Sporadic or “intermittent” fasting is a hot weight-control trend.
The concept was popularized physician/journalist Michael Mosley, M.D., in the 2012 BBC Horizon documentary Eat, Fast, & Live Longer.
There are several types of intermittent fasting ... Dr. Mosley tried the so-called “5:2” plan (more on that below), and wrote about it in his 2013 book, “The Fast Diet”.
Unlike typical diets, which require cutting the calorie count of every meal, intermittent fasting alternates periods of fasting (or steep calorie-cutting) with periods of normal eating.
Let’s review the most popular approaches to intermittent fasting, and the clinical research conducted to date on each type.
Alternate-day fasting calls for consuming about 25% of your normal calorie needs one day, then about 125% of your calorie needs the next day, in a repeating pattern.
Last year, researchers from Stanford University, the University of Illinois and Louisiana’s Pennington Obesity Research Center conducted a small clinical trial designed to compare the effects of alternate-day fasting (ADF) with those of a standard calorie-restricted diet
The research team recruited 100 obese adults aged 18 to 64 years, and randomly assigned them to one of three groups for this 12-month trial:
After one year, there were no significant weight-loss differences between the ADF and DCR groups, both of which lost more weight than the control group.
Nor were there any significant differences in blood measures of risk for cardiovascular disease and diabetes — with one exception.
Compared with the DCR group, the ADF group developed significantly higher levels of healthier, low-density LDL cholesterol particles by the end of the year-long trial — after a temporary rise in less-healthful high-density LDL particles.
It’s important to note that the people in the ADF group didn’t comply with their assigned plan closely: they consumed more calories than prescribed on fast days, and fewer calories than prescribed on feast days.
In contrast, those assigned to the DCR group generally consumed their prescribed calorie intakes.
Interestingly, the results of a 10-week University of Illinois study that involved 35 obese people showed that alternate-day fasting improved their cholesterol profiles over that rather brief period, regardless of whether their ADF diet was relatively high-fat/low-carb or vice versa (Klempel MC et al. 2013)
The 5:2 intermittent fasting diet
The 5:2 fasting diet allows you to eat as much of anything you want five days each week — provided that you stay within a “normal” range of calories for those days.
Then, for two non-consecutive days, you’re restricted to consuming 25% of your normal calorie needs: that is, about 500 calories for women and 600 for men during each of the two fasting days.
Last month, researchers from Germany, Seattle, and Utah, published the results of a 12-week clinical trial that compared the effects of the 5:2 diet to a daily calorie-restricted or DCR diet — one that involved a 20% drop in those participants’ usual calorie intake (Schübel R et al. 2018).
After 12 weeks, the 5:2 fasting group had lost slightly more weight than the DCR group.
And when the researchers followed up with the participants almost one year (50 weeks) later, they found that the people in the 5:2 fasting group had maintained a slight weight-loss advantage over people in the DCR group.
There were no significant differences between the groups in terms of the expression of 82 genes in fatty tissue that have been linked to chronic diseases — nor were there any significant differences in the levels of various biomarkers for metabolic disorders such as diabetes.
As the researchers wrote, “Our results on the effects of the “5:2 diet” indicate that ICR [intermittent calorie restriction] may be equivalent but not superior to CCR [i.e., DCR or daily calorie restriction] for weight reduction and prevention of metabolic diseases.”
Likewise, a recent German study found no advantage to 5:2 fasting over conventional calorie-restriction diets.
Scientists from the German Cancer Research Center and Heidelberg University Hospital recruited 150 overweight and obese people for a year-long trial.
They assigned the participants to one of three groups:
After 12 months, both diet groups experienced virtually identical weight loss and loss of belly fat.
However, as lead author Tilman Kühn said, “… for some people it seems to be easier to be very disciplined on two days instead of counting calories and limiting food every day.”
16:8 intermittent fasting
This daily plan bans all food and calorie -containing beverages for 16 hours every day.
That pattern resembles — but reverses the schedule of — the dawn-to-dark fast practiced by Muslims during Ramadan, which has also been the subject of some research.
Some practitioners of the 16:8 plan follow it for extended periods of time, while others follow it for just a few days each week.
Earlier this year, scientists from the University of Illinois, Indiana University, and the Salk Institute published the results of a 12-week clinical trial designed to test the effects of the 16:8 diet (Gabel K et al. 2018).
The researchers recruited 46 obese people (average age 45) with an average body mass index (BMI) of 35, and divided them into two groups:
Compared with the control group, the 16:8 group averaged about 350 fewer calories daily, lost about 3% of body weight and lowered their blood pressure — but there were no significant differences in body composition or risk markers for cardiac or metabolic disorders.
As the authors wrote, “These findings suggest that 8-hour time restricted feeding produces mild caloric restriction and weight loss, without calorie counting. It may also offer clinical benefits by reducing blood pressure.”
Somewhat surprisingly, fewer people in the 16:8 group dropped out of the study, compared with the participants in trials that have tested other types of intermittent fasting.
As study co-author Krista Varady, Ph.D. said, “The results we saw in this study are similar to the results we've seen in other studies on alternate day fasting, another type of diet, but one of the benefits of the 16:8 diet may be that it is easier for people to maintain. We observed that fewer participants dropped out of this study when compared to studies on other fasting diets.”
Keep these tips in mind
Want to give intermittent fasting a try?
It may be worth a shot if you’ve haven’t been able to stick to a typical calorie-restricted daily diet.
No matter which way you try fasting, Harvard Health Publishing offers these suggestions: