Little is known about the effects of a SAA restricted diet on body weight and we have thus based power calculations on data from generic vegan interventions [38]. The online tool GLIMMPSE (https://v2.glimmpse.samplesizeshop.org) was used for power calculations [39]. With two groups and three repeated measures we will test for a significant group by time interaction in the study, indicating differences in body weight over time between the groups. Desired power is 0.8 and alpha was set to 0.05. A body weight difference of 3 kg between groups at the end of the study is considered clinically meaningful. The average body weight at baseline was set to 90 (SD 6) kg. A mean body weight difference of 1 and 3 kg between the SAAlow and SAAhigh after 4 weeks and 8 weeks was set in the power calculations, respectively. Baseline body weight is generally strongly correlated with body weight at subsequent visits. We hypothesize a correlation of 0.9 between repeated measurements of body weight in each subject at each study visit and a decay rate of this correlation coefficient of 0.5 per visit. This yielded a total sample size of 46. Accounting for 25% dropout (46/1 − 0.25), the total sample size is 60 corresponding to 30 participants per group. The design file used for sample size calculations is available in the supplementary material and can be uploaded to GLIMMPSE to view results and design choices (Additional file 3).

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