Dr Michael Mosley discusses the latest intermittent fasting research

Hi everyone,

You may have been a bit shocked and surprised (I certainly was) to have seen recent reports in newspapers and circulating on social media that doing intermittent fasting, something we advocate at the Fast 800, could almost double your risk of having a fatal heart attack.

So, is this really the case? Well, experts are extremely sceptical about the study that generated these headlines, particularly as there have been hundreds of studies carried out on different forms of intermittent fasting over the last ten years that have consistently shown that intermittent fasting can be hugely beneficial.

Intermittent fasting research

A review of all the best available intermittent fasting research, published in the New England Journal of Medicine (the world’s most prestigious medical journal) in 2019, concluded that ‘intermittent fasting has broad-spectrum benefits for many health conditions, such as obesity, diabetes mellitus [type 2 diabetes], cardiovascular disease, cancers, and neurologic disorders [such as dementia]’. You can follow the link below if you want to read the report, titled “Effects of Intermittent Fasting on Health, Aging, and Disease” in detail.

https://www.nejm.org/doi/full/10.1056/nejmra1905136

One of the benefits of doing intermittent fasting – whether you are doing the 5:2 diet (where you cut your calories two days a week) or Time Restricted Eating (TRE), where you simply reduce the hours within which you eat –  is that you switch from burning sugar to burning fat as your main fuel, which helps in multiple ways. You are also giving your body a break from digesting and processing food, which in turn helps trigger something called autophagy, a form of cellular ‘spring cleaning’, where old cells are broken down and recycled.

A deeper dive into the latest (unpublished) intermittent fasting study

The fact that this new study flies in the face of so much previous research makes it immediately suspect. And the fact that this particular study has not yet been published or peer reviewed (all we have is an abstract and a press release), are further reasons to be doubtful.

What the press release claims is that a new study suggests that following a 16:8 pattern of time restricted eating (fasting for 16 hours and eating during an eight-hour window) is linked to a 91 percent higher risk of death from cardiovascular disease.

The so-far unpublished study was produced by researchers from the Shanghai University School of Medicine. They looked at data from the U.S. National Health and Nutritional Examination Surveys, collected between 2003 and 2018.

In these surveys Americans are asked about their eating habits. What the researchers did was pick out those who had ticked a box saying that on two particular days (across an 8 year study) they’d restricted their food consumption to an eight-hour window or less. The researchers then cross referenced these people to the US National Death Index database to see if they were more or less likely to live long and healthy lives.

And it turned out, to everyone’s surprise, that those who ticked the box had almost twice the risk of dying from heart disease than people who hadn’t.

The trouble is we don’t know anything about the people who ticked the box. How old were they? How healthy?  How accurate are their recollections of when they ate? And why did the researchers just look at two days, across an 8 year period, and decide those were typical of what they did on all the other days?

As Kevin McConway, Emeritus Professor of Applied Statistics at the Open University, pointed out, ‘We don’t know whether their eating times over those two 24-hour periods was typical of the times they usually ate. So to relate those patterns to a deliberate long-term time-restricted eating intervention seems to be going far beyond the data’.

Sir David Spiegelhalter, Emeritus Professor of Statistics at the University of Cambridge agreed, adding somewhat scathingly:  ‘This abstract should not have been graced with a press release’.

This study is based on data collected from people nearly 20 years ago, at a time when no-one was talking about the health benefits of doing time restricted eating. So, as other experts have pointed out, it is far more plausible that anyone eating that way, back then, was doing so because they had existing health problems (like cancer of heart disease) or because they were doing shift work  (ie truck drivers, security guards etc), something which is strongly linked to an increased risk of heart disease and type 2 diabetes.

Finally, we have no idea what these people were consuming during their eight-hour eating window. Was it a healthy and varied Mediterranean-style diet, the sort we advocate, or was it takeaways and highly processed junk food – which is going to be bad for you, regardless of how big or small your eating window is?

So, no, my belief in the benefits of intermittent fasting has not been shaken by this abstract, and nor should yours.

Looking into years of research

The fact is that many, much more rigorous randomised controlled studies, have shown the benefits of time restricted eating for heart and metabolic health. One example is a study published in Cell Metabolism on October 4, 2022, where researchers from the Salk Institute in the US randomly allocated 150 firefighters from the San Diego Fire-Rescue Department to either eating all their calories within a 10 hour period (14:10) or continuing as normal. After three months those who were doing 14:10 saw significant improvements in their cholesterol, their mental health, their blood sugars and their blood pressure.

Similarly, a big study, published last year [2023] in the journal Nature Communications, which looked at the impact of eating times on heart health in more than 100,000 adults, found the people who did time restricted eating had the lowest risks of heart disease, stroke and type 2 diabetes.

Interestingly, in this particular study, the healthiest people were those who had an early evening meal, rather than a late breakfast (probably because this works best with your body clock rhythms, and the production of hormones such as insulin).

So my advice remains: if you want to try Time Restricted Eating start with a pattern of 12:12, where you fast overnight for 12 hours and eat within a 12 hour window. The simplest way to do this is to stop eating 2-3 hours before bed and then not eat again until 12 hours later. Above all, follow our recipes and you can be sure you are eating a nutrient rich diet.

And don’t worry too much about the occasional newspaper headline! The reason they provoke so much interest is not because they are true or accurate but because they are provocative.