Do You Lose Muscle or Fat First When Sick? A Nutritional Guide

October 9, 2025 •

4 min read

During a serious illness, some studies show individuals in intensive care can lose a significant percentage of their muscle mass in just over a week. This rapid depletion of lean tissue causes many to question, do you lose muscle or fat first when sick? The surprising metabolic response of your body prioritizes breaking down muscle, not fat, for vital functions.

Quick Summary

Illness alters metabolism, causing the body to prioritize glucose and protein over stored fat for energy. Inflammation and inactivity accelerate muscle protein breakdown. Mild illness may not cause significant muscle loss, but severe or chronic conditions can result in substantial muscle wasting.

Key Points

Muscle Before Fat: When ill, especially with reduced food intake, your body burns muscle (protein) more readily than fat for energy.
Catabolic State: Illness triggers a hypermetabolic and catabolic state, breaking down body tissues to fuel the energy-intensive immune system.
Severity Matters: Significant muscle loss is most likely during severe or prolonged illness, especially with prolonged bed rest and poor nutrition.
Cytokine Effect: Inflammatory signals, like cytokines, inhibit muscle-building processes while promoting muscle breakdown during sickness.
Nutritional Defense: A high-protein diet and sufficient calories are the best defense to spare muscle tissue and provide essential amino acids for healing.
Mobility Benefits: Light, gentle movement, when medically appropriate, can help reduce disuse atrophy and preserve muscle mass.
Recovery Strategy: Regaining muscle mass after illness requires time, proper nutrition, and a gradual return to physical activity.

Sickness, Metabolism, and the Fight for Fuel

When you're fighting an illness, your body is in a state of stress, which triggers a complex metabolic response to support the immune system. This includes fever, lethargy, and a loss of appetite, which are all part of a coordinated effort to optimize immune function. The body enters a hypermetabolic and catabolic state, meaning it increases its energy needs and begins to break down complex molecules to release energy. The immune system is incredibly energy-intensive and can consume up to 30% of the body's total energy during an infection.

The Body's Energy Hierarchy

So where does this energy come from? The body accesses its fuel sources in a specific order. This hierarchy explains the initial weight loss and the long-term risk of muscle wasting:

Carbohydrate Stores (Glycogen): The body first taps into its most accessible energy, stored carbohydrates (glycogen) in the liver and muscles. These stores are limited and can be depleted within 12 to 24 hours of low food intake. The initial weight loss seen in mild illness is often due to water loss and the use of this glycogen.
Protein Stores (Muscle): After glycogen is gone, the body turns to its next most accessible energy source: muscle protein. During illness, inflammatory cytokines and hormonal changes significantly increase muscle protein breakdown. The amino acids from this process are used by the liver to create glucose (gluconeogenesis) to feed the immune system and other vital organs. This happens because converting muscle to energy is more efficient and faster than converting fat.
Fat Stores: Fat tissue is the body's long-term energy reserve. However, breaking down fat for energy is a slower and more energy-intensive process than using protein. While fat is still used for fuel during illness, especially during prolonged starvation, muscle is often prioritized initially, particularly in severe conditions.

Acute vs. Chronic Illness: A Tale of Two Responses

The severity and duration of an illness are key factors in determining whether you lose muscle or fat first and how much. A mild, short-term illness is very different from a severe, long-term one.


Feature	Acute (Mild) Illness (e.g., common cold)	Chronic (Severe) Illness (e.g., intensive care)
Metabolic State	Mildly hypermetabolic, lasts a few days.	Severely hypermetabolic, persistent catabolism.
Energy Sources	Primarily uses glycogen stores; some fat and muscle are used but balance is minimal.	Significantly elevated muscle protein breakdown to fuel the immune response.
Fluid/Water Weight	Often the most noticeable weight loss due to decreased intake and fever.	Fluid shifts and imbalances are common, affecting overall weight readings.
Muscle Loss	Not usually significant. Muscle protein synthesis and breakdown remain relatively balanced.	Substantial, rapid muscle wasting due to prolonged inactivity, inflammation, and malnutrition.
Fat Loss	Minimal fat loss, often overshadowed by initial water and glycogen depletion.	Gradual fat depletion as a secondary energy source after muscle breakdown.
Recovery	Often swift with a return to normal weight and muscle mass.	Can be a long process requiring targeted nutrition and physical therapy.

