The Ultimate Guide to Intermittent Eating

by Dr. Jacob Wilson, Ph.D., CSCS*D

June 5, 2017

What is Intermittent Fasting/Eating?


If you have ever heard of intermittent fasting or intermittent eating, it is described as periods of food restriction and feeding. Intermittent eating can be broken into two different categories. This includes time restricted feeding such as 12/12, 16/8, 20/4, 24/24; for clarification, the ratio is fasting/feeding, or alternate day fasting, which I will cover later on in the article. Fasting is interesting because you can do whichever protocol fits your lifestyle. Many use Intermittent Fasting (IF) as a weight maintenance and/or weight loss strategy, as well as technique to improve cognitive function. One big concern is the hunger that comes along with fasting. I will go into this topic in detail in this article. I will also discuss the mechanisms that cause fasting to be so effective!

Time Restricted Feeding and Body Composition

When it comes to dieting we are all aware of the general equation for fat loss. If calories in are less than calories out, we lose fat. CALORIES IN < CALORIES OUT = Weight Loss (hopefully fat). However, in bodybuilding, we often refer to dieting as body re-composition. We’re looking for the most optimal method at losing the most amount of fat possible while preserving lean muscle mass; better yet increasing lean muscle mass while getting leaner. In fact, our lab has found in several studies that at maintenance calories, individuals are able to gain muscle and lose fat at the same time! intermittent-fasting-before-and-after

To support this, in a recent study by Trabelsi and colleagues (2013) researchers investigated this exact question. These researchers used 16 bodybuilders who were practicing Ramadan fasting for 29 days, fasting approximately 15 hours a day. In this study, the bodybuilders maintained their normal calorie consumption. However, they were only allowed to eat and drink during the night time, which was approximately 9 hours. Furthermore, they were broken into two groups; the first group trained during the feeding period (FED) and the other group trained at the end of the fast (FAST).

As one can see, in this study the researchers found that over a 29-day period, athletes on Ramadan who maintained calories didn’t lose any fat. However, there certainly was a visual trend for fat loss, which possibly may result in true body composition changes had they sustained the diet for 12-16 weeks. This is interesting because as we stated before, to lose weight the calories consumed must be less than the calories burned in a day’s time. However, the individuals in this study ate maintenance calories and still had a slight trend towards fat loss!

In addition to the study I just mentioned, there was another great study by Karli et al. (2007) that looked at the effects of intermittent fasting on body composition. This was once again another Ramadan study; however, this one was conducted in elite power athletes. Once again, the guys in this study were consuming maintenance calories and performing a maintenance training stimulus. What the researchers found here is that from pre-Ramadan to post-Ramadan (29 days) to one-month post-Ramadan, the subjects actually gained lean mass and maintained fat mass.


Another study by Tinsley et al. (2016) looked at the effects of intermittent eating constrained to a four-hour window, four days per week. This group was compared to a group that ate at will during the study. Participants in both groups performed resistance training three days per week for eight weeks and were able to eat whatever they wanted during the study. The final results showed that the group that was restrained to a four-hour eating window, four days per week did not lose or gain any lean muscle mass. In addition, the group that was able to eat whenever they wanted did not lose or gain any muscle mass. These results show that constraining your eating to a four-hour window may not be optimal for muscle growth. However, the group that performed fasting consumed ~667 calories less on fasting days, which may have contributed to the absence of change in muscle mass.

Alternate Day Eating

Another form of intermittent fasting one can use during their dieting endeavors, which can be an alternative to long-term calorie restriction (i.e., being in a calorie deficit for extended periods of time) is to calorie cycle. Calorie cycling is when one has periods of calorie restriction (losing weight) and calorie maintenance (maintaining weight). An easier solution to calorie cycling is to avoid the need to track calories and change macronutrients every day; this is a technique called Alternate Day Intermittent Fasting.

This type of fasting involves a fasting day in which caloric needs are reduced to 25% followed by a non-fasting day; this process is repeated throughout the week. For example, if one normally consumes 2000 calories a day, on the “fasting” days they would only consume 500 calories. As you may notice, one may not technically be fasting on their very low-calorie days. However, due to the severe calorie restriction, this procedure can mimic a fasting state. Clearly, this is a more extreme example of calorie cycling.

Alternate Day Eating and Body Composition

With that being said, what does the research say on alternate date intermittent fasting? Well, it appears to work really well. Relative to this topic, Dr. Varady had two major questions when conducting a 2013 study. Specifically, in normal weight, non-training individuals. Does alternate day fasting cause muscle loss and can it help in fat loss? The study used 32 non-obese middle-aged men divided into a non-fasting and alternate day fasting groups.

The subjects in the fasting group could eat normally on non-fasting days. Unfortunately, we do not have much more information on the diets of these subjects, but we can assume the ones fasting were in calorie deficit overall for the duration of the study. With that, we can make the assumption that the calorie restriction caused the substantial weight loss. What we can say is that using the alternate day intermittent fasting resulted in over 7 pounds of fat loss in the 12-week period and a small amount of muscle loss (about 3 pounds).

Another study looking at the effects of alternate day eating was done by Heilbronn et al. (2005). However, this study was much more extreme than the one mentioned previously. In this 22-day study, participants fasted every other day for 24 hours. After the study, the subjects lost an average of ~6-pounds of total body weight, which was a mix between lean muscle and body fat.

The alternate day intermittent fasting approach does seem extreme and to some maybe even unhealthy. However, if you look at the science behind the idea it makes sense. In order to lose weight, one must be in a calorie deficit. Alternate day intermittent fasting does simply that. It causes a long-term calorie deficit, which allows for continued weight loss.

