What do we know about the timing of intake?

By Hassan S Dashti, PhD

When we describe our habitual diets, we often find ourselves talking about its nutritional composition (i.e. what) and quantity (i.e. how much), however novel research suggests that timing of intake might be yet another important component of diet we want to pay attention to. This was the main focus of discussion at the ASN Scientific Sessions at EB 2015 symposium titled, “Is ‘When’ We Eat as Important as ‘What’ We Eat? – Chronobiological Aspects of Food Intake” (read more here: Biologically, this makes sense as an endogenous clock, commonly termed the circadian clock, regulates a constellation of biologic processes, including metabolism (1). If up to 30 percent of genes in the intestines, liver, and kidney fluctuate throughout the day, yielding varying temporal functional profiles, doesn’t it make sense that there ought to be a time when dietary intake is optimal? Well, if the effect of a calorie on health is dependent on timing, what we all would like to know next is at what time should we be eating?

What currently determines our timing of intake is our culture and lifestyle for the most part. For instance, kids’ lunchtime is predetermined by school cafeterias, adults’ dinnertime is predetermined by rush-hour traffic, but even breakfast also seems to determine when we’ll have our next meal, lunch (2). History also played a role in determining meal times. In certain parts of the world, lunchtime was set for noon to enable workers to cope with long working hours in factories during the Industrial revolution. Perhaps it’s time to have science determine our meal hours.

Preliminary evidence suggests that earlier meal times tend to be healthier and “better aligned” with our biological clock. In one study, it was found that calories consumed after 8:00pm significantly predicted higher BMI (3). Meanwhile results from a 20-week weight loss intervention among overweight and obese individuals suggested that late eaters (lunch after 3:00pm) were less successful at weight loss compared to early eaters (lunch before 3:00pm), independent of 24-hour energy intake (4). Another trial assessing overweight and obese women further identified that high-calorie breakfasts, as opposed to high calorie dinners, were more beneficial for various cardiometabolic traits (5). Consistent with the findings from these trials is a cross-sectional analysis of a diverse cohort in the Los Angeles area that suggested that participants who consumed over a third of their calories by noon were less likely to be overweight and obese (6).

While these findings generally suggest that earlier hours of intake are generally healthier, they are not without their many limitations. One limitation worth noting is the high interrelatedness between timing of intake and other aspects of diet and life that also impact overall health and particularly sleep timing and duration, frequency of intake, and hours of fasting. Therefore, future studies should account for these strongly related dimensions when elucidating the timing of intake that best aligns with our internal clock.

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2.Kant AK, Graubard BI. Within-person comparison of eating behaviors, time of eating, and dietary intake on days with and without breakfast: NHANES 2005-2010. Am J Clin Nutr. 2015 Sep;102(3):661–70.
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