Within-group comparisons led to unsubstantiated conclusions in “Low-phytate wholegrain bread instead of high-phytate wholegrain bread in a total diet context did not improve iron status of healthy Swedish females: a 12-week, randomized, parallel-design intervention Study”

Errors in the implementation, analysis, and reporting of randomization within obesity and nutrition research: a guide to their avoidance

“…Yet it is possible that assigning participants to cut only 300 kcal would not have produced the same benefits if they once again achieved only half of that assigned. In another example, the random assignment of high phytate bread did not lead to a statistically significant difference in whole body iron status as compared to dephytinized bread when missing data was imputed, but it was significantly higher when dropouts were excluded [98,142,143]. A difference cannot be concluded from these data based on the causal question of the assignment of high phytate bread, particularly because dropout was significantly higher in one group, which may create an artificial effect.…”

“…The DINS error was identified in an RCT testing isomaltulose vs. sucrose in the context of effects of an energyreduced diet on weight and fat mass, where some conclusions, such as the outcome of fat mass, were drawn from within-group comparisons but the between-group comparison was not statistically different [86]. We observe this error frequently in nutrition and obesity research [87][88][89][90][91][92][93][94][95][96][97][98][99][100][101][102][103]. Sometimes using this logic still reaches the correct conclusions (i.e., the between-group and within-group comparisons are both statistically significant or not), but often it does not, and therefore it is an unreliable approach for inferences.…”