The Fundamental Law of Fat Loss
You must burn more energy than you consume to achieve meaningful fat loss. This is called a caloric deficit. Over the course of a period of time (usually measured on a daily basis), a caloric deficit is achieved when the sum of calories expended is greater than the sum of calories consumed.
So you definitely need to be eating fewer calories than you burn every day to lose fat, but when should you eat calories for best fat loss results? Should you graze throughout the day, or should you stick with 3 main meals? Should you eat breakfast or skip it? Does eating late at night cause you to pack on the pounds?
All of these questions relate to meal timing. Meal timing refers to how your meals, calories, and macros are distributed throughout the day.
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Nibbling vs. Gorging
So, the question that we need to answer to determine the optimal meal timing for fat loss is: do you burn more calories over a 24-hour period by eating a bunch of small meals or a few large meals?
Well as early as the 1960s, scientists began observing an inverse relationship between eating frequency, defined as the number of eating occasions per day, and weight gain. In other words, people who ate smaller, more frequent meals tended to be thinner.
Since then, the majority of cross-sectional studies on eating frequency and body weight have shown similar results. All of this research has fueled the idea that snacking or grazing throughout the day boosts your metabolism and causes you to burn fat more easily and stay lean. However, if you peel back the research a bit more, the truth is not so simple.
A key problem of these studies is that most people are notorious for underreporting their energy intake. In 1997, scientists at the French National Institute of Health and Medical Research analyzed all pertinent studies available at the time comparing the thermic effect of food in a wide variety of eating patterns, ranging from 1 to 17 meals per day (1). The thermic effect of food is the energy required for digestion, absorption, and disposal of ingested nutrients.
The scientists discovered that the greater the magnitude of underreporting of energy intake, the less frequently individuals also report eating. In other words, these scientists discovered that the apparent inverse relationship between eating frequency and weight gain can be attributed to the underreporting of energy intake, which is correlated with meal frequency. The people who ate fewer meals consumed far more food on average than they reported.
We see this again and again in scientific literature. Self-reporting is a major pitfall of poorly designed meal timing studies. People often underestimate their bad habits like eating too much or exercising too little. For example, a 2004 study from Tufts University analyzed data from a nationwide survey conducted by the USDA that provided the food intakes of 20,607 US adults on two nonconsecutive days (2).
The Tufts University researchers found evidence that eating frequency, eating portions, and calorie density were all underreported across the board in this survey. Again, we can’t rely on self-reporting studies.
Thankfully, there are many studies conducted over the past 2 decades in which food was provided either in a laboratory environment or pre-packaged by researchers for consumption at home (3). In other words, the subjects in these studies were given the EXACT portions of food to eat for each meal and therefore do not rely on flawed self-reporting.
If you lock people in a room, feed them fewer calories than they burn, and control all other factors, everyone loses weight as predicted. So what did these tightly controlled meal timing studies find?
Well according to a 2018 study published by researchers at the Department of Nutrition Science at Purdue University, the more times per day people eat, the more they tend to increase their daily caloric intake (4). So eating many small meals throughout the day does NOT help you burn body fat. It mostly comes down to how many calories you are consuming regardless of how many meals you disperse those calories across.
However, the latest meal timing research clearly shows that WHEN you eat your meals can have a measurable effect on your rate of fat loss. And it all comes down to your body’s circadian rhythm.
Circadian Rhythms (AKA chronobiology)
A circadian rhythm is a roughly 24-hour cycle in the physiological processes of living beings. It’s your body’s natural clock. And it has a huge impact on how your body functions.
For example, approximately 82% of all protein-coding genes in the human body exhibit circadian rhythms in expression according to a 2018 study from the University of California (5).
So What Runs Your Circadian Rhythm?
The human circadian system consists of a central clock and peripheral clocks.
The central clock is located in your brain (specifically, the hypothalamus) and synchronizes itself to light exposure (namely, sunlight), which allows your body to synchronize behavioral and metabolic rhythms to the light and dark cycles associated with normal 24-hour periods (6, 7). Peripheral clocks are influenced by physiological and behavioral factors such as sleeping and eating.
So basically, a normal circadian rhythm is about 24 hours because a day is about 24 hours and our bodies are largely programmed by sunlight. There is a rapidly growing interest in circadian biology, which has birthed a relatively new scientific field known as “chronobiology.”
Why Does Your Circadian Rhythm Matter for Fat Loss?
Our central biological clock plays a role in physiological processes in the body that impact energy regulation and metabolism. The two processes that your circadian rhythm impacts, which have an effect on fat loss, include:
Your body’s resting metabolic rate (which is the total number of calories burned when your body is completely at rest - this is generally what people are referring to when they are talking about “metabolism,” though this is a bit over-simplistic).
The thermic effect of food (which is the total number of calories required for digestion, absorption, and disposal of ingested nutrients).
