What happens to my body if I only eat one meal a day?

October 21, 2025 •

4 min read

According to a 2022 study, fasting for a two-hour window in the evening led to significant weight and fat loss in healthy adults. But beyond weight loss, many wonder: what happens to my body if I only eat one meal a day? This practice, known as OMAD (One Meal A Day), triggers several physiological changes that impact metabolism, energy, and overall health.

Quick Summary

This article explores the physiological effects of eating one meal a day, detailing metabolic shifts, potential benefits like weight loss and autophagy, and significant risks such as nutrient deficiencies, hormonal imbalances, and fatigue. It outlines how the body transitions from burning sugar to fat and provides guidance on how to perform OMAD safely, including tips on nutrition and hydration. Considerations for different populations and the importance of professional medical advice are also covered.

Key Points

Metabolic Switch: Extended fasting depletes glucose stores, forcing the body to burn stored fat for energy (ketosis), leading to potential weight loss.
Nutrient Deficiencies: It is extremely challenging to get all necessary vitamins, minerals, and nutrients from a single meal, increasing the risk of deficiencies.
Hormonal Impact: Extended fasting can disrupt hormonal balance, especially in women, potentially causing menstrual irregularities and increased stress.
Potential for Muscle Loss: Extreme calorie restriction may lead to the loss of lean muscle mass, which can negatively affect overall metabolism.
Caution for Specific Groups: The OMAD diet is not recommended for pregnant or breastfeeding individuals, children, teens, or those with a history of eating disorders or diabetes.
Requires Strategic Meal Planning: If attempting OMAD, the single meal must be exceptionally nutrient-dense and well-balanced to counteract the risk of deficiencies.
Unsustainability: For many, the strict and restrictive nature of OMAD makes it difficult to maintain long-term, often leading to potential binge eating or weight regain.

The Science Behind One Meal A Day

Eating one meal a day, a practice known as OMAD, is an extreme form of intermittent fasting that compresses all daily calorie intake into a short, typically one-hour, eating window. This creates a 23-hour fasting period, which triggers several significant metabolic and physiological changes in the body. During a normal eating pattern, your body primarily uses glucose from food for energy. However, after an extended fast of 12-17 hours, your body's glycogen stores are depleted, forcing a metabolic switch to using stored fat for fuel, a process called ketosis.

Metabolic Changes

One of the most notable changes is the body's increased metabolic flexibility, its ability to switch between fuel sources efficiently. Research has shown that a single evening meal can increase fat oxidation during exercise and lead to a reduction in total body and fat mass, without impairing physical performance. This metabolic adaptation is a key reason for the reported weight loss benefits of OMAD. However, the extreme caloric restriction can also have negative metabolic impacts, including a potential long-term slowdown of the metabolic rate.

Potential Benefits

Weight Loss: By significantly reducing the eating window, most people naturally consume fewer calories, leading to a calorie deficit and subsequent weight loss.
Autophagy: Extended fasting periods can trigger autophagy, a cellular process where the body cleans out old, damaged cells and replaces them with new, healthy ones.
Improved Insulin Sensitivity: OMAD may lead to lower blood sugar levels and improved insulin sensitivity, which is beneficial for managing and preventing type 2 diabetes.
Enhanced Focus: Many people report improved mental clarity and focus during the fasting period, attributing it to a more stable energy supply without glucose spikes and crashes.
Gut Health: Giving the digestive system a prolonged break can improve gut health and reduce inflammation.

Significant Risks and Side Effects

Despite the potential benefits, OMAD is not suitable for everyone and carries significant risks, especially over the long term.

Nutritional Deficiencies: It is extremely difficult to consume a day's worth of essential vitamins, minerals, and other nutrients in a single meal, which can lead to deficiencies. Common deficiencies include B12, iron, and vitamin D.
Hormonal Imbalances: For women, particularly those of reproductive age, extended fasting can disrupt hormones related to stress, metabolism, and menstruation. This can result in irregular periods, fatigue, and mood changes.
Fatigue and Low Energy: The initial adjustment period can cause low energy levels, dizziness, and irritability due to blood sugar fluctuations.
Increased LDL Cholesterol and Blood Pressure: Some studies have shown that individuals practicing OMAD may experience an increase in LDL ('bad') cholesterol and blood pressure.
Muscle Mass Loss: Extreme calorie restriction can lead to the loss of lean body mass, which can negatively affect metabolism and overall health.
Disordered Eating: For those with a history of eating disorders, the restrictive nature of OMAD can trigger unhealthy eating patterns like bingeing.

