Why Intermittent Fasting May Not Be Your Best Bet for Weight Loss

An Arivale Hot Topic

Jennifer Lovejoy, Arivale Chief Science Officer, PhD
Jennifer Lovejoy
Arivale Chief Science Officer, PhD

Intermittent fasting may not help you lose more weight than a standard diet, according to a new study published in the American Journal of Clinical Nutrition and covered by People.

The Study

The study compared the effects of intermittent calorie restriction (ICR) to those of a standard calorie restricted weight-loss diet (CCR) on a wide range of metabolic factors.

Researchers recruited 150 participants from Heidelberg, Germany, and randomly assigned them to one of three study arms: ICR, CCR or control. Participants in all groups were provided general information on healthy dietary guidelines by a dietitian, but those in the ICR and CCR groups got more comprehensive counseling and a personalized diet plan, as well as biweekly dietitian phone calls for the first 10 weeks.

Participants in the ICR group were advised to restrict their calorie intake to 25 percent of their daily energy requirement on two non-consecutive days per week. On the remaining five days, they were counseled on a calorie-neutral balanced diet. In the CCR group, participants were counseled on a 20-percent reduction in daily calorie intake.

Although the ICR and CCR diets were designed to be comparable in average weekly calorie reduction (20 percent), when researchers assessed compliance, the ICR group actually restricted more (35 percent) than the CCR group (25 percent) in the first 12 weeks of the study.

Over those 12 weeks, the average weight loss in the ICR group was 7.1 percent of initial weight vs. 5.2 percent in the CCR group and 3.3 percent in the control group – a difference likely due to the greater caloric restriction in the ICR group.

However, by week 24 and week 50 the ICR group had regained weight such that there was no difference in weight loss between ICR and CCR (5.2 percent vs. 4.9 percent) when the study ended. The regain in the ICR group probably relates to poor compliance after 12 weeks. During the initial 12 weeks, over 90 percent of participants in the ICR group reported performing the two calorie-restricted days as designed. However, at week 24 only 33 percent of participants reported doing the two days of restriction – and this decreased even further by week 50 to just 21 percent of participants.

In terms of biomarkers related to cardiometabolic health, researchers observed no difference in insulin, lipids, leptin, insulin resistance, inflammatory markers, or liver function between the two diets. The only biomarker that differed significantly between diets was fasting glucose, which improved to a greater extent in the CCR group compared to the ICR group at 12 weeks (despite less weight loss in that group by that point). This difference did not persist at later study time points.

Arivale’s Take

This study adds to literature showing caloric restriction – whether achieved by intermittent fasting (specifically the “5:2 Diet”) or more modest daily calorie restriction – is effective for sustained weight loss and improvements in cardiovascular risk. As with previous studies (such as this one), the new study finds no difference between the approaches, not only in weight loss but also in genetic and biochemical markers of health.

An advantage to this study is that participants were followed for nearly a year. While compliance with both calorie-restricted diets was very good in the first 12 weeks when people were motivated and receiving biweekly support calls from a dietitian, compliance with the ICR diet waned over the following months. By the end of the year, only about one in five participants were still “fasting” twice a week. Not surprisingly, this is also the group that regained the most weight over time.

One limitation to this study is the inability to generalize its results to a wide-ranging population since it was performed solely on individuals of European ancestry with no chronic disease. The authors note that the results may not apply to those with chronic diseases ­– such as type 2 diabetes, cardiovascular disease, or cancer – where there may be safety concerns around fasting.

Calorie restriction is central to any weight-loss plan and can be achieved by either limiting calories daily or through more extreme restriction two days per week. However, weight-loss diets only work when you follow them and when they can be sustained for the long term.

If you’re considering intermittent fasting, think carefully about whether such extreme restriction – even if it’s only a couple times per week – is something you can maintain for a long period and how you’ll transition to a weight-loss maintenance plan that requires making daily choices to prevent regain.

Further Reading



[Arivale Hot Topics address health stories currently in the news. The Arivale Clinical Team’s commentary on these news articles is not a review of the scientific evidence, nor an endorsement of a specific study, and is not meant as official medical opinion.]