Time-Restricted Feeding: The Low-Hanging Fruit

Photo by Fumiaki Hayashi on Unsplash

In today’s society, everybody is seeking instant gratification. They want immediate results and it appears they will not stop their pursuit until satisfied. The pursuit for weight loss is no different.

Whether it be shedding excess pounds to look good on the beach, fit into a wedding dress, or just improve one’s health, individuals want it done, and they want it done fast! Well I hate to break it to you, but there is no magic potion for weight loss (but if you find one, I think they have a Nobel Prize with your name on it). For most people, basic comprehension of the human metabolism is out of the question, and in fact, I wonder how many people would actually be able to list the three main macro-nutrients and how many calories are packed into one gram of each? Yet, people will spend their hard-earned money on nonsense such as fat-burning pills or liposuction without batting an eye. These particular approaches (and many others) are not only harmful to your wallet, but also may cause harm to your body (and you only get one of those, so treat it with care, dammit!). Additionally, these approaches are simply bandages to cover the problem. A short-term solution to a long-term problem. So tell me this, honey, what are you going to do when you run out of magic fat-burning pills? And honestly, if you care that much about what others think regarding your appearance, I urge you to read Mark Manson’s “The Subtle Art of Not Giving a F*ck” and then report back to me.

However, let’s just pretend that instead of trying to improve our body composition to impress Sally from Accounting, we are instead pursuing this goal to improve various bio markers (measurable indicators of the severity or presence of some disease state), thereby pushing us further away from metabolic dysfunction and closer to optimization. Ah…wishful thinking :)

Now what I am about to propose might sound quite revolutionary, but bear with me. What if, instead of searching for quick fixes to enhance our weight loss progress, we take some time to sit down and refresh ourselves on the basics of nutrition and human metabolism. Ah, there it is, that oh-so familiar buzz-word “metabolism.” We hear that word thrown around all the time, but do we really know what it means? Metabolism can be thought of as the collection of biochemical reactions in an organism that converts chemical energy into work (1). Quite a broad and vague definition, huh? Metabolic regulation is such a complex field of study that many will spend their whole lives dedicated to just a small portion of human or animal model metabolic function, let alone attempt to understand human metabolism as a whole. So, no, I am not going to explain glycolysis, gluconeogenesis or denovo-lipogenesis in this article (if you are confused, that’s good…thank you for proving my point), nor am I expecting my readers to understand these processes. Thus, you, like many of the individuals who are pursuing weight loss, probably wouldn’t even have been able to put together a working definition of the word “metabolism” prior to reading this article, but I am sure you have used the word before. It is this learning-gap that seems to lead us toward the quick fixes, rather than relying on evidence-based approaches rooted in science.

In fact, what I am about to argue is so basic, that you don’t really need to understand anything about metabolism. Oftentimes, when an individual is trying to lose weight, they may take an unsolicited dive into the dark web or sell their soul to an “internet nutrition guru.” Most of what these individuals find during their quest will revolve around attempting to count calories (which if you want it to be done “right” should involve the brutally tedious act of meticulously examining nutrition labels, weighing all of your food, recording every bite, and eating in a sustained caloric deficit…trust me I’ve been there). Don’t get me wrong, calories are in fact a huge part of the puzzle and are of utmost importance, so I am by no means dismissing their importance to weight loss and metabolic function. The point I am trying to make, is that the average individual who is pursuing weight loss has no idea where to start, and most of the “typical” weight loss procedures involving counting calories are just too complicated and not practical for an average individual to implement successfully.

