Why Are Marathon Runners So Skinny (A Fitness Coach Weighs In)

why are marathon runners so skinny

I was standing before the starting line when I noticed a bunch of elite runners that look so skinny. I’ve been running a lot lately so I wanted to find out why I can maintain my muscular physique while some runners become so skinny. So I called my long-time friend and a professional coach and here’s what I learned.

Marathon runners are skinny because they fail to match their calorie expenditure with their calorie intake. However, not all marathon runners are skinny. The ones who become skinny tend to be elite runners who are more likely to accumulate more miles and consistently burn more calories.

But calories are not the only factors to consider as to why long-distance runners are so skinny. The amount of strength training and how the body uses fuel also come into play. Ahead, we will look at those factors in detail.

Factors That Make Long Distance Runners So Skinny

After asking David, my long-time friend and fitness coach who also happens to be a medical doctor, I went on and continued to dive a little deeper into the topic.

Here’s what I found:

Insuficient Calorie Intake

The biggest factor that plays into why marathon runners are so skinny is a lack of calorie intake.

Running burns a tremendous amount of calories. As a matter of fact, according to a paper published by the American Council on Exercise, running burns the most calories per minute out of all the other exercises and sports on the list.

Activity Calories/min120 lb.140 lb.160 lb.180 lb.
Cycling (10 MPH)
Skiing (cross country)7.58.810.11.3
Swimming (moderate pace)7.8910.311.6
Weight Training6.
Source: American Council on Exercise

Side note: I intentionally left out some activities from the table. If you want to find out more about how many calories are burned per minute of a certain activity that is not on this list, visit the source.

Based on the data above, we can assume that marathon running burns the following calories depending on the pace of your run (mins/mile) and your body weight.

Calories burned in a marathon

Pace Mins/mile120 lb140 lb.160 lb.180 lb.
15’/mile4,441 5,187 5,934 6,681
14’/mile4,114 4,842 5,439 6,236
13’/mile 3,849 4,495 4,143 5,790
12’/mile 3,553 4,150 4,747 5,345
11’/mile 3,256 3,804 4,352 4,899
10’/mile 2,960 3,458 3,956 4,454
9’/mile 2,665 3,113 3,560 4,009
8 ‘/mile 2,368 2,767 3,165 3,563
7’/mile 2,072 2,4202,769 3,118
6’/mile 1,776 2,075 2,374 2,672
5’/mile 1,480 1,729 1,978 2,227
Estimated calorie burned in a marathon depending on pace and bodyweight.

Here are the average calories burned of each individual when running a marathon

  • Elite runners: 1,854 cal
  • Average male: 3,707 cal
  • Average female: 4,078 cal
  • Beginner: 4,934 cal

The standard American diet is only 3,600 calories and the average calorie expenditure in a day without any vigorous activity is between 1,600 to 2,400 calories a day.

Although one marathon won’t make you skinny, the training leading up to the marathon will. A marathon training plan can take 3-6 months and a typical training session could last about 2-3 hours depending on what phase you are in. That’s somewhere around 1800 to 2700 calories per training session for a 160 lbs runner.

It doesn’t take a lot other than basic math to figure out that a 3600 calorie diet is not enough to support your basic requirements plus a marathon training program almost every day.

That said, If you’re just eating a standard American diet (which is a lot for sedentary individuals), you’ll definitely end up being skinny.

If you’re not eating enough to replace your calorie expenditure, you will become skinny.

Runners Getup

Lack of Strength Training In The Program

Because training for a marathon is so vigorous and long, athletes tend to neglect including whole-body strength training in their program.

It is especially hard and time-consuming to do a strength session every day to preserve or grow your muscle mass. This is why marathon runners tend to focus on just the aerobic aspect of fitness which is the most important aspect in running a marathon.

I remember asking a friend who regularly runs a marathon whether he is still able to lift when he’s training for a race. He said, “Sometimes I will, but given the time it takes for me to finish my training session and the other aspects of my life that’s going on (job, kids, and chores), it’s hard to squeeze in a strength session”.

“Sometimes I do have the energy but not the time, in some cases I force myself to make time just because I don’t want to end up being skinny”. He added.

He usually trains only 3-4 times a week, so most of his workouts have to be marathon training if he wants to perform well on race day.

Some runners who join marathon events while maintaining great physique alternate strength days and cardio so they can maintain muscle mass while still being able to train for a marathon.

It Breaks Down Protein

A marathon training plan is usually composed of long-distance running. Long exercise duration can lead to protein breakdown and a decreased testosterone level.

Protein breakdown happens because of two things: increased levels of cortisol and insufficient glycogen and fat stores in your body.

When you run, your body’s primary source of energy is glycogen found in your muscles. After about 90 mins of low to moderate intensity running, your body switches over to fat stores which ideally should burn long enough to finish a training session.

