Our bodies have an internal “clock” that anticipates and prepares for food intake at regular times each day.
This was demonstrated in a new scientific study that manipulated meal timing in participants.
Key Facts:
- Participants were given either 2 large meals per day or 14 small meals evenly spaced through the waking period.
- In those eating 2 large meals, blood sugar levels dropped in the hours leading up to mealtimes.
- This anticipatory decline in blood sugar was not seen in people eating small frequent meals.
- The internal body clock regulated these mealtime responses even without any time cues from the environment.
- Feelings of hunger also increased right before the usual large mealtimes, further evidence of anticipation.
Source: Current Biology 2023 Apr 10;33(7):1321-1326.e3.
Our Bodies Predict Mealtimes
The human body follows daily, or circadian, rhythms that sync up with sunrise and sunset.
These circadian cycles regulate hunger, metabolism, blood sugar, body temperature, and sleep.
Research has shown that the timing of meals can shift the phase of circadian rhythms.
But until now, it was unclear whether humans physiologically anticipate upcoming meals when food intake is restricted to set times of day.
To test for meal anticipation, researchers at the University of Surrey in the UK conducted a laboratory experiment with 24 healthy young men.
The participants lived in a controlled environment for 9 days.
For the first 6 days, one group was given two large meals per day, while the other group ate 14 small meals spaced evenly throughout the 16-hour waking period.
Then for 2 days, all meals were removed and participants had constant blood sugar monitoring.
During the first part of the study, the group eating two daily meals showed declining blood sugar levels in the hours leading up to each mealtime.
Their blood sugar would spike after eating, then start dropping again in anticipation of the next meal.
In contrast, the frequent small meal group maintained steady blood sugar throughout the day.
Evidence of Anticipation
When meals were taken away during the 2 day constant monitoring phase, some fascinating effects emerged.
Blood sugar rhythms were 12 hours out of sync between the two groups, despite only a 7 hour difference in their initial meal timing.
The large meal group had lowest blood sugar levels at the times they previously ate meals.
Their feelings of hunger also increased right before the expected mealtimes.
These results provide the first evidence that human physiology anticipates temporally restricted food availability.
Circadian anticipation of meals has been demonstrated in animal studies, but not yet shown in humans.
The findings support the existence of a food entrainable oscillator that drives food anticipatory activity, even in the absence of environmental time cues.
The Brain’s Mealtime Alarm Clock
The master circadian clock is located in an area of the brain called the suprachiasmatic nucleus.
This clock synchronizes to light/dark cycles and drives 24 hour rhythms in hormone levels, body temperature, and sleep.
But other brain and body tissues also contain circadian clocks that can be synchronized by food intake rhythms.
The drop in blood sugar levels before scheduled mealtimes, persisting even without any meals, suggests an anticipatory mealtime alarm clock somewhere in the body.
This mechanism seems to prepare metabolic processes for upcoming food utilization and energy demands at consistent times each day.
As the study was tightly controlled with no time cues, these anticipatory effects must be driven by internal biological clocks rather than external behavioral or environmental factors.
However, the exact locations and underlying nature of food-entrainable oscillators remains a mystery to be solved.
Health Impact of Meal Timing
These findings help increase our understanding of how meal patterns impact circadian biology.
Other research suggests that eating at abnormal circadian times can lead to obesity and metabolic disorders.
The study reinforces that our bodies are adapted for regimented mealtimes within the natural daylight period.
Large calorie consumption at odd hours of the night disrupts healthy metabolic rhythms.
Night shift workers run into this problem when eating on an abnormal schedule dictated by their work hours.
Our physiology evolved assuming food availability during daytime activity periods.
But modern lifestyles often involve snacking and drinking calories throughout the day and night.
The researchers suggest that limiting food intake to set mealtimes within the active day period may help maintain robust circadian cycles and metabolic health.
Next Steps in Meal Timing Research
This study was conducted in healthy young men under highly controlled laboratory conditions.
The researchers point out some remaining unanswered questions:
- Will these anticipatory effects be even stronger if meals are limited to an even shorter window that’s further from normal lunchtime?
- Are there gender differences in meal anticipation that should be explored in future studies?
- What molecular signals drive the lowering blood sugar before meals? Cortisol and other hormones are likely involved.
- Can we pinpoint the anatomical locations of food-entrainable oscillators in both animal and human brains?
- Will meal anticipation differ in metabolically unhealthy obese or diabetic populations?
Addressing these research questions will reveal more about how metabolism interacts with our circadian systems.
This knowledge can be applied to develop nutrition recommendations that optimize health within our innate 24 hour biological rhythms.
Circadian Rhythms and Diet
This research adds to growing evidence on how meal timing impacts circadian clocks throughout the body.
Other studies show that eating late at night is associated with increased risks of obesity, high blood sugar, fatty liver disease, and heart disease.
Our internal circadian cycles evolved to synchronize with natural cycles of sunlight, activity, and food availability.
Disrupting this circadian order through erratic eating patterns can negatively affect metabolic health.
Limiting food intake to daytime hours within a consistent schedule may be the best practice for maintaining healthy circadian rhythms.
The field of chrono-nutrition examines how meal timing interacts with our internal 24 hour rhythms to impact weight, metabolism, digestion, and disease risk.
More research is illuminating this critical relationship between circadian biology and diet.
With further study, chrono-nutrition recommendations can be optimized to help people eat in better alignment with their innate body clocks.
References
- Study: Human glucose rhythms and subjective hunger anticipate meal timing
- Authors: Cheryl M Isherwood et al. (2023)