So what if I told you, that there could be a way to begin the weight loss process without any of that mumbo-jumbo? I am sure that many of my listeners are familiar with the phrase “intermittent fasting.” Just like “metabolism” it seems to be thrown around a lot these days. Personally, I like to think of intermittent fasting as an umbrella term for a variety of ways to restrict the time in which nutrients are consumed. For instance, you may be familiar with the popular 16:8 intermittent fasting protocol (where one restricts nutrient intake for 16 hours and has an 8 hour window to consume their nutrients for the day). Other popular intermittent fasting protocols include 20:4, one-meal-a-day (OMAD) or even alternate day fasts and prolonged fasts where individuals refrain from nutrient ingestion for 24+ hours. As the title of this article suggests, I want to focus on time-restricted feeding (TRF), which is when eating is limited to a certain number of hours each day, thus being a specific form of intermittent fasting (2). TRF is a fascinating component of “chrononutrition” which is a field of study that investigates the relationship between the time of day and food intake (3). I plan on writing an article about chrononutrition and circadian rhythms (chronobiology) in the near future.

Researchers at the University of Surrey conducted a pretty cool TRF pilot study that piqued my interest (4). This was a ten week intervention trial that included thirteen participants. The participants undertook a two week baseline period to orient their sleep-wake cycles and feed-fast cycles (these are components of one’s intrinsic circadian rhythms). Essentially, this step was taken to hopefully get all the participants on the some sort of consistent “internal rhythm” before breaking off into control and experimental groups. The control group was told to maintain their original eating habits whereas, the experimental group was instructed to push back their breakfast by 90 minutes and push their dinner forward by 90 minutes. Hence, if during the baseline period, a participant in the experimental group ate their first meal at 8 am and their last meal at 6 pm, they would now eat their first meal at 9:30 am and last meal at 4:30 pm for the duration of the ten weeks. What makes this study design interesting, is that the researchers did not restrict what the participants ate (in the literature this is referred to as ad libitum feeding, meaning as much or as often as necessary or desired). Therefore, these participants did not have to worry about counting calories, reading nutrition labels, or planning their meals several hours in advance. In fact, if a participant really wanted to, they could eat anything and everything in sight, as long as they recorded it in their diet diary, and ate it within the designated time frame if they were part of the experimental group. All these participants had to do was weigh their food on a food scale and record the value. Therefore, they could essentially mindlessly weigh the food, record it, and eat it without thinking twice.

The only thing that was restricted was the feeding window for the experimental group. Thus, in theory, this experiment should give some insight into whether the amount of time or time of day in which we consume our calories, matters for weight loss.

Upon completion of the ten week period, it was found that even with ad libitum eating, TRF participants reduced daily caloric intake and saw a reduction in adiposity (loss of body fat) when compared to their pre-intervention measurements. But why was there a reduction in body fat in the experimental group? The researchers proposed a hypothesis which elucidated that the TRF group was eating in better alignment with their endogenous circadian rhythms (4) and extending their daily fast when compared to the control (5).

There are two ideas to briefly unpack here: 1) Circadian rhythmicity (and its relation to nutrient timing), and 2) Fasting and its purported benefits. To begin with circadian rhythms, we can break the word down to its Latin roots. We see “circa” meaning around/approximately, and “diēm” meaning day. Thus, we can think of the circadian system as a multitude of repeating physiological processes that repeat approximately every 24 hrs. This gives us a fairly basic definition that will be functional for the purposes of this article (I am going to do a more in-depth article on circadian rhythms in the near future).The neat thing about our circadian system is that this “clock” can up-regulate and down-regulate certain aspects of our metabolism, such as our glucose (carbohydrate), lipid (fat), and energy metabolism systems depending on the time of day (6, 7). To put this into practical sense, in humans, β-cell responsiveness (pancreatic cells that secrete insulin), insulin sensitivity (hormone that regulates our blood glucose levels), and the thermic effect of food (the amount of energy required to break down, absorb, and excrete ingested food products) are all up-regulated in the morning (6–9). All together, these promising data suggest the possibility that our metabolisms may actually be “primed” to take in nutrients earlier in the day, rather than later in the day. In fact, several research groups report that consuming the majority of one’s caloric intake in the biological morning and reducing caloric intake in the biological evening results in improved blood sugar regulation, improved blood lipid (fat)levels, increased weight loss, and a reduction in hunger, blood pressure, and oxidative stress(damage to our cells and tissues via reactive oxygen-containing species; 10–15 ).

