Alternate day fasting may help aid weight loss

"Extreme 'caveman' diet of fasting every other day may help overweight patients lose nearly 8lbs in just four weeks," reports the Mail Online.

The website reports on a new study that investigated the effects of intermittent fasting on weight, metabolism and a number of general health markers, such as cholesterol levels.

Researchers recruited 60 healthy adults aged 35 to 65, who were not obese, to either continue their normal diet or eat every other day.

Those who fasted every other day could consume only water and unsweetened black or green tea or coffee on fasting days, but could eat what they wanted on other days.

The study found volunteers lost weight, reduced their body fat and lowered their blood pressure and heart rate.

Bone density was slightly lower at the end of the study, but the cells of the immune system were not affected. People's metabolisms did not change.

The study suggests intermittent fasting may be a useful treatment for people needing to lose weight.

But people in this study were all healthy and with a body mass index (BMI) that ranged from healthy to overweight, so it's unclear how well intermittent fasting would work in obese people.

Intermittent fasting has been a popular diet plan since the 5:2 diet, where you eat normally for 5 days and fast for 2, hit the headlines.

But intermittent fasting is not safe or suitable for all people, such as those with long-term health conditions or a history of eating disorders.

And the researchers involved in this study made the point of warning that alternate day fasting should not be tried without medical advice. Speak to a GP for advice before starting.

Find out more about tried-and-tested ways to lose weight that are suitable for everyone

Where did the story come from?

The study was carried out by a large group of researchers from institutions including the University of Graz and Joanneum Research Forschungsgesellschaft in Austria, the University of Fribourg in Switzerland, INSERM in France and the University of Freiburg in Germany.

It was published in the peer-reviewed journal Cell Metabolism on an open access basis and the study is available to read for free online.

The Sun reported: "Experts have claimed that alternate day fasting, also known as the 'caveman diet' is 1 of the best and 'healthiest' ways to lose weight quickly".

It's not clear which particular group of experts the newspaper is referring to as no such claim is made in the study.

The Sun also claimed that the diet allowed people to "gorge on whatever tasty food they wanted" when not fasting, but the study's authors said "a wholesome and balanced diet is likely crucial" to benefit from the practice.

Several media reports said people were aged 48 to 52, but this represented the average ages in the group, not the full range of ages.

What kind of research was this?

The researchers carried out a randomised controlled trial (RCT), which is the best way to see the effects of an intervention.

They also compared a group of people who said they had followed alternate day fasting for at least 6 months with a healthy control group.

They described this as a cohort study, although it was not a representative cohort of the population, but a self-selecting group.

What did the research involve?

Researchers recruited 60 volunteers who:

  • were aged 35 to 65
  • had a BMI of 22 to 30 (a BMI between 18.5 and 24.9 is considered healthy)
  • were a stable weight
  • had blood glucose, cholesterol and blood pressure at recommended levels
  • had no history of diabetes, heart disease or stroke, inflammatory disorder or cancer
  • had not used tobacco or recreational drugs within 5 years
  • drank no more than 15 alcoholic drinks a week
  • did not take hormonal medicine or antidepressants, or regular drugs for blood sugar, blood pressure or cholesterol
  • were not vegetarian or vegan
  • were not pregnant, breastfeeding or planning pregnancy

They randomly assigned half to continue their normal diet (control) and half to alternate day fasting for 4 weeks.

People in the alternate day fasting group were asked to eat and drink normally during only 12 in every 48 hours.

During fasting periods, they could drink only water and unsweetened black or green tea or coffee.

Everyone in the study filled in food frequency questionnaires to record what they ate each day.

Before the start of the study and again at the end, the researchers recorded a range of body measures, including:

  • BMI
  • body fat and lean mass composition
  • bone mineral density
  • blood glucose and lipids
  • circulating immune cells
  • markers of inflammation
  • blood pressure
  • resting energy expenditure
  • activity levels

A second group of volunteers was also recruited for investigation. These people all reported having followed the alternate day fasting regime for at least 6 months.

They were also measured, and their measurements were compared with those of the 60 volunteers at the start of the RCT.

What were the basic results?

Of the 30 people assigned to ADF, 2 people dropped out during the study and 1 dropped out of the control group.

People in both groups reduced their calorie intake and lost weight during the 4-week study:

  • the control group ate about 839 calories a week less on average
  • the alternate day fasting group ate about 4,271 calories a week less on average, a reduction of 37.4%
  • the control group lost an average 0.19kg
  • the alternate day fasting group lost an average 3.5kg

This weight loss translated to about a 1.2 point difference in BMI for the alternate day fasting group.

Bone mineral density (BMD) changed slightly, but not in a way that was statistically significant compared with the control group:

  • BMD at the spine changed from 1.25g/cm to 1.26g/cm in the control group
  • BMD at the spine changed from 1.23g/cm to 1.22g/cm in the alternate day fasting group

The alternate day fasting group lost more total fat mass, 2.1kg compared with 0.15kg, especially from the trunk region.

Their systolic blood pressure dropped from an average 121mm/Hg to 115mm/Hg, and their heart rate from 63 beats per minute to 60 beats per minute.

There was no change in cholesterol measures.

The changes seen in the alternate day fasting group were the equivalent of a 1.4% reduction in risk on the Framingham Risk Score, which gives the percentage risk that someone will have a cardiovascular event (such as heart attack or stroke) in the next 10 years.

Markers of immune function were not affected. People in the alternate day fasting group were not less active than the control group and their metabolic rate did not slow.

The second group, which had been using alternate day fasting for 6 months or more, showed no adverse effects and had comparable calorie intake, bone mass density, activity levels and immune cells to the control group.

They also had lower cholesterol levels and although their blood iron levels were lower, they were not deficient in iron.

How did the researchers interpret the results?

The researchers said: "Four weeks of strict alternate day fasting improved markers of general health in healthy middle-aged humans while causing a 37% calorie reduction on average. No adverse effects occurred even after more than 6 months.”

But they added: "Even healthy adults should not perform ADF [alternate day fasting] without consultation by clinicians to rule out adverse effects due to critical medical conditions.

"Importantly, although not directly assessed in this study, a wholesome and balanced diet is likely crucial to foster the beneficial effects caused by ADF.

"Thus, appreciable clinical support and a generally healthy lifestyle should be considered before starting ADF."


This study adds interesting new information to evidence about the possible effects of intermittent fasting on humans.

But the study has limitations. It was relatively small (30 people each in the alternate day fasting and control group) and lasted only 4 weeks.

People in the study were all healthy and with a BMI that ranged from healthy to overweight.

We do not know what people in the study actually ate on the days when they were eating.

The volunteers who took part in the study were likely to be already interested in health, diet and alternate day fasting.

The study also was not blinded. Everyone knew whether they were in the control group or alternate day fasting group, for obvious reasons.

And we do not know the long-term effects, such as whether people put the weight back on that they'd lost during the study.

The group of long-term alternate day fasting users were self-selecting and we do not know anything about them before they started using alternate day fasting, so the information about them is of little use for assessing long-term effectiveness or safety of this practice.

The researchers said alternate day fasting may be easier to stick to than trying to restrict calories all the time for people wanting to lose weight.

But other studies have shown that people do feel hungry during fasting periods.

Many people put weight back on after losing it on a diet, and there's no reason to think that alternate day fasting would be any different.

It's really important not to use an extreme diet if you have a long-term illness, or if you have an eating disorder or a previous history of eating disorders.

Find out more about achieving a healthy weight

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