Fasting 'no magic bullet' for weight loss, study suggests

Intermittent fasting has become a popular diet among people hoping to lose weight. (Stock, Getty Images)
Intermittent fasting has become a popular diet among people hoping to lose weight. (Stock, Getty Images)

Fasting has become a popular way of staying slim, with many Hollywood starlets crediting intermittent diets for their svelte frames.

Made popular by TV medic Dr Michael Moseley, the well-known 5:2 diet recommends eating normally for five days of the week and drastically cutting your calorie intake on the other two.

Many praise intermittent fasting for helping them to shed the pounds without completely overhauling their lifestyle, however, scientists from the University of Bath have warned that the diets are not a "magic bullet" for weight loss.

The team analysed 36 people who had a healthy weight at the start of the study.

Read more: Fasting lowers blood pressure in rats

After three weeks, the participants who had limited their calorie intake across all their meals lost more weight than those who fasted on alternate days, but then ate more over the 24 hours where their consumption was not restricted.

Fasting was also linked to more weight being lost from muscle mass than fat, potentially affecting a dieter's ability to exercise.

Female fitness still life. Scales and measuring tape on pink background. Mockup. Planning of diet and trainings. Top view with copy space. Healthy lifestyle concept. Slimming
Fasting may cause more weight to be lost from muscle mass, rather than fat stores. (Stock, Getty Images)

"Many people believe diets based on fasting are especially effective for weight loss or these diets have particular metabolic health benefits even if you don't lose weight," said lead author Professor James Betts.

"Intermittent fasting is no magic bullet and the findings of our experiment suggest there is nothing special about fasting when compared with more traditional, standard diets people might follow.

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"Most significantly, if you are following a fasting diet it is worth thinking about whether prolonged fasting periods are actually making it harder to maintain muscle mass and physical activity levels, which are known to be very important factors for long-term health".

The participants were divided into three groups. The first fasted on alternate days. The days when the dieters drastically cut their calorie intake were then followed by them eating 50% more than normal.

In the second group, the participants reduced their calories across every meal by 25%, with no intermittent fasting.

The third group fasted on alternate days, like group one. The day after they restricted their calories, however, their consumption increased by 100%.

Read more: Exercise 'partly reverses' a fatty diet

Groups one and three cut their calorie intake from the recommended 2,000kcal to 2,500kcal a day to around 1,500kcal to 2,000kcal.

The participants in groups one and two reduced their calorie consumption by the same amount, but in different ways. The third group fasted, but did not reduce their overall calorie intake.

Watch: What is intermittent fasting?

The participants in group two, the only ones not to have fasted, lost 1.9kg (4lb 3oz) in three weeks.

Body scans revealed this weight loss was almost entirely down to them shedding fat, as published in the journal Science Translational Medicine.

The participants in group one lost a similar amount of weight – 1.6kg (3lb 8oz) – but only half came from reduced body fat, with the remainder being lost from muscle mass.

Finally, group three lost a "negligible" amount of weight, likely due to the participants' bodies not having to draw on their fat stores for energy.

Concerns have been raised that people may overindulge on their "cheat days", potentially undoing the hard work put in when fasting.

Cutting calories across the week and upping your exercise routine may therefore be a more sure-fire way to stay slim.

Watch: Presenter Ryan Seacrest credits fasting for maintaining his energy levels