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New Study Saying Time-Restricted Eating Doesn’t Work, Raises Skepticism

Timothy Boyer Ph.D.'s picture
Problems with timed-eating study results.

Time restricted eating or intermittent fasting has been a popular weight loss method this past year. However, a new study states that it does not work. Here are some responses to the study after it was published and what you can take away from this confusing dieting news.


Fasting Methods for Weight Loss

Intermittent fasting typically allows a person to eat what they normally want five days a week after drastically slashing calories the other two days a week by either avoiding all foods or at the very least limiting your calorie intake to just 500 calories or less during each of the two days.

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One of the ideas behind this is that it overcomes the hurdle of food restriction common in many diets that build up uncontrollable cravings and backsliding on a diet. Another idea supporting intermittent fasting is that the body needs a true fasting period to reduce blood glucose levels rather than maintain constant levels of glucose that are usually too high and thereby add to weight gain.

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However, typical intermittent fasting has evolved into a fasting method that typically comes in two types: The 24-hour fast and the 16/8 fast. The 24 hour fast is one where you choose 2 days of the week that are non-consecutive and simply avoid all foods and stick to just water or some cleansing tonic recommended for weight loss.

The 16/8 fast is one in which you gradually shorten your eating period to an 8-hour time slot while leaving the remaining consecutive 16 hours as a time for fasting. And then you do this every day until you’ve reached your weight loss goal.

Presumably, dieters find the time-split fasting methods preferable due to the fasting period is shorter and easier to manage. Earlier, we reported one study that pitted a 20/4 versus a 18/6 fasting split to see if varying the splits to closer to a full 24-hour fast can make a difference.

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New Study States Intermittent Fasting Does Not Work

However a new study published in the JAMA Internal Medicine made the news just yesterday saying that time-restricted fasting does not work for weight loss.

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According to a news release from the University of California San Francisco, researchers involved in the study stated that, “While time-restricted feeding has been shown to prevent weight gain in mice on a high-fat diet and reduces weight and metabolic outcomes in already obese mice, most of the reported benefits of such fasting in humans has had limited scientific testing.”

To add to what is known and understood about the benefits (or lack of) from intermittent fasting with human study participants, the researchers performed their own time-restricted eating study as follows per the news release:

This study, which included 116 men and women with a body mass index (BMI) of 27 to 46, found that people who were randomly assigned to only eat during an eight-hour period each day lost 2 lbs. (0.94 kg) on average over the 12-week period, while those with consistent meal timing lost an average of 1.5 lbs. (0.68 kg).

The study found no significant difference between the two groups in total weight loss or in other markers, such as fat mass, lean mass, fasting insulin or glucose, HbA1C levels, energy intake, or total or resting energy expenditure.

In other words, after 12 weeks, a weight loss comparison was made between a consistent meal timing (CMT) group instructed to eat 3 structured meals per day, and a time-restricted eating (TRE) group instructed to eat from 12:00 pm until 8:00 pm and then completely abstain from eating from 8:00 pm until 12:00 pm the following day. The result was no observed statistical difference between the two groups; hence, the conclusion, “Time-restricted eating, in the absence of other interventions, is not more effective in weight loss than eating throughout the day.”

Skeptics Remark on the Study

But is this really true? Along with the study, JAMA Internal Medicine also posted comments from others who pointed out their perceived flaws of the study and its conclusion:

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1. September 29, 2020: Danielle Hammond

16 hours of fasting isn't often enough to lose weight, and the participants were allowed zero calorie beverages. As far as I'm concerned no one should have wasted time and money on this study to try to say that fasting doesn't work.

Try fasting with only black coffee, tea, or water. And for at least 18-20 hours.

Have the participants do this for 3 months to become fully adapted. Then present the results.

2. September 29, 2020: No Objective Measure of Compliance and Differential Attrition
Robert Kaestner, Ph.D.

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The study measured compliance through self-reports. According to these self reports, 92.1% complied with CMT and 83.5% complied with TRE.

Further, it is unclear what non-compliance in the CMT group means? Did it include partial intermittent fasting—e.g., skipping breakfast? Did it mean ever not complying? Or not complying consistently?

Similar questions apply about compliance for the TRE group? And which group is more likely to misreport compliance?

The absence of compliance information is potentially quite important. For example, if we scale the weight change of compliers was -1.13 (0.94/0.835) kg. Doing the same for CMT group yields -0.74 kg. The difference is -0.39 kg., which is 50% larger than the simple difference reported in Table 2 of article.

But the absence of accurate and reliable compliance information is a serious flaw that diminishes the value of the results reported in the article.

Then there is significant attrition and it is non-random by treatment arm. This problem too diminishes the value of the results.

Overall, this article provides little credible evidence of the effect of intermittent fasting.

3. September 30, 2020: Disease markers? Fips Alfredsson, PhD.

Fasting is not just about weight loss. The study did not measure parameters that could assess general health status, which, in the long run, might be more important than weight loss.

Apart from this, I find another comment a bit misplaced—more extreme forms of fasting for much longer times of course will have likely promoted statistically significant differences in weight loss; yet, the idea behind studies like this is that a rhythm of 16:8 is what most people can do it, while 18:6 or 20:4 is much harder to achieve and live by for a broad sector of the population, and therefore not very feasible as an intervention method.

Regardless, it would have been important to consider more measures than weight loss to assess the efficiency of 16:8.

4. September 30, 2020: Time-restricted Eating Misunderstood in Study, MH Tang, MSc.

The study states that 3 meals were consumed—obviously the participants will not lose weight if total caloric intake remains unchanged.

The study authors have also taken the time-restricted eating out of proper context i.e. time-restriction is to be seen in the context of not consuming 2 of the typical 3 meals in a day—usually breakfast and lunch is skipped, or just breakfast.

5. September 30, 2020: Why even do such a study without controlling for what a subject eats? Rod Erickson, DC, MS.

As long as the subjects eat the SAD (standard American diet) ad libitum but limit it to 8 hrs/day does not suggest anything different from what is already known.

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What Does This Really Mean?

So what do we make out of all of this?

The study may not be all that the news media has hyped it to be about. The commenters make some reasonable arguments against the study and its findings—some of which are common dieting good sense regarding questioning of caloric intake by the participants.

But what the study may actually be showing us—between the lines—is that an accurate distinction between just what does and does not qualify as intermittent eating versus fasting, perhaps is clouding the issue and thus the study results.

For a commonsense approach toward timed eating with useful advice regarding when to eat and when not to eat, here’s an informative weight loss story about a pregnancy pounds weight loss success story during COVID-19.

Timothy Boyer has a Ph.D. in Molecular and Cellular Biology from the University of Arizona. For 20+ years he has been employed as a freelance health and science writer. Today, with an eye on the latest news, Timothy continues writing about science with a focus on what you need to know for healthier living. For continual updates about health, you can also follow Timothy on Twitter at TimBoyerWrites.

Image Source: Courtesy of Sonja Langford on Unsplash


Time-Restricted Eating Doesn’t Work for Weight Loss” UCSF News 28 Sept. 2020.

Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity” Dylan A. Lowe, PhD et al. JAMA Intern Med. Published online September 28, 2020.