) In my view, we can't tell which outcome the study found based on what the journalist wrote. However, since they used the terms "both" rather than "especially", it seems like it might be Outcome A. My reading of the empirical study backs this up--it seems to be an additive effect (not interactive).
g) Outcome B and C both show interactions, but of different types. First, you can see the interactions in both because the lines are not parallel--there are differences of differences. That is, the effect of diet is different, depending on if we're talking about regular exercise or no exercise.
In Outcome B, there is an interactive, or multiplicative effect--I sometimes call this an "especially" interaction. It shows that if you do both diet AND exercise, you have an especially long lifespan (compared to doing either one alone).
In Outcome C, there is a different kind of interaction--I might call it an "only if" or an "it depends" interaction. You could explain it by saying that lifespan is reduced only if you don't exercise AND don't eat well. If you exercise, or diet, or both, you'll live longer.
