4 Common Types of Intermittent Fasting
By Annalouise O’Connor, PhD, RD
Intermittent fasting is a dieting style growing rapidly in popularity. We sat down with Annalouise O’Connor, PhD, RD to learn more about this eating option.
Q: What is the difference between traditional dieting and intermittent fasting?
When people go on a traditional diet, their primary focus is on what they eat and how to cut calories every day until their weight loss goal is met. Intermittent fasting differs in that the primary focus is on when you eat. Intermittent fasting focuses on the food timing pattern and builds in clear fasting windows of at least 12 consecutive hours.
Another difference lies in the potential benefits and how they are discussed. When we think “diet,” we generally think weight loss. But the benefits of intermittent fasting are thought of more broadly than this. Many studies have shown us that intermittent fasting leads to successful weight management, but there are examples of benefit beyond or independent of weight loss, making these protocols relevant to a broader group of people.1,2
Q: Why do you think there is so much interest in intermittent fasting?
Intermittent fasting protocols firstly provide another layer of personalization, helping people meet their individual health goals, beyond focusing on what they eat.
Intermittent fasting also applies what we are learning about the body’s need to repair and renew each day and the link to its circadian rhythms. If these circadian rhythms (processes that the body follows over 24 hours), become dampened or irregular, it can spell trouble for weight management, glucose control, and mood.3 Introducing short periods of fasting within a 24-hour period can help reset these circadian patterns and provide the body with a break from digesting food and storing nutrients to focus on critical repair activities.1
With no incoming nutrients, the body must dip into stored energy during these times of fasting and shifts from preferring glucose as an energy source to using fat or ketone bodies (an energy source made from fat)—a shift described as “flipping the metabolic switch,” which is proposed to underlie many of the health benefits of intermittent fasting.1 For someone having three meals and snacks throughout the day and into the evening, this may rarely happen. Intermittent fasting can help to “flip this metabolic switch” and provide the body with the time it needs to engage in repair.
Q: I understand there are four common intermittent fasting protocols. Can you take us through each one?
One popular protocol is time-restricted feeding (TRF), which sets the eating window to anywhere from 4-12 hours per day. A typical protocol is 16:8, which basically means all food is eaten during an 8-hour eating window. This is followed by a 16-hour fast (water or zero-calorie beverages only). This protocol applies our learning on how the feeding-fasting cycle can help to keep circadian rhythms regular.
Another well-studied protocol is alternate-day fasting (ADF). This involves a fast of at least 24 hours alternating with a nonfasting day, which is not restricted. This fasting period allows the body to dip into its stored energy and take a break from digestion in order to focus on repair.
An evolution of the ADF protocol is the modified alternate-day fasting protocol (mADF). During this protocol, instead of a total fast during each alternating 24-hour period, intake is restricted to 500-600 kcal on these days. Because of the very low intake, the body still needs to dip into energy stores, and because it is hard to make 500 kcal last, it indirectly introduces a fasting window of at least 12 hours (in practice this tends to be much longer) in between eating occasions.
The 5:2 protocol takes the concept of the very low intake (~500-600 kcal) from the mADF and applies it two days per week. The two restricted days can be consecutive or nonconsecutive. There are variations on this protocol, for example 6:1, where the 500-600 kcal intake is consumed just on one day of the week, and the remaining 6 days are nonrestricted.
Q: Doesn’t that seem a little drastic?
What we know about our ancestral populations indicates that we evolved experiencing time periods with little or no food. Fasting for religious reasons has been documented for thousands of years, so we evolved with an ability to cope with periods of food absence, and this was maintained culturally.1
In many ways we have been mentally programmed to think about having three meals plus snacks every day, but our current eating pattern may be very much at odds with how we how we as a species have eaten in the past, and this can impact our health status.
Q: Are there different patient types associated with each option? Or do practitioners tend to recommend them on a patient-by-patient basis?
We see in the scientific literature that intermittent fasting is effective for a range of outcomes. However, we don’t yet have head-to-head studies comparing their effects. Practitioners can use what we do know and layer in other aspects of personalization in a plan that can work for someone long-term—things like medical history, work schedule, family eating times, trigger points, and so on.
