A Runner's Guide to Protein (Part 1)

Americans are obsessed with protein.

I can’t eat at a restaurant or walk through a grocery store without being bombarded by high-protein claims. After the marketing blitz of the last few years, it’s become hard to think clearly about how much we actually need and why we need it. Almost everyone I meet is operating from a protein scarcity mindset: nobody thinks they’re getting enough.

Over the next two newsletters, I want to forget about the hype and answer the questions runners actually have. My hope is that you finish with a clear sense of what this macronutrient does and how you build it into your training and fueling.

What is protein actually doing for a runner?

Endurance runners have historically ignored protein, believing that they only need to focus on carbohydrates to fuel the work and that protein is just for gym rats looking to add bulk. While it’s true carbohydrates are essential for fueling, protein provides an equally valuable service. To understand that service, we need to explore what running costs you.

Every foot strike causes micro-trauma in skeletal muscle, and hours of accumulated impact add up to real damage. At the same time, prolonged running pushes your muscles to burn amino acids (the building blocks of protein) as a direct fuel source. Amino acid oxidation accounts for as much as 6% of the total energy cost of exercise.

The body does not have a storage depot for protein as it does for carbohydrates and fat. Every gram of protein in your body is already doing a job. So when demand for amino acids rises, and none are arriving from your diet, your body “raids” its own muscle tissue to pay for the metabolic demands of a run.

Only when you refuel with protein can the body begin using those building blocks for repair and to cover the losses incurred from metabolic expenditure.

This is the balance that governs everything. When muscle protein synthesis exceeds breakdown, you rebuild. When muscle protein breakdown exceeds synthesis, you lose ground. A hard run pushes you towards breakdown, and if you finish that run and eat nothing, you’ve tipped the scales unfavorably towards breakdown.

Practical Takeaway: Carbohydrates are your fuel, but protein is your shield. Adequate protein repairs the damage training causes and keeps your body from harvesting amino acids from muscles.

How fast does your body actually rebuild itself?

The body is in a constant state of turnover. Rates of muscle protein synthesis and breakdown oscillate to allow for tissue remodeling. What most people don’t realize is how fast that happens.

Every single day, you break down and re-synthesize 1% to 2% of your skeletal muscle. This means that the skeletal muscle is rebuilt from scratch every 50 to 100 days.

For most of the history of nutrition science, there was no way to prove this. You could measure what went into a person’s mouth, and you could measure what happened to their muscle, but you could not follow a single amino acid from one to the other. The Dutch scientist Luc van Loon found a way around it in what has become known as the “glowing cow study.”

He spent 40,000 euros on a rare form of carbon-13 and infused it into a lactating cow. The cow then built that traceable carbon into her own milk and meat, which meant every amino acid in that food carried a chemical signature that lab equipment could pick out from ordinary carbon. The protein was, in effect, glowing.

In the trial, van Loon’s team gave 20 grams of that labeled protein to 12 healthy young men, then drew blood and took muscle biopsies over the next five hours. About 45% of the protein never left the digestive system. The gut and liver took their cut first, using those amino acids to maintain the intestinal lining, which is one of the fastest-turning-over tissues in the body.

The other 55% entered the bloodstream. Within two hours, the glowing amino acids were already showing up inside the muscle tissue. By the end of five hours, 11% of everything the men had eaten had been directly built into new skeletal muscle protein.

Practical takeaway: You are, quite literally, what you just ate. A meal is not only fuel, but it is also the physical raw material your body reaches for when it goes to repair the damage from your run. This process starts within a few hours.

Will I bulk up if I eat a lot of protein?

This is a common fear that’s often cited by runners, but sports science has repeatedly shown that eating protein without progressive, heavy lifting will not make you look like a bodybuilder. Here’s why.

Firstly, the exercise stimulus itself drives physical adaptation, and protein merely supports it. Dietary protein does not possess a mind of its own. It cannot force a muscle to grow larger. For example, if you just lie in bed all day and eat copious amounts of protein, you won’t gain any muscle. The signal that drives muscle hypertrophy is lifting heavy weights to exhaustion in the gym.

Secondly, muscle remodeling is not the same as muscle growth. When a runner consumes protein after exercise, the response is fundamentally different than what a resistance training athlete experiences.

Endurance training stimulates mitochondrial protein synthesis, vascular remodeling, and the production of aerobic enzymes. Resistance training stimulates the muscles in a way that they grow bigger and stronger. Dietary protein supports each of these divergent processes in its own way.

Even though both groups have exceptionally high rates of muscle protein synthesis, elite marathon runners and elite bodybuilders are going to have completely different structural dimensions.

Practical Takeaway: Eating protein makes runners more conditioned athletes, not bulkier ones.

So, how much do I actually need?

Let’s start with some official recommendations from the US Food and Nutrition Board. The estimated average requirement for protein is 0.62 g/kg/day. That is the median. At that intake, half the population has enough, half does not. To buffer against the risk of deficiency, the Recommended Dietary Allowance is set high enough to cover 97.5% of people. In the United States, that number is 0.8 g/kg/day.

Many people feel that number is too low. The evidence suggests it is.

A lot of the confusion here is a naming problem. “Recommended Daily Allowance” sounds like a target, but it’s more of a floor (again, it’s the amount needed to keep 97.5% of the sedentary population out of deficiency). The RDA was never built to answer the question runners are asking: how much do I need to adapt to hard training?

So where should runners land? The most useful evidence I’ve seen comes from a clever measurement method called indicator amino acid oxidation, or IAAO. I will spare you the details of how this works (it’s complicated). What matters is that it pinpoints the intake at which your body stops wasting amino acids and whole-body protein synthesis is maximized.

A couple of studies map this out for endurance athletes:

• A 2016 study ran six male endurance-trained athletes through 34 metabolic trials following a 20 km treadmill run. It found an estimated average requirement of 1.65 g/kg/day and a recommended intake of 1.83 g/kg/day.

• A 2023 study repeated the design in 15 male and female athletes after the same 20 km run. The recommended intake was estimated to be 1.81 and 1.89 g/kg/day for male and female athletes, respectively.

See the chart below to see where your total protein intake can fall across the spectrum of recommendations.

Practical Takeaway: My read is that 1.8 g/kg/day is the defensible target for a serious runner, and I would err on the high side of the spectrum. Within the 1.6 to 2.2 range, there is no evidence of side effects or kidney harm, which was previously considered at risk.

Conclusion

Next week we’ll dig more into the protein science and tackle these questions:

• Is it better to consume a protein powder or whole food source of protein?

• Is animal versus plant protein better?

• Is there an anabolic window where I need to consume protein?

• Is protein intake before bed beneficial?

• How should I change my protein intake around an injury?