Is it okay to not eat every other day?

October 29, 2025 •

7 min read

Recent research found that alternate-day fasting, which involves eating little or no calories every other day, can be a tool for weight loss and improved cardiometabolic health. But is it okay to not eat every other day, and what does the science say about its long-term safety?

Quick Summary

This article explores the practice of alternate-day fasting (ADF), detailing its different forms, health benefits such as weight loss and improved metabolic health, and potential risks including side effects and nutritional deficiencies. It outlines how to approach ADF safely, highlighting critical considerations and who should avoid this eating pattern.

Key Points

ADF is a type of intermittent fasting: It alternates between days of regular eating and days of significant calorie restriction (often 500-600 calories) or no calories at all.
It can lead to weight and fat loss: Studies show ADF can be effective for losing weight, particularly reducing fat mass and visceral fat, with results often comparable to daily calorie restriction.
ADF offers metabolic benefits: It can improve heart health markers like triglycerides, LDL cholesterol, and blood pressure, and may help lower fasting insulin levels.
Side effects are common initially: Beginners may experience hunger, fatigue, irritability, and headaches, though these often diminish as the body adjusts to the fasting rhythm.
Not for everyone: ADF is not recommended for children, pregnant or breastfeeding women, underweight individuals, those with a history of eating disorders, or individuals with certain medical conditions.
Professional consultation is key: A healthcare provider should be consulted before starting ADF to ensure it is a safe and appropriate choice for your health profile.
Long-term safety is still being studied: While promising short-term results exist, more research is needed to fully understand the long-term safety and effects of alternate-day fasting.
Modified ADF may be more sustainable: The modified version, which allows for a small meal on fast days, is often considered more tolerable and sustainable for many people compared to a complete fast.

Understanding Alternate-Day Fasting (ADF)

Alternate-day fasting (ADF) is a popular form of intermittent fasting that involves alternating between days of normal eating (feast days) and days of calorie restriction (fasting days). This rhythm of eating and fasting is theorized to promote a "metabolic switch," where the body shifts from burning glucose for energy to burning stored fat. ADF isn't a single, rigid rule; there are two main approaches:

Complete Fasting: On fast days, no food is consumed, only calorie-free beverages like water, black coffee, and unsweetened tea. This is considered the more extreme version.
Modified Fasting: A more sustainable and commonly studied method, modified ADF allows for a small meal of around 500-600 calories (or 20-25% of energy needs) on fasting days. This approach often mitigates some of the unpleasant side effects of complete fasting.

The Science Behind the Fasting Cycle

When you fast, your body first uses up its readily available glucose stores, known as glycogen, which typically lasts for about 18 to 24 hours. Once these stores are depleted, your body enters a state of gluconeogenesis, where it begins to create its own glucose, primarily from fat. This process, along with ketosis, leads to the breakdown of stored body fat for energy. Proponents believe this metabolic rhythm offers several health advantages beyond simple calorie reduction, including improved insulin sensitivity and reduced inflammation.

Potential Health Benefits of Alternate-Day Fasting

Scientific research into ADF has explored its effects on various health markers, with promising results for many individuals, particularly those who are overweight or obese.

Weight and Body Composition

Numerous studies confirm that ADF is an effective strategy for weight loss, often yielding results comparable to daily calorie restriction. In 8 to 12 weeks, obese and overweight adults can experience a reduction of 3–8% of their body weight and a significant decrease in fat mass. A key finding is that ADF can be particularly effective at reducing visceral fat—the harmful fat surrounding internal organs. One study also suggested that ADF might be more beneficial for preserving lean muscle mass compared to other calorie-restricted diets.

Metabolic and Cardiovascular Health

ADF has been shown to improve several markers of metabolic health and heart disease risk. These improvements include:

Reduced Insulin Levels: Studies indicate a decrease in fasting insulin, potentially lowering the risk of developing type 2 diabetes.
Improved Cholesterol Profile: ADF can lower triglycerides and LDL ("bad") cholesterol levels, and in some cases, increase LDL particle size, which is a positive indicator for heart health.
Decreased Blood Pressure: Research has observed improvements in blood pressure in individuals practicing ADF.
Reduced Inflammation: Some findings suggest that ADF can lower inflammation markers, which are linked to various chronic diseases.

