Intermittent fasting and weight loss diets: what you need to know

We weigh up the pros and cons of intermittent fasting.
Published 4 August, 2019

The science of intermittent fasting

Two of the biggest buzzwords of the moment are ‘intermittent fasting’ due to its apparent efficacy in helping with weight loss and its benefits to the body. Plenty of diets come and go, but it seems the idea of limited periods of fasting offset by being able to eat what you like on other days is an eating regimen that’s slowly winning legions of followers. We examined the science behind this way of dieting and whether fasting can really improve everything from your weight to your health.

Different types of fasting diets

Intermittent fasting involves moving between cycles of fasting or a limited calorie intake and unrestricted eating, with the time frame of the fast dependent on the particular diet you’re following.

Some of the most popular intermittent fasting diets at the moment include:

5:2 diet - eating 500-600 calories two days a week and eating normally the other days.

16:8 diet - fasting for 16 hours and eating whatever you like for eight.

Eat Stop Eat - fasting for 24 hours once or twice a week.

Alternate-day fasting - fasting every other day.

The Warrior Diet - fasting throughout the day then eating one big meal at night.

All have varying time frames for the ‘fast’ and ‘feast’ window. However, all work off the same concept that eating should be restricted to a number of hours a week.

Benefits of fasting

  • It gives the digestive system a break
  • Easier mentally than dieting
  • Aids weight loss

It gives the digestive system a break

One of the benefits of intermittent fasting, says accredited practising dietitian Skye Swaney, is that it gives the digestive system a break and also encourages your body to use energy stored in fat cells. “When you eat, some of the energy from that food is stored in the liver as glycogen. After 10-12 hours of not eating, these glycogen reserves will have been mostly used, triggering fat cells in your body to release fats into your bloodstream. The fat cells head straight to your liver, where they’re converted to energy for your body and brain,” says Swaney.

It can be mentally easier

Additionally, for some, it can also be a way to lose weight rather than daily dieting, as mentally it can be easier to be strict on what you eat for a set number of days rather than all the time.

Ability to aid weight loss

One suggested pro of intermittent fasting is said to be its ability to aid in weight loss. Losing weight requires a reduction of calorie intake, so it stands to reason that intermittent fasting may result in weight loss. But it doesn’t necessarily mean it’s a better way of losing pounds.

Cons of fasting

  • Not superior to daily calorie restriction
  • Harder to sustain long-term
  • Concern over quantity of food
  • Insufficient reserach

Not superior to daily calorie restriction

A clinical trial conducted by scientists at the University of Illinois at Chicago into alternate-day fasting found that “it did not produce superior weight loss or weight maintenance compared with daily calorie restriction”.

Harder to sustain long-term

Even if being on an intermittent fasting diet results in weight loss, it might not be sustainable due to its restrictive nature, which can be difficult to maintain in the long-term. “It certainly can be [effective], but like most diets, it's only effective if you stick to it,” says Swaney.

Concern over quantity of food

The Dietitians Association of Australia has also expressed concern about the diet’s focus on the quantity of food eaten rather than the quality. While you may be hitting your calorie goal, you may not actually be meeting your required daily nutrient intake – something that could seriously affect your health in the long run.

Insufficient research

Although some studies suggest intermittent fasting may have health benefits, it’s important to note that most of this research has been done on animals and not humans, and therefore isn’t something we can rely on.

The bottom line is that, at this stage, there’s not enough robust data available to show the potential side effects of intermittent fasting on the human body over an extended period of time. “We don’t really know the long-term effects at this stage and whether the body is able to adapt to longer fasting periods over time,” says Swaney.

When is it not safe to follow a fasting diet?

Intermittent fasting is not suitable for everyone. Those who are pregnant or breastfeeding, teenagers, those with high-calorie needs, such as athletes or highly active people, and anyone with diabetes should avoid it.

Embarking on this eating regimen is essentially a lifestyle choice, much like being on a daily reduced calorie intake diet would be. And while it can lead to weight loss and may work seamlessly with some people’s lives, it might not be the right path for everyone.

There are some who might find it too restrictive, for example. “Many people have maintained a long-term intermittent fasting diet and made it a lifestyle. However, the sustainability of the diet depends on each individual’s personality and lifestyle, so that won’t be the case for everyone,” says Swaney.

As with most things to do with health, we have to be discerning about the trends we follow and which lifestyle changes we make. There’s no one-size-fits-all approach when it comes to our eating patterns and, perhaps instead of focusing on the way we eat, we should be turning our attention to what’s on our plate. Because regardless of whether you’re eating just at night or every other day, ultimately, it’s what you eat, not when you eat, that really counts.

Can fasting and WeightWatchers® programme work together?

The WW weight-loss programme doesn’t advocate fasting as there is no clear evidence of its potential benefits, or side effects to health both short and long-term. You can, however, take some principles from intermittent fasting and adapt it to work with the WW programme, if you choose.