Does eating unhealthy make you lose muscle? The hidden costs of a poor diet

October 15, 2025 •

4 min read

According to a study comparing dieting methods, those on a very low-calorie diet experienced significantly more muscle mass loss than those with a more moderate approach. This highlights a crucial point: an unhealthy diet, particularly one lacking in key nutrients, absolutely contributes to muscle loss, especially when combined with a calorie deficit.

Quick Summary

A poor diet, marked by insufficient protein and excessive calories from low-nutrient foods, can trigger muscle breakdown and hinder muscle growth. When combined with a calorie deficit or lack of exercise, the body may catabolize muscle tissue for energy, negatively impacting metabolism and overall body composition.

Key Points

Inadequate Protein Intake: An unhealthy diet often lacks sufficient protein, the crucial building block for muscle repair and growth, leading to muscle breakdown, especially during calorie restriction.
Excessive Calorie Deficits: Extremely low-calorie diets, often associated with unhealthy eating patterns, force the body into conservation mode, where it will break down muscle tissue for energy.
Poor Exercise Fuel: Junk food provides short-lived energy from simple sugars, leading to fatigue and poor performance during workouts, which hinders muscle-building progress.
Impaired Recovery: An unhealthy diet lacks the necessary nutrients, like protein and complex carbs, needed to properly fuel and repair muscles after a workout, slowing recovery and growth.
Long-Term Metabolic Decline: Loss of metabolically active muscle tissue due to poor nutrition can slow metabolism, making it easier to gain fat and harder to keep weight off in the long run.
Increased Fat Gain: When a diet of unhealthy food is consumed in a calorie surplus, the body is more likely to store the excess calories as fat rather than efficiently using them to build lean muscle.
Negative Health Consequences: Chronic muscle loss from an unhealthy diet can contribute to reduced bone density, an increased risk of osteoporosis, and negative impacts on mood and mental health.

A common misconception, particularly for those on a weight loss journey, is that any reduction in weight is a success. However, an uncomfortable truth is that not all weight loss is good weight loss, as a significant portion can come from muscle mass rather than fat. While eating unhealthy foods in a calorie surplus may lead to fat gain, eating unhealthily in a calorie deficit can directly contribute to losing muscle. Understanding the mechanics of how a poor diet affects your body is key to protecting your muscle health and achieving your fitness goals.

The Role of Protein and Calories

Protein is the foundational building block for muscle tissue. A diet rich in high-quality protein is essential for repairing the muscle fibers broken down during exercise and for stimulating the growth of new, stronger tissue. When your diet is low in protein, especially during weight loss, your body lacks the necessary materials for this repair process. Conversely, filling up on low-nutrient, high-calorie junk food means you're consuming empty calories that don't provide the protein needed to support muscle health.

Furthermore, an excessive calorie deficit forces the body into a state of conservation. In this mode, the body prioritizes survival over muscle maintenance. It burns through readily available energy stores, like glycogen, and then turns to muscle tissue for fuel through a process called gluconeogenesis. A diet of unhealthful foods makes this process more likely because it often fails to provide the balanced nutrition needed to fuel the body effectively without sacrificing muscle.

The Impact of Junk Food on Performance and Recovery

Consuming ultra-processed foods, high in sugar and trans fats, can have a detrimental effect on your energy levels and exercise performance. While a sugary snack might offer a temporary energy boost, it lacks the sustained fuel from complex carbohydrates and healthy fats needed for long, high-intensity workouts. This can lead to premature fatigue, reduced motivation, and less effective training sessions.

Recovery is when muscles rebuild and grow stronger, and proper nutrition is paramount during this time. A post-workout meal with a balance of protein and carbohydrates is essential to replenish glycogen stores and kick-start muscle repair. A diet of junk food deprives your body of these crucial recovery nutrients, leading to slower recovery, increased fatigue, and hindering your ability to build and retain muscle mass.