Cellular Repair

Fasting is known to trigger autophagy, a process where old or damaged parts of cells are degraded and recycled. This cellular clean-up has been linked to delayed aging and a reduced risk of neurodegenerative diseases, cancer, and heart disease. While promising, more extensive human studies are needed to fully understand ADF's long-term effects on these processes.

Risks and Considerations

Despite its benefits, ADF is not without its risks, and it's not suitable for everyone.

Common Side Effects

As your body adjusts to the fasting cycle, you may experience some unpleasant, but usually temporary, side effects. These often subside within a few weeks:

Hunger and cravings
Headaches and dizziness
Fatigue and low energy
Irritability and mood swings
Constipation
Bad breath (from ketosis)
Sleep disturbances

Potential Dangers

More serious risks must be considered, particularly for certain populations. The restrictive nature of ADF can be dangerous for some:

Eating Disorders: ADF can exacerbate or trigger disordered eating patterns, and should be strictly avoided by those with a history of anorexia, bulimia, or binge eating.
Metabolic Slowdown: While studies suggest ADF might not cause a significant drop in metabolic rate compared to continuous calorie restriction, long-term or extreme restriction could potentially lead to a decrease in resting metabolic rate.
Nutrient Deficiencies: Without careful planning on non-fasting days, it can be challenging to consume all necessary nutrients, potentially leading to malnutrition.
Cardiac Risks: Some research raises concerns that intermittent fasting could increase cardiovascular mortality, though more research is needed to understand this potential link.

Comparison of ADF vs. 5:2 Fasting

For those considering intermittent fasting, understanding the differences between popular methods is key.


Feature	Alternate-Day Fasting (ADF)	5:2 Fasting	Comment
Schedule	Normal eating on day 1, modified (500-600 calories) or zero calories on day 2, and so on.	Normal eating for 5 days, modified fast (500-600 calories) for 2 non-consecutive days.	Both use periodic calorie restriction but on different frequencies.
Adherence	Requires frequent switching between fasting and feeding days, which some find more difficult to sustain long-term.	Weekly two-day restriction may be easier for some to integrate into a routine.
Fatigue/Energy	Higher reported fatigue and hunger on fasting days, especially initially.	Fatigue and hunger are concentrated on the two fasting days, potentially making the other five days feel more normal.
Effectiveness	Shown to be effective for weight loss and improving metabolic markers.	Also effective for weight loss, with some studies showing similar outcomes to ADF.
Visceral Fat	Studies show reductions in visceral fat.	Some studies suggest better results for visceral fat reduction compared to ADF, though findings vary.

How to Approach Alternate-Day Fasting Safely

If you are considering ADF, it is crucial to do so safely and with proper medical consultation. The process involves more than simply not eating every other day. Here are some steps to follow:

Consult a Healthcare Professional: Before starting, speak with a doctor or registered dietitian to ensure it is appropriate for your specific health needs and goals. This is especially vital if you have pre-existing conditions like diabetes or a history of eating disorders.
Start with Modified ADF: For beginners, the modified approach (500-600 calories on fast days) is more manageable and sustainable than a complete fast. This helps your body adapt to the rhythm of fasting.
Stay Hydrated: Drink plenty of calorie-free fluids like water, black coffee, and tea, especially on fasting days, to prevent dehydration and manage hunger.
Prioritize Nutrition on Feast Days: On eating days, focus on nutrient-dense, whole foods to ensure you get all the necessary vitamins, minerals, and fiber. Avoid the temptation to binge on unhealthy, processed foods.
Listen to Your Body: Pay close attention to how you feel. If you experience severe side effects like prolonged dizziness, fatigue, or mood disturbances, it may be a sign that ADF is not the right approach for you.

Conclusion

For healthy, non-obese adults, studies show alternate-day fasting (ADF) can be a safe method to achieve weight loss and improve cardiometabolic markers in the short to medium term. However, ADF is a rigorous strategy that can cause side effects like hunger, fatigue, and irritability, particularly during the initial adaptation period. It is not recommended for children, pregnant or breastfeeding women, underweight individuals, or those with a history of eating disorders. Ultimately, its suitability depends on individual health, lifestyle, and sustainability. Finding an approach you can stick with long-term, whether it's ADF or another eating pattern, is the key to lasting success. Always consult a healthcare provider before making significant changes to your diet to ensure it aligns with your health and wellness goals.

