Does intermittent fasting raise the risk of fatal heart disease? What experts say

A new scientific study linking intermittent fasting (IF) with an increased risk of cardiovascular problems was greeted with interest this week.

For those who routinely eat only during an eight-hour daily window and fast for the rest of the day, the findings that they could be twice as likely to die of heart attacks and strokes as those who spread their food across 12 to 16 hours are clearly concerning.

IF has been popularised by celebrities such as Jennifer Aniston and Scarlett Johansson and scientific research has linked it to weight loss as well as short-term improvements in blood pressure, blood sugar and cholesterol, key contributors to cardiovascular disease.

However, new research involving a large group of people monitored for a several years have made what its researchers – and many others – described as a “surprise” finding; that the diet may actually harm rather than benefit people in the longer term.

The study involved 20,078 participants in the US with an average age of 49 and followed them for up to 17 years – and for an average of eight years.

But while this finding clearly highlights a danger that needs much more thorough investigation there are a number of shortcomings which mean we shouldn’t take the findings as gospel, according to scientists not involved in the research.

‘Huge doubts’ over results

To begin with, the research has yet to be peer-reviewed – a process in which scientists not involved in the work scrutinise its methodology and results.

Another fundamental issue with the research is that it was an “observational” study meaning that it identified an association rather than demonstrating cause and effect.

The researchers did ‘control’ for factors that might have skewed the results, such as the participants’ age, income, body mass index, blood pressure and whether they drank or smoked but there could still be some unknown cause rather than fasting that they hadn’t thought of.

Having said that it is relatively rare to demonstrate cause and effect where diet is concerned because of the difficulty of controlling and monitoring food the food intake of a large group of people over any length of time.

But perhaps the biggest difficulty with the study is that the researchers only looked at people’s eating habits for a total of two days and assumed they followed the same schedule for an average of eight years.

Sir David Spiegelhalter, Emeritus professor of statistics at the University of Cambridge, said: “So much is unclear about this study.

“In particular, why were those particular two days chosen to measure times of eating? And how do they know whether food was eaten outside the eight-hour window and just not entered in the questionnaire? This [conference presentation] abstract should not have been graced with a press release.”

Kevin McConway, Emeritus professor of applied statistics at the Open University, added there were “huge doubts on whether the study can show what it purports to show.”

“For example, 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,” he said.

Fasting ‘not always for health reasons’

It may also be that many people who conducted IF did so out of necessity rather than by design because their hours of work demanded it – and, in turn, this could bias the findings in other ways, scientists said.

The participants were part of a wide ranging and long-running national nutritional survey representative of the US population, rather than people who had specifically signed up to take part in an IF study.

“It is likely that individuals reporting time-restricted eating may be working antisocial hours – such as truck drivers, security personnel, health professionals, night workers. This is important because there is evidence that this type of working practice is associated with increased risk of type 2 diabetes and cardiovascular disease,” said Tom Sanders, professor emeritus of nutrition at King’s College London.

It is also likely that at least some were put on strict diets by a doctor because they were overweight and others choose to restrict eating due to ill health – both categories putting people at higher risk of heart attacks and strokes.

People with cancer, for example, might struggle with appetite loss and therefore eat during a more restricted time period and may have a shorter life expectancy.

Duane Mellor, registered dietitian and senior lecturer at Aston Medical School, Aston University, said: “It is impossible to say if how a person restricts the time that they eat is linked to risk of health outcome.

“Some people might be doing this for health reasons, whilst others due to stressful work environments or poverty, which are both risk factors for cardiovascular death.”

‘More research needed’

But scientists said the study definitely has a role to play in the rapidly but little known field of IF – both in its own right, once it has gone through the peer-review process as well as in highlighting the uncertainties that still exist in this dietary habit.

Professor Keith Frayn, of Oxford University, said: “Time-restricted eating is popular as a means of reducing calorie intake, although its proponents claim other benefits such as ‘ramping up metabolism’. This work is very important in showing that we need long-term studies on the effects of this practice. But this abstract leaves many questions unanswered, and further research will be needed.”

And the study’s lead researcher professor at Shanghai Jiao Tong University agrees “it is too soon to conclude that people should avoid time-restricted eating”.

While much is still to be learned about IF, a key study from King’s College London, found that eating all food in a 10-hour window can significantly improve your mood, boost energy levels and reduce hunger.

Nearly three-quarters of participants reported improvements in energy, while 57 per cent saw their mood improve and 47 per cent felt less hungry, according to a study of 37,535 intermittent fasters.

Researchers say that IF gives your body, gut lining and gut bacteria more time to rest, recharge and repair itself, which can’t be done so effectively when a person is eating and digesting food.

IF has also been linked to weight loss, although, as with all research into fasting, more research is needed.