Intermittent Fasting May Not Aid Weight Loss, Large Review Finds

The authors of this review highlight the problem of obesity and how it is a risk factor for health problems like cancer. Intermittent fasting involves a number of different eating patterns, where there are timeframes of food intake and periods with little or no intake.

There is variation, and people may also pair this with special diets when they are eating. For this review, researchers wanted to specifically examine the relationship between intermittent fasting and weight. In addition to weight loss, other critical outcomes included adverse events and quality of life.

This review specifically focused on randomized controlled trials and cluster-randomized controlled trials, where the fasting intervention was at least four weeks, and the minimum follow-up was at least six months.

In all, the authors included 22 studies using data from 1,995 participants between them. All participants were at least 18 years old and had overweight or obesity.

Researchers looked at studies that compared intermittent fasting to traditional dietary advice, as well as those that compared it to no intervention or being on a waiting list to receive treatment.

Most studies compared intermittent fasting to other strategies, such as reducing calorie intake. The most common intermittent fasting method was time-restricted feeding, which involves only eating within a specific timeframe each day. Thirteen of the studies combined intermittent fasting with other interventions like nutritional guidance.

Compared to regular dietary advice, researchers concluded that intermittent fasting may have little to no impact on weight loss, quality of life, or adverse events.

However, researchers they also noted that there were fairly low levels of certainty regarding these conclusions. Additionally, intermittent fasting may have little to no effect on lipids, such as total cholesterol.

Researchers were more confident about their findings comparing intermittent fasting to being on a waiting list for treatment or receiving no treatment. However, this analysis only included six studies.

They concluded with moderate certainty that intermittent fasting has little to no effect on weight loss. They concluded with very low certainty that it has little to no impact on the “general physical health domain of quality of life,” and a similar finding for adverse events.

Intermittent fasting also may not have a major impact on total cholesterol or triglycerides. However, it might increase levels of good cholesterol. Sensitivity analyses revealed similar findings to the initial analyses.

In the subgroup analyses, researchers did find some differences for adverse events. For example, compared to regular dietary advice, men may be more likely to experience adverse events from intermittent fasting than women, although there is a need for more research in this area.

They also found some differences when it came to triglycerides and different methods of intermittent fasting when compared to no intervention or being on a waiting list.

This review does have limitations, both from the data and methods used. Researchers faced challenges from high workloads and having to label some information as awaiting classification due to a lack of information.

There is a possibility that this review does not provide complete information, and there is likely a need for more research.

Researchers assessed each study’s risk for bias “for the results of the primary outcomes.” They note that for most outcomes, there was an overall high risk of bias.

Furthermore, there was a lack of data in a number of areas. For example, the authors noted that none of the studies included information about diabetes status, participant satisfaction, or an overall measure of comorbidity, or co-occurring conditions. Additionally, less than half of the studies reported on participant adherence to the intervention.

There are also the limitations of the individual trials included in the review. For instance, most of the studies did not have a follow-up timeframe that lasted longer than a year.

Most were single-center studies, seven trials focused mainly on female participants, and a majority were conducted in high-income countries, accounting mainly for white participants.

In some of the studies included, intermittent fasting was often combined with other interventions, and it is unclear how these impacted trial results. The authors noted that most of the studies also “reported results that were inconclusive.”

Finally, body mass index (BMI) also has limitations as a weight measurement. The authors of the review themselves highlighted that BMI “is not a reliable indicator of obesity at the individual level,” though it is what they used in this review to determine overweight or obesity.

Priya Kalia, MD, a family medicine and obesity medicine board-certified doctor, and assistant professor in family medicine, who was not involved in this research, commented the following regarding the review:

“According to this Cochrane review, from the available randomized evidence, there’s not much of a difference [between] intermittent fasting compared to standard caloric restricted diet. One key point is that major metabolic outcomes — like diabetes prevention, diabetes remission, or long-term cardiometabolic risk — were not well studied or reported in these trials. Also, 5 lb [pounds] of muscle is not the same as 5 lb of fat when it comes to metabolic health. It would be nice to see studies incorporate body compositional analysis to have better measured outcomes.”