Alternate Day Fasting (ADF) Research

Research on ADF has explored its effects on weight loss, heart health, and metabolic factors. Studies have shown ADF to be effective in reducing body weight and body fat, sometimes outperforming continuous calorie restriction in certain metrics. For instance, a meta-analysis found ADF to be highly ranked for efficacy, and another study in healthy individuals reported improved cardiovascular markers and reduced fat mass after just four weeks. However, findings on long-term adherence and comparative effectiveness against other intermittent fasting methods like time-restricted feeding are still evolving. Some research suggests ADF may lead to a slight increase in LDL cholesterol after a year, though others find reduced LDL over shorter periods. Adherence tends to be high in short-term studies but decreases over longer durations. While ADF has shown positive outcomes in many studies, it is considered an extreme dietary intervention, and long-term research is still limited. The ultimate success depends on personal sustainability and should be pursued with medical oversight.

Potential Long-Term Considerations:

Bone Health: Some studies on calorie restriction suggest potential impacts on bone mineral density, though long-term ADF studies in healthy individuals have not shown this.
Immune Function: Similar to bone health, animal studies on calorie restriction have raised questions about immune function, but long-term ADF research in humans has not confirmed negative effects.
Sustainability and Adherence: Maintaining a very restrictive pattern can be challenging over extended periods. Drop-out rates in studies can be a limiting factor in determining long-term feasibility.
Monitoring and Supervision: Given the potential risks, especially for vulnerable populations or those with specific health concerns, ongoing medical supervision is highly recommended for long-term ADF.

What to Know Before You Begin ADF

Before embarking on an ADF protocol, it is essential to understand the full scope of what it entails, including potential benefits, risks, and necessary precautions. By prioritizing safety, listening to your body, and consulting with healthcare professionals, you can determine if this eating pattern is a suitable and sustainable option for your health journey. For those who find ADF too challenging, other forms of intermittent fasting or standard calorie restriction remain viable and effective alternatives.

NIH: Safety of alternate day fasting and effect on disordered eating symptoms in obese adults

Frequently Asked Questions

For most healthy adults, modified alternate-day fasting (ADF) appears safe in the short to medium term, but it is not recommended for everyone. Individuals who are pregnant, breastfeeding, underweight, have a history of eating disorders, or have certain medical conditions should avoid it.

The potential benefits include weight loss, improved body composition, better metabolic markers like reduced insulin and cholesterol, and a boost in cellular repair processes through autophagy.

Your body switches from burning glucose for energy to burning stored fat (ketosis), particularly on fasting days when carbohydrate sources are depleted. This metabolic shift, combined with overall calorie reduction, leads to weight and fat loss.

Common side effects, especially in the beginning, include increased hunger, headaches, fatigue, dizziness, irritability, and digestive issues like constipation. These symptoms often decrease over time.

Consult a healthcare professional first, especially if you have underlying health issues. Start with a modified plan (500-600 calories on fast days), prioritize nutritious meals on non-fasting days, and stay well-hydrated.

Research suggests ADF is effective for weight loss, often showing results comparable to continuous calorie restriction. Some studies indicate superior efficacy for certain weight and fat loss metrics, but its effectiveness depends heavily on individual adherence.

Long-term data on the safety and adherence of strict ADF is limited. Some find it unsustainable, and concerns exist about long-term metabolic or nutritional impacts. Medical supervision is essential for anyone considering it over an extended period.

On fasting days, stick to the 500-600 calorie limit with nutrient-dense foods high in protein and fiber to promote fullness, like eggs, Greek yogurt, or fish. On feeding days, focus on whole foods, vegetables, lean proteins, and healthy fats, avoiding processed foods and overeating.

Yes, for many healthy adults, this strategy can be an effective method for weight loss by creating a calorie deficit over time. However, success hinges on consistency and not compensating for lost calories by overeating on non-fasting days.