Fasting is actually an ancient practice, followed in different formats by various groups of people around the world. But intermittent fasting was popularised in 2012 by Dr Michael Mosley. It’s claimed to promote weight loss and improve your metabolic and long-term health. But is there actually any evidence to support these claims?

Rosie Saunt, registered dietitian and author of nutrition bible Is butter a carb? shares her expert advice for the Thriva newspaper.

What is intermittent fasting?

Intermittent fasting involves alternating periods of fasting and eating. It’s a form of eating which focuses on when to eat, not what to eat.

The main forms of intermittent fasting

There are a couple of different ways to do intermittent fasting.

Complete alternate-day fasting

This involves restricting all food and calorific drinks one day and alternating with regular eating days. Arguably this is much trickier to stick because of hunger and irritability. 

Modified fasting regimens

This form typically allows you to eat a very low amount of calories on a fasting day (500-600kcal), alongside regular eating days. The 5:2 diet fits into this category. 

Time-restricted feeding

This involves eating within a window of time (like 10 hours per day), in sync with your body’s 24-hour clock. There’s considerable scientific interest in this eating pattern. 

Intermittent fasting and weight 

Much of the interest in intermittent fasting was originally sparked by its potential to lead to weight loss.

Intermittent fasting regimens are essentially calorie restriction because of the shortened eating windows. So it’s not surprising that some people lose weight. But research has found the weight loss achieved is fairly varied and can be quickly regained once you stop fasting. 

Research has also shown that weight loss via intermittent fasting is the same compared to more traditional diets. Most people who go on a diet eventually regain the weight they have lost and may end up in a cycle of yo-yo dieting. 

There are also so many other complex factors related to body weight that are outside our control, like genetics and the environment. 

Intermittent fasting and metabolic health

More recently, there’s been emerging interest in the possible benefits of time-restricted feeding. To understand this, you first need to understand body clocks.

We all have a central body clock in our brain. This is connected to what are called peripheral clocks, located in other tissues and organs. All these clocks are involved in your body’s metabolism — chemical processes in your body that keep you alive, like breathing and breaking down food into energy. And they run over a 24-hour period. 

It’s thought that these circadian rhythms have evolved to make sure your vital bodily processes are performed at optimal times —  like sleep, blood sugar regulation, hunger hormones, gut function, inflammation, and so on. Your body relies on external cues to synchronise these processes to the right time of day. Light is the most powerful of these, followed by food intake.

Historically, we’ve evolved to eat during the day when active, and fast during the night when at rest. Consuming most of your energy needs while you’re most active makes sense. And it seems that’s what our natural rhythms expect. 

But modern-day life now means we tend to work more, travel internationally, have access to food on-demand, and expose ourselves to artificial light. Meaning our meal patterns are less and less resembling this ideal evolutionary way of eating.

It’s thought that consuming large amounts of energy when your body isn’t expecting it sends unfamiliar messages to your clocks. The same goes for unnatural light, like just before bed or during the night — disrupting your body’s natural metabolic rhythms. 

Through animal studies, it’s thought that this might increase your risk of a shortened life span or the development of a chronic disease like heart disease, cancer, and type 2 diabetes. These diseases are also more common in shift workers who have to alter their eating periods during the nighttime.

Time-restricted feeding aims to keep eating to a daytime window — leading to better functioning of your circadian rhythm and bodily processes. And evidence currently indicates that starting eating earlier in the day is better. But this doesn’t mean eating everything at breakfast, it means eating the majority of your calories before the evening. 

Limitations of intermittent fasting

Does intermittent fasting improve important health behaviours like eating more vegetables, drinking alcohol moderately, and sleeping better? We don’t know. And as any sort of fasting regimen is essentially a restrictive diet, it might not be safe if you’re at risk of an eating disorder or have a history of eating disorders.

There’s also the argument that a biohacking eating regimen can miss the wider picture when it comes to your health. There are many complex factors that increase the risk of disease — food timing is likely to be only a small piece of the puzzle. 

For those who don’t have an adequate income or enough food to eat, they don’t have the ability to be picky about their diet.

Key take-homes

The timing of food intake is possibly important when it comes to your health and disease risk. So it could be beneficial, particularly if you’re a night shift worker, to have a 10ish hour eating window (like 9 am to 7 pm). And aiming to have more of your daily calorie intake earlier in the day.

But much of the evidence for the health benefits of intermittent fasting is from studies on animals. Evidence in humans is limited and possibly over-exaggerated. So, we’re not yet at a stage where we have enough evidence to make any recommendations. We’re likely to see more research on lifestyle interventions to support our circadian rhythms in the future.

Recommended listening for you

Share Article

Aisling Moran BSc (Hons)

Written by Aisling Moran BSc (Hons)

19th May 2021 • 5 min read