For weight management the four common protocols discussed have been shown to be effective.4-7 Interestingly, these protocols have been shown to be as effective for weight management as traditional diets, but not more effective—at least in the research setting. In the real world we know that weight management can be an ongoing challenge, and one of the benefits of intermittent fasting protocols is that they offer more flexible tools in a person’s toolbox.
Protocols such as 5:2 may be particularly relevant depending on what an individual thinks about “going on a diet,” and what his or her specific pain points are when it comes to weight management. One quote that’s stuck with me from a study of women’s experiences when following a 5:2 protocol was, “For me it’s about not feeling like I’m on a diet.”8 For the women in the study, the intermittent fasting protocol was seen to be sufficiently flexible to fit in with the demands of life and didn’t lead to all-or-nothing thinking when it came to dieting, which can be a barrier to adherence.8
Another theme that’s emerged from work on intermittent fasting is that participants tend to eat less on the unrestricted days than they did before the study started. We see this in studies on 5:2 and mADF studies where although the restriction days over time may not be as low as planned, there is a real carry-over effect, meaning that rather than wanting to overcompensate, participants were naturally eating less on the nonrestricted days.
Some research has indicated that postmenopausal women lost more weight with mADF than premenopausal women.9 So age and hormonal status are other factors that practitioners may take into account when developing a personalized protocol.
Finally, when it comes to healthy insulin function, another study that sticks out is a recent study in men following an early time-restricted feeding protocol, where all food was eaten earlier in the day (ate all food in the morning and early afternoon) compared to eating the same amount of food over a standard 12-hour window. Even without any weight change, having a strict eating window of early in the day led to better insulin function.2
Q: Do people tend to stay with one method or switch among them as they get closer to reaching their goals?
This depends on the goals of each individual patient. One of the major benefits of having these additional protocols is that they provide more flexibility for people, as well as more options that a practitioner can work with to get people to where they need and want to be. These can certainly be combined or switched over time. The only word of caution is to work with a practitioner to ensure that the protocol is being applied appropriately before switching to something else. Small tweaks within a protocol may be more effective than bouncing from protocol to protocol.
Q: What else should people know about intermittent fasting before talking to a healthcare practitioner?
Know that the foundations of healthy food choices still apply. This plan is not just about skipping meals. The quality of food, such as including plenty of raw fruits and vegetables, ensuring you are getting enough fiber, watching your refined sugar intake—these are all still relevant in this eating pattern.
Continue to build physical activity into your week, aiming for at least 150 minutes per week and building from there. Consider activity such as High-Intensity Interval Training (HIIT), which has been shown to be a time-effective tool to reduce abdominal and visceral adiposity, as well as improving insulin sensitivity and building muscle, along with cardio and resistance training.
Don’t forget to stay hydrated with plenty of plain water and herbal teas or other zero-calorie beverage options throughout the day.
Finally, see a practitioner who can guide you on the intermittent fasting protocol that will help you reach your health goals. Your practitioner can also make sure that intermittent fasting is right for you. There are certain groups, for example women who are pregnant or breastfeeding, whom intermitting fasting is not right for. But there are other important things to keep an eye on when embarking on any new protocol, especially something like intermittent fasting. A practitioner can track hormones and other biomarkers, or for women track menstrual cycle changes, and make any necessary protocol tweaks to keep these parameters where they should be.
- Anton SD et al. Obesity (Silver Spring). 2018;26(2):254-268.
- Sutton EF et al. Cell Metab. 2018;27(6):1212-1221 e1213.
- Panda S. Nat Rev Endocrinol. 2019;15(2):67-69.
- Headland ML et al. Int J Obes (Lond). 2018.
- Trepanowski JF et al. JAMA Intern Med. 2017;177(7):930-938.
- Gabel K et al. Nutr Healthy Aging. 2018;4(4):345-353.
- Catenacci VA et al. Obesity (Silver Spring). 2016;24(9):1874-1883.
- Donnelly LS et al. J Hum Nutr Diet. 2018;31(6):773-780.
- Barnosky A et al. Nutr Healthy Aging. 2017;4(3):255-263.