One Fasting Day a Week: The Kyoto University App Study

A pilot trial from Kyoto University tested a 12-week online program built around one fasting day a week, with meal replacements delivered to participants and encouragement sent through a phone app. The group on the program lost a modest amount of weight compared with a control group, most participants stuck with it, and no serious adverse events were reported. The trial also found a signal of muscle loss, which the authors say the program needs to address.

The paper was published on 26 January 2026 in JMIR mHealth and uHealth. Kyoto University announced the work on its research news page on 4 August 2026, which is why it is getting attention now. The findings are the same ones published in January.

What the program was

The researchers call it a Brief Online Intermittent Fasting Program. According to the paper's abstract and the university announcement, it combined:

• one fasting day each week, self-administered at home
• very low-calorie meal replacements (407 kcal) delivered for that day
• online guidance by video call
• app messages encouraging fasting and healthy habits, with weight management in the app

Note what "fasting day" meant here. It was not a day with no food at all. Participants ate the supplied meal replacements, so it was one very low-calorie day a week.

What the study did

This was a 12-week randomized controlled trial with two groups, described by the authors as a pilot. Adults with overweight or obesity (BMI from 23 to under 35) were recruited at 1 university, 1 hospital and 2 company offices in Japan.

Design	Pilot randomized controlled trial, 12 weeks
Participants	57 adults with overweight or obesity in Japan
Intervention group (28)	Weekly fasting day with 407 kcal meal replacements, online guidance, app messages
Control group (29)	App messages promoting healthy lifestyles only
Completed follow-up	25 in the intervention group, 27 in the control group
Main outcome	Change in body weight after 12 weeks
Also measured	Body composition, blood pressure, blood markers, quality of life, physical activity, adherence, adverse events

The program was delivered by a researcher who knew which group each person was in, so the trial was not blinded.

What it found

Weight. The mean change over 12 weeks was -0.9 kg in the intervention group and +0.6 kg in the control group. After adjusting for sex, age and starting weight, the difference between groups was -1.6 kg (95% CI -2.5 to -0.8), which was statistically significant. The authors describe the result as "modest weight loss".

Body composition. This is the part that deserves attention. The change in fat mass was not statistically significant (-0.1 kg, 95% CI -1.3 to 1.4). The abstract says a reduction in muscle mass "was implied" (-1.3 kg, 95% CI -2.5 to -0.2). So the trial cannot show that the weight lost was fat, and it points toward some of it being muscle.

Adherence and safety. Adherence in the intervention group was 79% (22 of 28); the university announcement rounds this to about 80%. No serious adverse events were reported. There were no significant changes in key blood markers such as hemoglobin A1c, or in quality of life.

The authors conclude that the program was feasible and safe over 12 weeks, and that the observed muscle loss "indicates a need for program refinement, such as incorporating exercise guidance."

What it does not show

• It is a pilot. 57 people is a small trial. Its purpose was to test preliminary effectiveness, feasibility and safety, not to settle the question.
• Twelve weeks only. The abstract reports nothing about whether the weight stayed off afterward.
• The effect is small. A difference of 1.6 kg over three months is real in the statistical sense, but it is modest, and the authors say so.
• The muscle finding cuts against it. A weight-loss method is usually judged by fat lost and muscle kept. Here the fat change was not significant and the muscle estimate went down.
• You cannot separate the parts. The intervention group got delivered meals, video guidance and fasting messages together. The trial cannot say which of these produced the difference.

There is also a disclosure to be aware of. The paper's conflict of interest statement says that a company funded the study and had no role in its design, conduct, analysis or reporting; that the study app was provided by a company with ties to members of the research team; and that the first author owns a dietary guidance business involving fasting. The statement says these were reported to and audited by Kyoto University.

What this means if you fast

The trial tested a specific, structured program: supplied 407 kcal meals on the fasting day, guidance from the research team, and screening of who could take part. It did not test fasting on your own, and it did not test QoFast or any other consumer app. Results from that supervised setup should not be assumed for a self-directed weekly fast.

With that said, a few ideas from the study are reasonable to take away as ideas, not as promises:

• A once-a-week pattern was something most participants kept up for 12 weeks. If daily windows do not suit you, weekly patterns exist; 5:2 and Eat Stop Eat are the common ones, and they differ from the program in this trial.
• The muscle result is a caution. The authors' own suggestion is to add exercise guidance. If you are losing weight by any method, it is worth asking a doctor or dietitian how to protect muscle.

This is general information, not medical advice. If you are pregnant or breastfeeding, have diabetes or take blood-sugar medication, have a history of eating disorders, take other regular medication, or are under 18, talk to a doctor before fasting. If you live with obesity or a related condition, a weight-loss plan is something to set up with your doctor.

QoFast is a fasting tracker app for iPhone and Android with intermittent plans (16:8, 18:6, 20:4, OMAD and custom), a timer that works offline, and water and mood logging with a fasting history. It was not part of this study and it does not deliver meals or coaching. It gives general information, not medical advice, and is free to download with an optional Pro subscription: App Store and Google Play.

FAQ

Does fasting one day a week help with weight loss? In this pilot, a program with one very low-calorie day a week led to a statistically significant but modest difference of -1.6 kg against the control group over 12 weeks.

Did participants eat nothing on the fasting day? No. They received very low-calorie meal replacements of 407 kcal for the weekly fasting day.

Was the weight lost fat? The trial could not show that. The fat mass change was not statistically significant, and the data implied a reduction in muscle mass.

Sources

• Noda T, Shimamoto T, Kiyohara K, Imanaka M, Nishioka N, Tateyama Y, Iwami T. "A Brief Web-Based and Mobile Intervention of Intermittent Fasting With Meal Support for Weight Loss Among Adults With Overweight and Obesity in Japan: Pilot Randomized Controlled Trial." JMIR mHealth and uHealth, 26 January 2026. DOI: 10.2196/58930, PubMed 41587446
• Kyoto University research news, 4 August 2026 (in Japanese): デジタル×週1日断食で減量へ