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OMAD and 20:4 Intermittent Fasting

There are three similar intermittent fasting protocols that have quite short daily eating windows: 18:6, 20:4, and one-meal-a-day (OMAD). In general, these involve abstaining from calories for 18-23 hours a day, and then eating freely within the remaining time. The eating window can occur at any point of the day, but most people choose to eat between 2 p.m. and 8 p.m.

Our evidence-based analysis on omad and 20:4 intermittent fasting features 13 unique references to scientific papers.

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Summary of OMAD and 20:4 Intermittent Fasting

Primary information, health benefits, side effects, usage, and other important details

What are 20:4 & OMAD?

As a note, the scientific community tends to restrict the term “intermittent fasting” to diets that drastically restrict calories for 2-4 days of the week (such as not eating at all for a day, or eating very few calories for a couple days of the week). For daily fasting of set durations, the term “time-restricted eating” is preferred. For the sake of this page, the terms will be used interchangeably.

Time-restricted eating is often described as the ratio of hours spent abstaining from food and the hours where eating is permitted.

In the 18:6, 20:4, and one-meal-a-day (OMAD) protocols, individuals abstain from any caloric intake for 18 to 23 hours a day and then eat freely for the rest of the day. Those who follow the OMAD protocol eat all their food at one sitting, which typically lasts an hour or less, while the other protocols typically involve more than one meal or a meal and a snack.

Colloquially, these types of eating schedules fall under the category of “intermittent fasting”, but in the scientific literature that term generally refers to eating schedules that involve abstaining from or drastically lowering energy intake for more than 24 hours. Time-restricted eating can involve a caloric deficit[1] to lose weight, but doesn’t have to.[2]

The 18:6, 20:4, and one-meal-a-day (OMAD) protocols are types of time-restricted eating where you abstain from eating for 18 to 23 hours, and then eat freely for the rest of the day. They can be combined with a caloric deficit for weight loss, but don’t have to be.

What are the benefits of 20:4 & OMAD?

There aren’t many studies on these particular intermittent fasting regimens, and the findings are a bit contentious.

In general, they can be a viable method for weight loss when paired with a caloric deficit and may be easier to follow than a standard diet that requires caloric restriction at each meal. That being said, the more restrictive protocols can present a considerable challenge to some. When paired with resistance exercise, there doesn’t seem to be strength or muscle losses, but further research is required.[3]

Most of the purported benefits of fasting come from the shift into an energy-scarce state, which upregulates metabolic pathways associated with cell-cycle regulation and energy mobilization.[4] Shorter fasts probably don’t provoke these changes, but fasting for 18 hours or more might. Although there isn’t a lot of data on extended fasts, one 24-hour fasting study saw considerable improvements in blood pressure and glycemic control.[5]

For some people, these protocols may be easier to follow than standard weight loss diets. Additionally, they may lead to cardiometabolic benefits.

Are 20:4 & OMAD safe? Are there side effects?

No studies have specifically investigated the safety of these protocols, but most studies report a small number of adverse effects that typically include hunger, headaches, slight dizziness, or gastrointestinal upset.[5]

Following these protocols, especially OMAD, can be quite difficult for some. Studies have reported high rates of participant dropout, likely due to the challenge of adhering to such a limited eating window on a sustained basis.[6]

For those who are looking to optimize muscle growth, long fasting periods may not be ideal, but this isn’t completely clear yet.[3]

Finally, fasting can be contraindicated in some diseases such as type 1 diabetes, pregnancy, and preexisting eating disorders. For individuals with these or similar conditions, speaking with a well-informed physician is warranted.

Although no serious side effects have been reported in existing studies, some individuals with medical conditions may want to exercise more caution.

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How to Take

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How to follow 20:4 & OMAD

Simply allocate an 18 to 23 hour window in the day during which you will abstain from calories completely. Many choose their fasting window to begin around 8 p.m. and extend to dinnertime the next day, but it’s just as possible to eat earlier in the day and stop eating earlier.

