Why Are Runners So Skinny? Top Reasons

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Why Are Runners So Skinny

Why are Runners So Skinny? | Muscles worked | Benefits | Sprinters vs long-distance runners | Marathon training without weight loss

Many people are intrigued by the fact that runners tend to have slim and lean physique. This has led to a common question: “Why are runners so skinny?”

The slim bodies of runners can be attributed to various factors, but primarily, their training helps them develop lean muscle mass. These muscles can effectively burn calories at a higher rate leading to a very low body-fat ratio. 

In this article, we will delve into the top reasons behind runners’ slimness, shedding light on the interplay between running, metabolism, calorie expenditure, and body composition.

Why are Runners So Skinny?

Several factors can explain why runners, especially most marathon runners and elite long-distance runners, often have slender physiques.

Let’s analyze the top factors that contribute to this phenomenon:

1. They exercise often, and their workouts are long.

Most runners, especially elite marathon runners, dedicate hours to training and engage in long-distance running. This high level of physical activity results in significant energy expenditure and can lead to a reduction in body fat.

The demanding nature of running can also cause protein breakdown, contributing to skinny muscles and a leaner physique.

Specifically speaking, elite runners train for at least about 2 hours per day for 5-6 days per week. The body’s glycogen stores are depleted in the first few minutes, with fat burning taking over after about 90 minutes. 

This means that the body relies more on fat oxidation for energy expenditure during longer runs.No wonder, then, that marathon runners and other distance runners have such low body fat percentages and lean body mass.

2. Their workout has no strength training

Strength training exercises, such as weightlifting, promote muscle growth and bulk. Since runners focus on aerobic training, they tend to have less muscle mass compared to individuals who incorporate strength training into their routines.

3. Energy Expenditure in Running

Running is a highly calorie-intensive activity, burning a significant amount of calories per minute compared to other exercises. 

Running is an aerobic exercise that primarily relies on fat metabolism as a fuel source. The high caloric demands of running, combined with its ability to promote fat burning, can lead to weight loss and a lower body fat percentage.

According to the American Council on Exercise [1]American Council on Exercise: Calorie Burners: Activities That Turn Up the Heat, an individual weighing 160 pounds can burn approximately 15.1 calories per minute while running. At this rate, running tops the list in terms of calorie burn. 

In the context of a marathon, the calorie expenditure varies based on pace and body weight, with elite runners burning an estimated 1,854 calories and beginners burning around 4,934 calories during the race. 

When considering the average daily calorie intake and expenditure, it becomes apparent that runners require a higher caloric intake to support their training and basic energy needs.

Table 1: Caloric Expenditure in Different Activities

ActivityCalories/minute120 lb140 lb160 lb180 lb
Cycling (10 MPH)
Skiing (cross country)7.58.81011.3
Swimming (moderate pace)7.8910.311.6

Estimated Caloric Expenditure in a Marathon based on Pace and Body Weight

Pace (Mins/mile)120 lb140 lb160 lb180 lb

Table 2: Estimated Caloric Expenditure in a Marathon based on Pace and Body Weight

Pace (Mins/mile)120 lb140 lb160 lb180 lb

4. Muscle efficiency

Running can help develop lean muscle mass. It doesn’t typically contribute to significant muscle hypertrophy or an increase in bulk. 

The emphasis on slow twitch muscle fibers(more endurance-oriented), in long-distance running may result in a leaner physique compared to activities that target fast twitch muscle fibers associated with explosive power.

5. Calorie intake

Marathon runners expend a substantial amount of energy during their runs, depleting their glycogen stores and utilizing carbohydrates and fats as fuel sources.

Carbohydrate depletion peaks within the first 90 minutes of long-distance running. This can contribute to weight loss. Additionally, the “afterburn” effect can elevate metabolism and calorie expenditure for up to 24 hours post-run.

Other Factors Affecting Runners’ Body Composition

The other factors that may contribute to the lean-to the skinny physique of runners are: 

  • Nutritional considerations for runners.
  • Training strategies that contribute to weight management.
  • Psychological factors and their impact on body composition.
  • Genetic factors.

What muscles does running work

Running is a dynamic activity that requires the activation of several muscle groups. The primary muscles involved in running are predominately lower body muscles

The upper body is also engaged during running but to a lesser degree. Core muscles are activated to maintain balance and posture, while the arms and shoulders assist in generating momentum and driving the body forward. 

Here’s a list of the muscles worked while running:

  • Hip flexors
  • Glutes
  • Quadriceps
  • Hamstrings
  • Calf muscles
  • Core muscles
  • Upper-body muscles

Benefits of Long Distance Running as an Exercise

Long-distance running provides a multitude of benefits, not only for physical fitness but also for mental well-being. Let’s explore the scientific link between running and the following advantages:

1. Reduces belly fat

Long-distance, endurance running can burn belly fat, otherwise known as abdominal fat or visceral fat.

One study [2]National Library of Medicine: Effect of exercise training intensity on abdominal visceral fat and body composition found that doing endurance training with low to moderate intensity, equalling about 12 miles per week of walking or running, can effectively decrease body fat mass and waist circumference.

2. Cardiovascular health

Running is a good aerobic exercise. Long-distance running is known to have a profound impact on cardiovascular health.

According to a study published in the Journal of the American College of Cardiology [3]ScienceDirect: Leisure-Time Running Reduces All-Cause and Cardiovascular Mortality Risk, regular running provides significant cardiovascular benefits. It was found that compared to non-runners, regular runners had a 29% lower risk of all-cause mortality and a 50% lower risk of cardiovascular mortality.

