When people want to lose weight, the potential benefits of fasting can seem very appealing, because only having a certain window to eat means you potentially consume fewer calories.

But what is fasting? According to registered dietitian Marcela Fiuza, a spokesperson for the British Dietetic Association, fasting means consuming no calories in a certain time frame. It can be ‘intermittent’, meaning you switch between eating and fasting, or ‘prolonged’, which generally means fasting from two days onwards.

“Intermittent fasting, in particular time-restricted eating like the 5:2 or 16:8, has become popular in recent years,” she adds. “It involves eating within a time-restricted window each day, usually eight to ten hours.”

As well as weight loss, lots of people do fasting purely for the potential benefits, including better gut and heart health as well as lower blood pressure. But it’s important to note that fasting doesn’t guarantee these results, and it’s not suitable for everyone — particularly anyone with disordered eating, pregnant women, diabetics, seniors and children.

In this article we talk more to Fiuza about the potential benefits of fasting, and take a look at some of the cons too.

What does fasting do to the body?

Fiuza explains: “During fasting the body makes a number of metabolic adaptations to keep functioning optimally in the absence of external fuel (food). In the first few hours of fasting, the body resorts to its glycogen stores for energy. Once these are depleted, there is a metabolic switch, in which the body starts breaking down fatty acids into ketones that are then used as a source of energy.