Safely Implementing One Meal A Day

If you and your healthcare provider decide that OMAD might be appropriate for a short period, a cautious approach is essential.

Gradual Transition: Do not jump straight into a 23-hour fast. Start with a less restrictive intermittent fasting method, like a 16:8 or 18:6 schedule, and gradually shorten the eating window.
Nutrient-Dense Meal Planning: Focus on a balanced meal rich in lean protein, complex carbohydrates, healthy fats, and a wide variety of vegetables and fruits. Aim for at least 1,200 to 1,400 calories to meet minimum nutrient needs, depending on your body size and activity level.
Prioritize Hydration: Drink plenty of water throughout the day, as hunger can often be mistaken for dehydration. Non-caloric beverages like black coffee or herbal tea are also allowed during the fasting period.
Listen to Your Body: Pay close attention to how you feel. If you experience persistent fatigue, dizziness, irritability, or other negative symptoms, it is a sign to stop or adjust the approach.
Consider Supplements: Because meeting all nutritional requirements in one meal is challenging, supplementation with a multivitamin or specific minerals might be necessary under medical guidance.

OMAD vs. Traditional Calorie Restriction


Feature	One Meal A Day (OMAD)	Traditional Calorie Restriction (TCR)
Mechanism	Restricts all daily calories to a single, narrow eating window (typically 1 hour).	Spreads reduced daily calorie intake across multiple meals throughout the day.
Metabolic Shift	Induces deeper ketosis and increases metabolic flexibility by extending fasting periods.	Relies primarily on reducing overall calorie intake without extended fasting.
Satiety	Can lead to intense hunger and cravings during the 23-hour fast.	More consistent energy and less intense hunger due to regular, smaller meals.
Nutrient Intake	High risk of nutrient deficiencies as it is challenging to fit all micronutrients into one meal.	Easier to achieve a balanced nutritional intake by spreading meals and nutrients throughout the day.
Sustainability	Often considered unsustainable and difficult to maintain long-term due to its restrictive nature.	Typically more sustainable for most people as it involves less extreme dietary changes.
Social Impact	Can be socially isolating as many activities revolve around meals.	Less disruptive to social activities and family routines involving food.

Conclusion

For those considering the One Meal A Day diet, it is vital to understand the significant physiological changes involved. While OMAD can lead to rapid weight loss and improved metabolic markers like insulin sensitivity in the short term, it also carries substantial risks, including nutrient deficiencies, hormonal disruptions, and a potential loss of muscle mass. For most people, a less extreme form of intermittent fasting, like the 16:8 method, or traditional calorie restriction may be a safer and more sustainable approach to achieving long-term health goals. The decision to pursue OMAD should always be made in consultation with a healthcare provider, and careful, nutrient-dense meal planning is crucial to mitigate its potential dangers.

This article provides general information and is not a substitute for professional medical advice. Always consult with a healthcare provider before making significant dietary changes.

Frequently Asked Questions

No, OMAD is not safe for everyone and is not recommended for certain groups, including pregnant or breastfeeding individuals, children, teenagers, older adults, and people with a history of eating disorders or diabetes.

Yes, it is possible to lose weight on OMAD because it often leads to an overall reduction in calorie intake. However, this method is not necessarily more effective than traditional calorie restriction and may come with significant health risks.

Your single meal should be nutrient-dense and balanced, including lean protein (like chicken or fish), complex carbohydrates (quinoa or sweet potatoes), healthy fats (avocado or nuts), and plenty of fruits and vegetables to maximize nutrient intake.

Common side effects include fatigue, dizziness, irritability, headaches, and increased hunger, especially during the initial adjustment phase.

Yes, you can typically drink zero-calorie beverages like black coffee and herbal tea during the fasting period. However, adding sugar, milk, or cream will break the fast.

Some research suggests that long-term, extreme calorie restriction could potentially slow metabolism, although OMAD's impact on metabolism is complex. In the short term, the metabolic switch to burning fat can be efficient.

To minimize muscle loss, it is important to ensure your one meal contains sufficient protein and to continue engaging in moderate physical activity. However, building significant muscle mass may be challenging on this restrictive diet.