Boring Critical Details in a Flashy Intermittent Fast Study

At first glance, it’s a fascinating study. An intermittent fast, gut microbiota, and an effect on the risk for heart disease – the title of this new study in JCEM pushes all the right buttons. But a look under the surface tells a different story. The primary outcome measures for this study were BMI and fat mass. However, you’ll find nothing about those outcomes in the abstract.

Instead, the findings are all about secondary outcomes – an association between the functioning of gut microbes and cardiometabolic risk factors. In an 11-part tweetorial, Nicola Guess describes this paper as an exemplar for “everything that’s wrong with the publishing side of science.”

All that sizzles is not steak.

Pay Attention to Primary Outcomes

At the top of the list of details that matter in a study, it’s important to know what the primary aim was. A good paper will provide this detail straight up. In this paper, the wording is vague. The authors describe a main outcome measure:

“Cardiometabolic risk factors including body composition, oxidative stress, inflammatory cytokines, and endothelial function were assessed at baseline and 8 weeks. The diversity, composition, and functional pathways of the gut microbiota, as well as circulating gut-derived metabolites were also measured.”

Indeed, this list is a smattering of things the researchers studied. But it does not contain the primary outcome measures of the study. For that, you need to look up the clinical trial registration on That is where you will find that when they started this study, the intent was to find if an eight-week course of fasting two days per week would cause a reduction in BMI or fat mass. In the control group, people ate their regular diets. The control group received no dietary counseling.

Subjects in both groups lost weight. But the authors do not present evidence that either BMI or fat mass improved more in the intermittent fast group than in the control group. They just gloss right over the findings on these two primary outcomes. If you read closely, you can find something about the differential effect on weight. That’s all.

Sexy Secondary Outcomes

This study protocol specified 15 secondary outcomes and seven “other outcome measures.” Guess describes this as a dubious tactic:

“This way, if you don’t find any effect of your intervention on your primary outcome[s] (or your primary outcome is pretty boring), you can pick and choose whichever one does change significantly!”

Indeed, everything in the title, abstract, and conclusions of this paper is all about selected non-primary outcome measures. For added appeal, it includes lots of language to suggest the research demonstrates cause and effect. Even though it does not.

Bottom Line

If you read very carefully, you can find the truth buried within this paper. It is an “exploratory” study with essentially “observational findings.” The small sample size means that the findings have limited application. In fact, the bottom line is simple. When you ask people to fast two days per week for eight weeks, they will eat less. So they lose weight and some of their metabolic parameters change.

But no, this study does not demonstrate important effects on the microbiome and cardiometabolic risk factors with an intermittent fast. For that, you need a well-controlled study with that as a primary objective.

Click here for the paper and here for the tweetorial on its flaws. For more on intermittent fasting, click here and here.

Bored, photograph © Matt Deavenport / flickr

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October 12, 2020