While millions flock to church every Sunday to feel themselves filled with the power of the Lord, a new study claims that all that fullness may be translating to their waistlines as well.

This week, a group of researchers from Northwestern's Feinberg School of Medicine published a study reporting that religious people were 50 percent more likely to become obese. It's the kind of study that lends itself happily to such clickable headlines as, "Faith Makes You Fat." The researchers found a correlation between how often participants attended religiously oriented activities—like mass or church groups—and how likely they were to become obese.

But on closer inspection, it's not clear what this study really tells us. The conclusions are based on a cohort of subjects who were examined for the first time more than 20 years ago. At the onset of the study, they were asked about their religious predilictions. They were never asked again. Their obesity rates were re-checked in 2005. Their religious habits were not. Therefore, the Northwestern research is linking religious habits as reported by a cadre of twentysomethings in 1987 with obesity rates nearly 20 years later. This opens the door to a whole host of alternative variables that could explain the study's findings.

Matthew Feinstein, a fourth-year medical student at Northwestern and the lead author of this research, readily admits that it would have been preferable to have an up-to-date understanding about how religious the subjects of his study actually were.

He and his colleagues were examining data taken from a group of roughly 5,000 Americans who were signed onto a longitudinal study in the 1980s focused on cardiac health. From the beginning, the study's stated focus was heart health and the factors that affect it over time.

The study participants were examined regularly for various health-related markers. It was in year two of their participation that they were also asked how often they attended religious events. Only a fraction of the total sample chose to respond to the religious question at all. Those who said they attended at least one religious-oriented activity per week were classified as "religious."

In 2005 and 2006, researchers did their year-20 assessment of the study participants' cardiovascular health, looking at such factors as their body mass index. It was from this pool of data that Feinstein and his colleagues pulled a statistical link between what the participants reported about their religious habits and how much they weighed two decades later.

The researchers found a correlation between how often participants attended religiously oriented activities and how likely they were to become obese.

The idea for the study came in the context of previous research that establishes a link between religiousness and obesity rates, says Feinstein. Most of the existing research, however, is what's known as "cross-sectional," meaning a snapshot of a given sample of people at one moment in time. This type of study can be hard to interpret. One problem, Feinstein points out, is that, as others have proposed, it's possible that a "cross-sectional association between religiosity and obesity occurs because obese people are more likely to seek the welcoming environment of religious groups," adding, "Our finding of a longitudinal association suggests that this is not the case."

Part of the logic at work here for Feinstein and his team is that religiosity in early adulthood likely predicts religiosity in later adulthood and is therefore relevant to a finding made 20 years after the fact. Perhaps this is true. But it isn't exactly research to hang your hat on.

Casey Schwartz is a graduate of Brown University and has a Masters Degree in psychodynamic neuroscience from University College London. She has previously written for The New York Sun and ABC News. Currently, she's working on a book about the brain world.