Although we need much more research in this area, especially in lean and/or trained individuals, alternate day intermittent fasting may be a very useful tool for those trying to lose weight quickly or maintain a certain level of leanness. Furthermore, if restricting calories so drastically seems hard every other day, then they can start by restricting calories once or twice per week preferably on their off days from the gym to promote fat loss.

Do You Need to Intermittent Eat Every Day?

A big question that I normally get is, “Do I need to Intermittent Eat Every Day?” Let’s see what the research says. A study by Chaix et al. (2014), looked at the effects of three different interventions on mice. Yes, this was a mouse study, however, the results are intriguing and they open the door for future research in this topic. Time restriction, in this case, was a 15 hour fast and a nine-hour feed. As you can see in the picture, obese mice that were subjected to eating when they wanted became more obese, however, when they were restricted, they became lean. In addition, when lean mice ate when they wanted they became obese, but when they were restricted they stayed lean. Furthermore, when lean mice were restricted during the week (five days) and ate when they wanted on the weekend (two days), they stayed lean. Interestingly enough, the calories consumed between each group were exactly the same!


This study interestingly seems to contrast with a study done by Trabelsi et al. (2013). These researchers found that over a 29-day period athletes on Ramadan, who maintained calories, didn’t lose any fat. However, there are a few things we need to consider:

1) The mouse study gave a typical fast food diet, whereas the Ramadan study was more of a bodybuilding “clean” diet. It is suggested that intermittent fasting may cause fat loss by increasing insulin sensitivity. Thus, the effects from the mice study can further be increased with the combination of training and a cleaner diet, as seen in the Ramadan study.

2) The length of the studies; the mice study was carried out for 12 weeks whereas the Ramadan study only lasted a month. It is fair to note that the Ramadan study had a visual trend for fat loss. Perhaps these changes would have been significant if it were to extend to 12 weeks. Interestingly, in the mouse study, when they fed the mice a cleaner diet over the course of 20 plus weeks, they got leaner with the same caloric intake; which supports my contention.

What is the Mechanism of Intermittent Eating?

If you have read the article on insulin sensitivity, you may recall an enzyme called AMPK. AMPK is the cells fuel gauge that helps regulate the energy used in the cell. During periods of fasting, AMPK increases causing your body to break down fat for energy and increase insulin sensitivity. Therefore, during the fast you are utilizing stored body fat as energy and when you eat you are more sensitive to the food (specifically carbohydrates) that you consume. This will allow you to store the food as energy and not body fat. This shows that intermittent fasting may be great for overall improving body composition, but how do you overcome the hunger pains?

Intermittent Eating and Hunger

One question that we commonly hear regarding intermittent fasting is how it affects hunger. Most people cannot imagine only eating during scheduled times because they fear that they will experience intense hunger that will affect their day. This thought often prevents people from giving intermittent fasting a try, but is hunger actually an issue? Maybe it is more of a psychological relationship with food that makes the idea of not being able to eat scary. I am here to present some information on why intermittent fasting gets easier as one progresses.

In Dr. Varady’s study, the researchers also evaluated the effects of intermittent eating and fullness and the feeling of satisfaction in healthy active people. Participants in this study were divided into a control group that didn’t fast and an experimental group that fasted every other day by lowering calories to 500 on fasting days; what we term alternate day intermittent fasting. These participants followed this protocol for 12 weeks and were asked to rate their fullness and satisfaction throughout this period. The results found are expressed in the graph below.

Intermittent fasting appears to have an adaptation period as well. Researchers from the study above found that fasting became easier over time! This is likely due to the individuals adapting to the diet! This means that when you try intermittent fasting, you need to be patient. As you become more and more adapted to your fasting period, your desire to eat will likely lower. Additionally, you will probably begin to consume fewer calories than you previously did. Not only does intermittent fasting offer weight loss benefits, but it has been speculated that fasting also improves cognitive function. There are a number of ways that this can happen, however, one of the most obvious is the fact that once you have become adapted to not eating for extended periods of time you tend to not think about food. Ask yourself this question, “how productive could I be if I didn’t have to worry about eating every three or four hours?” Like I said before, this idea does take some time to adjust to, however, you’ll be surprised with the benefits and results!

Practical Application

Obviously, there is still a lot of research that needs to be done in the field of intermittent fasting on its effects on strength, power, hypertrophy, performance and overall body composition. Up until now, many have only researched its health benefits, which include the ability to reduce markers of cardiovascular disease (high blood pressure, high total cholesterol, high LDL cholesterol, elevated triglycerides, atherosclerosis), as a cancer therapy with calorie restriction, and neurological and cognitive benefits. The research is quite promising when it comes to intermittent fasting and improvements on quality of life.

You can utilize intermittent fasting as an approach to maintain lean muscle mass and decrease fat mass. If you are doing intermittent fasting every day, I would refrain from fasting longer than 16 hours in fear of potentially losing muscle mass. However, if you are only fasting a few times per week, then you can utilize longer fasts to help increase fat loss. In addition, a way that one can make this intermittent fasting adaptation period easier is to start with a small fast and progress to a larger fast. For example, start with a simple 12-hour fast and schedule the majority of the fasting period while sleeping. Then increase the fasting period each day in small increments until you become adapted to the diet. From that point you’ll be acquainted with the idea of having periods of fasting, therefore it will be easier to fast for longer durations of time.

Stay tuned because my team of scientists and myself are in the process of developing a study where we will investigate the direct effects of meal timing on body composition. This seems to be one of the biggest concerns when it comes to intermittent fasting; people want to know what is the BEST protocol. That is something that we want to figure out for YOU!