Why Are Marathon Runners So Skinny?

Have you ever watched a marathon and noticed the lean, slender builds of the runners crossing the finish line? The typical marathon runner often appears skinny and lightweight, unlike the larger physiques in other sports. This common observation has led many to wonder: “Why are marathon runners so skinny?” 

We understand this stereotype. However, the relationship between marathon running and low body weight is more complex than it may appear. The purpose of this article is to take an in-depth look at the physiology, nutrition, genetics, and other factors that influence the physiques of marathon runners. 

We aim to provide research-backed explanations for this perpetuated image of the skinny marathon runner. While marathon training burns massive calories and lowers body fat, runners come in all shapes and sizes. The best marathoners balance intense training with proper fueling and strength work. 

A lean, lightweight physique can improve running economy, but substantial muscle mass and power are also critical. The skinny runner stereotype overlooks the importance of strength and reduced injury risk. Not all marathon champions are skinny, and not all skinny people are gifted runners.

The Physiology of Marathon Running

Training for and completing a marathon places immense physiological demands on the human body. The ability to run 26.2 miles requires optimizing several bodily systems and adapting them to the rigors of long-distance running. Let’s explore some key biological factors and how they impact a marathoner’s physique.

VO2 Max (Cardiorespiratory Fitness)

VO2 max is the maximum amount of oxygen the body can utilize during intense exercise. It measures a runner’s cardiorespiratory fitness and aerobic capacity. VO2 max levels correlate strongly with marathon performance times, as a 2020 study in Science Daily demonstrated. 

Marathoners have an average VO2 max of around 70 ml/kg/min for males and 60 ml/kg/min for females. A high VO2 max allows runners to sustain faster paces with less effort. 

It signifies the body’s ability to deliver oxygen to working muscles and the muscles’ ability to use that oxygen to generate energy aerobically. High-mileage marathon training boosts VO2 max by increasing the number of oxygen-carrying red blood cells and the density of capillaries supplying the muscles. 

VO2 max is a major limiting factor for endurance performance. A marathoner’s speed and race time correlate directly with their VO2 max. Training can raise it by 15-20%, but VO2 max is also influenced by genetics. 

The greater aerobic capacity allows runners to be competitive at lower body weights since their cardiovascular and respiratory systems can efficiently deliver oxygen with less muscle mass to move.

Lactate Threshold and Running Economy

According to Physio Pedia, the lactate threshold indicates the running intensity where lactic acid accumulates exponentially in the muscles. It marks the transition between comfortable running and unsustainable harder efforts. The higher the lactate threshold, the faster a marathoner can run before muscular fatigue sets in. 

A review study published by Springer Open shows that running economy refers to the oxygen demand to sustain a given running pace. It indicates how efficiently a runner’s body can use oxygen while running. A better running economy means less energy expenditure per mile. Running economy accounts for a large portion of the variation in performance between elite runners. 

Both lactate threshold and economy are enhanced through consistent marathon training. High mileage and tempo runs extending past the lactate threshold adapt the muscles to resist fatigue and rely on aerobic pathways. The economy is improved by ingraining an optimal stride length and form. 

Marathoners who improve their lactate threshold and economy can run faster splits on less training mileage. They can stay competitive on higher weekly volume but with less emphasis on high-intensity speedwork. It allows them to achieve a leaner physiological look. However, lactate threshold and economy are also tied to one’s genetics and muscle fiber composition.

The Impact On Physique

The phenomenal aerobic capabilities and running efficiency of elite marathoners allow them to be competitive at remarkably low body weights. Male marathon champions have an average BMIs of only 20.5 (18.5 to 24.9). 

However, caution must be exercised when pursuing low body weight because severe depletion can compromise power, increase injury risk, and impair health. Beyond VO2 max, several other physiological factors influence marathon performance and the ideal marathoner physique:

Energy Expenditure and Weight Loss

Running a marathon burns an enormous number of calories. The 26.2-mile distance requires 2-3 hours of continuous exercise. The energy expenditure can be staggering. 

A marathon burns about 2,600 to 2,800 calories for a 125 to 155-pound person. A combination of stored carbohydrates, fat, and protein fuels the massive calorie expenditure. The longer the event, the more significant percentage of calories burned comes from fat stores. 

The heavy calorie expenditure of marathon training provides a tremendous opportunity to lose weight through an energy deficit. Weight loss occurs when the body burns more calories than it takes in, forcing it to use fat stores as fuel. 

According to Very Well Fit, a deficit of 3,500 calories must be created to lose one pound of fat. If a marathoner burns 500 extra calories per day through training, that adds up to 3,500 calories of fat loss in just one week. 

Additionally, sustained aerobic exercise stimulates increased metabolism and builds lean muscle mass. The net result is that body fat percentage drops dramatically as the muscles become more defined. 

While genetics play a role, the physiological adaptation to the training stimulus is the primary reason marathon runners experience rapid weight loss and develop a skinny physique.

Genetics and Body Type

An individual’s genetic makeup and body type can influence their propensity to become skinny as a marathon runner. However, with proper training and fueling, runners of all body types can succeed in the marathon. 

Genetics impact our metabolism, muscle fiber composition, fat storage patterns, and other physiological factors tied to body weight and composition. Some people are predisposed to being tall and slender. For them, staying slim as a marathoner may come easier. 

However, human genetics is complex. Genes also influence metabolic efficiency, running economy, and biomechanics. Runners with these attributes can excel at the marathon distance regardless of size. 

