The current study aimed to investigate the potential bias induced by the order of 24-hrs recalls, in a population of obese men and women, who were at the time of the investigation under medical supervision, under ad libitum diets of their choice (no dietary intervention).
The reduction in reporting, or a true reduction of intakes in consecutive days of evaluation, has been reported in some occasions (15), but has never been approached in obese individuals yet, to the best of our knowledge. In our dataset, energy and carbohydrates intakes (in which a reduction of reporting was observed), and fat and protein intakes (in which similar intakes were observed), support the results found by Arab et al in series of eight days of evaluation (15). The reduction of reporting in series of 24-hrs recalls could be explained by subjects becoming aware of their intakes in the process of declaring the intakes, also known as training bias (the “big brother” effect). These findings could also be due to reporting fatigue, or could reflect genuine changes in eating habits, induced by the fact that subjects become more aware about their diets and the importance of healthy eating. Underreporting of energy intakes was previously associated with dissatisfaction with body image and dieting practices, especially in women (7).
In this study, the only differences in energy and macronutrient reporting (carbohydrates) were identified between the first two recalls (Table 1 and Fig. 1). These systematic differences, specific to second recall, prompted us to ascertain whether the second recall could play a role in modifying the average intakes, if this time point would be eliminated from the averages constructed using all recalls. The exclusion of the second recall led to the occurrence of differences in energy and intakes for many nutrients, when compared to averages across all four recalls.
In this context, several questions deserve more scrutiny: 1) If a systematic bias exists in the reported values of one specific ordered recall, and this bias cannot be ascertained to other known factors, should this recall be included in further analyses?; 2) If this specific recall is eliminated from calculations, how does this change the reported energy and nutrient intakes that are considered further in the subsequent analyses? Our study suggests that the elimination of the recall identified with order-specific bias would significantly change the reported intakes, and this could potentially create further methodological issues when intake data would be used for further analyses (Table 4).
On the other hand, this study has also identified lower intakes for several micronutrients (vitamin C, calcium, fiber, folates, potassium), and for total sugars, which were specific for either the second or fourth recalls (Table 1 and Fig. 2). Due to the design of our study, it is difficult to speculate whether such differences are truly systematic and specific to the order of recalls, or if, within the FDR limits, these are spurious.
The origin of differences in energy and carbohydrate intakes, specific to the second recall, is not clear. One hypothesis could be that such differences reflect true differences in intakes. The follow-up recalls (second to fourth) were performed, for each individual, in different days of the week, with no obvious reason to consider that such differences could arise from a different distribution between weekends and work week days. Therefore our analysis indicated that these differences could not be ascertained to the distribution of weekends in first two recalls. However, we did identify differences in some of the nutrient intakes between weekend days and work days (Table 2), which could be due to differences in eating habits. Studies performed in other countries reported that the day of the week had little impact on the variance of reported values (4, 16). Recently, Gibson et al discussed many causes of misreporting and measurement errors in self-administered 24-hour recalls (17), However, our recalls were all administered by an interviewer.
Another hypothesis is that the decrease in energy and carbohydrate reported intakes, in the second recall, could reflect underreporting of intakes due to training bias. Although the 24-hrs recall method has the lowest underreporting bias (18) as compared with other food intake capturing methods, it is known that this method is still prone to misreporting energy intakes by up to 15% (18, 19).
Another reason for the systematic differences observed between the first two recalls could be due to changes in eating behavior, which then subsides or diminishes during subsequent recalls (third and fourth).
Although the weekend/weekday discrepancies had been tackled before in studies aiming children (20, 21), young women (22), middle life women (23) or general population (24), this assessment has not been previously done in an obese population, when looking at the systematic differences between the order of recalls. In this population, the increase in the energy difference between weekend and weekday intake was related to an increase in both carbohydrates and fats, indicating higher non-specific macronutrient food intake in weekend.
The trend observed in men (Tables 2 and 4) was similar to other results published, suggesting that weekend intakes are higher than weekday intakes, with mean differences in energy of 195.1+/-832.1 kcal, in fat of 0.9+/-30.8 g, and in carbohydrates of 13.1+/-102.1 g, but not reaching significance threshold in our sample after FDR correction (24–26). For females there was no difference observed between weekdays and weekends, with mean differences in energy − 2.1+/-503.6 kcal, fat 10.2+/-52.4 g and carbohydrates − 1.1+/-69.6 g, which is in contrast to results published in other studies, where similar trends as in males were seen (23, 24). Vitamin K and vitamin C intakes were lower in weekend for men, and vitamin A and riboflavin, folates and calcium had lower intakes during weekend in women, denoting potentially a lower quality diet during weekend for both men and women, in agreement with other published results (22–25, 27, 28).
In order to improve the overall quality of ordered 24-hrs recalls used to capture the energy and nutrient intake, and to reduce the exacerbation of bias observed between different sessions, our study suggests the possibility of systematic differences in the reported intakes for energy and macronutrients, which can be specific to the second reporting session. Whether these differences are due to true lower intakes or due to reporting bias, this study indicates that repeated 24-hr recalls can inherently present systematic differences between specific sessions. In our study, these differences were specific to the second session.
The results obtained when comparing the average of four 24-hrs recalls with the average of three evaluations made up from the same 24-hrs recalls (minus the second evaluation) indicated an increase of the averaged energy intake, all macronutrient intakes and most of micronutrients, with small or small to medium size effects (Table 4).
To overcome this potential bias, we suggest that, prior to averaging specific intakes across all sessions of reporting, a preliminary analysis should be done in order to identify if a certain time point had systematic, order-specific differences form all other time points. Once identified, a decision should be made about whether to include this time point or not in further analyses for nutrient intakes.
One of the limitations of this study was that the true intake of nutrients was not assessed, nor biomarkers of available nutrients were available, so the true cause of the differences found for the second time point could not be identified. Another limitation was that the study did not use a control sample and, therefore, one cannot ascertain whether such differences were specific or not to individuals with obesity and associated morbidities.