By: Sara Zipf

Balancing weight loss goals with marathon training can be a significant challenge because the two goals partially conflict. Marathon training emphasizes fuel availability and adaptation, whereas weight loss promotes energy conservation. That said, excess weight can interfere with performance. One study published in the Journal of Sports Medicine found that a high body fat percentage can slow race times. Within lower-body fat ranges, in contrast, speed and weekly distance are more important than body fat. If you wish to shed excess pounds, there are specific strategies you can embrace to lose fat while maintaining muscular strength and stamina.

Weight Loss and Body Composition

The key diet and lifestyle approaches to weight loss are calorie restriction and exercise, both of which can have adverse effects on body composition, metabolic health, and quality and quantity of life. However, there are vital differences between weight and fat loss. In an ideal world, weight loss would come exclusively from fat mass, as fat mass is the main driver of metabolic disease. However, several studies have shown that when people lose weight, between 20% and 40% of the weight loss is due to muscle loss. In some cases—for instance, when people consume very restrictive diets or are sedentary—muscle loss can exceed 50%. Muscle reduction can occur with short-term energy restriction, long-term weight loss regimens, and even moderate caloric deficits. Researchers have also found that weight loss through nutrition alone is inherently inefficient if the goal is to lose fat exclusively.

Why Muscle Loss Can Interfere with Your Training Goals

A significant reduction in muscle mass can affect training goals by reducing strength, lowering insulin sensitivity, and decreasing resting metabolic rate. Restrictive diets can also increase one’s risk of sarcopenia (muscle loss), particularly in older adults. The loss of muscle tissue can also lead to muscle twitching and cramps, which every runner seeks to avoid. It is therefore vital to select appropriate workouts and to prioritize protein-rich nutrition. This is especially true for athletes completing long weekly mileages while aiming to lose fat.

Aerobic and Strength Training

Aerobic exercise is essential for achieving fat loss goals, as aerobic workouts increase total energy expenditure, enhance fat oxidation, and contribute to overall weight loss. However, this type of exercise does not provide sufficient mechanical stimulus to maintain muscle and does not strongly activate anabolic signaling pathways (such as mTOR) that regulate muscle protein synthesis and support muscle growth and maintenance. Resistance training is the most effective means of preserving muscle mass during weight loss, as it increases muscle protein synthesis, activates key pathways, enhances neuromuscular efficiency, and prevents muscle breakdown when caloric intake is low. For best outcomes, however, it is also vital to focus on dietary intake.

Standard Protein Recommendations May Fall Short

Typically, the recommended dietary intake of protein is approximately 0.8 grams per kilogram per day. However, this amount may be insufficient to stimulate muscle protein synthesis during a weight loss diet. It is recommended that those training for serious running events, such as marathons, consult a clinical dietitian for personalized protein intake recommendations. Overall, most marathon runners aiming to reduce fat while training should target approximately 1.6-2.2 grams of protein per kilogram of body weight per day, although individual variation is common and personalization is essential. If protein intake is inadequate, the body compensates by losing lean mass. Overall, runners can aim for approximately four meals per day, with 25 to 40 grams of protein per meal. They also need to ensure that each meal provides approximately 2 to 3 grams of leucine to activate mTOR signaling. Timing also matters; it is vital to consume approximately 25-40 grams of protein after runs and combine it with carbohydrates to reduce muscle wasting and improve glycogen restoration. Some athletes consume an additional 30 to 40 grams of slow-digesting protein to reduce the risk of overnight muscle breakdown.

Losing fat while training for a marathon is a challenging feat. Typically, weight loss involves a significant loss of both muscle and fat. To help maintain muscle, it is vital to prioritize strength training and a high-protein diet. Because the precise protein requirement for each athlete varies with activity level and other factors, it is advisable to consult a clinical dietitian or sports nutritionist.