Does Intermittent Fasting Actually Work? Study Finds Meal Frequency Matters More Than Timing

Intermittent fasting is an eating pattern in which people alternate between eating and fasting. It has grown in popularity in recent years as a weight loss strategy, with proponents claiming it can help improve metabolism and reduce inflammation.

A recent six-year study published in the Journal of the American Heart Association found no association between the timing of first and last meals and weight loss.

New research published in Journal of the American Heart Association showed that the frequency and portion size of meals had a greater impact on weight loss or gain than the time frame between the first and last meal.

According to senior study author Wendy L. Bennett, Ph.D., of the Johns Hopkins University School of Medicine, despite the popularity of intermittent fasting, no studies have determined whether restricting total eating windows helps control weight.

This study assessed the association between the time from first to last meal and weight change. Nearly 550 adults (age 18 or older) with electronic health records from three health systems in Maryland and Pennsylvania participated in the study. Participants registered weight and height measurements at least once in the two years preceding the study enrollment period (February-July 2019).

Overall, the majority of participants (80%) identified themselves as white adults; 12% identified themselves as black adults; and about 3% identified themselves as Asian adults. Most participants indicated college or higher education; mean age was 51; mean body mass index was 30.8, which is considered obese. The average follow-up time for weights recorded in electronic health records was 6.3 years.

Participants with a higher BMI at enrollment were more likely to be black adults, older, have type 2 diabetes or high blood pressure, be less educated, exercise less, eat fewer fruits and vegetables, be Adults with longer meal times were more likely to fall asleep and had a shorter duration from their first meal to their last meal than adults with a lower BMI.

The research team created a mobile app called Daily24 for participants to record in real time when they sleep, eat and wake up for each 24-hour window. Emails, text messages, and in-app notifications encouraged participants to use the app as much as possible during the first month, and to use the app again during “momentum weeks” — one week per month for the six-month intervention part of the study .

From daily sleep and meal times recorded in the mobile app, the researchers were able to measure:

• The time from the first meal to the last meal of the day;
• Time elapsed from waking up to first meal; and
• Time interval from last meal to bedtime.

They calculated the average of all data for the number of days completed by each participant.

Data analysis found:

• Meal timing was not associated with weight change during the six-year follow-up period. This includes intervals from first to last meal, from waking up to first meal, from last meal to bed, and total sleep time.
• During six years of follow-up, the total number of daily large meals (estimated at more than 1,000 calories) and medium meals (estimated at 500-1,000 calories) were associated with weight gain, while fewer small meals (estimated at less than 500 calories) were associated with weight gain related to mitigation.
• The average time from first meal to last meal was 11.5 hours; the average time from waking up to first meal was 1.6 hours; the average time from last meal to bed was 4 hours; the average sleep time was calculated to be 7.5 Hour.
• The study found no association between meal timing and weight change in people with a wide range of weights.

As Bennett reports, while previous research has suggested that intermittent fasting may improve the body's rhythms and regulate metabolism, this study of a large group of people with a wide range of weights found no such link. Clinical trials of large-scale, rigorous intermittent fasting effects on long-term weight change are extremely difficult; however, even studies of short-term interventions may help guide future recommendations.

Although the study found meal frequency and total calorie intake to be stronger risk factors for weight change than meal timing, the findings could not prove direct cause and effect, said the study's lead author, Dr. Di Zhao, an associate scientist at Johns Hopkins. Department of Cardiovascular and Clinical Epidemiology, Bloomberg School of Public Health, University of Pennsylvania.

The researchers noted that the study was limited in that they did not assess the complex interplay of timing and frequency of eating. Also, because the study was observational in nature, the authors were unable to draw causality. The authors note that because the majority of the study's participants were white, well-educated mid-Atlantic U.S. women, future studies should strive to include a more diverse population.

The researchers also could not determine the study participants' weight loss intentions before enrollment, nor could they rule out any additional variables of pre-existing health conditions.

According to the American Heart Association's 2022 statistics, 40% of adults in the United States are obese; the association's current diet and lifestyle recommendations to reduce the risk of cardiovascular disease include limiting total calorie intake, eating healthy foods, and increasing physical activity.

2017 Scientific Statement from the American Heart Association: Meal Timing and Frequency: Effects on Prevention of Cardiovascular Disease No Clear Preference for Frequent Small Meals or Intermittent Fasting. It states that irregular patterns of total caloric intake appear to be detrimental to maintenance of body weight and optimal cardiovascular health. Also, changing meal frequency may not help with weight loss or improve traditional cardiometabolic risk factors.

Reference: Di Zhao, Eliseo Guallar, Thomas B. Woolf, Lindsay Martin, Harold Lehmann, Janelle Coughlin, Katherine Holzhauer, Attia A. Goheer, Kathleen M, “Association of diet and sleep intervals with body weight over time: The Daily24 cohort”. McTigue, Michelle R. Lent, Marquis Hawkins, Jeanne M. Clark, and Wendy L. Bennett, January 18, 2023, Journal of the American Heart Association.
DOI: 10.1161/JAHA.122.026484

The study was funded by the Johns Hopkins University School of Medicine through the American Heart Association's Strategic Grants Research Network.