Intermittent fasting is an eating pattern that involves periods of eating followed by periods of fasting. Two popular forms of intermittent fasting are Alternate-Day Fasting (ADF) and One-Meal-A-Day (OMAD). Both of these methods involve significantly reducing the amount of food consumed on certain days or at certain times, but they differ in the specific schedule of eating and fasting that is followed.
ADF involves alternating between days of normal eating and days of very limited food intake or complete fasting. On fasting days, people may consume only a small number of calories, such as 500-600 calories, or they may abstain from food completely. On non-fasting days, people can eat as they normally would.
OMAD involves consuming all of the day’s calories in a single meal, typically a large meal in the evening and abstaining from food for the rest of the day.
Both ADF and OMAD can be effective for weight loss and other health benefits, but they may not be suitable for everyone. In order to get a better understanding of both methods, let’s have a closer look at them first.
What is Alternate-Day Fasting?
Alternate-day fasting is a type of intermittent fasting diet where fasted days occur every other day and alternate with eating days. The most common form of alternate-day fasting is referred to as modified alternate-day fasting, which is the form that we will be discussing as there is a wealth of research on this diet. In modified alternate-day fasting, individuals can consume 25% of their daily caloric needs [2, 3, 7]. Often, most people consume those calories in one meal on their fasting day.
How to do Alternate-Day Fasting
Alternate day fasting can take your body some time to adapt to. You may begin with easier intermittent fasting diets to help your body adjust to fasting, such as 16:8 intermittent fasting or 20:4 intermittent fasting.
To do alternate-day fasting consists of alternating fasting days and ad libitum days, meaning that you can eat without restrictions. On fasting days, you can consume approximately 25% of your recommended daily calories for weight maintenance, about one small meal [2, 3, 7].
Sample Alternate-Day Fasting Schedule:
- Day 1: ad libitum (eat without restrictions)
- Day 2: 25% kcal (of normal daily intake)
- Day 3: ad libitum
- Day 4: 25% kcal
- Day 5: ad libitum
- Day 6: 25% kcal
- Day 7: ad libitum
The difference in fasting hours can vary depending on what time you eat your small meal on your fasting days. Fasting hours could range from 30 to 40 hours of fasting time, not just 24 hours as it may appear 
For example, if you eat your last meal on day one at 11 PM and then break your fast on day three at 5 AM, your fasting duration would be 30 hours. If you eat your last meal on day one at 6 PM and break your fast on day three at 10 AM, your fasting duration would be 40 hours. The varying durations of time and when your fasting meal (25% kcal) is eaten can alter metabolism and benefits .
To find out how many calories equals 25% of your daily recommended calories, you can use a calorie calculator. Calculate how many calories you need to eat for weight maintenance. This should be how much you eat on ad libitum days. Then times the daily amount of calories by 25% to calculate how many calories you can eat on fasting days.
What Can You Eat When Alternate-Day Fasting?
Even though you are allowed to eat without restrictions on non-fasting days, it does not mean that you should load up on junk food. There would be no point in alternate-day fasting if you consumed junk foods on non-fasting days because that would sabotage the potential benefits you could experience.
It would be best to eat your daily recommended caloric intake for weight maintenance on non-fasting days and consume nutritious foods. On fasting days, the calories that you do consume in one small meal should be nutrient-dense. The USDA states that a nutritious diet consists of eating fruits, vegetables, dark leafy greens, whole grains, and lean protein . They also recommend that you limit added sugars, refined grains, and saturated fats .
On fasting days, you can drink as many calorie-free beverages as you would like, such as water, black coffee, or tea.
Benefits of Alternate-Day Fasting
Alternate-day fasting appears to be effective for weight loss.
Many studies have found that those who participated in alternate-day fasting for 3 to 12 weeks lost between an average of 2.5% to 8% of their body weight [1, 2, 3, 5, 6, 7].
One study correlated that the weight reduction experienced was due to the participants having a caloric deficit on fasting days .
However, even though many participants experienced weight loss while alternate-day fasting, many studies concluded that alternate-day fasting is not superior to continuous caloric restriction [1, 3].
Weight loss benefits can be seen regardless of when the small meal is consumed on fasting days .
It is important to note that these trials did not study the effects of their participants engaging in physical activity while alternate-day fasting, as they were diet-focused studies. One source hypothesized that combining alternate-day fasting with physical activity may lead to increased weight reduction .
