My family spent the last week of the year visiting my wife’s family in the Seattle area, filled with the warmth of family. Even though we were also filled with lots of great food, it didn’t lead to lots of satisfaction with said food. Maybe I would feel full (or overfull) temporarily, but soon enough, I came circling back to the snack table or was planning the next place we’d stop for a coffee and muffin. In Seattle, it’s the darkest part of winter (and was a rainy one at that!) as it is elsewhere in the northern hemisphere. Our mammalian bodies know that heaps of food help us stay warm and hedge against the increased scarcity of food in the winter. The squirrels waddling, chubby, and cheerful across my lawn in late autumn know all about this.
According to the British Nutrition Foundation (nutrition.org.uk “Energy density approach”), researchers in recent years have found that our bodies give the sensation of fullness in response to the total mass of food we eat, versus the calorie content. Calorie content can be measured in Joules (J), the SI unit of energy; foods are usually measured in kilojoules (kJ = 1000 joules) or megajoules (MJ = 1 million joules, 1000 kJ) for a full day. Saying the findings in a different way, we tend to feel full when we have eaten enough grams of food, regardless of the MJ contained in that food, so it’s important to consider the “energy density” of foods, which is measured in MJ/kg.
The BNF page above explains that lower energy density foods, which tend to be high in water content like fruits and vegetables, help us to feel full after consuming fewer calories, versus high energy density options such as fried foods and candies. Low-density foods are commonplace in healthy dietary recommendations.
This blog is about a related body of work into how satisfied we feel from the food we eat and how that varies over the course of the day. In 2004, de Castro analyzed data previously collected for other studies. These studies had participants record the when and the what of all the foods and drinks they had throughout the day. With this data in hand, de Castro could extract a relationship between the time that intake occurred and the overall caloric intake, in kilojoules (kJ), over the course of the day.
He found that “…although the meal size increased over the day up until 2200h [10 pm], the time until the next meal (after meal interval) actually decreased over the same period.” This can be quantified by what de Castro calls the “satiety ratio” of eating something, which for a given meal is computed as
(time until next meal eaten / calories eaten in current meal)
and so has units of (min / MJ) where again 1 MJ = 1000 kJ. When I found myself circling back to have more snacks, despite having eaten plenty during meals, that’s an indication that my satiety ratio was very low and matches my experience of not being satisfied with all the eating I was doing.
Satiety ratios are highest in the morning, and fall dramatically during the hours of the evening. Since the satiety ratio involves units of energy, like megajoules, you might think as I did that what’s happening is that foods eaten are higher density in the evening. However, Figures 5 and 7 from the same paper shows that the rate of intake, measured in kJ/min, was fairly steady during meals over the course of the day in the study but that the “duration spent eating” increases in the evening.
In other words, as the day goes on, you spend a bigger chunk of every successive hour eating. These shorter gaps between your later-in-the-day food intakes contribute to your satiety ratio going down over the day. One way to look at this is that you, like me, don’t feel as satisfied by what you last ate later in the day, so you go looking for your next snack sooner than you otherwise would.
Folks familiar with time-restricted eating are already clued into the fact that your body is expecting food during its sense of “daytime” and so limiting eating to these times can help improve satiety and avoid unhealthy weight gain. In Arcashift, this meal window is one of the biologically-driven pieces of advice offered by Today’s Schedule.
de Castro’s work suggests that limiting eating to this window is just the first dimension of a time-informed view of eating. By moving more eating to the morning, and decreasing the energy density of foods in the later hours of the day, we can feel more satisfied with the foods we are eating while maintaining a healthier total calorie intake.
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- de Castro JM. The time of day of food intake influences overall intake in humans. J Nutr. 2004 Jan;134(1):104-11. doi: 10.1093/jn/134.1.104. PMID: 14704301. https://pubmed.ncbi.nlm.nih.gov/14704301/