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Fasting continues to trend as an effective weight loss method and health promoter. Fasting is popular because the science behind most methods is strong, it’s a fan favorite amongst celebrities and health enthusiasts, and it usually does not require you to change the way you eat – it only changes the frequency. These are all positive aspects, but perhaps the most impressive outcome is what is happening internally. Fasting creates an environment in which the body is forced into a metabolic switch from its primary and preferred fuel, glucose, to fat. This may translate to weight loss, better blood sugar management, reduction of metabolic biomarkers and of course, a biologic journey to ketosis. All of this can be summed up in one message – that fasting is our evolutionary manner in which to preserve and improve our cellular health.

A 2019 study in The New England Journal of Medicine cited the abundance of research showing this concept. At the cellular level, fasting can reduce inflammation, improve blood sugar, and reduce the risk for several chronic diseases. While weight loss is also a wonderful aspect of fasting, it is truly only the icing on the cake.

In order to fully understand fasting, you first to recognize the various forms that are detailed in the last 5 years of research. Many of the following forms of fasting can be considered types of intermittent fasting – a dietary pattern that cycles between consuming nourishment in any form with periods of complete abstinence. However other forms, such as a short fast, offer one-time commitments to fasting. Additionally, while the timing of an each fast varies by protocol, the type of food consumed is not (in our next article, we’ll tell you why the type of food you consume is just as important).

Finally, it’s important to realize that there are not one size fits all approach to fasting. What may work for you may not work for your family and friends. In fact, some forms of fasting for your individual makeup may even be considered dangerous, while others would be seen as safe. Choosing the right fast often means assessing your overall lifestyle and goals first.

Here’s a breakdown of the most popular types of fasting.

Time restricted eating (TRE)

TRE is a method in which food is limited to either an 8 hour or 10-hour window. Multiple benefits have been associated with TRE. A 2019 study found that limiting nourishment to ten hours a day resulted in improvements in blood pressure, belly fat and lipid profile. Further, a 2020 study found that TRE can improve metabolic health, even before weight loss occurs. Changes in metabolite profile and gene expression were observed that resulted in the diet not negatively impacting the circadian rhythm of the body. Finally, a 2018 study found that when participants consumed breakfast 90 minutes later, and dinner 90 minutes earlier than usual, the results was fat loss without change to actual diet within a ten-week intervention. In addition to these benefits, a TRE approach often results in reduction of overall calorie despite the approach not being a calorie restricted diet. This factor may also help create an energy deficit which in turn can increase metabolism and promote weight loss.

The perfect candidate for a TRE approach is: someone who has the ability, both personally and professionally to consumer an earlier dinner and forgo breakfast first thing in the morning. TRE can also be a great option for athletes and individuals who struggle with late night snacking. For athletes, a 2019 study showed that exercise before eating breakfast increased fat loss and resulted in better insulin response. For individuals who struggle with late night eating, TRE creates a literal stop on the clock for eating, creating an environment that, over time, eliminates the craving for late night eating. A 2020 study found that when overnight fasting times were the same, individuals in a group that ate breakfast, and skipped a late-night meal or snack fared better in burning fat and losing weight.

Finally; you may think that the longer you go in between meals, the better. Think again. Recent data indicates that long-term fasting periods more than 16 hours may be harder to sustain over time and could negatively impact lipid panel. More studies are needed however, as other lipid markers have been shown to improve.

5:2 approach

The 5:2 diet includes 5 days of “normal” eating coupled with 2 days of fasting in the form of 500 calories for women or 600 calories for men. Some health experts cite the discovery of the 5:2 approach (almost ten years ago) as the moment the world saw fasting as viable method for weight loss. The pattern of the 5:2 approach can be a split of calories between breakfast and dinner or, consuming just one moderate sized meal during the day. The timing of these meals, and the exact amount of calories per meal may vary, however the total caloric intake throughout the day does not. On “off” days, individuals are encouraged to eat healthy however the breakdown of meals and timing is not structured. A 2018 study found that a 5:2 approach could be especially beneficial for blood sugar and triglyceride management after meals. As with other forms of intermittent fasting, the 5:2 has been cited to potentially reduce oxidative stress, inflammation, and biomarkers for disease as well.

