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Does Intermittent Fasting Work?

>> Monday, May 15, 2017




One of the diet approaches that has really taken off in popularity is Intermittent Fasting.  Essentially, this means that certain days/times you restrict eating (or don’t eat at all) and other days/times, you feast.  This can take the form of Alternate Day Fasting (fast one day and feast the next), restricting on some days (eg weekdays) and feasting on others (eg weekends), or restricting eating to only a few hours each day.

Most studies showing benefit of intermittent fasting have been of very short duration (less than 12 weeks) – and let’s face it, just about anything can work over this very short term.  Now, a one year randomized controlled clinical trial has investigated whether intermittent fasting works.

The study, published in JAMA Internal Medicine, is quite a beautifully conducted trial (in my opinion), randomizing 100 people with obesity to one of three groups:

  • Alternate day fasting: 25% of energy needs on fasting days, and 125% of energy needs on non fasting days
  • Daily calorie restriction: 75% of energy needs on all days
  • Control group: no intervention (they received 3 months of free weight loss counselling and a 1 year free gym membership at the end of the study)

Participants followed the above for the first 6 months of the study, which was the weight loss phase. 

For the second 6 months, the focus was on weight maintenance. Calorie needs were reevaluated (because we need less calories to maintain weight following weight loss), and the groups proceeded as follows:

  • Alternate day fasting: 50% of energy needs on fasting days, and 150% of energy needs on non fasting days
  • Daily calorie restriction: 100% of energy needs on all days
  • Control group: no intervention


For the scientists in the audience: Total daily calorie needs were assessed using doubly labeled water, assessed at baseline and again at the start of the weight maintenance phase (t=6 months).  Analysis was by intention to treat.

At 12 months, the rate of dropout from the study was highest in the alternate day fasters at 38%, compared to 29% in the daily calorie restriction and 26% in the control group.

They found that the weight loss between the alternate day fasting and daily caloric restriction were no different at 6 months or 12 months.  Weight loss was 6.0% greater than the control group at one year in the intermittent fasting group, vs 5.3% greater than the control group in those on daily calorie restriction.   Other than a slightly higher bad cholesterol (LDL) in the intermittent fasters, there were no differences in any metabolic parameter.

While the study is small, it is the longest and largest clinical trial of alternate day fasting to date. 


BOTTOM LINE: The results of this study suggest that alternate day fasting is no better than daily calorie restriction for weight loss, and that the likelihood of sticking to the diet is lower with alternate day fasting.


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