Image taken from Sutton et al., 2018 (16). The demonstrated benefits of eating in an early TRF manner, thereby better aligning food intake with metabolic circadian rhythms.

In addition, by condensing the feeding window (1.5 hour delay in breakfast and 1.5 hour advance in dinner), the participants in the experimental group added an additional three hours onto their fasting period as opposed to if they were eating unrestricted. Much more fasting research is needed in human models, however, reported benefits include: reduction in oxidative damage and inflammation, enhancement of energy metabolism, improved cellular protection, and improvement in a slew of biomarkers (5).

So could the combination of eating in better alignment with our circadian rhythms and an extension in our fasting-window explain the significant difference in fat loss seen in the University of Surrey study? Potentially, but the researchers came to a less extravagant conclusion. Based on the fact that individuals in the experimental group were consuming significantly less calories per day than they were prior to the intervention, it is postulated that the TRF group had sustained enough of a caloric deficit to result in a reduction of body fat (11,17,18). Thus, TRF has the potential to be an appetite suppressant, to put it simplistically.

Now before you go singing the praises of TRF and implement a similar protocol into your own life, I urge you take the research with a grain of salt (and if you are so inclined, you can read the study for yourself). Every study has its limitations, and this study has plenty! For instance, the sample size was extremely small (n=13) and predominately female (n=12), thereby limiting the ability to generalize the findings. Also, because this was a ten week study and allowed for ad libitum feeding, researchers did not tediously watch every morsel of food enter the participants mouth, and instead relied on a self-reported diet diary…nothing could go wrong with that right? Additionally, the participants were asked to maintain their baseline physical activity, but there was no formal testing of physical activity throughout the ten weeks. This then causes me to beg the question, “Did this TRF pattern of eating also cause an increase in energy expenditure, thereby favoring a negative energy balance and accelerating fat loss?” There are plenty more limitations to highlight, but do you see my point? No study is perfect, and you have to be extra careful when it comes to nutrition studies, because there are so many moving parts, regardless of how controlled you try to make the experiment.

Lastly, I will point out that the researchers had the participants fill out a questionnaire after completing the study. The answers to the questionnaire brought into question the sustainability of this pattern of eating. For instance, 57% of the TRF participants stated that this eating pattern was not compatible with their social lives/family. The social implications are definitely something you must consider before trying this for yourself. Of course you aren’t going to be perfect, and if you attempt this protocol, I do suggest easing yourself into it. You could potentially try eating in a restricted time-frame from Monday-Friday and then easing back to your “normal” schedule for the weekend to allow for more social time. That of course is up to you.

So what does this all mean? I threw a lot at you, but when we distill it down, it is quite straightforward. The study from the University of Surrey demonstrates the feasibility of implementing a TRF protocol into daily living. The findings in this study suggest that by condensing your feeding window by as little as three hours could potentially result in a reduction of body fat. There are several possible reasons as to why one could potentially lose body fat by adopting a similar protocol, but three predominant: 1) Eating in better alignment with our circadian rhythms, 2)Extending our fasting window, and 3) Decreasing appetite which leads to a reduction in caloric intake. Obviously more research needs to be conducted to elucidate exactly what is going on here, but the main point I want to hammer home is that if you so choose to embark on a weight/fat loss journey, you don’t necessarily need to dive right into calorie counting (and the oh-so painful reduction in calories). I like to think of this situation as a lever: When one is just starting out on their fat loss journey they can chose to focus on a set number of calories allotted for each day, or a set time in which they are “allowed” to consume calories for the day. How you play with that lever is up to you…but if I were just starting out with my journey, I know which way I would pull that lever!

  1. Miesfeld, Roger L., and Megan M. McEvoy. Biochemistry. W.W. Norton & Company, 2017.

Post-Baccalaureate research assistant in the Molecular and Clinical Nutrition Lab at the National Institutes of Health