One study found that alternate-day fasting for 3 to 12 weeks can effectively reduce body fat by 3 to 5.5 kilograms, depending on the individual . Fat loss results were seen in those who had a normal BMI and those who were overweight or obese .
They also found that eating one small meal on fasting days improved lean muscle mass retention during fasting, as fat-free mas stayed consistent throughout the trial .
Another study found that alternate-day fasting reduced waist circumference and body fat in obese adults, but the results were no greater than what we see from continuous caloric restriction .
Type 2 Diabetes
Alternate-day fasting shows promise for improving markers for type 2 diabetes.
While a few studies found that alternate-day fasting reduced insulin and glucose concentrations [1, 3], another found no change in insulin sensitivity after eight weeks of alternate-day fasting .
One study, more specifically, found that insulin was reduced after fasting . However, there was an increase in plasma-free fatty acids, which could impair glucose disposal .
While we can see some promise that alternate-day fasting may improve markers for type 2 diabetes, more research is needed before we can make conclusions.
Alternate-day fasting may improve risk factors for cardiovascular disease and improve heart health [1, 2, 3, 7, 9].
Many studies found that alternate-day fasting reduced total cholesterol, LDL (low-density lipoprotein), and triacylglycerols, as well as increased HDL (high-density lipoprotein) [1, 2, 7]. Many also found that alternate-day fasting reduced heart rate and blood pressure [2, 3, 7].
However, even though markers of cardiovascular disease were reduced through alternate-day fasting, these improvements were no different than they are for daily calorie restriction .
Alternate-day fasting may provide greater results if combined with physical activity .
Other Potential Benefits
One study aimed to research if alternate-day fasting could improve symptoms of asthma . After two weeks of alternate-day fasting, participants were found to have significant improvement in their asthma symptoms and pulmonary functions, and a reduction in inflammation 
Risks of Alternate Day Fasting
Overall, alternate day fasting appears to be safe and effective. However, some side effects can be experienced .
On fasting days, you may experience extreme hunger because your body may not be used to calorie restriction . Other unpleasant side effects may include feeling dizzy or lightheaded, weakness, irritability, constipation, trouble sleeping, and bad breath [5, 10].
Overall, the side effects that can occur from alternate-day fasting are similar to those experienced in daily calorie restriction .
Even though alternate-day fasting only causes mild side effects, some individuals should be cautious.
Individuals with type 2 diabetes should be cautious as they could experience hypoglycemia on fasting days .
There is no research on how alternate-day fasting could affect women who are pregnant or breastfeeding, so for safety, it should be avoided .
If you have any pre-existing health concerns, talk with your primary health provider before beginning an alternate day fasting diet.
What is OMAD?
OMAD stands for one meal a day. OMAD is an intense version of an intermittent fasting diet, where you fast for 23 hours of the day and eat within a 1-hour window. The general guideline is to consume your daily caloric intake within this 1-hour window or to at least consume as much food as you can without restriction.
While some books have been written about this diet, there is very little clinical research that has been conducted specifically about the OMAD diet.
How to do OMAD fasting?
Fasting for 23 hours of a day and then eating as much as you please in 1 hour may sound simple, but it could be difficult to maintain.
You may ease yourself into intermittent fasting by practicing shorter interval intermittent fasting diets first, such as 16:8 or 20:4 intermittent fasting. This will allow your body time to adjust and reduce the potential undesirable side effects that calorie restriction can cause.
The 1-hour eating window each day should be scheduled at the same time every day to remain consistent. For example. If you choose to eat your daily meal at 5 PM, you should eat your daily meal at 5 PM every day (for the duration of your fasting diet period). The same rule applies no matter what time you choose to eat your one meal each day. Consistency in having a 23-hour fasting period is what matters.
The fasting time frame is the only strict rule for this diet. In the 1-hour eating window, you are technically allowed to eat whatever you want. There are no caloric guidelines defined for OMAD, but you should aim to consume your daily caloric intake within the 1-hour window to make up for the skipped calories throughout the day.
If you do not meet your daily recommended caloric intake when following OMAD, you could be in a substantial caloric deficit at the end of the week. For example, if your daily recommended caloric intake is 2,400 calories per day, but you only consume 1,600 calories within your 1-hour eating window, that would put you in a substantial caloric deficit. Over time this would become very unhealthy, and you may experience many side effects.
