Headline Fantasies: Coffee and Obesity

“Coffee could slash obesity,” says the New York Post. Now you might think that cynical folks at the Post just make this stuff up because it’s so obviously false. But in fact, they have help from PR by the BMJ, and they’re not alone. The BMJ managed to induce quite a few news outlets last week to promote a paper from their journal with headline fantasies about coffee and obesity. For good measure, diabetes was part of the story.

Actually, the study was a good piece of work. But the PR spin was out of control – exaggerating the effect that caffeine might have on obesity and diabetes. In fact, this research did not specifically study coffee, it was all about caffeine. And there was no “slashing” – only the detection of a likely effect that’s likely modest.

Mendelian Randomization to Detect Causality

Make no mistake, this study has merit because Susanna Larsson, Benjamin Woolf, and Dipender Gill use Mendelian randomization to detect a causal effect of caffeine exposure on adiposity, type 2 diabetes, and cardiovascular disease. They crunched through genetic and outcomes data on individuals that numbered into the millions. In other words, they had a lot of statistical power to detect modest effects.

And they did find modest effects. Caffeine exposure in this analysis appears to have a modest, favorable effect on both adiposity and on the development of type 2 diabetes. It had no detectable effect on cardiovascular outcomes.

Modest Effects, Inconsistent Results

Given the modest effects Larsson et al detected, it’s hardly surprising that other studies have reported inconsistent findings. For example, a 2019 meta-analysis found only a modest association of coffee intake with less obesity in men, but not in women. A 2020 Mendelian randomization study found no evidence that caffeine intake could have a protective effect against either type 2 diabetes or coronary artery disease.

Sensationalism and Journal PR

Perhaps for the BMJ, this exercise in spin was worth it. Their press release, promoting a potential role for calorie-free caffeinated drinks to prevent obesity and type 2 diabetes, sparked some sensational headline fantasies about coffee. It generated 105 news articles.

But it didn’t do much to enlighten the public about caffeine, obesity, and diabetes. Instead, it sowed the seeds for confusion and magical thinking.

Click here for the study in BMJ, here and here for further perspective.

Fruit and Coffeepot, painting by Henri Matisse / WikiArt

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March 20, 2023