Fasting – does it really help you lose weight?

Various fasting diets claim you can lose weight, improve your health and potentially extend your lifespan. We find out what the evidence is.

Helen Foster
March 2018

What is fasting? Traditionally, fasting meant totally abstaining from food, with the only thing consumed being water – a method that can be extremely harsh on the body.

“Now what we mean by the term is modified fasting where you simply restrict calories for a period of time,” explains Dr Jane Bowen, research dietitian at CSIRO.

“That might mean you limit calories for a couple of days a week or that you abstain from food for a set period each day. So you might have your last meal at 7pm and then not eat again until noon the next day.”

This form of fasting effectively gives the body a break from processing food. Supporters of fasting believe that when you do this it causes knock-on effects within the body’s cells that produce a range of health benefits.

Can fasting improve your health?

A recent study from Harvard University found fasting may alter how parts of each cell, called the mitochondria, behave. During their lifespan, mitochondria can switch back and forth between an intact state, which is harmless, and a ‘fragmented’ one that is linked with faster ageing. The trial discovered that during periods of fasting, the mitochondria remained in that helpful intact state, which could help slow how fast you age.

And research at the University of Southern California showed that when people cut kilojoules to between 3,130 and 4,600 (instead of the 8,700kJ consumed by the average Australian) for 5 consecutive days a month over 3 months, they reduced levels of inflammation in the body. Inflammation is increasingly becoming associated with faster ageing and chronic diseases such as heart disease. The people in the study also lowered their blood pressure and blood sugar level.

“The short-term evidence is definitely exciting,” says accredited practising dietitian Joel Feren. “But what we don’t yet know is how long these results actually last.”

Can fasting help you lose weight?

Some people are adopting fasting not for its potential long-term health benefits, but a more immediate one – weight loss. In the University of Southern California trial, for example, fasters lost on average 2.7kg and their waistlines shrank 2.5–5cm with effectively only 15 days of dieting. 

“Fasting diets definitely can lead to weight loss but it’s no greater than what you can achieve by simply restricting [food] intake day to day,” says Dr Bowen. “When fasts have been directly measured against this more traditional form of energy restriction they create the same results.”

Feren agrees, adding that it’s very likely that the weight will go back on if you don’t make some longer-term changes once you reach your goal, such as cutting back on treats, eating sensible portions and increasing your intake of fruit and vegetables. “It’s not something I like to see my clients do for that reason,” he says.

Are all fasting regimens the same?

If you do decide to take up fasting, it’s important to understand that it’s unclear whether all fasting regimes produce the same positive results, and it’s unlikely one approach will work for everyone.

While the University of Southern California study did show good results for cutting back food intake for 5 consecutive days a month, it’s not known if you’ll get the same results from regimens which just see you fasting a couple of non-consecutive days a week.

Who is fasting suitable for?

Fasting plans aren’t suitable for everyone. Feren says anyone who has had an eating disorder in the past shouldn’t use them; pregnant or breastfeeding women shouldn’t fast; and for people with health conditions such as diabetes or heart disease, fasting should only be done under medical supervision. Anyone on long-term medication should speak to their doctor before fasting as some drugs shouldn’t be taken on an empty stomach.