Why Do You Have to Eat More When Working Out?

December 21, 2025 •

4 min read

According to research from the Mayo Clinic, a person weighing 160 pounds can burn over 600 calories during just one hour of running. This significant energy drain is a key reason why you have to eat more when working out, providing the necessary fuel and nutrients to support your body's increased demands.

Quick Summary

Increased physical activity from working out requires a higher caloric intake to replenish energy stores, repair muscle tissue, and support metabolic processes. Fueling adequately prevents low energy availability, enhances performance, and aids in efficient recovery. Macronutrients like carbohydrates and protein are especially crucial for active individuals.

Key Points

Energy Demands: Exercise, especially intense or prolonged activity, significantly increases your body's total daily energy expenditure, requiring more calories to maintain balance.
Glycogen Replenishment: Workouts deplete muscle glycogen stores, and consuming carbohydrates post-exercise is essential for refilling these energy reserves for future performance.
Muscle Repair: Protein provides the amino acids needed to repair the microscopic muscle tears that occur during strength training, which is the foundation of muscle growth.
Hormonal Response: Exercise impacts appetite-regulating hormones like ghrelin and peptide YY, leading to increased hunger signals as your body demands refueling.
Preventing Under-fueling: Eating enough prevents low energy availability (LEA), which can cause serious health issues like hormonal imbalances, reduced bone density, and increased injury risk.
Macronutrient Balance: A strategic balance of carbohydrates for energy, protein for muscle repair, and fats for sustained fuel and overall health is crucial for active individuals.
Optimized Recovery: Combining carbohydrates with protein after a workout can enhance both muscle glycogen resynthesis and muscle protein synthesis, improving overall recovery.

The Science of Energy: Why Your Body Needs More Fuel

When you engage in physical activity, your body's energy requirements increase dramatically beyond its normal resting state. This is not a simple matter of 'calories in versus calories out.' The type of exercise, its intensity, and its duration all play a role in determining your specific nutritional needs. The body is an incredible machine, and when subjected to the stress of exercise, it requires specific resources to perform optimally, repair damage, and adapt to become stronger. Insufficient fueling can lead to serious consequences, including performance decline, increased risk of injury, and even metabolic issues.

The Role of Macronutrients

Your body relies on macronutrients—carbohydrates, protein, and fats—to function. For active individuals, the balance and quantity of these macros are even more critical. Carbohydrates are your body's preferred and most readily available source of energy. They are stored in your muscles and liver as glycogen and are broken down to fuel your workouts, especially during intense exercise. Protein is the building block for muscle repair and growth, a vital process that happens after exercise. Fats provide a source of energy for lower-intensity, longer-duration workouts and are essential for hormone production and nutrient absorption.

Replenishing Glycogen Stores

During exercise, especially endurance training or high-intensity interval training (HIIT), your body rapidly depletes its stored glycogen. Glycogen is like the high-octane fuel for your muscles. After a workout, your body needs to replenish these stores to ensure you have enough energy for your next session. Consuming carbohydrates post-exercise is crucial for this process. For optimal and rapid replenishment, studies show that combining protein with carbohydrates can be particularly effective, especially when carbohydrate intake is not maximized.

Muscle Repair and Growth

Strength training and other forms of resistance exercise cause microscopic tears in your muscle fibers. This is a normal part of the process of building strength and muscle mass. To repair and rebuild these fibers, your body requires amino acids, which it gets from the protein you consume. By increasing your protein intake, you provide your body with the necessary materials to synthesize new muscle tissue, leading to a stronger, more resilient physique over time. This is not about just a single protein shake; it's about a consistent and adequate intake throughout the day.

Hormonal Changes and Appetite Regulation

Exercise affects the hunger hormones in your body, particularly ghrelin (which increases hunger) and peptide YY (which suppresses it). After a workout, your body cools down, and your metabolic rate increases, prompting a genuine need for energy replenishment. Sometimes, especially after lower-intensity workouts, this can manifest as an increased appetite. However, other times, intense exercise can temporarily suppress appetite before a surge of hunger hits later. Acknowledging these hormonal shifts is vital for managing your energy intake effectively.