The problem is that when you fail to replace your depleted glycogen stores with high-quality carbs. After some time of training for a marathon, your body is depleted and the only way it could fuel your run is by using protein as a fuel source.

Now your body starts to “burn” muscles which will make you look skinnier.

Add that to the fact that exercise itself increases cortisol level (causes breakdown) and exercising too long leads to a decreased testosterone level (a hormone that helps build muscle mass), you can expect muscle wasting if you’re not able to train the right way.

Your body will start to burn your muscles if you don’t have enough glycogen stores and fat in your body.

Runners Getup

Not All Marathon Runners Look Skinny and Unhealthy

You’ve probably seen pictures of elite marathon runners looking incredibly thin with sunken eyes and dry skin. If you’re like me, you could’ve misjudged the sport as the reason behind the skinny and “unhealthy” look. But when I was searching through the internet and looked at other participants of different marathon events, I observed something different in MOST of their bodies: They look incredible.

So I asked the question: Why do some marathon runners look unhealthy?

In general, only a small percentage of marathon runners look unhealthy. That’s because of multiple factors including dehydration, overtraining, and stress. The timing of when you see them could also be a factor as to why they look unhealthy.

Think about it, most marathon runners you see on the internet are Olympians performing on a very high level or someone who is going through a rough time during the race. No wonder why they look so unhealthy in the pictures.

But if you glance through other marathon event photos, especially those events where the intermediate and advanced runners are present, most of them look perfectly healthy and athletic.

Skinny elite marathon runners look that way because they spend a considerable amount of time training. Another factor is dehydration which could make their skin look dry and their eyes sunken.

To prove my point that not all marathon runners look unhealthy, let’s look at some examples of athletes who are known to participate in very long-distance running but look jacked and healthy.

David Goggins is famous for his hard work mentality. He is known for running incredibly long, 100-mile races back to back. And from the looks of it, he looks incredibly fit despite his hobby of running incredibly long distances.

Nick Bare is another incredible runner with an unbelievable physique. As a matter of fact, in one of his youtube videos, he addressed the questions of his followers whether or not he’s losing muscle while training for a marathon which he answered no.

Does Marathon Runners Have Low Body Fat?

You may have noticed that marathon runners are skinny, but do they actually have a low body fat percentage?

In general, most marathon runners have low body fat. Elite runners’ body fat percentage usually ranges from 5 to 11 percent for men and 10 to 15 percent for women. That’s because long-distance running tends to use fat stores for fuel.

But again, the percentage of body fat differs from person to person. Other factors may be in play like the type of food intake, the amount of food intake, and genetics.

Most runners in the elite category intentionally try to reduce body weight to become “lighter”. So they make changes to their diet plan to reduce weight.

For most of us, an aim of 14-17% of body fat is a more realistic goal.

Will Training For A Marathon Make You Lose Muscle

Now that you’ve identified not every marathon runner looks skinny, your next question must be if marathon training results in muscle loss.

Training for a marathon will make you lose muscle if you do not eat enough calories to replace the calories you spent. You could also lose muscle if you neglect to include strength training in your program.

A lot of incredibly fit and muscular athletes actually run the marathon for its mental and physical benefits. Just make sure you do not overdo it.

How To Train For A Marathon Without Becoming Skinny

If you are able to train for a marathon, I highly encourage that you do it.

It’s possible to train for a marathon without becoming skinny.

Here are a few tips that could help you:

  • Eat lots of carbs after training (even sodas, ice cream and orange juice are okay)
  • Don’t skip strength day, make sure lifting is a part of your program
  • Increase your overall calorie intake (increase carbs, fats and protein)
  • Don’t overdo it (If the program says 5 mile run, just do 5 miles)

It’s important to point out that if you’re lifting weights very often and you decided to reduce the volume and intensity to make space for a marathon training plan, you are going to lose some of that muscle. There’s no way around it.

But once your body reaches its optimal weight where you are able to run a marathon and lift heavy weights, your body will adapt to that weight and will likely stay in that weight and body composition unless you make changes in your training and diet.

The human body is very smart. Whatever you do with it, it will do its best to adapt for that activity.


To conclude, not all marathon runners are skinny. Those who are skinny are the ones who fail to replace the calories they spent with their calorie intake. Most elite runners also try to become skinny intentionally to gain some advantage.

Training for the marathon without becoming skinny is possible. The key is to increase your calorie intake to match with your calorie expenditure and by doing a strength program.

Nicho Mauricio

Running wasn't always my favorite sport. I was a CrossFit athlete and I loved every bit of it. But since the pandemic began, I was forced to stay away from the gym and train at home instead. Things got boring. That's when I decided to trail run with my friends. I instantly got hooked. So I started training and researching all things running. As a beginner, I want to buy only the best running gear and do only the best practices. This blog is where I share what I've learned in my journey and my experiences as a runner.

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