Thanks in large part to Satchidananda Panda of the Salk Institute, time-restricted eating is now in the popular lexicon, and diet-aficionados the world over are trying it out. Panda suggests that the world of nutrition, which has mostly been focused on macronutrient or calorie restriction, has been missing a key element of the diet puzzle: time.
What's an Eating Window?
In time-restricted eating, people restrict the total eating period of each day, called the eating window. The eating window is defined as beginning from the time that a person consumes their first calorie until the time they consume their last calorie. For example, if you have a coffee with cream at 7 am and a late-night snack at 11 pm, your eating window is 16 hours. Dr. Panda recommends an initial eating window of 12 hours, followed by reductions to 11 and then 10 hours. He believes that the health benefits that arise from TRE double with every hour reduction from 12, judging by animal studies. The scientific reasoning for restricting the eating window boils down to the circadian rhythms of pancreatic function.
Circadian Rhythms, Light, and Food
Every day, blue light in our environment is absorbed by melanopsin cells in our retina, which then communicate that it is daytime to our suprachiasmatic nucleus (SCN), located in the hypothalamus. The SCN is our body’s master circadian clock, it programs our body’s daily processes in accordance with light. While light governs most of our biological circadian rhythms, Panda discovered that there are several exceptions. Unlike most of our body’s processes, the liver and pancreas’s circadian function is initiated not by first light, but by our first calorie consumed. These organs, critical to digestive processes, operate optimally for only a few hours.
Blood Glucose Control and Nutrient Partitioning
Food is processed by the body less efficiently when consumed outside of the optimal working hours of the pancreas and liver. The pancreas is less able to produce insulin after optimal eating hours, and our bodies also appear more insulin resistant. This leads to higher blood glucose levels for longer periods after eating, as well as worse nutrient partitioning. This means that we store more fat from calories consumed more distantly from our first calorie consumed. For those that use continuous blood glucose monitors like the Dexcom 6, you will already have noticed that you have worse glucose control later in the day. This is not because it is nighttime, but because you are eating a meal that is many hours after your first meal.
The Body is a Complex System
Panda’s work is more evidence for the argument against the belief that weight loss is only about caloric reduction. The physics-influenced ‘calorie in – calorie out’ model that is exclusively concerned with inputs and outputs, instead of complex biological systems, should be a thing of the past. If eating an apple 15 hours after a first meal isn’t the same as eating it 5 hours after the first meal, a calorie is no longer a calorie.
 Panda, S. (2020). The Circadian Code: Lose Weight, Supercharge Your Energy, and Transform Your Health from Morning to Midnight. Rodale Books.