The Dangers of Under-fueling

Consistently under-eating while training hard leads to a state known as low energy availability (LEA). The body is an expert at survival and, when faced with an energy deficit, it prioritizes immediate needs like exercise over 'non-essential' functions like reproductive health, bone density, and a strong immune system. The potential negative consequences of LEA include:

Decreased performance and strength
Increased risk of injury, such as stress fractures
Hormonal imbalances, including loss of menstruation in women (RED-S)
Reduced muscle protein synthesis
Increased susceptibility to infections
Impaired mood and cognitive function

Balancing Your Nutrition

Proper nutrition for working out involves a strategic approach to fueling. It requires consuming enough calories to meet your total daily energy expenditure (TDEE), which accounts for your basal metabolic rate plus the calories burned during activity. The key is not only quantity but also quality and timing. Prioritizing whole foods and spreading macronutrient intake throughout the day is more effective than cramming calories into a few large meals.

Macronutrient Importance: Active vs. Sedentary


Macronutrient	Role for Sedentary Individual	Role for Active Individual
Carbohydrates	Primary energy source for daily functions.	Primary fuel for workouts; replenishes muscle glycogen for future performance.
Protein	Maintains existing muscle and tissue.	Crucial for repairing and building new muscle fibers damaged during exercise.
Fats	Important for hormone regulation and vitamin absorption.	Sustained energy for lower-intensity exercise; supports cellular health and hormone function.

Conclusion

Understanding why you have to eat more when working out is foundational to achieving your fitness goals and maintaining long-term health. Exercise is a catalyst for physical change, but proper nutrition is the fuel that powers that transformation. By prioritizing adequate caloric intake, focusing on a balanced ratio of carbohydrates, protein, and healthy fats, and paying attention to meal timing, you can optimize your energy levels, maximize muscle repair, and enhance overall performance. For many, this means reframing the relationship with food from a simple calculation to a strategic part of a healthy, active lifestyle. For personalized guidance, consider consulting with a sports dietitian who can help tailor a nutritional plan to your specific needs and goals.

The Three R's of Post-Workout Nutrition

Refuel: Replenish depleted energy stores with carbohydrates after a workout. This process is most efficient within the first 30-60 minutes post-exercise.
Repair: Consume protein to provide the amino acids necessary for repairing and rebuilding muscle tissue damaged during training.
Rehydrate: Drink plenty of fluids and replenish electrolytes lost through sweat to prevent dehydration and muscle cramps.
Recovery Optimization: Combine carbohydrates and protein in your post-workout snack or meal to maximize both glycogen replenishment and muscle protein synthesis.
Consistency over Timing: While immediate post-workout fueling is beneficial, research suggests that consistent daily intake of sufficient macronutrients is most critical for muscle growth and repair over time.

Frequently Asked Questions

Yes, but it is not recommended. Consistently under-eating while training hard can lead to low energy availability (LEA), which can cause performance decline, muscle loss, and health problems like hormonal disruption and weakened bones.

The ideal post-workout meal or snack contains a combination of carbohydrates and protein. This helps to replenish muscle glycogen stores and provides the necessary amino acids for muscle repair and growth. Examples include a protein shake with a banana, Greek yogurt with berries, or chicken with brown rice.

Not necessarily. When you eat more to fuel increased activity, those calories are often used for energy and muscle repair, not stored as excess fat. The goal is to eat enough to support your body's increased needs, not to create a calorie surplus large enough to cause fat gain.

High-intensity exercise can temporarily suppress your appetite due to hormonal changes, such as a decrease in the hunger hormone ghrelin. This effect is usually temporary, and hunger signals will return shortly after your body begins to cool down.

For light to moderate exercise lasting under an hour, water is often sufficient for refueling. However, for longer, more strenuous workouts, or if you feel consistently hungry, a snack containing carbohydrates and protein is beneficial. Lightly active individuals will have a smaller increase in caloric needs compared to elite athletes.

Without sufficient protein, your body will have a difficult time repairing and rebuilding muscle tissue. This can lead to slower recovery times, limited muscle growth, and potentially muscle loss over time, as the body struggles to keep up with the demands of training.

No, carbohydrates are not the enemy. For active individuals, they are the body's primary energy source. The key is to consume the right types and amounts of carbohydrates, focusing on complex carbs from whole foods to provide sustained energy, rather than simple sugars from processed snacks.