The perfect candidate for the 5:2 approach is: Individuals who can control their off day eating habits. This is because despite the fact that multiple studies neglect to look at the diet quality during eating window’s, plenty of evidence shows that binging on off days may reset the hard work done on fasting days. For example, a 2021 study found that fasting was beneficial to metabolic health however fasting coupled with a Mediterranean diet protocol on off days showed benefits to the gut microbiota as well. The 5:2 approach is also useful for individuals that cannot eat early meals, either due to work, or child commitments, individuals who travel (fasting days would not coincide with travel days) and individuals that simply can’t limit the window of eating every day of the week. An alternative method to a 5:2 approach is a 6:1 approach which utilizes only one day a week of fasting. Yet another, however more extreme, alternative is to couple a 5:2 approach as a form of alternate day fasting.

You may not be a good candidate for 5:2 if severe restriction of calories, even for just two days a week may be too great of stress for blood sugar, or medications prescribed (such as blood pressure medication).

Alternate day fasting

Alternate day fasting cycles between days of “normal” eating followed by days where abstinence of eating occurs. Several news studies have shown promise in fasting every other day. A 2019 study showed that alternate day fasting was safe alternative to counting calories (a method proven to lack sustainability). Additionally, the study showed that ADF individuals achieve ketosis and stay in ketosis even on eating days. A 2009 animal study found that alternate days fasting reduced both oxidative stress and inflammation. Alternate day fasting and the 5:2 method have often been done at the same time as well (cycling between a 500-calorie day and a fasting day). Alternate day fasting (as well as OMAD) may not be a viable methods for athletes, especially those who are in training.


The one meal a day plan, or OMAD has been gaining traction lately as a potentially effective, yet more extreme form of fasting. Typically, the meal of choice is dinner, however the individual can choose breakfast, lunch, or an even a specific time frame (3:00 – 5:00 for example) as their meal window. As with other forms of fasting, the breakdown of macronutrients is not structured in the plan. The OMAD diet was made popular by another plan, known as the warrior diet which encompasses many phases surrounding the one meal a day concept. Though OMAD may produce the same general benefits that other forms of fasting have, it may also come with some risk. Because the OMAD pattern may involve one large meal, achieving ketosis and maintaining normal blood sugar levels may be challenging. Further, the sustainability of the diet may also be hard to stick to on a day-to-day basis. OMAD may be especially harmful for individuals with a history of disordered eating and studies are still needed to truly asses its value in an individuals with cardiovascular disease.

Short Fast

A short fast is exactly what it sounds like – short. These fasts are usually one day a week of fasting where nourishment is not provided the entire day outside of water, coffee, and zero calorie electrolyte replacement. This type of fast will make sense for an individual that only wants to fast one day a week. Further, during a short fast, the body’s cellular material will still sense, and need to adapt to stress which ultimately brings metabolic benefit. This approach may be more sustainable as it only requires a minimal amount of fasting on any given week.

Water Fasting

Water fasting is a method in which all nourishment outside of water is consumed for a minimum of 3 days. Though water fasting stresses cells to the point of autophagy, it can be dangerous due to impact on blood sugar, blood pressure and electrolyte balance. What is autophagy? It’s the body’s way of getting rid of old and useless cellular material to make way for stronger, and more resilient cells (in addition to fasting, very low-calorie diets may also induce autophagy according to a 2018 study). Autophagy may be an essential way to reduce inflammation, thus reducing the risk for multiple chronic diseases. However, because water fasting can be dangerous, an alternative method to induce autophagy was developed that includes nourishment and significantly reduces the risks found in traditional water fasting. The fasting mimicking diet is the name of this method.

Fasting mimicking diet

The fasting mimicking diet is a 5 day highly structured approach developed by Valter Longo at USC. The diet has been shown in studies to trick the body into thinking it is in water fast while providing nourishment, and components necessary to retain lean muscle mass. Studies have demonstrated that the 5-day meal plan may help to reduce the risk for cardiovascular disease, diabetes, and cancer. Further, participants who do the FMD lose an average of 6 pounds and often report renewed energy. Animals studies have shown cellular regeneration and rejuvenation (similar to autophagy that would be seen on a water fast) as well. To find out how to do the Fasting Mimicking diet, click here.

Consult your physician before starting a fast

Fasting is not the right answer for everybody. Fasting may not be a heathy pattern for the following:

  • Pregnant
  • Breastfeeding
  • Underweight
  • Under 18 years old
  • Type 1 diabetic or type 2 diabetic on insulin
  • Certain chronic conditions, especially renal and other chronic digestive illness
  • Elderly
  • Individuals with a history of eating disorders

Before starting any new regimen dietary regimen, individuals should speak to their physician first.

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