Studies using 16:8 fasting protocols have found that insulin sensitivity (and thus overall glycemic response to food) tends to be better in the morning, so having an early eating window may be preferable for some people, although many opt to eat later in the day because it’s more conducive with social eating.[7][8][9][10] Adherence to these protocols is typically easier for people who have familial support, and for some the practice becomes easier with time.[11][6][12]

Generally speaking, water, tea, seltzer, and coffee are considered permissible to eat during the fasting window. While other things such as chewing gum, exogenous ketones, MCT oil, supplements, apple cider vinegar, and artificial sweeteners are hotly debated, a good rule of thumb is that if the product has more than 5-10 kcal then it should be avoided until the fasting window ends.

It’s simple: don’t consume anything except for water, coffee, or tea for 18 to 23 continuous hours, and then eat freely for the rest of the day.

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Human Effect Matrix

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The Human Effect Matrix summarizes human studies to tell you what effects OMAD and 20:4 Intermittent Fasting has on your body, how much evidence there is, and how strong these effects are.

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Grade Level of Evidence
Robust research conducted with repeated double-blind clinical trials
Multiple studies where at least two are double-blind and placebo controlled
Single double-blind study or multiple cohort studies
Uncontrolled or observational studies only
Level of Evidence
? The amount of high quality evidence. The more evidence, the more we can trust the results.
Outcome Magnitude of effect
? The direction and size of the supplement's impact on each outcome. Some supplements can have an increasing effect, others have a decreasing effect, and others have no effect.
Consistency of research results
? Scientific research does not always agree. HIGH or VERY HIGH means that most of the scientific research agrees.
Notes
grade-d Notable High See all 3 studies
These protocols reliably increase cholesterol levels. This could be partially related to the fact that fatty acid oxidation could be increased, but the details remain to be seen.
grade-d Notable High See all 3 studies
These intermittent fasting protocols can lower fasting insulin. In fact, one study saw fasting insulin decrease in as few as 24 hours.
grade-d Notable High See all 3 studies
These protocols reliably increase cholesterol levels. This could be partially related to the fact that fatty acid oxidation could be increased, but the details remain to be seen.
grade-d Notable Very High See all 3 studies
These protocols reliably increase cholesterol levels. This could be partially related to the fact that fatty acid oxidation could be increased, but the details remain to be seen.
grade-d Minor Moderate See 2 studies
These protocols probably reduce glycemic response to food. One study found otherwise, but practically nullified their findings because they drew each group's blood at different times of day.
grade-d
Minor
- See all 3 studies
Various studies have found these protocols to increase, decrease, and have no effect on blood pressure. Further research is required to understand this relationship.
grade-d Minor - See study
When paired with a caloric deficit, these protocols can lower body fat.
grade-d Minor - See study
These protocols can probably lower fasting glucose, both acutely and over time.
grade-d Minor High See all 3 studies
Insulin resistance seems to be improved by these protocols, particularly when the feeding window occurs early in the day.
grade-d
Minor
- See 2 studies
One study that investigated this outcome found that 18:6 increased triglyceride levels, but a 24-hour fasting study found the opposite to be true.
grade-d Minor High See all 3 studies
When paired with a caloric deficit, these protocols can effectively reduce body weight.
grade-d - - See 2 studies
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grade-d - - See 2 studies
grade-d - - See study
grade-d - - See study
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grade-d - - See study
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grade-d - - See study
grade-d - - See study
grade-d - - See 2 studies
grade-d - - See 2 studies
grade-d - - See study
grade-d - - See study
grade-d - - See 2 studies
grade-d - - See study
grade-d - - See study
grade-d - - See 2 studies
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Things to Note

Is a Form Of

  • Diet

Primary Function:

Also Known As

Time-restricted Eating, Time-restricted Feeding, The Warrior Diet

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Click here to see all 13 references.