The extreme ectomorphic body type seems ideal for marathon running. However, elite marathoners can come from any somatotype. Training adaptations and fueling strategies can help overcome one’s natural tendencies.

Diet and Nutrition

The dietary habits of marathon runners are optimized to fuel training, recovery, and body composition goals. Proper nutrition provides the raw materials to perform at a high level while facilitating fat loss. 

What and when a runner eats can tremendously impact achieving a lean physique. Marathon runners require additional calories and nutrients to support their heavy training demands. Key diet strategies include: 

Complex carbohydrates: Whole grains, legumes, starchy vegetables and fruits supply energy for working muscles. Carbs get stored as glycogen. 

Protein intake: Consuming protein post-workout and at regular intervals helps rebuild damaged muscle fibers. Shoot for 20-30g of protein every 3-4 hours. 

Healthy fat sources: Unsaturated fats from nuts, seeds, avocado, olive oil, and fatty fish provide energy, aid recovery, and help absorb vitamins. 

Hydration: Water and electrolyte drinks are vital during long runs to replace fluids and minerals lost through sweat. 

Nutrient density: Runners require more vitamins and minerals for energy metabolism and immunity. Eat plenty of colorful fruits, vegetables and lean proteins. 

Portion control: Additional calories are needed to fuel training but beware of overeating. Use a food journal to find the optimal caloric intake for your body and training level. 

Moderating sugar and alcohol: While carbohydrates are essential, runners should limit sugar as well as alcohol which impairs recovery. 

The strategic consumption of complex carbs, lean proteins, and healthy fats feeds the muscles while avoiding sugar crashes and hunger spikes. Nutrients are timed around key workouts to optimize performance, recovery, and metabolic response. 

In addition, runners tend to consume very nutrient-dense foods like vegetables, fruits, whole grains and lean proteins. These foods provide vitamins, minerals, fiber and water with fewer calories per bite. Runners emphasize quality and satisfaction over the sheer quantity of food. 

The result is the ability to eat until satisfied while maintaining a caloric deficit. This deficit forces the body to increasingly tap into fat stores for the extra calories needed to complete marathon training. Any excess body fat melts away, revealing the lean muscle beneath.

The "Skinny" Stereotype

The common stereotype is that all marathon runners have extremely lean, skinny physiques bordering on underweight. This stereotype stems from the observation that many competitive marathoners possess very low levels of body fat required for performance. 

However, the stereotype that all marathoners must be rail-thin to complete the 26- mile distance is inaccurate and unhealthy. For some runners, the pressure to conform to this stereotypical body type can contribute to poor self-image and risky dieting practices. 

Running message boards are filled with questions from average runners, wondering if they are too big or need to lose weight. This fixation on being skinny can develop disordered eating patterns or exercise addiction tendencies in susceptible individuals. 

However, many experienced marathoners point out that while leanness helps performance, being underfueled is detrimental. They aim for an optimal racing weight that balances power and lightness while avoiding extremes. 

Most importantly, they focus on health and fitness above all else. While some naturally thin runners do exist, runners come in all shapes and sizes. The goal should be developing a physique that safely supports completing the marathon distance rather than matching some unrealistic ideal. 

For most runners, the highest priority is training consistently with adequate nutrition to become fit enough to finish strong. The truth is the number of calories burned through high-mileage training would make it extremely difficult for most marathoners to stay emaciated without consciously starving themselves. 

For many, the challenge is keeping weight on, not losing it. The long miles instead make them strong and muscular. Each runner needs to find a healthy physique that fits their body. Toning comes naturally through SMART training. No one should feel pressured to conform to one body type promoted by stereotypes. Runners should aim for body positivity and dispel those dangerous misconceptions.

The Benefits of Being Lean

There are clear biomechanical advantages to having a leaner physique for marathon running. Additional body weight in the form of fat requires more energy expenditure to move that mass over 26.2 miles. 

The less excess weight a runner has to carry, the better their running economy. According to the Journal of Sports Medicine, a 10% reduction in total body weight can improve running economy by up to 8%. 

So this has enormous performance implications over the marathon distance. Excess body weight can also increase injury risk, especially with the high mileage training required for marathons. 

The impact forces experienced during running get multiplied with each additional pound carried. Each foot strike stresses the bones, joints, and connective tissues more. Bulking up with too much upper body muscle contributes to injury risk. 

The idea is to develop lean, functional muscle for speed and power. As such, many competitive marathoners aim to optimize strength-to-weight ratios. Having a lean physique helps create durability. 

Moreover, the ability to dissipate heat is another critical factor. Leaner runners have an advantage in thermoregulation thanks to a higher surface area to mass ratio. They can shed heat more readily due to having less insulation from excess body fat. Staying cool is vital for maintaining pace in warmer conditions. 

The lean marathon physique also maximizes capillary density and blood flow to the skin. Enhanced vascularity helps transfer core heat to the skin’s surface. These adaptations contribute to improved heat tolerance on race day.

A Final Recap

Running marathons requires immense physical endurance and training. The long distances burn substantial calories, and sprint intervals build lean muscle. This intense regimen transforms marathoners’ bodies into optimized running machines. 

So through their intense commitment to running, marathoners sculpt ultra-fit, super-lean bodies. The skinny marathoner build reflects the sport’s exceptional demands. Their skinny bodies show years of hard work and passion. 

Commit to the training and see your body evolve if you want to reveal your inner marathoner. The marathon awaits any runner determined to push themselves to new heights! 

Scroll to Top