Both your resting metabolic rate and the thermic effect of food vary throughout the day in a predictable manner because they correspond with your body’s circadian rhythm. In other words, we see a circadian variation in a person’s Total Daily Energy Expenditure due to predictable variations in their resting metabolic rate and the thermic effect of food.
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Collectively, your resting metabolic rate and the thermic effect of food contribute approximately 80% to your total daily energy expenditure, so any changes to these two components of daily energy expenditure have the potential to significantly impact energy balance. In other words, your resting metabolic rate and the thermic effect of food really matter when it comes to fat loss.
And here’s the interesting part when it comes to meal timing: energy expenditure both at rest (which is your resting metabolic rate) and after eating (which is increased by the thermic effect of food) is typically higher earlier during the day. This phenomenon is what ultimately influences your rate of fat loss.
(Side note: Having a sleep routine is important for maintaining your body’s peripheral clocks. You don’t want to fight your body’s natural circadian rhythm by having a chaotic or unnatural sleep schedule. This is the main reason why having a poor sleep schedule hinders your ability to burn body fat.)
So let’s now dive into a bit more of the science behind how your metabolic rate and the thermic effect of food vary along with your body’s circadian rhythm.
The Circadian Rhythm of Resting Metabolic Rate
First, let’s discuss how your body’s circadian rhythm affects your metabolic rate. Your resting metabolic rate follows a predictable circadian rhythm as demonstrated by a 1994 study conducted by researchers in Switzerland who found that the subjects’ metabolic rates peaked in the morning between 9 AM and 12 PM and dipped to their lowest point during normal nighttime hours between 12 AM and 6 AM (8).
These findings are supported by a 2000 study published in the Journal of Physiology and a 2018 study conducted by researchers at Harvard University that both demonstrated a similar circadian rhythm in metabolic rate (9, 10). All of these studies reported minimum average core body temperature values at approximately 5 AM.
And body temperature has been found to be a practical and effective proxy measure of circadian rhythms and metabolic rate according to a paper published in the International Journal of Obesity in 2014 (11). In other words, your metabolism is running at full speed in the morning and tends to slow down through the afternoon and evening, dropping to its lowest levels between 12 AM and 6 AM.
The Circadian Rhythm of the Thermic Effect of Food
Now, let’s discuss how your body’s circadian rhythm affects the thermic effect of food. A team of researchers at Harvard Medical School studied the effect of meal timing on the magnitude of the thermic effect of food and found that the thermic effect of food was 44% lower at 8 PM compared to 8 AM, following an identical meal (12).
Assuming that your total daily energy expenditure is 2,000 calories (which is the case for the average adult) and the thermic effect of food is about 10% of total energy expenditure, which is largely affected by the macronutrient composition of your diet, that would mean that the thermic effect of food totals 200 calories per day (13).
As a brief though relevant aside, protein causes the highest thermic effect of food (with 20-30% of the energy content of ingested protein being burned) compared to carbohydrate (5-10%) and dietary fat (0-3%) (14).
If we take the 44% difference in the thermic effect of food found in the Harvard study and apply it to the approximate thermic effect of food for the average adult of 200 calories, we’d get a difference of 88 calories. These findings were supported by authors of a similar study published in the International Journal of Obesity in 2015 who reported a 90-calorie difference in the thermic effect of food between daytime and nighttime eating (15).
There is also evidence to suggest that some of the caloric burn benefit to eating more in the morning is because our bodies bulk up our muscles with glycogen, the primary fuel our muscles burn for energy (16).
So What Does The Circadian Rhythmicity of Your Resting Metabolic Rate and the Thermic Effect of Food Have to Do With Fat Loss?
Well again, fat loss mostly comes down to energy balance. Energy in versus energy out. You absolutely must be consuming fewer calories than you burn every day to achieve meaningful fat loss results. But the human body is incredibly complex and there are many confounding factors that can affect energy digestion or energy expenditure.
The research that we’ve covered definitively proves that a calorie is not always just a calorie. It depends on WHEN the calorie is eaten. So why do we burn more calories eating a morning meal than eating an evening meal? Because our metabolic rates and the thermic effect of food follow our body’s natural circadian rhythm.
Energy expenditure both at rest (resting metabolic rate) and after eating (thermic effect of food) is typically more efficient earlier in the day. And the scientific literature that we’ve discussed suggests that prioritizing energy intake earlier during the day may help with body weight maintenance and fat loss (17).
What are the practical implications?
Your largest meal of the day should be breakfast (ideally) or lunch, with little eaten in the evening. It’s okay to eat dinner, but try not to eat past 6 PM. If you don’t follow the typical 3-meal eating plan, then just aim to consume most of your calories earlier in the day.
Find out how many calories you should be eating to meet your health and fitness goals! Use our Free Vegan Nutrition Calculator to break down exactly how much protein, carbs, and dietary fat you should include in your daily meal planning.