Cutting calories is a widely used strategy for weight loss, but when combined with regular exercise, overly aggressive reductions in food intake may lead to unintended muscle loss and reduced performance, health experts warn.
A calorie deficit occurs when the body consumes fewer calories than it expends, forcing it to use stored energy for fuel. According to UK clinical guidance from NICE (CG189), a daily deficit of around 600 calories is commonly recommended as a starting point for weight management, typically resulting in a gradual weight loss of 0.5 to 1 kilogram per week.
However, experts note that adding consistent exercise to this equation significantly increases energy demands. When calorie intake is not adjusted to match activity levels, the resulting deficit can become too large, shifting the body from fat burning to breaking down lean muscle tissue for energy.
When fat loss begins to affect muscle
Health professionals explain that the body prioritises survival during prolonged energy shortages. In such conditions, it reduces non-essential functions, including muscle protein synthesis, particularly when energy intake is insufficient to support both exercise and recovery.
For individuals training several times per week, experts suggest a more moderate deficit of 250 to 400 calories per day may be more appropriate. This range can support gradual fat loss while still preserving energy for physical performance and muscle maintenance.
Protein intake is also a key factor in protecting muscle mass. Guidance from the British Dietetic Association recommends 1.2 to 2 grams of protein per kilogram of body weight daily for active individuals, with higher intakes often required for those engaged in resistance training. Spreading protein intake across meals is considered more effective for muscle preservation than consuming it in a single sitting.
Carbohydrates also play a critical role, particularly for those exercising at moderate to high intensity. Nutrition specialists caution that excessive carbohydrate restriction can impair workout performance, increase fatigue, and slow recovery. Strategic timing of carbohydrate intake around training sessions is often recommended instead of complete elimination.
Risks of excessive energy restriction
Sports medicine experts warn that prolonged and severe calorie restriction combined with high physical activity can lead to a condition known as Relative Energy Deficiency in Sport (RED-S). The condition is not limited to professional athletes and can affect anyone who consistently under-fuels their activity levels.
Symptoms may include ongoing fatigue, reduced performance, frequent illness, prolonged muscle soreness, difficulty concentrating, mood changes, and increased focus on food. In women, disrupted or absent menstrual cycles may also occur, while men may experience reduced libido and hormonal changes. In severe cases, bone stress injuries such as stress fractures can develop.
Medical attention is advised for anyone experiencing severe symptoms such as chest pain, dizziness, palpitations, or collapse during exercise.
What safe weight loss approaches involve
Public health guidance recommends combining dietary changes with at least 150 minutes of moderate-intensity physical activity per week, along with muscle-strengthening exercises on two or more days.
Very low-calorie diets (below 800 calories per day) are not recommended outside of medical supervision due to the risk of nutrient deficiencies, muscle loss, and metabolic disruption. Similarly, large unsupervised deficits exceeding 1,000 calories per day can negatively affect long-term weight management and overall health.
Adequate sleep is also considered essential for recovery and metabolic health, with seven to nine hours per night recommended to support muscle repair and hormonal balance.
Certain groups, including individuals with diabetes on insulin or sulfonylurea medication, those with a history of eating disorders, pregnant or breastfeeding individuals, and those under 18 or with kidney disease, are advised to seek professional guidance before attempting calorie restriction.
Health professionals recommend consulting a registered dietitian or GP to ensure that weight loss strategies are both effective and safe, and do not compromise muscle health or overall wellbeing.
