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New research suggests intermittent fasting increases the risk of dying from heart disease. But the evidence is mixed

Intermittent fasting has gained popularity in recent years as a dietary approach with potential health benefits. So you might have been surprised to see headlines last week suggesting the practice could increase a person’s risk of death from heart disease.

The news stories were based on recent research which found a link between time-restricted eating, a form of intermittent fasting, and an increased risk of death from cardiovascular disease, or heart disease.

So what can we make of these findings? And how do they measure up with what else we know about intermittent fasting and heart disease?

The study in question

The research was presented as a scientific poster at an American Heart Association conference last week. The full study hasn’t yet been published in a peer-reviewed journal.

The researchers used data from the National Health and Nutrition Examination Survey (NHANES), a long-running survey that collects information from a large number of people in the United States.

This type of research, known as observational research, involves analysing large groups of people to identify relationships between lifestyle factors and disease. The study covered a 15-year period.

It showed people who ate their meals within an eight-hour window faced a 91% increased risk of dying from heart disease compared to those spreading their meals over 12 to 16 hours. When we look more closely at the data, it suggests 7.5% of those who ate within eight hours died from heart disease during the study, compared to 3.6% of those who ate across 12 to 16 hours.

Read more: Does intermittent fasting have benefits for our brain?

We don’t know if the authors controlled for other factors that can influence health, such as body weight, medication use or diet quality. It’s likely some of these questions will be answered once the full details of the study are published.

It’s also worth noting that participants may have eaten during a shorter window for a range of reasons – not necessarily because they were intentionally following a time-restricted diet. For example, they may have had a poor appetite due to illness, which could have also influenced the results.

Other research

Although this research may have a number of limitations, its findings aren’t entirely unique. They align with several other published studies using the NHANES data set.

For example, one study showed eating over a longer period of time reduced the risk of death from heart disease by 64% in people with heart failure.

Another study in people with diabetes showed those who ate more frequently had a lower risk of death from heart disease.

A recent study found an overnight fast shorter than ten hours and longer than 14 hours increased the risk dying from of heart disease. This suggests too short a fast could also be a problem.

But I thought intermittent fasting was healthy?

There are conflicting results about intermittent fasting in the scientific literature, partly due to the different types of intermittent fasting.

There’s time restricted eating, which limits eating to a period of time each day, and which the current study looks at. There are also different patterns of fast and feed days, such as the well-known 5:2 diet, where on fast days people generally consume about 25% of their energy needs, while on feed days there is no restriction on food intake.

Despite these different fasting patterns, systematic reviews of randomised controlled trials (RCTs) consistently demonstrate benefits for intermittent fasting in terms of weight loss and heart disease risk factors (for example, blood pressure and cholesterol levels).

RCTs indicate intermittent fasting yields comparable improvements in these areas to other dietary interventions, such as daily moderate energy restriction.

A group of people eating around a table.
There are a variety of intermittent fasting diets. Fauxels/Pexels

So why do we see such different results?

RCTs directly compare two conditions, such as intermittent fasting versus daily energy restriction, and control for a range of factors that could affect outcomes. So they offer insights into causal relationships we can’t get through observational studies alone.

However, they often focus on specific groups and short-term outcomes. On average, these studies follow participants for around 12 months, leaving long-term effects unknown.

While observational research provides valuable insights into population-level trends over longer periods, it relies on self-reporting and cannot demonstrate cause and effect.

Relying on people to accurately report their own eating habits is tricky, as they may have difficulty remembering what and when they ate. This is a long-standing issue in observational studies and makes relying only on these types of studies to help us understand the relationship between diet and disease challenging.

Read more: Does it matter what time of day I eat? And can intermittent fasting improve my health? Here's what the science says

It’s likely the relationship between eating timing and health is more complex than simply eating more or less regularly. Our bodies are controlled by a group of internal clocks (our circadian rhythm), and when our behaviour doesn’t align with these clocks, such as when we eat at unusual times, our bodies can have trouble managing this.

So, is intermittent fasting safe?

There’s no simple answer to this question. RCTs have shown it appears a safe option for weight loss in the short term.

However, people in the NHANES dataset who eat within a limited period of the day appear to be at higher risk of dying from heart disease. Of course, many other factors could be causing them to eat in this way, and influence the results.

When faced with conflicting data, it’s generally agreed among scientists that RCTs provide a higher level of evidence. There are too many unknowns to accept the conclusions of an epidemiological study like this one without asking questions. Unsurprisingly, it has been subject to criticism.

That said, to gain a better understanding of the long-term safety of intermittent fasting, we need to be able follow up individuals in these RCTs over five or ten years.

In the meantime, if you’re interested in trying intermittent fasting, you should speak to a health professional first.

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