Cutting calories feels like the obvious shortcut when you want to lose weight. Eat less, burn more, simple right? But when you eat too little for too long, your body doesn’t just melt away fat. It looks for other energy sources, and muscle is one of the things it will use. That matters, because muscle is not just about how you look, it helps your strength, mobility, long-term calorie burn, and overall health. 

Below is a guide to what actually happens to muscle when calories are too low, why that matters, and what to do so you lose fat instead of strength. 

The Body’s Energy Hierarchy and What it Burns First  

When you reduce calories, your body follows a predictable plan: 

1. Glycogen and water. Short term, the body uses stored carbohydrate (glycogen). Glycogen is bound to water, so you’ll often see quick weight loss at first.
2. Fat. As the deficit continues, stored fat becomes a main fuel.
3. Protein (muscle). If calorie restriction is severe or prolonged, and especially if protein intake and resistance exercise are low, the body will break down muscle tissue for essential amino acids and to support blood glucose.

That last step, muscle breakdown, is the one most people miss. It’s not inevitable, but it’s common unless steps are taken to prevent it. 

Why Losing Muscle is a Problem  

• Lower resting calorie burn. Muscle is metabolically active tissue. Losing lean mass reduces your resting metabolic rate, which means you burn fewer calories at rest. That makes future weight loss harder and increases the chance of regaining weight.

• Weaker and more injury-prone. Less muscle equals less strength and resilience, daily tasks feel harder and injury risk can rise.

• Metabolic and hormonal effects. Muscle helps regulate blood sugar and supports metabolic health. Losing it can worsen insulin sensitivity and energy regulation.

• Aging risk. Long-term loss of muscle contributes to sarcopenia (age-related muscle loss), which has serious health consequences.

Medical and nutrition authorities warn that overly aggressive calorie cutting often produces a mix of fat, water, and muscle loss, not just fat. The old “3,500 calories = 1 lb” rule is also an oversimplification, energy balance changes as you lose weight. 

How and When Your Body Starts Breaking Down Muscle  

Two factors make muscle loss more likely: 

• Calorie intake below basic needs. Everyone has a basal metabolic rate (BMR) — it’s the energy required to keep the heart beating, lungs working, and basic organs functioning. If your daily intake drops well below your BMR for prolonged periods, the body looks for alternate fuel sources, including muscle protein. Recommended “floor” calories vary by person, but many guides caution women not to go below 1,200 kcal/day and men not below 1,500–1,800 kcal/day without medical supervision.

• Insufficient protein and no strength stimulus. When protein is low and you aren’t doing resistance training, the body has little reason to keep muscle. Protein supplies the amino acids that protect muscle during calorie restriction, resistance training signals the body to preserve muscle. Meta-analyses show that combining resistance training with careful dietary strategies helps keep fat-free mass even during weight loss. 

How Much Muscle Loss Can Happen? 

There’s no single answer because it depends on the deficit size, how long it lasts, protein intake, training status, and genetics. But studies in controlled settings show that large, rapid deficits (very-low-calorie diets) cause greater losses of lean mass. Small-to-moderate deficits combined with higher protein and resistance training preserve lean mass far better. Athletes in intense calorie restriction may use protein intakes up to 1.6–2.4 g/kg/day to protect muscle, general guidance for people aiming to retain muscle while losing fat is commonly in the 1.4–2.0 g/kg/day range depending on activity and goals.  

Research on Intermittent Fasting and Muscle  

Intermittent fasting (IF) is popular and can work for some people’s weight goals. Research suggests that IF combined with resistance training can reduce body fat while largely preserving lean mass, compared with non-IF approaches provided protein intake and training are adequate. However, certain IF protocols (very low-calorie “fast” days or very short eating windows) can increase the risk of muscle loss if protein and training are neglected. 

Practical Rules to Keep Muscle While Losing Fat  

If your aim is fat loss without losing muscle, apply these evidence-backed steps: 

1. Aim for a modest calorie deficit 

Large, rapid deficits raise the risk of muscle loss. A moderate deficit often around 200–500 kcal/day from maintenance, gives steady fat loss and better muscle preservation. Also remember the calorie-loss rate slows as you lose weight; simple rules like “500 kcal = 1 lb/week” are only rough starting points. 

2. Prioritisedaily protein 

Make protein a priority across the day. A practical target for most people trying to keep muscle while losing fat is about 1.4–2.0 g/kg body weight per day, spread across meals. Athletes or those in steep deficits may push toward the higher end. High-quality protein (eggs, dairy, lean meats, fish, soy, legumes) supports muscle repair and maintenance. 

3. Lift weights consistently

Resistance training is the strongest signal to the body to keep muscle. Aim for at least 2–4 sessions per week of progressive resistance work (compound moves, progressive overload). Even modest lifting prevents muscle loss much better than dieting alone. 

4. Don’tignore carbs around workouts 

Carbohydrates help fuel hard sessions and allow you to maintain training intensity. Low energy plus low carbs makes it hard to lift heavy, and that reduces the muscle-preserving stimulus. Time carbs around workouts when possible. 

5. Keep recovery and sleep non-negotiable

Poor sleep increases stress hormones and makes recovery worse. Aim for consistent, restful sleep and manage chronic stress so cortisol and recovery stay balanced. 

6. Monitor progress sensibly

Track strength (are reps and loads holding up?), energy levels, and how clothes fit more than daily scale fluctuations. If strength or energy drops sharply, reassess the deficit, protein, and training load. 

Precautions, If You Must Lose Weight Faster  

Some people choose a rapid approach for short periods (example, to meet an event). If you do, accept that risk of muscle loss rises and take extra precautions: 

• Keep protein on the high side (near 2.0 g/kg).

• Preserve resistance training volume and intensity as much as possible.

• Consider slight reductions in cardio if it competes with recovery from strength work.

• Work with a coach or clinician if calories fall near or below BMR for extended periods. 

Key Takeaways  

Cutting calories can reduce weight, but an excessively low intake or very aggressive dieting puts you at risk of losing muscle. Muscle matters for strength, aging, daily function, and long-term metabolism. The best approach to fat loss that protects muscle is a moderate calorie deficit, enough protein, and regular resistance training, along with attention to sleep and recovery.