Unhealthy Diet vs. Healthy Diet: A Comparison


Aspect	Unhealthy Diet	Healthy Diet
Calorie Source	Primarily empty calories from added sugars and unhealthy fats.	Nutrient-dense calories from whole foods.
Protein Intake	Often low or inadequate for muscle repair and growth.	Prioritizes sufficient high-quality protein for muscle building and maintenance.
Energy Levels	Provides short-lived energy spikes followed by crashes, leading to fatigue.	Offers sustained energy throughout the day, supporting consistent workouts.
Macronutrient Balance	Imbalanced, with excessive unhealthy fats and simple carbs, and insufficient healthy fats and fiber.	Balanced, with appropriate ratios of protein, complex carbs, and healthy fats.
Metabolic Health	Impairs metabolism, promoting fat storage and increasing the risk of metabolic syndrome.	Optimizes metabolic function, which helps with weight management and muscle retention.
Inflammation	Increases systemic inflammation due to processed ingredients.	Reduces inflammation with antioxidant-rich whole foods.

How to Prevent Muscle Loss with a Healthy Diet

To prevent muscle loss, particularly during a weight loss phase, it's crucial to shift your focus from simply eating fewer calories to consuming higher quality, nutrient-dense foods. Here are some practical steps:

Prioritize protein: Ensure you consume enough protein throughout the day, spreading intake evenly across meals to support muscle protein synthesis. Focus on lean meats, fish, eggs, dairy, and plant-based sources like legumes.
Maintain a moderate calorie deficit: Avoid extreme calorie restriction, which signals your body to burn muscle for energy. A moderate deficit, combined with proper macronutrient ratios, promotes fat loss while preserving muscle.
Include complex carbohydrates: Don't eliminate carbs entirely. Healthy, complex carbs from sources like brown rice, oats, and sweet potatoes are vital for fueling workouts and replenishing glycogen stores for recovery.
Incorporate resistance training: Strength training is the most effective way to signal your body to retain and build muscle, even in a calorie deficit. Aim for 2-4 sessions per week.
Refuel after exercise: A meal or snack containing both protein and carbohydrates after a workout helps optimize recovery and muscle repair.

The Long-Term Consequences

Beyond immediate physical performance, chronic unhealthy eating that leads to muscle loss has significant long-term consequences. Reduced muscle mass slows your metabolism, making it harder to maintain a healthy weight and potentially leading to weight regain. It also impacts bone density, as stronger muscles place stress on bones, strengthening them. Loss of muscle can increase the risk of osteoporosis and frailty in later life. Lastly, there is evidence linking low muscle mass with a negative impact on mental health, including mood and overall wellbeing.

Conclusion

In short, the answer to "does eating unhealthy make you lose muscle?" is a resounding yes, especially when combined with a calorie deficit and lack of physical activity. An unhealthy diet starves your body of the essential building blocks for muscle repair and provides a poor fuel source for exercise. This, in turn, can trigger the breakdown of your own muscle tissue for energy, leading to a host of negative health outcomes. By focusing on a balanced, nutrient-dense diet rich in protein, combined with regular resistance training, you can protect your muscle mass, boost your metabolism, and build a stronger, healthier body for the long run. The key is to prioritize quality nutrition and smart, sustainable lifestyle choices, not just a number on the scale.

Frequently Asked Questions

While it's possible to gain weight by eating junk food in a calorie surplus, this typically results in fat gain rather than lean muscle growth. For optimal muscle building, a diet rich in high-quality protein and nutrient-dense foods is essential.

Not necessarily. While some muscle loss can occur during weight loss, it can be minimized by maintaining a moderate calorie deficit, consuming enough protein, and incorporating resistance training into your routine.

Signs of muscle loss include a rapid drop in weight (more than 1 kg per week), feeling more tired or sluggish during exercise, and mood changes like stress or depression. Monitoring your strength and body composition, rather than just the number on the scale, can provide a more accurate picture.

For active individuals or those aiming to build muscle, protein needs are higher than for sedentary people. Recommendations can range from 1.1-1.7 grams per kilogram of body weight per day. Spreading this intake evenly throughout the day is also beneficial.

A 'clean bulk' involves eating in a calorie surplus with a focus on nutrient-dense, whole foods to maximize lean muscle gain. A 'dirty bulk' involves eating excess calories from junk food, which typically results in significant fat gain alongside some muscle.

Yes, chronic unhealthy eating that contributes to muscle loss can also lead to bone weakening. Strong muscles stimulate bones to become denser, and without this, bone density can decrease over time, increasing the risk of conditions like osteoporosis.

An occasional, moderate indulgence won't derail long-term progress, as consistency is what matters most. The key is moderation and ensuring the majority of your diet consists of nutrient-dense whole foods.