Why Your Body Prioritizes Muscle Loss During Illness

The rapid loss of muscle is not a failure of the body but a survival mechanism. Several factors contribute to this process:

Inflammatory Cytokines: When the immune system is activated, it releases signaling proteins called cytokines, such as tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6). These cytokines block the pathways that build muscle and promote those that break it down.
Nutrient Deficiency: Illness often causes a loss of appetite, leading to inadequate intake of calories and protein. Without enough dietary protein, the body must break down its own muscle tissue to obtain the amino acids needed for immune function and to synthesize glucose.
Immobility: Bed rest, even for healthy individuals, leads to disuse atrophy where protein breakdown exceeds synthesis. This effect is amplified during severe illness, further accelerating muscle loss.

The Crucial Role of Nutrition in Preserving Muscle

Proper nutrition, even when you don't feel like eating, is the single most important defense against muscle loss. It helps your body maintain protein synthesis, providing the necessary building blocks for repair and immune function without cannibalizing your muscle tissue.

Strategies to Minimize Muscle Loss While Sick

Prioritize Protein: Aim for 1.2 to 2.2 grams of protein per kilogram of body weight per day during recovery, as recommended for athletes and those with illness. Choose easily digestible sources like lean poultry, eggs, fish, and Greek yogurt.
Stay Hydrated: Fever and diarrhea can lead to fluid loss. Proper hydration is essential for all cellular functions, including muscle repair. Electrolyte-rich fluids can also help replenish minerals lost during illness.
Eat Nutrient-Dense Foods: Focus on fruits, vegetables, and whole grains to get key vitamins and minerals. These support your immune system and overall energy levels.
Consider Supplements: For those with critical illness or poor appetite, supplements like protein powders (whey, casein, or plant-based) or a nutrient-rich oral liquid can help meet caloric and protein needs. The compound HMB may also help hinder muscle breakdown.
Gentle Movement (if tolerated): For mild illness, light activity like walking can help prevent disuse atrophy. For more severe cases, moving from bed to a chair or performing gentle mobility exercises can make a difference. Always listen to your body and consult a healthcare professional.

Conclusion

While the goal is to lose fat, when you are sick, your body's survival mechanisms prioritize the breakdown of muscle to fuel the immune system, particularly during severe or prolonged illness. Initial weight loss is often water and glycogen, but with persistent illness, muscle tissue is significantly at risk. A proactive nutrition diet focused on adequate protein and hydration, combined with light activity when appropriate, is the best strategy to minimize muscle wasting and support a quicker recovery. For those recovering from critical illness, rebuilding lost muscle is possible with professional guidance and a dedicated plan. For further reading on regaining strength after illness, consult resources like Harvard Health's article on the subject: "Don't let muscle mass go to waste".

Frequently Asked Questions

Yes, it is common to lose weight when you are sick. This is often due to a combination of reduced food intake, water loss from fever or sweating, and your body using energy reserves to fight the infection.

The rate of muscle loss depends on the severity and duration of the illness. In critically ill patients, significant muscle loss (up to 15-20%) can occur in as little as 7 to 10 days due to factors like inflammation, inactivity, and poor nutrition.

The body can convert muscle protein into glucose more quickly and efficiently than converting stored fat. This makes muscle a more readily available source of immediate energy to fuel the immune system and vital organs during the stress of illness.

Catabolism is the metabolic process of breaking down complex molecules, like muscle protein and fats, into simpler ones to release energy. During sickness, the body enters a catabolic state to meet the heightened energy demands of the immune system.

Focus on consuming adequate protein and maintaining caloric intake, even if your appetite is low. Prioritize easily digestible sources and stay well-hydrated. Engage in gentle movement if your condition allows, and rest sufficiently to recover.

For a mild, short-term illness like a cold, you will likely not experience significant muscle loss. The body primarily uses glycogen stores first, and a few days of rest typically won't result in major muscle atrophy, especially if you maintain proper nutrition.

Focus on protein-rich foods like chicken, eggs, fish, dairy, and legumes. Aim to spread your protein intake throughout the day. If solid food is difficult, consider protein shakes or smoothies to get the nutrients your body needs to fight infection and spare muscle.