This form of intermittent dieting may also help reduce body fat, lower blood pressure and improve other markers of health, the study reports.
“Why exactly calorie restriction and fasting induce so many beneficial effects is not fully clear yet,” says Dr. Thomas Pieber, one of the study’s authors and an endocrinologist at the Medical University of Graz in Austria, in a released statement.
“The elegant thing about strict ADF is that it doesn’t require participants to count their meals and calories: they just don’t eat anything for one day,” he adds.
The people who participated in this study were all healthy, however, and with a body mass index (BMI) in either the “normal” or “overweight” category. It’s therefore not clear if this form of intermittent fasting would work for people with obesity or if it would be safe for those with medical conditions. And, as Pieber and his colleagues warn in the study, intermittent fasting should not be tried — even by healthy people — without first consulting a physician.
Still, the findings are interesting, for this is the largest clinical trial to date to look at the effects of strict alternate-day fasting on weight loss and various health measures.
How the study was done
For the four-week study, the researchers recruited 60 volunteers between the ages of 35 and 65. Their BMIs ranged from 22 to 30. (A BMI between 18.5 and 24.9 is considered “normal” or healthy, and one between 25 and 29.9 is considered overweight.)
The participants’ blood glucose, cholesterol and blood pressure were at recommended levels, and none of the people in the study had a history of diabetes, heart disease, stroke or cancer. In addition, none had used tobacco or recreational drugs within five years, and none reported drinking more than 15 alcoholic drinks per week.
The participants were randomly assigned to either continue with their usual eating habits (the “control” group) or to do alternate-day fasting — specifically, 12 hours of usual eating followed by 36 hours of fasting. While fasting, people were permitted to drink water or unsweetened coffee or black or green tea.
The participants were also instructed to keep food diaries throughout the study. In addition, they wore continuous glucose monitors (which show spikes in blood glucose when people consume calories) to make sure they stayed with the fasting regimen.
The basic results
People in both groups consumed fewer calories and lost weight during the four weeks of the study. But the fasting group took in far fewer calories and lost significantly more weight.
Here are the specifics:
- The control group consumed, on average, about 840 fewer calories per week.
- The fasting group consumed, on average, about 4,270 fewer calories a week — a reduction of 37 percent from their pre-study weekly calorie count.
- The control group lost an average of slightly less than half a pound during the study.
- The fasting group lost an average of 7.7 pounds — an amount that lowered their BMI by an average of 1.2 points.
- The control group lost a third of a pound, on average, in total body fat.
- The fasting group lost an average of 4.6 pounds in total body fat. Most of that reduction involved abdominal fat, which, in excess amounts, is considered a greater threat to health than hip or thigh fat.
Other markers of health also improved for the fasting group. For example:
- Their systolic blood pressure fell from an average of 121 mm/Hg to 115 mm/Hg.
- Their heart rate dropped from an average of 63 beats per minute to 60 beats per minute.
- They experienced a reduction in levels of soluble intercellular adhesion molecule-1, a marker linked to inflammation and age-related disease.
- They experienced a drop in the thyroid hormone triiodothyronine, without encountering any problems with how their thyroid functioned. Other research has linked lowered levels of this hormone with longevity in humans.
As part of this study, the researchers also examined health data from a group of 30 patients who had been following an alternate-day fasting regimen for six months or more. They found no adverse effects among this group.
Not for everyone
Although this small clinical trial had positive results for alternate-day fasting, its authors stress that intermittent fasting is not for everyone.
“We feel that it is a good regime, for some months, for obese people to cut weight, or it might even be a useful clinical intervention in diseases driven by inflammation,” says Frank Madeo, a study co-author and a microbiologist at the University of Graz, in a released statement. “However, further research is needed before it can be applied in daily practice.”
“Additionally, we advise people not to fast if they have a viral infection, because the immune system probably requires immediate energy to fight viruses,” he adds.
The researchers underscore the need to consult a physician before going on any kind of fasting diet. “Even healthy adults should not perform [alternate-day fasting] without consultation by clinicians to rule out adverse effects due to critical medical conditions,” they write in their paper.
They also point out that “a wholesome and balanced diet is likely crucial to foster the beneficial effects caused by [alternative day fasting]. Thus, appreciable clinical support and a generally healthy lifestyle should be considered before starting” this form of fasting.
FMI: You can read the study in full on Cell Metabolism’s website.