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Does intermittent fasting work?

Weight of evidence suggests it’s the food that matters, not timing

Fasting has been around for centuries – one of the earliest Biblical stories describes Jesus being tempted by the Devil after fasting for 40 days and nights in the Judaean Desert.

However the last decade has seen a new spin on an age-old practice, often associated with religious rituals or political protests, with the concept of intermittent fasting. The practice has been said to have a range of benefits, from weight loss to blood sugar balance, reduced inflammation and even improved brain function.

The BBC Horizon programme in 2012 – Eat Fast and Live Longer – presented by doctor and journalist Michael Mosley, saw the introduction of a new and “genuinely revolutionary” diet.

Mosley’s book, The Fast Diet, was published in 2013, followed rapidly by BBC journalist Kate Harrison’s book, The 5:2 Diet Book. The recommendations in both books vary slightly, though the general principles remain the same.


The era of intermittent fasting was born.

Intermittent fasting is a term used to describe a variety of eating patterns that have alternating periods of fasting – abstinence from foods – and eating.

The 5:2 diet involves fasting for two non-consecutive days of the week and eating normally for five days. The alternate-day fasting diet involves fasting days alternating with days where food and beverages are consumed normally, without restrictions.

Time-restricted eating is a type of intermittent fasting that limits the ‘eating window’ to 4-12 hours, inducing a daily fasting period of 12-20 hours. People can eat until they are full without any restrictions on calories during the eating window.

Much of the evidence around intermittent fasting considers the impact of fasting on the body’s circadian rhythm that represents the 24 hours cycle of metabolism in the body. This includes the control of the sleep-wake cycle, blood pressure, mood regulation and hormonal balance.

A review of the evidence in Nutrients (2020) suggested that eating for lengthy periods during the day may disrupt the circadian rhythm and increase the risk of chronic disease, including heart disease, cancer, and type 2 diabetes.

Findings across animal and human studies showed favourable changes in cholesterol levels. Animal studies have found that fasting in a 14:10 pattern – eating within a window of 10 hours – can act on the risk factors for metabolic syndrome, including reducing waist circumference, body fat percentage and visceral fat.

Despite the many touted benefits, there are some potential downsides. A review in Behavioural Science (2017) identified possible negative effects to include: hunger, irritability, worsened mood, fatigue, fears of feeling out of control about food, overeating during eating windows and difficulty concentrating.

In addition, most of the research on intermittent fasting is based on animal research with little long-term human research available.

A review in JAMA Netw Open (2021) found that only six of 104 alleged health benefits of intermittent fasting were supported by moderate- to high-quality evidence, and most findings were based on low-quality research.

The strongest evidence was found for improved anthropometric (scientific measurements and proportions of the human body) and cardio metabolic outcomes for adults with overweight and obesity.

The review stated that more rigorous human research on the long-term benefits of intermittent fasting is warranted.

The most recent evidence provides exactly that.

Research in the Journal of the American Heart Association in January indicates that the number of daily meals, but not the timing between first and last daily meals, was significantly associated with weight changes over a six-year period in a prospective study of 500 adults.

The participants downloaded an app to record the timing of their meals and sleep. Electronic records were used to obtain information on weight and comorbidities of the participants.

The mean interval from first to last meal was 11.5 hours and was not associated with change in weight. The mean interval from waking up to eating was 1.6 hours and the mean time from last meal to sleeping was 4 hours and these were not associated with weight changes over the follow up period. Sleep duration was not associated with weight changes over time.

However, the total daily number of large- and medium-sized meals was associated with weight gain over time, while those who reported more smaller meals showed weight loss.

“Animal studies have shown benefits for time-restricted feeding, but there are still questions about whether it helps prevent weight gain or promotes weight loss in humans,” said study author, Dr Wendy Bennett. In terms of the current study findings “we were not surprised; humans are more complicated than animals and we have complicated behaviours, especially with eating,” she concluded.

The research did not support time-restricted eating as a long-term weight loss strategy and the authors concluded that more studies are required with a longer follow-up period. However, there may be a role for time-restricted eating as a method of total calorie control.

Less focus on worrying about squeezing meals into a restricted window and more focus on the type and amount of food on the plate may be a more effective strategy for long-term health.

- Dr Catherine Conlon is a Cork public health doctor and safefood’s former director of human health and nutrition

Read: What I’ve learned from two years of intermittent fasting