What are the pros and cons of intermittent fasting?

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  • Intermittent fasting is an eating pattern involving not eating anything for large parts of the day.
  • The most common form of intermittent fasting is the 16:8 split – where you may eat for a period of eight hours and then fast for 16 hours.
  • There are several factors to consider when trying intermittent fasting for weight loss or management.

The practice of intermittent fasting has been gaining popularity as it is believed by many people to be an easy weight loss strategy.

According to the World Health Organization (in 2016), 1.9 billion adults were classified as overweight and 650 million as obese. This means that almost 34% of the global population have unhealthy eating and activity habits.

What is intermittent fasting?

Intermittent fasting is an eating pattern involving alternating periods of eating and fasting. There are different methods of intermittent fasting, which split either the day or week into eating periods and fasting periods. Some people may implement it unknowingly when skipping breakfast or eating breakfast later in the morning. 

How does intermittent fasting work?

The most popular form of intermittent fasting is the “16/8” method, also known as “time-restricted fasting”.

With this method, people usually stop eating at 8pm and resume eating after 12pm the next day, thus fasting for 16 hours, while allowing an eating window of eight hours.

Other forms include “alternate day fasting”, which involves rotating between eating days and fasting days, and “modified fasting”, which involves two versions:

  • Eating small amounts of food on fasting days, and as a result, cutting energy intake down to roughly 25% of the amount consumed on an average day.
  • The "5:2 fast", where you follow your normal eating pattern for five consecutive days, while fasting for the remaining two days of the week.

How intermittent fasting helps you lose weight

Intermittent fasting works on a dual concept of reduced glucose (and consequent insulin levels) and energy (kilojoule) restriction.

Carbohydrates, are broken down into glucose, which is used by our cells for energy. Glucose can, however, only enter our cells with the help of insulin – a hormone produced by the pancreas.

Insulin binds to the glucose in the bloodstream and transports it into the body cells for energy, but when consumed excessively, it is stored as fatty acids in the fat cells.

Thus, with intermittent fasting, when we don’t snack between meals or don’t eat for prolonged periods of time, less glucose enters the bloodstream to be stored as fat. Our fat cells can, however, release the stored fat to be converted to glucose and used for energy.

Thus, the first concept of intermittent fasting is to lower blood glucose levels and consequently the insulin response, enabling the body to burn stored fat for energy.

With regard to the second concept, it is important to know that the only way to lose fat is to get into an energy (kilojoule) deficit. This means burning more energy than we eat.

We should maintain a restricted energy intake from all foods, together with increased activity to ensure an energy deficit. Practising the intermittent fasting method of only eating two meals a day instead of three also helps to create an energy deficit.

Does intermittent fasting have any side effects?

Studies have found that binge eating and overeating are common downfalls of intermittent fasting.

In addition, side effects like irritability, poor concentration, fatigue, and headache have been observed in some individuals. 

Does intermittent fasting have metabolic advantages?

An analysis of previous research found that people with type 2 diabetes who did intermittent fasting had a slightly better weight-loss outcome (1kg) than the control group.

They found no difference in blood glucose control between the two groups. The HbA1c values were the same in the intermittent fasting group as in those who followed a healthy diet consisting of three meals a day.

The HbA1c value is an indication of the average blood glucose levels over a period of three months.

Another analysis of research assessed the effect of intermittent fasting on the prevention of cardiovascular disease (CVD).

They found no significant evidence to support intermittent fasting for the prevention of CVD when comparing people who ate a three-meal energy restricted diet to those who practised intermittent fasting.

It is important to remember that as long as you maintain an energy (kilojoule) deficit, you will lose weight and benefit from the metabolic advantages associated with a healthy weight – whether you enjoy three meals a day or practise intermittent fasting.

Intermittent fasting is an individualised approach to restricting energy (kilojoules), which might, however, not suit everyone’s lifestyle.

*Raffaella Braga is a registered dietitian

READ | Intermittent fasting for weight loss: Two types work, according to new review

READ | How snacking before bedtime piles on the kilos

READ | 10 health benefits of intermittent fasting diets

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