Monday and non-Monday concentrations of lifestyle-related blood componentsin the Oslo Diet and Exercise Study

Objective. We hypothesized that participants of intervention studies have a n unfavourable lifestyle at the weekend compared with the rest of the week, thus affecting the concentrations of components in samples drawn on Monday s. Design, The hypothesis was examined using data from the Oslo Diet and Exerc ise Study, a 2 x 2 randomized intervention trial on diet and exercise invol ving 209 participants. Each person chose which day of the week to attend fo r 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 an d 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 n on-Monday values, all Monday values differing from the non-Monday values in a cardiovascularly unfavourable direction. Participants starting and endin g on a Monday showed the unfavourable profile both times, and those who wer e examined both times on a non-Monday showed a consistently favourable prof ile, whereas those who changed the day of examination at the start and end changed profile accordingly. Conclusion. The lifestyle-related components examined here showed differenc es 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 weeken d compared with ordinary working days, If such effects are not recognized a nd properly taken into account, they may markedly affect the outcome of int ervention studies.