Stability of the timing of food intake at daily and monthly timescales in young adults

The global rise in obesity, particularly among college-aged individuals, necessitates investigating factors contributing to weight gain. Emerging evidence suggests that the timing of food intake plays a crucial role in cardiometabolic health. Previous research indicated a correlation between later eating (closer to dim-light melatonin onset, DLMO) and higher body mass index (BMI) in non-lean individuals. However, these studies were often short-term and cross-sectional, using clock time rather than physiological measures. This study addresses the gap in knowledge regarding the stability of meal timing, both in relation to clock time and circadian rhythm, over a longer period, assessing both the timing and stability of food intake.

Existing literature highlights the association between the timing of eating and cardiometabolic health. Studies have shown that individuals with higher BMIs tend to consume more calories later in the day, closer to their DLMO. However, these findings are based on short cross-sectional studies using clock time as a reference. The stability of meal patterns, defined as the regularity of meal consumption, size, and number, has also been linked to cardiometabolic health. Previous research on meal stability primarily focused on a few days within a week, neglecting longer-term stability. This study aims to address these limitations by evaluating the stability of eating timing over a semester, using both clock time and circadian timing as reference points.

This study included 14 college undergraduates (5 female) aged 19.1 years (± SEM 0.3, range 18-21) with an average BMI of 22.9 kg/m². Participants used a photographic mobile phone application (MealLogger) to record their food intake for 7 consecutive days near the beginning, middle, and end of the spring semester. Circadian phase (DLMO) was determined during three overnight laboratory visits. Data analysis included the Composite Phase Deviation (CPD) to assess day-to-day stability and intraclass correlation coefficients (ICC) to determine individual stability across months. Participants were also categorized as lean or non-lean based on sex-specific body fat percentage criteria. Mixed-effects models and Pearson correlations were used to analyze group differences and individual consistency.

The study revealed poor day-to-day stability in the timing of eating, with a ~3-h variation in the timing of the first, midpoint, and last daily caloric events. The ICC values for day-to-day stability ranged from slight to fair (0.12–0.34). In contrast, the timing of eating showed greater stability across months (~1-h variation), with ICCs ranging from 0.54 to 0.63 when measured using clock time. However, stability decreased when measured relative to circadian timing (ICC = 0.33–0.41). The timing of snacks showed a significant main effect of month, while breakfast, lunch, and dinner timing did not. Individual consistency in meal timing varied depending on the meal, with dinner showing the highest consistency and breakfast and lunch showing the lowest. Non-lean individuals exhibited higher stability in meal timing between months compared to lean individuals. Regarding caloric and macronutrient intake, daily caloric intake showed a significant main effect of month but no significant differences between specific months. Individual consistency in caloric and macronutrient intake was substantial to almost perfect. The number of meals consumed daily did not show a significant effect of month, with lean individuals showing greater consistency in meal number across months.

This study demonstrates that despite significant day-to-day variability, the timing of eating shows considerable stability across months when assessed using clock time. This finding suggests that week-long assessments of dietary habits are likely to represent habitual behaviors. The greater stability observed with clock time compared to circadian time may be attributed to external factors like social schedules influencing the timing of main meals. The unexpected finding that non-lean individuals showed higher meal timing stability warrants further investigation. This could be related to how stability is defined, as previous studies often focused on meal frequency, which is associated with poorer health. The study's findings highlight the importance of considering both daily and monthly timescales in assessing eating habits and their relationship with health outcomes. Future research should examine the role of social and other external factors in modulating eating patterns.

This study provides evidence for the stability of eating timing across months, suggesting that week-long dietary assessments can capture habitual behaviors. Day-to-day variability is significant, highlighting the importance of longer-term assessments. The unexpected higher stability of meal timing in non-lean individuals requires further investigation. Future studies with larger samples and diverse populations are needed to confirm these findings and further elucidate the complex relationships between eating timing, stability, body composition, and health outcomes.

The relatively small sample size limits the generalizability of the findings and restricts the power of subgroup analyses (lean vs. non-lean). The study was conducted within a college setting, potentially limiting the generalizability to other populations. While the study design attempted to account for real-world influences, exogenous factors like exams and seasonal changes may have impacted the results. Furthermore, the seven-day recording period may have increased day-to-day variability due to differences between school/work days and free days.