Meal Timing and Size: Impact On Weight Loss

Why Meal Timing Matters

Whether you realize it or not, your whole life is dictated by the circadian rhythm (sleep-wake cycle). Simply put, the circadian rhythm is an intrinsic biological clock that runs every 24 hours. The circadian rhythm (CR) is synchronized with our body clocks in response to light and food intake. The CR releases neurotransmitters (melatonin and cortisol) in response to these cues. For example, when it is dark out you feel sleepy due to melatonin secretion, or in the morning you find it easier to wake up when it is light out due to cortisol spikes. Food also influences the circadian rhythm, for example, eating breakfast can help promote metabolic homeostasis by regulating blood sugar, and improving metabolism, and eating breakfast can actually help you sleep better (1), more on this later. It is important to note that the circadian rhythm runs on a 24-hour circuit even without input from outside factors (1). We want to make sure our bodies are synchronized to this internal clock, to do this the body’s biological clocks must be entrained daily (1) (ie. through sun exposure and meals). 

So What Happens If Your Clocks Are Off? 

Circadian system misalignment has been associated with various metabolic issues, such as disordered glucose tolerance and lower insulin sensitivity, higher instances of non-alcoholic fatty liver disease, elevated levels of proinflammatory cytokines, higher blood pressure, and the reduced ability to burn calories, leading to weight gain (1). To sum it up nothing good. 

Chronotypes 

Before we get into it we need to talk about chronotypes. There are two chronotypes a lark (morning type) and an owl (evening type), you probably already know which one you are (Here is a quiz you can take if not). Your chronotype can affect how your internal clock works and how you feel at different times of the day. Your chronotype can also change how your body reacts to when you eat meals and how that might impact your weight (2). You can’t change your chronotype but you can influence it through light exposure and meal timing (like the CR). So let's get into it. 

Breakfast 

You’ve definitely heard the saying “Breakfast is the most important meal of the day” and I’m here to tell you this is actually backed by science. Research has found that ghrelin (the hunger hormone) peaks around 8 am, and the hormone that helps us burn fat, adiponectin peaks around 11 am (1). This means our bodies are better at burning sugars and fats for fuel in the morning. As the day goes on, our bodies become more prone to storing fat, especially after dinner. Not only this but eating an early breakfast is essential for the synchronization of central and peripheral clocks and promoting metabolic homeostasis (1). Breakfast also seems to be important for having a good night’s sleep, which can contribute to earlier bedtimes and wake times and promote a healthier circadian rhythm (1). Simply put, eating breakfast helps our bodies stay in sync, keeps our metabolism in check, and makes it easier to control weight. One study found that participants who ate bigger breakfasts (within two hours of waking up) had lower chances of being overweight or obese when compared to those who ate the majority of their calories at night (2). However, when comparing chronotypes this study found that among morning types, those who ate the most in the morning had a much lower chance of being overweight or obese than those who ate the least. However, for evening types, eating a lot in the morning didn't make much of a difference in whether they were overweight or not. To sum it up, eating breakfast is important for everyone, but might be even more important if you have a morning chronotype. 

Lunch 

I’m convinced lunch is the most skipped, and sad (ie. uninspiring) meal of the day for a lot of people. Skipping lunch or eating a late lunch could be what is stopping you from meeting your weight loss goals. For example, a Spanish study looking at 420 obese people trying to lose weight, found that those who ate lunch (the main meal in Spain) after 3 pm didn't lose as much weight during their treatment compared to those who ate lunch earlier, (before 3 pm). These groups were similar in age, how hungry they felt, how much energy they used, how long they slept, and what kinds of foods they ate (3). To sum it up, eating lunch early (before 3 pm) might help people lose more weight, regardless of other factors. Other studies have found similar results- ie. late eaters lose less weight and lose weight at a slower rate than early eaters, and even that transferring 100 calories from dinner to breakfast or lunch lowered LDL (bad) cholesterol (3). There doesn’t seem to be an impact on chronotypes and lunch, so everyone please eat lunch! 

Dinner 

It is thought that 150% more food energy is eaten in the evening relative to the morning (4). As you have just read this might be the norm but isn’t necessarily a good thing. Eating late at night (after 8 pm) and going to bed soon after dinner causes blood sugars to stay high and take longer to go back to normal. One study found that both healthy people and people with type 2 diabetes had higher levels of blood glucose and insulin after eating meals at night (1). Studies have also found that late-night eating was linked to lower HDL (good) cholesterol (1). Having dinner earlier in the evening has been shown to positively influence blood glucose levels and metabolism (1). When it comes to chronotypes, eating more food at night was linked to a higher chance of being overweight or obese, especially for people who are evening types. Among evening types, those who ate the most at night were about five times more likely to be overweight or obese compared to those who ate the least at night. However, for morning types there wasn't as strong a link between eating late at night and being overweight or obese (2). To sum it up, eat dinner earlier (before 8 pm), especially if you are an evening chronotype. 

TLDR

The circadian rhythm runs the show! Meal timing and light exposure influence the circadian rhythm and your chronotype. If you have a morning chronotype a bigger breakfast can help with weight loss. Night eating (past 8 pm) does not affect your health as much as evening types. If you are an evening type breakfast does not affect your health as much as it does for morning types, however eating dinner late is linked to weight gain. Everyone should eat lunch! To really sum it up, eat most of your calories during breakfast and lunch, as a rule of thumb eat when the sun is up and stop when the sun is down. 

Resources 

1. BaHammam, Ahmed  S, and Abdulrouf Pirzada. “Timing Matters: The Interplay between Early Mealtime, Circadian Rhythms, Gene Expression, Circadian Hormones, and Metabolism-A Narrative Review.” Clocks & Sleep, U.S. National Library of Medicine, 26 June 2023, pubmed.ncbi.nlm.nih.gov/37754352/. 

2. Xiao, Qian, et al. “Meal Timing and Obesity: Interactions with Macronutrient Intake and Chronotype.” International Journal of Obesity , U.S. National Library of Medicine, Sept. 2019, www.ncbi.nlm.nih.gov/pmc/articles/PMC6669101/. 

3. Lopez-Minguez, Jesus, et al. “Timing of Breakfast, Lunch, and Dinner. Effects on Obesity and Metabolic Risk.” Nutrients, U.S. National Library of Medicine, 1 Nov. 2019, www.ncbi.nlm.nih.gov/pmc/articles/PMC6893547/. 

4. Spence, Charles. “Explaining Diurnal Patterns of Food Consumption.” Food Quality and Preference, Elsevier, 1 Feb. 2021, www.sciencedirect.com/science/article/abs/pii/S0950329321000252.