What is intermittent fasting and does it really work for fat loss?

From the 16/8 approach to the 5:2 Diet and time-restricted eating, clock-watching while you eat is catching on, but what's the science behind intermittent fasting and does it really work for fat loss?

Intermittent fasting has exploded onto the mainstream, with everyone from Insta influencers, to high-profile sportspeople and even Twitter CEO, Jack Dorsey, singing its praises.

In one extreme example, filmmaker Kevin Smith reportedly went from being so overweight he was not allowed on an airplane, to dropping 23kg through intermittent fasting.

But does intermittent fasting really work for fat loss and if so, what’s the safest way to do it?

What is intermittent fasting?

You’ve probably heard of intermittent fasting in one form or another. The principle is simple: you divide each day into two periods of time, one for eating and one for fasting.

One of the original intermittent fasting approaches is the 16/8 plan, which only allows you to eat during an 8-hour window of time each day. You can choose the eight-hour window that best fits your schedule and hungher levels.

The most appealing thing about the 16/8 approach compared to most other diets? There is no calorie counting involved meaning you could technically go wild and eat whatever you want in your chosen eight-hour window. Some people follow the 16/8 schedule every day, while others only practice it a few times per week. 

But the most popular form of intermittent fasting is probably the 5:2 Diet. Unlike the 16/8 ratio, which refers to fasting and eating windows in one day, the 5:2 Diet ratio refers to multiple days. For five days a week you eat whatever you like and on the other two days men are limited to 600 calories, while women are only allowed 500 calories. 

Does intermittent fasting work for fat loss?

Why would intermittent fasting help with fat loss? The typical explanation is based around insulin; the hormone in your body that regulates metabolism and fat storage. Fasting advocates point out that the less often you eat, the less often you raise your insulin levels, theoretically helping you to lose fat. 

Keep insulin really low – by fasting for extended periods, or cutting out foods that spike it (which is what the ketogenic diet aims to do) – and your body switches to using stored fat and ketone bodies (hence keto) as its main fuel. 

To think about this another way, imagine being an early-era hunter-gatherer who has a (literally) mammoth feast one day and then goes a while without eating. Your body, the theory goes, will release fuel from its fat stores to let you go out and find more food, leaving you alert and ready for action. 

And there appears to be some truth to the theory. One (admittedly small) study observed 23 obese participants who adhered to a 16/8 fasting schedule and lost between 3% to 8% of their bodyweight in just eight weeks. While a 2017 study in the International Journal of Obesity found the 5:2 Diet resulted in more weight and fat loss compared to day-to-day calorie restriction.

Can intermittent fasting help with more than just fat loss?

There’s some research to back up the idea that fasting works for more than fat loss - a recent study, for instance, suggests that fasting can ‘reset’ your immune system to some extent, providing improvements in blood pressure, resting heart rate and markers of systemic inflammation. 

Supervised fasting, fans claim, can help to combat or even reverse diabetes, while the more low-key version might help you to change your relationship with hunger by, for instance, teaching yourself not to reach for a bagel the instant you feel a bit peckish. 

And while older studies used extreme forms of fasting - including multiple days without food – to get these results, more recent research suggests that the 16/8 diet can offer the same benefits.

What about time-restricted eating (TRE)?

Here's where things get a little confusing. You might have also heard of something called time-restricted eating (TRE) and wondered how it's different from intermittent fasting.

TRE is simply a form of intermittent fasting. Like intermittent fasting, every day has an eating window and a fasting window. Unlike intermittent fasting, however, TRE shifts the focus to how many hours per day you get to eat versus how long you have to fast.

You may, for example, start out with a 12-hour eating window from 8am to 8pm. You won't eat a thing outside of that designated window. TRE adherents typically start reducing their eating windows by an hour at a time until the window is as small as just six hours.

Fans of TRE claim that food intake is a crucial factor in regulating your body’s enzyme activity, and that grazing for more than 12 hours a day keeps your system over-occupied with digestion without giving it time for much else.

Restrict your eating window just a bit, says Dr Satchin Panda, creator of the Mycircadianclock app, which automatically tracks food intake and sleep, and you’ll benefit from increased energy, improved endurance, and even better body composition.

The downside of this approach is that, unlike traditional 16/8, TRE doesn’t even allow black coffee – the theory goes that anything outside of water is technically a ‘xenobiotic’ which your system goes online to deal with, breaking your fast.

So, although you could start eating at 8am and finish your dinner by 8pm, you can't have a 7am espresso to kick-start your day.

And, while fasting’s being studied by an array of scientists with fresh results coming almost monthly, TRE’s still largely reliant on self-reported results from a few thousand enthusiastic participants and some experiments conducted on rats and mice.

Should you try intermittent fasting?

Intermittent fasting definitely isn't for everyone, and should be avoided if you are: diabetic, hypoglycaemic, suffering from blood sugar regulation issues, pregnant, breastfeeding, susceptible to eating disorders. 

It also, like many things in life, sounds a lot easier on paper than it is in practice. Everyone is different, of course, but for many of us getting through a day with just 500 calories, or going 16 hours without eating can be torturous. 

It’s also worth considering that some of the benefits of intermittent fasting could simply come from people consuming fewer calories overall, snacking on less sugar early in the morning or late at night, or just being more mindful in general about how much they eat.

If you feel like your daily energy levels could use a boost, or you’re interested in losing fat, it’s worth investigating intermittent fasting further - but don’t leap into a new eating plan without doing your research. And whatever you do, always start gradually and only ever try what feels right for you.