Fasting continues to boom in popularity, thanks to science showing it can help us maintain a healthy weight and live longer. But are the benefits overstated? And is it right for everyone? We investigate.
Intermittent fasting (IF) is the practice of going without food for a period of time. Fasts come in many forms, the most extreme being water-only. These involve abstaining from food entirely and can last upwards of 48 hours. Time-restricted eating (TRE) is a more popular model that includes alternate-day fasts (eating one day and not the next), OMAD (one meal a day) and the ‘warrior diet’ – eating within a four-hour window, followed by a 20-hour fast.
The most popular (and perhaps practical) TRE approach is 16:8, which involves eating within an eight-hour window before fasting for the remaining 16 hours of the day (or night). Other fasts make use of low-calorie days rather than the absence of food altogether. These include the 5:2 (popularised by Dr Michael Moseley), which stipulates two fasting days per week, with calories limited to 500-800 kcal on those days.
Research has shown we tend to eat within a 12-hour window when not fasting. Most also eat a large number of calories at the end of the day – 45% of our average daily energy intake is made up of dinner and post-dinner snacks, to be precise. Advocates of fasting stress this is not a natural way for humans to eat. Our hunter-gatherer ancestors experienced feast and famine cycles, they say, going for long stretches of time without food. But, with the average lifespan of hunter gatherers said to be 31, it’s unclear whether the ‘natural’ approach is always best. There is evidence, however, that fasting may indeed have benefits for lifespan as well as for our waistline, heart health and gut.
Let’s dig into the data to see which benefits need further investigation and which stand up to scrutiny.
Weight loss
There’s plenty of evidence to suggest that fasting results in short-term weight loss. Studies show this is true of all different types of fasts, including TRE, water-only fasts, as well as a 5:2 approach. Greatest weight loss tends to be seen in those who are overweight.
Anecdotally, many also report finding fasting an easier weight-loss strategy than traditional calorie restriction, as there are fewer opportunities to eat. It’s harder to overeat in a four-hour window than it is over 24 hours. Yet studies have shown that fasts don’t seem to be any more effective than calorie-cutting diets for weight management. There’s also mixed data on which diet is the easiest to stic
