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 to lose weight, but doesn’t have to.
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.
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.
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. 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.
For some people, these protocols may be easier to follow than standard weight loss diets. Additionally, they may lead to cardiometabolic benefits.
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.
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.
For those who are looking to optimize muscle growth, long fasting periods may not be ideal, but this isn’t completely clear yet.
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.