Objective: We hypothesized that participants of intervention studies have an unfavourable lifestyle at the weekend compared with the rest of the week, thus affecting the concentrations of components in samples drawn on Mondays.
Design: The hypothesis was examined using data from the Oslo Diet and Exercise Study, a 2 x 2 randomized intervention trial on diet and exercise involving 209 participants. Each person chose which day of the week to attend for blood sampling, both before and after the 1 year of intervention.
Main outcome measures: Comparison of mean concentrations of the components measured, in samples drawn on Mondays vs. non-Mondays, both at the start and at the end of intervention.
Results: At the start, nine components, from haemostasis, carbohydrate and lipid metabolism, showed a difference of more than 10% between Monday and non-Monday values, all Monday values differing from the non-Monday values in a cardiovascularly unfavourable direction. Participants starting and ending on a Monday showed the unfavourable profile both times, and those who were examined both times on a non-Monday showed a consistently favourable profile, whereas those who changed the day of examination at the start and end changed profile accordingly.
Conclusion: The lifestyle-related components examined here showed differences between Monday and non-Monday values, which were not due to a selection bias. We suggest they may be related to a different lifestyle at the weekend compared with ordinary working days. If such effects are not recognized and properly taken into account, they may markedly affect the outcome of intervention studies.