Intermittent fasting (IF) is an extremely popular method of dieting that many people use for weight loss. It involves alternating between specified periods of fasting and feeding. Some of the most popular intermittent fasting protocols include:
- The 16:8 method, where individuals fast for 16 hours and feed for 8 hours throughout the day.
- Alternate-day fasting, where individuals fast for 24 hours followed by a full day of eating.
- The 5:2 method, where individuals eat normally throughout the week but restrict their calories to 500-600 kcal for two days per week.
- Some people even do 20:4 fasting protocols where they completely fast for 20 hours and only eat for four hours every single day.
In this article, we will examine the current available scientific evidence on intermittent fasting. Furthermore, we will discuss the potential benefits and downsides of intermittent fasting in a more practical and implementable fashion, and provide recommendations for those wanting to consider intermittent fasting for weight loss.
By the end of this article, you should have a better understanding of the science behind intermittent fasting to determine whether or not it's a suitable weight loss tool for you.
Intermittent fasting and fat loss: what does the research show?
There are a good number of studies investigating the effects of intermittent fasting on fat loss. Overall, the data show that when people go from eating all day to restricting their eating to pre-specified time windows, they lose weight.
For example, a 2021 study showed that women with PCOS lost ~3 lbs over six weeks when they restricted their eating window to only 8 hours per day (8am to 4pm). Similarly, a meta-analysis including data from 6 intermittent fasting trials found that intermittent fasting resulted in a loss of ~10 lbs over 12 months. The studies included in this meta-analysis implemented different forms of IF as well, which indicates that overall, IF is an effective way to help people reduce body fat.
The question is, is IF superior to continuous energy restriction? In other words, if calories are equated, are there any additional benefits of fasting for fat loss?
That’s where the “magical benefits” of IF start to dwindle away. The same meta-analysis that found that IF results in a loss of 10 lbs in a year also found that when compared to continuous energy restriction, IF doesn’t result in any additional weight loss. These findings have been reproduced several times in other studies as well so the evidence is pretty clear that the main reason that intermittent fasting is beneficial for weight loss is that it helps people reduce their overall caloric intake.
Of course, this makes sense. If you go from eating for 16 hours per day to just 6 or 8 hours, you’re likely going to eat less overall. It would be really difficult to eat the same amount of food in less than half the time.
Interestingly though, there is some evidence showing that IF may actually have some slight negative effects compared to traditional caloric restriction when it comes to weight loss and improving body composition. In a randomized controlled trial published in 2021, participants were placed into one of three groups:
Group 1 (0:150): Alternate day fasting. Complete fasting on one day followed by 150% of maintenance calories on alternate days.
Group 2 (CER): Continuous energy restriction. 75% of maintenance calories on a daily basis.
Group 3 (0:200): Alternate day fasting. Complete fasting on one day followed by 200% of maintenance calories on alternate days.
Both groups 1 and 2 had the same degree of energy restriction, 25% below maintenance, and group 3 served as a control to see if there are any benefits of fasting without caloric restriction. The results showed that both CER and 0:150 IF resulted in similar weight loss (~3-4lbs) over 3 weeks. However, the weight lost in the CER group was almost exclusively fat mass while half of the weight came from lean mass in the 0:150 group.
In other words, while both groups lost the same amount of weight, CER resulted in more loss of fat mass compared to lean mass than the 0:150 group, which is not good if your goal is to optimize your body composition.
Now, we can’t say that all types of intermittent fasting are inferior to normal caloric restriction for body composition because that’s not true. This study investigated the effects of alternate-day fasting, specifically. It makes sense that if you go an entire 24 hours without eating, you’re going to be spending a ton of time in a catabolic state, which wont be optimal for building muscle, let alone retaining it. It's logical to assume that intermittent fasting protocols that simply limit the amount of time you spend eating in one day (e.g., 16:8) are not as detrimental to muscle as alternate-day fasting since you still eat food every single day.
Potential benefits of intermittent fasting
Although intermittent fasting is not superior to traditional caloric restriction, there are some situations in which it can be a helpful tool for some people. At the end of the day, some form of dietary restriction is necessary for fat loss and the best form of restriction for you is going to depend on your personal preferences.
If you’re someone who doesn’t necessarily mind fasting and enjoys eating bigger meals in a short period of time, intermittent fasting may be useful for you. Intermittent fasting fasting is simple and easy to implement. All you do is limit the amount of time you spend eating. Some people may not want to go through the “hassle” of tracking calories and therefore, intermittent fasting may seem easier. Second, intermittent fasting does seem to result in reduced caloric intake without tracking calories. If your goal is to lose weight and you can reduce your calories by spending less time eating, then it can certainly be helpful for weight loss.
Again, while it may not be “optimal” for fat loss, it doesn’t mean that it’s completely useless. Many people enjoy eating this way, it fits their lifestyle, and they have tremendous success with it.
Potential downsides of intermittent fasting
Just as there are potential benefits, there are also potential negative impacts of intermittent fasting. First off, limiting your food intake to only 6-8 hours per day can be tough for some people. Although some claim that “you get used to it”, the transitionary period can be rough and you might experience intense hunger throughout your fast. An easy way to mitigate that, however, would be to slowly transition into your fasting window. Nonetheless, some individuals just prefer traditional caloric restriction rather than fasting.
Aside from the fact that fasting may feel difficult for some people, it can also negatively impact your performance in the gym. If you’re someone who really cares about building muscle, then fasting may not be optimal. There’s ample research showing that training in a fed state with nutrients flowing through your blood is superior for training performance, and if you perform better in the gym, you’ll likely build more muscle too. This is especially important for athletes who like to train in the morning. If you don’t eat anything overnight and then go train in the morning in a fasted state, you’ll almost certainly perform worse than if you had breakfast 1-2 hours prior to training. Even just a small snack of some simple digesting carbs, like a piece of fruit, would be beneficial if you train early in the morning.
Lastly, in theory, fasting is sub-optimal for muscle protein synthesis. As shown earlier, alternate-day fasting strategies appear to be suboptimal for muscle mass retention during weight loss. Since you’re not consuming protein during your fasting window, levels of MPS are going to be relatively low compared to when you’re adequately consuming protein. Theoretically, you want to be eating protein on a regular basis to maximize the amount of muscle you can gain.
It’s important to emphasize that, aside from alternate-day fasting, there isn’t really strong evidence that consuming protein evenly throughout the day is far superior to having all of your protein in a short period of time, as long as you eat an adequate amount of total protein. That being said though, it does make logical sense to eat protein on a regular basis throughout the day if you want to squeeze out every ounce of muscle that you can from your training. An easy way to work around this is to do what some people call a “protein fast,” which means that you’re allowed to eat protein during your fast, but nothing else. Of course, this isn’t really intermittent fasting anymore.
Intermittent fasting can be a useful tool for fat loss, but it isn’t “magic.” Many of its purported benefits, such as improved insulin sensitivity and increased fat oxidation, are simply due to its effects on overall caloric intake and weight loss.
Compared to traditional caloric restriction, intermittent fasting is not superior for weight loss and, in some instances, may actually be worse for body composition. That being said, some individuals really do benefit from intermittent fasting because they find it easy to implement and it helps them reduce their overall caloric intake. Like with all things though, there are some potential downsides to fasting, specifically when it comes to hunger/satiety regulation, physical performance, and muscle growth.
At the end of the day, it’s important to weigh the pros and cons of intermittent fasting and decide if it’s a useful tool for you.
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