The repetitive nature of long-distance running improves stroke volume (the amount of blood pumped with each heartbeat) and cardiac output. This results in a lower resting heart rate, reduced blood pressure, and enhanced overall cardiovascular function in any elite runner or marathon runner.

3. Enhance stamina

Endurance and stamina are crucial for professional marathon runners or elite long-distance runners. As runners consistently engage in aerobic activities, their bodies undergo physiological adaptations.

Slow twitch muscle fibers, which are predominant in endurance athletes like marathon runners, become more efficient in utilizing oxygen and metabolizing fuel sources such as fat and carbohydrates.

This optimization of slow twitch fibers allows runners to sustain prolonged physical exertion, leading to enhanced stamina and improved performance in running and other endurance-based activities.

4. Better cognitive health

The mental health benefits of long-distance running are well-documented. Regular aerobic exercise, including long-distance running, stimulates the release of endorphins, serotonin, and dopamine, which are neurotransmitters responsible for regulating mood and reducing symptoms of depression and anxiety.

Additionally, aerobic training increases blood flow and oxygen supply to the brain, promoting neurogenesis and enhancing cognitive function. Long-distance runners, like elite marathon runners, often report improved memory, attention, and overall cognitive performance.

5. Lower stress levels

Long-distance running serves as an effective stress management tool. Running has been scientifically linked to mood regulation and enhancement [4]Springer Nature: Benefit of human moderate running boosting mood and executive function coinciding with bilateral prefrontal activation. Engaging in regular running sessions helps regulate the production of stress hormones like cortisol, leading to a reduction in stress levels.

Additionally, the rhythmic and repetitive nature of running, especially in natural environments, induces a meditative state that promotes relaxation, calms the mind, and alleviates psychological tension.

6. Relaxation and clear thinking

Running provides an opportunity for relaxation and clear thinking. The solitary nature of running allows individuals to disconnect from distractions, promoting introspection and mindfulness.

Running in natural settings further enhances the relaxation response, reducing mental fatigue and fostering a sense of calm. This state of focused attention and relaxation facilitates clearer thinking, problem-solving, and decision-making.

Why are sprinters muscular and long-distance runners skinny?

The differences in body composition between sprinters and long-distance runners can be attributed to several factors, including the physiological demands of their respective events, training methods, and genetics.

Here’s a detailed table outlining the differences in various aspects between sprinters and long-distance runners:

AspectSprintersLong-Distance Runners
Physiological DemandsExplosive, anaerobic activityEndurance-based, aerobic activity
Primary Energy SystemATP-PC (Adenosine Triphosphate-Phosphocreatine) SystemAerobic System (Oxidative Phosphorylation)
Muscle Fiber TypeFast-twitch muscle fibers (Type II)Slow-twitch muscle fibers (Type I)
Fiber CharacteristicsLarger diameter, more potential for hypertrophySmaller diameter, less potential for hypertrophy
Training FocusStrength and power developmentEndurance and cardiovascular fitness
Strength TrainingEmphasized to enhance power and muscle growthLess emphasis, focused more on endurance and stamina
Typical ExercisesWeightlifting, plyometrics, explosive drillsExtensive running mileage, lower-intensity exercises
Body CompositionMuscular and powerful appearanceLean and slender appearance
Genetic FactorsPotential for greater proportion of fast-twitch fibersPotential for greater proportion of slow-twitch fibers
Performance GoalsSpeed, acceleration, explosive powerStamina, endurance, cardiovascular efficiency

Marathon training without weight loss?

Here are some tips for marathon training or long-distance running while aiming to maintain your weight, bulk, or muscle mass:

  1. Consume a well-rounded diet that includes a good mix of macronutrients (carbohydrates, proteins, and fats) to support your training and maintain muscle mass. Prioritize complex carbohydrates, lean proteins, and healthy fats.
  2. Ensure you’re consuming enough calories to fuel your training and maintain your weight. 
  3. Consider consulting a registered dietitian or nutritionist to determine your daily caloric needs based on your activity level and goals.
  4. Consume adequate amounts of high-quality protein sources such as lean meats, poultry, fish, dairy products, legumes, and plant-based protein options.
  5. Incorporate regular strength training sessions into your routine. Focus on compound exercises that target multiple muscle groups, such as squats, deadlifts, lunges, and bench presses. This helps preserve muscle mass and overall strength.
  6. Drink sufficient water throughout the day, especially during training sessions. Electrolyte-rich sports drinks can also be beneficial for longer runs to replenish electrolytes lost through sweat.
  7. Allow adequate time for rest and recovery between training sessions. Include rest days in your schedule to prevent overtraining and promote muscle repair.
  8. Engage in cross-training activities such as swimming, cycling, or low-impact cardio exercises to improve cardiovascular fitness without putting excessive stress on your joints.
  9. Adequate sleep is crucial for muscle recovery and overall performance. Aim for 7-9 hours of quality sleep each night to support your training efforts.


Maintaining your weight and muscle mass while training for long-distance running is possible with the right nutrition habits, strength training regimen, and recovery practices. 

Eating a balanced diet, incorporating regular strength training sessions into your routine, and allowing adequate time for rest and recovery are essential components of marathon training while avoiding weight loss. 

Consult with a qualified healthcare professional to learn more about the best nutrition and exercise strategies for your individual needs.

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