Intermittent fasting
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  • New observational study suggest 8-hour time-restricted eating linked to “91 per cent higher risk of cardiovascular death”
  • Experts say new study, currently only in abstract form, needs closer scrutiny to analyse quality of data, on which surprise conclusions were based.

Would you drop dead sooner if you do intermittent fasting (IF) than if you didn’t do it? The answer is “yes”, according to a controversial new study.

The news is everywhere: This popular IF “fad diet” is purportedly more dangerous than everyone initially thought.

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But hang on: the study is "observational", which by nature cannot show direct causation. And it's still in preliminary, “abstract” form, initially presented at the EPI|Lifestyle Scientific Sessions in Chicago, which runs until March 21, 2024.

Still, it had already kicked up a storm.

What does the 'abstract' say?

The “abstract” was released with this surprise conclusion: restricting food to a brief eight-hour window daily could result in a higher risk of death from heart attack and stroke than those who didn’t do it.

Key findings: 

  • Researchers observed that participants adhering to 16:8 intermittent fasting were 91 per cent more likely to succumb to cardiovascular disease compared to non-practitioners of this fasting regimen.
  • Individuals with pre-existing cardiovascular conditions who confined their calorie intake to an 8 to 10-hour window faced a 66 per cent elevated risk of mortality from heart disease and stroke.
  • Among individuals with cancer, those who consumed their daily calories over a 16-hour period without fasting exhibited a lower risk of cancer-related mortality than their counterparts cadhering to a more restricted eating schedule.

What type of study was it?

It was an “observational” study.

Intermittent fasting
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Why is it controversial?

The findings challenge the presumed health benefits of IF – which in recent year has gained popularity as a means to help people lose weight, enhance cholesterol levels, boost metabolism, bring blood sugar back to normal levels and potentially mitigate the onset of certain chronic ailments.

The initial study suggests that adherents to the popular 16:8 IF regimen (restricting food intake to an 8-hour window) might lead to heightened risks of cardiovascular disease – and death – compared to those who do not fast.

It claims that compared with a standard schedule of eating across 12-16 hours per day, limiting food intake to less than 8 hours per day was not associated with living longer.

91 %

Increased risk of death from cardiovascular disease for IF practitioners compared to non-practitioners, according preliminary research according to preliminary research

The preliminary results were presented at the American Heart Association’s Epidemiology and Prevention│Lifestyle and Cardiometabolic Scientific Sessions 2024, March 18- 21, in Chicago.

The meeting offers the latest science on population-based health and wellness and implications for lifestyle.

What are the conclusions based on?

Researchers analysed data from the annual National Health and Nutrition Examination Surveys (NHANES), and tabulated the answers given by 20,000 adults to a set of survey questions.

Clinical vs experiment vs observational study

It is important to note that the researchers didn’t base their conclusions on clinical data.

It was NOT a result of a randomised, double-blind and placebo-controlled trial, the “gold-standard” of medical research that can inform a direct causation conclusion. So no information was taken from lab, or blood work.

Instead, researchers poured through survey responses taken from those 20,000 adults. Though the 20,000  respondent size is huge. The questions covered the respondents' best "recollections" of their dietary habits from 2003-2018.

As the study had a “longitudinal” design – looking at individuals over an average of 8 years (the questions covered the respondents' best "recollections" of their dietary habits from 2003-2018) – the researchers poured through dietary information from the respondents on their eating habits. The responses were collected by two 24-hour “recalls” at the time of enrollment.

The 20,000 respondent size, however, is a decent number for a survey sample. 

What did health groups say about the study?

On March 18, the American Heart Association (AMA), in a release about the study, published this caveat: “As noted in all American Heart Association scientific meetings news releases, research abstracts are considered preliminary until published in a peer-reviewed scientific journal.”

That means it is hard to draw definitive conclusions. However, this study it does add to the growing body of studies on the pros and cons of time-restricted eating.

What did other experts say?

Michelle Routhenstein, RD, CDN, a preventive cardiology dietitian who wasn’t involved in the study, told Medical News Today, that it being an observational study can limit the accuracy of findings – given that dietary patterns can change over a period of time.

Why is it a big surprise?

The findings are totally unexpected. The complete study has not been released. It’s possible that huge variances in the baseline traits of the time-restricted eating cohorts (survey respondents) could explain these unexpected results.

Intermittent fasting
Intermittent fasting Image Credit: Shutterstock

Dr. Cheng-Han Chen, an interventional cardiologist (who was not involved in the study) and medical director of the Structural Heart Program at MemorialCare Saddleback Medical Center in California, also cited another limitation of the research.

“These findings run counter to many previous studies which find benefits of time-restricted eating to cardiovascular and metabolic health,” he told Medical News Story.

Discrepancies in baseline demographics and characteristics between the time-restricted (TR) eating groups, particularly between those adhering to less than an 8-hour window and others – as well as potential “recall bias” in participants reporting their eating patterns, may confound the results, explained Dr. Cheng-Han.

What is an observational study?

In general, an observational study draws inferences from a sample to a population where the independent variable is not under the control of the researcher because of ethical concerns or logistical constraints.

In addition, “observational studies”, by their very nature, cannot prove cause-and-effect.

What does ‘association does not imply causation’ mean?
Statistical association between two variables merely implies that knowing the value of one variable provides information about the value of the other.

It does not necessarily imply that one causes the other.

But why is this study significant?

  • Despite the study limitations, the findings warrant further investigation, according to experts.
  • The long-term effects of IF on humans is not yet fully understood.
  • Currentl, it is the only 8-year study on intermittent fasting. A study such as this is crucial to helping deepen the medical community’s understanding of the practice.
  • It also shows IF may not be right for everyone.

“This study is a great example of how short-term outcomes don’t always predict the long-term impact,” said Dr. Avantika Waring (not involved in the study), the chief medical officer at cardiometabolic care telemedicine company 9amHealth, told Medical News Today.

“It may be that intermittent fasting helps people lose weight in the short term and improves some metabolic markers, but doesn’t provide lasting cardiovascular benefits,” Dr Waring added.

In the end, your decision to attempt any form of intermittent fasting will rely on your personal preferences, your doctor's advise, and your short- and long-term objectives.

“Exciting study”

Waring said: “This study is overall quite exciting, and I know that many of my patients have had success losing weight by limiting their eating window, but the focus should remain on the type of food consumed — i.e., unprocessed and nutrient-dense,” she said.

“And until we have more details about this study, and ideally a randomised controlled trial to study the actual effect of intermittent fasting beyond associations, I would not recommend this over other eating patterns.”

Dr Chen said he would still recommend IF for his patients as a way to lose weight, improve blood sugar control, improve cholesterol, and for overall cardiovascular health.