There are no guidelines on what to eat for your one meal. However, to try and consume your daily calories and gain enough nutrients, it would be best to eat nutrient-dense foods such as healthy fats, protein, whole grains, fruits, and vegetables .
You may drink an unlimited amount of calorie-free beverages while in the fasting period.
Benefits of OMAD
Weight Loss and Body Composition
Since there is a very limited amount of research on eating one meal a day without calorie reductions, the only common benefit found is weight loss and body composition.
Many studies found that OMAD resulted in significant weight loss, body composition modification, and fat loss, even when participants consumed their daily caloric intake for weight maintenance [14, 15, 16, 17].
No changes were found in fat-free mass, lean mass, body water, or bone mineral density [14, 15].
Risks of OMAD
Binge eating disorder is classified as eating an excessive amount of calories within a short time window . Those susceptible to eating disorders, specifically binge eating disorders, should avoid OMAD as it may promote binge eating behaviors.
Heart Health Risk
Many studies found that OMAD caused a significant increase in total cholesterol, LDL cholesterol, and HDL cholesterol [14, 15, 16, 17]. Increased LDL cholesterol is associated with an increased risk of cardiovascular disease .
One study found that participants had elevated systolic and diastolic blood pressure during meal consumption , but other studies stated that heart rate was unaffected .
Impaired Glucose Tolerance
Consistently, studies found that OMAD elevated fasting plasma glucose levels and caused a delayed insulin response [15, 16].
While this is important to note, it is also significant to know that those participants were able to reverse their impaired glucose tolerance when they returned to eating three meals a day . This means that over a short duration of OMAD fasting, these effects would not cause permanent damage. However, those already at risk for developing type 2 diabetes should be cautious.
Other Side Effects
You will likely experience extreme hunger when following an OMAD fasting plan due to the long duration of fasting .
Like any other fasting diet, you may also experience weakness, dizziness, irritability, gastrointestinal issues, and trouble sleeping [5, 10].
Anyone with pre-existing health concerns should be cautious of OMAD fasting as it may cause complications.
Alternate-Day Fasting Vs. OMAD: Winner
It is clear that the better diet is alternate-day fasting. Overall, research on alternate-day fasting benefits and risks is abundant compared to OMAD (one meal a day without daily calorie restriction).
Alternate-day fasting is proven to have many benefits and a low level of risk. Whereas OMAD only had one benefit (weight loss) and many risks. While alternate-day fasting decreased risk factors for heart disease and type 2 diabetes, OMAD may increase the risk of heart disease and type 2 diabetes.
Weight loss was the only shared benefit between the two diets. However, there is more sound evidence for alternate-day fasting and weight loss.
ADF is more sustainable for a longer duration of time than OMAD. When alternate-day fasting, you are allowed to eat ad libitum on non-fasting days and eat 25% of your calories on fasting days. When doing OMAD, you are restricted to consuming all of your daily calories within a 1-hour window every day. OMAD is much more restrictive than alternate-day fasting.
Frequently Asked Questions
Should I do OMAD or alternate-day fasting?
If you are looking to lose weight very quickly, then OMAD may give you better weight loss results due to the longer duration of fasting and meal timing. However, both OMAD and alternate-day fasting weight loss benefits are not any more superior to daily calorie restriction. Also, alternate-day fasting poses less risk, based on research.
Is it healthy to eat one meal a day?
For most people, there are no risks in limiting eating to one meal per day. However, those who need a steady calorie intake, such as type 2 diabetics, adolescents, and pregnant women, should be cautious in limiting themselves to one meal a day.
How long can I do the OMAD fasting diet?
It is not recommended to do OMAD longer than a few weeks. Research is limited on what the long-term effects are.
How long can I do alternate-day fasting?
You can safely do alternate-day fasting for up to 3 months. The most important thing to remember is to listen to your body and pay attention to any side effects you may feel. If your body does not respond well to alternate-day fasting, there is no pressure to continue the diet.
Research provides support that Alternate-day fasting is superior to OMAD, and there are very few associated risks. Overall, the better diet to choose would be alternate-day fasting. But, no matter which diet you choose to follow, remember to seek the opinion of your primary care provider or a registered dietitian before beginning any fasting diet.
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