A controversial study says intermittent fasting may shorten your lifespan. It shows how little we really know about the long-term effects.

• A new study links intermittent fasting to a higher risk of early death instead of longevity.
• Some experts are skeptical, citing major limitations with the study like other lifestyle variables.

Intermittent fasting may not be the key to longevity after all — and may even be linked to dying earlier, according to controversial new research.

Once one of the hottest topics in longevity research, limiting your eating to a specific window of time each day with intermittent fasting has been touted as a way to reduce your "biological age" and extend your life.

That's why the authors of a new, as-yet-unpublished study about intermittent fasting were surprised to find it was linked to a higher risk of dying of cardiovascular disease, the lead researcher told Business Insider.

While there's not yet enough evidence to say intermittent fasting is risky, the findings suggest we may have a lot more to learn about whether it's good for our health in the long run.

Fasting is a popular anti-aging diet, but research shows mixed results

Fasting is popular among longevity enthusiasts who believe it can help you stave off age-related diseases, so you feel younger for longer. They are backed by studies that found fasting has health benefits such as reducing blood pressure, helping manage weight, balancing blood sugar, and more.

However, some rigorous studies on intermittent fasting have found mixed results: some suggest it's no better for you than other diets, and isn't worth the side effects like hunger.

The long-term effects of fasting hadn't been as well studied, said Victor Wenze Zhong, co-author of the most recent study and professor at Shanghai Jiao Tong University School of Medicine in China.

The new study reported a link between fasting and poor heart health

Zhong's team looked at data from more than 20,000 American adults, comparing their self-reported eating habits to their rates of illness and death over an average of eight years.

The researchers expected to find that people who only ate within an eight-hour window — i.e. people who follow the 16:8 diet — would have a lower risk of dying during the study, and better heart health. That's what other studies had found, after all.

What they found was that people who self-reported fasting for 16 hours a day were 91% more likely to die of cardiovascular illness during the study than their peers who ate during a longer window of time or didn't fast.

It's too early to say if intermittent fasting causes a higher risk of cardiac death since the study is observational and so far only suggests a link

It's controversial

The findings are part of preliminary research that will be presented at the American Heart Association Scientific Sessions this week in Chicago. The study has also been submitted to but has not yet been published in a peer-reviewed journal.

Judging by the summary alone, independent experts are skeptical.

For one thing, the study analyzed participants' diets based on just two days worth of surveys on their eating habits.

"Two days of diet record data are not at all reflective of an individual's regular eating pattern — this is a major limitation to the study," Krista Varady, a professor of nutrition at the University of Illinois Chicago who has published extensive research on fasting, told Business Insider.

Varady also noted that the study didn't account for lifestyle factors like exercise, socioeconomic status, alcohol and tobacco use, and other variables that can make a major difference in heart disease risk.

Other researchers were more blunt in their assessment of the study.

"So much is unclear about this study. In particular, why were the particular two days chosen to measure times of eating? How do they know whether food was eaten outside the 8-hour window and just not entered in the questionnaire?" David Spiegelhalter, emeritus professor of statistics at the University of Cambridge, said in a statement. "This abstract should not have been graced with a press release."

Too much fasting may cause you to lose muscle mass

Some experts believe this may be a thread worth pulling.

More specific data could help scientists understand how fasting may affect our health over time, according to Christopher D. Gardner, a Stanford professor, and chair of the writing committee for the American Heart Association's 2023 scientific statement, who was not involved in the study.

"Overall, this study suggests that time-restricted eating may have short-term benefits but long-term adverse effects," Gardner said in a press release.

For example, Zhong's team said people on fasting diets tended to have lower muscle mass than those who didn't fast. Maintaining lean muscle is a crucial factor in healthy aging, and eating too little, or fasting for too long, can make it harder to build or maintain muscle.

"Loss of lean body mass has been linked to a higher risk of cardiovascular mortality," Zhong said. "This will be an exciting research question, but we really do not know much right now."

To follow up, Zhong wants to look at what people eat, not just when they eat, since the study results suggest people who fasted tended to have a lower diet quality than people who didn't fast.

There are ways to fast safely

For now, there's good evidence that focusing on eating the right things, like plenty of protein, fiber, and other nutrients, can be a major tool in living a long, healthy life as we wait for future fasting research to offer some clearer answers.

Getting enough of these nutrients could be key to mitigating potential side effects to make fasting safer.

As for proven methods to eat for a healthy heart, the Mediterranean diet and DASH diet are evidence-based ways to lower your blood pressure and reduce your risk of heart attack, stroke, and heart disease.