Background Some studies on weight loss promotion using smartphone apps have shown a weight loss effect but not an increase in physical activity. However, the long-term effects of smartphone apps on weight loss and increasing physical activity have not been rigorously examined to date. Objective The aim of this study was to assess whether the use of a smartphone app will increase physical activity and reduce body weight. Methods In this parallel randomized clinical trial, participants recruited between April 2018 and June 2019 were randomized in equal proportions to a smartphone app group (n=55) or a control group (n=54). The intention-to-treat approach was used to analyze the data from December 2019 through November 2021. Before the intervention, an hour-long lecture on weight loss instruction and increasing physical activity was conducted once for both groups. Participants in both groups were instructed to weigh themselves immediately after waking up at least once daily from the start of the intervention. Monthly emails were sent advising the participants in both groups on how to lose weight and increase physical activity in order to maintain or increase motivation. Participants in the smartphone app group were instructed to open the app at least once a day to check their step count and rank. The primary outcome was daily accelerometer-measured physical activity (step count) and the secondary outcome was body weight. Since there was a significant difference in the wear time of the accelerometer depending on the intervention period (P<.001), the number of steps and moderate-to-vigorous physical activity were also evaluated per wear time. Results The mean age of the 109 participants in this study was 47 (SD 8) years. At baseline, the mean daily total steps were 7259 (SD 3256) steps per day for the smartphone app group and 8243 (SD 2815) steps per day for the control group. The difference in the step count per wear time between preintervention and postintervention was significantly different between the app group and the control group (average difference [95% CI], 65 [30 to 101] steps per hour vs –9 [–56 to 39] steps per hour; P=.042). The weight loss was –2.2 kg (SD –3.1%) in the smartphone app group and –2.2 kg (SD –3.1%) in the control group, with no significant difference between the groups. In addition, when divided into weekdays (Monday through Friday) and weekends (Saturday and Sunday), there was a significant interaction between step counts (P=.004) and MVPA (P=.003) during the intervention, with the app group showing higher interaction on weekends than the control group. Conclusions In this trial, the group with the smartphone app intervention showed increased physical activity, especially on weekends. However, this increased physical activity did not lead to increased weight loss. Trial Registration University Hospital Medical Information Network UMIN000033397; https://center6.umin.ac.jp/cgi-open-bin/ctr_e/ctr_view.cgi?recptno=R000037956

BACKGROUND Some studies on weight loss promotion using smartphone applications (smartphone app) including mobile applications have shown a weight loss effect, but not an increase in physical activity, and they have not been rigorously examined for longer periods. OBJECTIVE To assess whether the use of a smartphone app will increase physical activity and reduce body weight. METHODS In this parallel randomized clinical trial, participants recruited between April 2018 and June 2019 were randomized in equal proportions to a smartphone app group (n=55) or control group (n=54). The intention-to-treat approach was used to analyze the data from December 2019 through November 2021.　Before the intervention, an hour-long lecture on weight loss instruction and increasing physical activity was conducted once for both groups. Participants in both groups were instructed to weigh themselves immediately after waking up at least once daily from the start of the intervention. Monthly e-mails were sent advising the participants on how to lose weight and increase physical activity in order to maintain or increase motivation in both groups. Participants in the smartphone app group were instructed to launch the app at least once a day to check their step count and rank. The primary outcome was daily accelerometer-measured physical activity (step count) and secondary outcomes was body weight. RESULTS The 109 participants had a mean (standard deviation) age of 47 (8) years. At baseline, the mean (standard deviation) daily total steps were 7259 (3256) for the smartphone app and 8243 (2815) for control groups, respectively. After the 32-week intervention period, the step count per wear time was significantly higher in the smartphone app group than in the control group [average difference (95%CI): 65 (30 to 101) vs. -9 (-56 to 39), p=0.042]. The weight loss was -2.2 kg (-3.1%) in the smartphone app group and -2.2 kg (-3.1%) in the control group, with no significant difference between the groups. During the intervention period, the step count per wear time on Saturdays [615 (545 to 684) vs. 554 (483 to 624), p=0.006] and Sundays [623 (553 to 694) vs. 556 (485 to 627), p=0.004] was significantly higher in the smartphone app group than in the control group. CONCLUSIONS In this trial, the smartphone app group showed increased physical activity, especially on weekends. However, this increased physical activity did not lead to weight loss. CLINICALTRIAL UMIN000033397 https://upload.umin.ac.jp/cgi-open-bin/ctr/ctr.cgi?function=brows&action=brows&recptno=R000037956&type=summary&language=J