What Happens When You Only Eat Lunch?

November 28, 2025 •

4 min read

According to a 2023 study, eating just one meal a day is associated with an increased risk of all-cause and cardiovascular mortality. This practice, often linked with the One Meal A Day (OMAD) approach, can have significant and complex effects on your body, both positive and negative, which is why understanding what happens when you only eat lunch is crucial before adopting such a restrictive eating pattern.

Quick Summary

Consuming only one meal per day, such as just lunch, can lead to weight loss and improved insulin sensitivity in some individuals. However, this restrictive eating pattern also carries substantial risks, including nutritional deficiencies, digestive problems, fluctuating energy levels, and potential metabolic slowdown. The safety and long-term effects of this practice vary greatly among individuals.

Key Points

Nutritional Deficiency: Eating only one meal, even a large one, makes it difficult to consume all essential vitamins and minerals, potentially leading to weakened bones, poor immunity, and chronic fatigue.
Metabolic Impact: The body may slow down its metabolism as a survival mechanism in response to prolonged fasting, potentially leading to rebound weight gain.
Blood Sugar Fluctuations: Extended fasting can cause significant spikes and dips in blood sugar, which can lead to low energy, irritability, and may increase the risk of type 2 diabetes.
Cardiovascular Risk: Some research links one-meal-a-day diets to increased LDL cholesterol and higher blood pressure, raising concerns about long-term heart health.
Psychological Strain: Severe hunger and restriction can lead to increased irritability, difficulty concentrating, and a higher risk of developing disordered eating behaviors like binge eating.
Not for Everyone: This eating pattern is not suitable for everyone, including pregnant or breastfeeding women, children, older adults, and those with a history of eating disorders or diabetes.

The Physiological Response to Only Eating Lunch

When you restrict your caloric intake to a single meal, your body enters a prolonged fasting state for the remaining 23 hours of the day. Your body's primary fuel source is glucose from food. In its absence, it shifts to burning stored fat for energy, a metabolic state known as ketosis. This can initially lead to weight loss, particularly a reduction in body fat, and has been associated with enhanced mental clarity for some. However, the body's response is not solely fat-burning. Hormonal changes, such as reduced leptin and increased ghrelin (the hunger hormone), also occur, which can intensify hunger signals and influence food choices.

Significant Health Risks of Only Eating Lunch

Despite some initial weight loss, relying on a single meal, especially lunch, carries numerous health risks that often outweigh the benefits.

Nutritional Deficiencies

One of the most significant dangers is the high probability of developing nutritional deficiencies. Consuming a single meal makes it extremely challenging to ingest all the necessary macronutrients (proteins, fats, and carbohydrates) and micronutrients (vitamins and minerals) required for optimal bodily function. This can lead to:

Weakened immunity: A lack of essential vitamins like C and D can compromise your immune system.
Poor bone health: Deficiencies in calcium and vitamin D can weaken bones over time.
Hair loss and fatigue: Insufficient iron and B vitamins can lead to symptoms like hair loss, fatigue, and low energy.

Metabolic Slowdown

Contrary to boosting metabolism, prolonged and extreme caloric restriction can cause the body to enter "survival mode." In an effort to conserve energy, the body may slow down your resting metabolic rate (RMR), making it harder to lose weight in the long run. This metabolic adaptation can lead to rebound weight gain once normal eating patterns resume. Research has also shown inconsistent results regarding the long-term effects of OMAD on metabolism, with some studies suggesting potential negative impacts.

Increased Risk of Cardiovascular Issues

Several studies have linked restrictive eating patterns and skipping meals with adverse cardiovascular outcomes. Skipping breakfast, for example, has been associated with a higher risk of heart disease and increased mortality. One study also found that subjects on an OMAD diet experienced increased levels of total and LDL (“bad”) cholesterol and higher blood pressure compared to those with traditional eating patterns.

Psychological and Social Impact

The demanding nature of a one-meal-a-day diet can have significant psychological and social consequences. The intense hunger and preoccupation with food can lead to increased irritability, anxiety, and difficulty concentrating. For some individuals, this restrictive pattern can also trigger disordered eating behaviors, such as binge eating, as a compensatory response to prolonged food deprivation. Furthermore, since many social interactions revolve around food, adhering to an OMAD schedule can lead to social isolation and complicate relationships.

Comparison: Only Eating Lunch vs. Balanced Eating


Feature	Only Eating Lunch (OMAD)	Balanced, Multi-Meal Eating
Nutrient Intake	Risk of significant deficiencies due to difficulty packing all required nutrients into one meal.	Easier to meet daily nutritional needs across varied meals and snacks.
Energy Levels	Prone to fluctuations; high after eating, but low energy, fatigue, and weakness during long fasting periods.	More stable and consistent energy throughout the day by providing a regular supply of glucose.
Metabolic Health	Potential for metabolic slowdown as the body adapts to conserve energy.	Supports a healthy metabolic rate and steady blood sugar regulation.
Weight Management	Can cause rapid initial weight loss, but may lead to metabolic adaptation and rebound weight gain.	Promotes sustainable weight loss by focusing on a steady caloric deficit and balanced nutrition.
Cardiovascular Risk	Associated with increased LDL cholesterol and higher blood pressure in some studies.	Generally supports better cardiovascular health markers.
Social Impact	Can complicate social life and lead to feelings of isolation due to rigid eating schedule.	Allows for more flexible social engagement around meals.

Conclusion

While the prospect of eating only lunch might seem like a simple solution for weight loss or a busy schedule, the practice comes with serious trade-offs. The potential for rapid weight loss and some metabolic benefits is overshadowed by significant risks, including severe nutritional deficiencies, metabolic slowdown, cardiovascular strain, and negative psychological effects. For most people, a more balanced, less restrictive eating pattern is safer and more sustainable for long-term health. Before adopting such an extreme dietary change, it is highly recommended to consult with a healthcare professional or a registered dietitian. The safest and most effective approaches to health involve consistency and balance, not deprivation. For more details on the metabolic effects of intermittent fasting, a study published in Frontiers in Physiology provides further insight into time-restricted feeding.

Frequently Asked Questions

While eating one meal a day (OMAD) can lead to short-term weight loss by creating a calorie deficit, it is often not a sustainable long-term solution. It can also lead to metabolic slowdown, meaning your body burns fewer calories, potentially causing rebound weight gain.

Common side effects include intense hunger, fatigue, irritability, difficulty concentrating, headaches, dizziness, and constipation. Many of these are related to significant fluctuations in blood sugar levels.

It is extremely challenging to consume all the necessary vitamins, minerals, and macronutrients in a single meal. This puts you at a high risk for developing nutritional deficiencies over time, which can impact your bone health, immune system, and energy levels.

Research suggests that eating later in the day may be associated with less weight loss success and higher cardiometabolic risk factors. While the best time to eat is individual, skipping breakfast can cause higher blood sugar spikes after lunch, and late eating is generally discouraged.

When you fast until lunch, your blood sugar and insulin levels drop significantly. After your large lunch, you can experience a significant spike in blood sugar. Over time, this constant fluctuation can impair insulin response and may increase the risk of type 2 diabetes.

While the OMAD diet is not classified as an eating disorder, its restrictive nature can increase the risk of developing disordered eating patterns, including binge eating, for some individuals.

Individuals with diabetes or hypoglycemia, pregnant or breastfeeding women, older adults, children and teens, and people with a history of eating disorders should avoid such a restrictive diet. It is crucial to consult a healthcare professional before making any drastic dietary changes.