The goal of a “cutting” phase is to achieve a caloric deficit, forcing the body to reduce stored body fat while maintaining muscle mass. This requires a delicate balance between energy intake (diet) and energy expenditure (activity). Running is an effective tool for creating or deepening this energy deficit, but its success depends on how strategically it is combined with resistance training and a high-protein diet.
Running as a Tool for Energy Expenditure
Running contributes to fat loss by increasing energy expenditure, widening the gap between calories consumed and calories burned. This is achieved through two modalities: Low-Intensity Steady State (LISS) or High-Intensity Interval Training (HIIT). LISS involves maintaining a consistent, moderate pace (50 to 65 percent of maximum heart rate) for a longer duration, often 45 to 60 minutes. This lower intensity allows the body to rely more heavily on fat as the primary fuel source during the session.
In contrast, HIIT involves alternating short, near-maximal bursts of sprinting with brief periods of low-intensity recovery. Though sessions are shorter (10 to 30 minutes), they result in a high number of calories burned per minute. HIIT also stimulates the Excess Post-Exercise Oxygen Consumption (EPOC) effect, where the body’s metabolism remains elevated post-workout, continuing to burn calories during recovery.
Though HIIT burns more calories per minute and activates a greater EPOC, LISS allows for a higher total weekly volume with less recovery strain. Because LISS is less taxing on the central nervous system, it can be performed more frequently without negatively impacting subsequent resistance training sessions. Ultimately, both running styles are effective for fat loss, and the choice depends on an individual’s available time and recovery capacity.
Minimizing Muscle Loss During Cardio
The primary concern with integrating running into a cutting phase is the risk of muscle catabolism, which can be triggered by excessive endurance exercise in a caloric deficit. This risk is linked to the activation of the enzyme AMP-activated protein kinase (AMPK). Prolonged, intense endurance activities activate AMPK, signaling that cellular energy stores are depleted.
Activation of AMPK can inhibit the molecular signaling pathway known as mTOR (mammalian target of rapamycin), the main regulator of muscle protein synthesis and growth. When mTOR is inhibited, the signal to maintain muscle tissue is suppressed, increasing the likelihood of muscle loss. To mitigate this, manage the total volume and frequency of running sessions.
Ensuring adequate protein intake is a foundational strategy for muscle preservation during this time. A daily protein intake of 1.6 to 2.2 grams per kilogram of body weight is often recommended to support muscle retention. This high intake provides the necessary amino acids to fuel the body and directly counteract the potential catabolic signals from running. Strategic post-run fueling, particularly with protein and carbohydrates, helps to quickly replenish glycogen stores and switch the body from a catabolic state back toward recovery.
Structuring a Cutting Program
An effective cutting program places running in a supportive, rather than primary, role. Resistance training, focused on compound movements and heavy to moderate loads, provides the necessary stimulus to retain muscle mass despite the energy deficit. The hierarchy of importance for maintaining muscle while losing fat is:
- A controlled caloric deficit (diet).
- Consistent resistance training.
- Cardio as an accelerator.
For optimal results, running sessions should ideally be separated from resistance training sessions by at least six hours, particularly when performing intense lower-body work. Running can also be utilized on designated rest days from lifting to maintain the caloric deficit without interfering with muscle recovery from strength training.
Monitoring progress is important, and adjustments to running volume should be made based on recovery and strength levels. If strength begins to drop rapidly or recovery is severely inhibited, the volume or intensity of running should be reduced. The most sustainable approach for most individuals is a foundation of LISS running, which is less taxing, with a small, manageable dose of HIIT incorporated for efficiency.