- Cardiologist Dr. Ethan Weiss has long been an advocate of intermittent fasting, recommending it to patients for weight loss and health benefits and practicing it himself for over five years.
- But Weiss said he quit fasting after his new study showed it has no apparent benefits compares to a normal eating patterns, and has rethought how he shares his personal experiences with patients.
- Weiss said it's evidence of how complicated nutrition science can be, and why it's important to be wary of anecdotal evidence and focus on rigorous data.
- Visit Insider's homepage for more stories.
A new study this week found that intermittent fasting may not live up to the hype its garnered in recent years.
The results prompted the lead author to quit the practice, which he's been following for more six years, and rethink recommending it to patients.
Dr. Ethan Weiss, a cardiologist at the University of California, San Francisco, had been interested in intermittent fasting since promising research about the topic emerged in 2013.
Within a year, he was trying it himself, limiting calorie intake to between noon and 8 pm each day. While he found it to be personally beneficial, there was little data yet of how well it worked in humans, prompting Weiss to instigate a clinical trial.
The study was intended to compare intermittent fasting with a normal eating pattern of three meals a day with snacks, without any prescriptions as to what or how much people would eat. This, researchers thought, would help see how fasting would perform in a real-world setting, and what the practical benefits could be.
When Weiss looked at the data, he was so surprised he asked his expert statistician to analyze it again.
"We didn't believe it at first," Weiss told Insider. "He'd send us the data and we would just scratch our heads."
But the numbers were unequivocal: there was no statistically significant benefit from intermittent fasting in this study either for weight loss or for other health outcomes.
The only distinction for fasting, in fact, was a negative side effect. Participants on the intermittent fasting diet lost significantly more lean mass than fat mass, compared to those who had regular meal times.
"What the study showed was that this is a lousy tool for weight loss for most people, and it may not even be the right kind of weight loss even if you get the pounds to come off," Weiss said.
This study illustrates the risks of relying on anecdotal evidence, and how powerful biases can be, even for experts
Weiss, who has long been an advocate for fasting because of the personal benefits he experienced, said the study's results highlight the pitfalls of anecdotal evidence and personal biases, and why good, rigorous data on nutrition is crucial.
"I had done it and become an advocate for it because it worked for me," Weiss said. "That's a good lesson — just because it works for me, doesn't mean it will work for everyone."
After seeing the study results, Weiss quit fasting. It was tricky at first, but he said it's since been beneficial, if only because he (and his family) no longer has to schedule activities around his meal timing. Previously, a mid-morning hike was challenging because Weiss would be extremely hungry, having not yet eaten.
"Over the past few months, everything has changed. I actually enjoy eating breakfast," Weiss said. "My wife was very happy for this to stop."
But the bigger lesson, he says, is that relying too much on one's own experience can be limiting. The experience has made him more cautious about extrapolating his personal perspective to broad generalizations about what works for health.
"Almost anything can work if you start paying attention to what you're eating and being more thoughtful and careful," he said. "We just have to be more careful about over-interpreting our own anecdotal experience. That's one of the biggest takeaways of the study for me. There's no excuse not to do good science."
Dietary dogma is hard to shake
The response to Weiss' study has been dramatic. He said he's been overwhelmed by emails, and had to take a break from using Twitter as replies to the study became "out of control."
And many of those messages have been skeptics, people questioning Weiss' work, trying to reinterpret his data, or even outright suggesting his study is an orchestrated attempt to undermine intermittent fasting.
That includes people with little formal background in nutrition science. Many people who are staunch proponents of particular dietary regimens, such as fasting, keto, or veganism, base their arguments on their personal success stories with the diet, what they've read on the internet, or both.
"The problem, which is also a good thing, about nutrition is that anyone can do it. You don't need a prescription or fancy tools, you can just do it, and then you become a spokesperson and an advocate," Weiss said.
That can have the positive effect of leading the general public to explore the world of nutrition science and potentially learn more. But it can also stifle informed debate or objective scientific inquiry when people become over-confident in their conclusion, and unaware of their own biases.
"People assume they have expertise in areas where they actually have none," Weiss said. "It's not that we're not teaching people science, it's that we're not teaching people to be humble about science."
Evidence shows we all have cognitive biases that make us overestimate how well we understand a given subject. The more familiar something is, the more likely we are to misjudge our own expertise on it.
"I think there's a similar problem with nutrition. It's something we do every day so we think we're experts," Weiss said.
As yet, though, there's no good answer for how to combat rampant misinformation or intellectual hubris online.
"It's a very difficult problem, I wish I knew how to solve it but I don't," he said. "The one thing we can do is do good rigorous science."
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