April 11, 2025

 

Your Gut Hates the Heat—Even When You Don’t Feel It

We all know running in the heat feels harder—but what’s happening inside your body might be even more intense than you realize, especially in your gut.

To test how the GI system responds to hot-weather running, researchers involved in a new study had competitive triathletes run for an hour at a hard pace (90% of their anaerobic threshold) in two settings: cool temperatures (~19°C / 66°F) and moderate heat (~29°C / 84°F) They measured core temperature, sweat loss, and a panel of blood markers tied to gut integrity and systemic stress.

Core body temperature rose dramatically in the heat—finishing at 39.6°C (103.3°F) compared to 38.7°C (101.6°F) in cooler conditions. Heart rate and perceived exertion also spiked, but more striking were the changes in internal biomarkers. Markers of gut cell damage and systemic stress (cell-free DNA) both increased significantly more during the hot run. In fact, the higher the runner's core temp climbed, the more significant the rise in these markers, suggesting a direct link between internal heat load and gut barrier disruption.

Immune cell counts (neutrophils and lymphocytes) rose more under heat stress, too, pointing to amplified systemic strain. Meanwhile, levels of D-lactate and lipopolysaccharide-binding protein (LBP)—both tied to increased gut permeability—spiked after both runs but tended to be higher after heat exposure. Here’s what’s surprising: None of the athletes reported GI symptoms, even though their blood told a different story.

What this means for runners

Running hard in moderately warm weather—not just extreme conditions—can compromise your gut lining and dial up your body’s stress responses. These changes may not produce immediate symptoms, but over time, they could impact how well you recover, absorb nutrients, or tolerate race-day nutrition. This reinforces the importance of treating heat exposure as a real training variable—something to consider with the summer months looming on the horizon.

RELATED ARTICLE: 10 Expert Tips For Running In The Heat And Humidity


🎧 What is "Heart Rate Drift?" You Asked, We Answered: The Mailbag Episode

🎙️ This week’s podcast mailbag is bursting at the seams: We’re dishing out marathon training tipsdebating super shoes for “slow” runners, solving the case of the spiking heart rate, and revealing what “speed goggles” actually are. Plus, the sunscreen mistake that’ll ruin your race and why Puma fans came for us.

WATCH NOW

Or listen and subscribe to the show wherever you get your podcasts:


Here's When You Start Burning Protein During a Run

When you run long or hard, your body doesn't just burn carbs and fat—it may start tapping into protein, too. That means your muscle tissue can become fuel. But when does this actually happen?

A new meta-analysis offers the most straightforward answer yet.

Researchers analyzed 36 studies tracking protein oxidation during aerobic exercise (mostly cycling and running). They looked at how much muscle protein was used depending on how intense, how long, and how much total work people did.

The study found that both moderate- and high-intensity exercise significantly increased protein oxidation compared to low-intensity exercise. Moderate-intensity aerobic work (defined as 50–70% of VO₂max) had a more substantial effect on protein use than even high-intensity work (>70% VO₂max). Protein oxidation increased by 1.02 mg/kg/min during endurance exercise from the level of 0.81 mg/kg/min measured without exercise. Overall, protein contributed 3.28% of the total energy expenditure.

Duration mattered, too: Sessions lasting more than 60 minutes led to significantly higher protein oxidation than shorter workouts. When the researchers looked at total volume (a combination of intensity and duration), they found that moderate- and high-volume sessions caused significantly more protein use than low-volume sessions. However, as with intensity, there was no meaningful difference between moderate and high volume—most of the increase in protein oxidation occurred in the shift from “easy” to “moderate” training. Importantly, the majority of protein breakdown happened during the workout itself, not after.

What this means for runners

If you’re regularly running hard or long—especially sessions over an hour—your body may start burning small amounts of protein for energy. That doesn’t mean you’re wasting away, but it does mean your muscles are under more metabolic strain than during easy runs. If you tend to do fasted training, run low on carbs, or don’t refuel properly afterward, the impact could be even more significant. Over time, this could interfere with muscle maintenance, recovery, and adaptation.

Aim for a protein-rich snack or meal within an hour after moderate-to-long runs or high-intensity efforts. This helps repair any protein breakdown and supports recovery and adaptation. While it’s not necessary to obsess over protein for every jog, higher-volume or higher-intensity days call for more attention to fueling.

RELATED ARTICLE: How Much Protein Do Runners Need? The Ultimate Guide To Protein For Runners


Do Intense Runs Hurt Your Efficiency? Not for These Female Athletes

Running economy—the energy it takes to move at a given pace—is a major predictor of performance. But how stable is it after a typical hard or easy workout? And does it shift across the menstrual cycle?

This new study from the FENDURA Project gives female endurance athletes some good news.

Researchers recruited 24 naturally menstruating, endurance-trained women and had them complete a 60-minute low-intensity run (LIT: 45–55% of max velocity) and a 55-minute high-intensity interval session (HIT: 5 x 4 minutes at ~90% VO₂max). Each workout was performed in three different menstrual cycle phases (early follicular, ovulatory, and mid-luteal). Running economy was measured before and after each session by measuring each athlete’s energy and oxygen cost.

After the low-intensity sessions, running economy didn’t change. Both energy cost and oxygen cost remained stable. No surprise—easy runs are meant to be low-stress.

After the high-intensity intervals, oxygen cost increased slightly (by 1.8%), and VO₂ (oxygen consumption) was 2.4% higher, suggesting a small dip in efficiency. But energy cost (the more comprehensive measure) didn’t change. So, while breathing demands went up a bit, overall energetic demand stayed the same.

Most importantly, the menstrual cycle phase had no effect on these outcomes. Whether the women were in the follicular, ovulatory, or luteal phase, their ability to maintain their running economy stayed stable.

What this means for runners

For female athletes (and their coaches), this is a reassuring finding: a typical 1-hour easy or hard session doesn’t impair running economy, even during different menstrual cycle phases. That means you can plan training with more confidence—without worrying that a specific phase of your cycle will tank your efficiency on everyday workouts.

Also worth noting: While there was a small increase in oxygen use after the hard session, it was below the level of normal day-to-day variation in elite runners. That suggests no meaningful fatigue-related dip in efficiency, at least for sessions of this duration. If you’re targeting durability or want to stress running economy more deliberately, you may need longer sessions, higher volume, or fewer rest periods than what was used here.

RELATED ARTICLE: Running Economy: How To Become A More Efficient Runner


Here's what else you would have received this week if you were a subscriber to the complete, full-text edition of “Run Long, Run Healthy.” SUBSCRIBE HERE.

• Sprinting just once per week—can it make you a more efficient runner?

• Here’s what happens to your legs during a half marathon (and how long it takes to recover)

• Higher activity levels are linked to coronary artery calcium

Thanks for reading. As always—Run Long, Run Healthy

~Brady~


Subscribe to Run Long, Run Healthy Premium for as little as $3/month

March 13, 2025

Here's the free but abridged version of this week's RLRH newsletter. Please consider subscribing, below, to receive the full-text version. Brady

Underfueling Before the Season Can Ruin Your Performance

Many runners believe that being leaner equals being faster, leading us to try to manipulate our weight or body composition before or during key competitions. If you’ve got weight to lose, this could be a good strategy, but a new study on collegiate female distance runners shows that starting a season in an energy-deficient state leads to worse performance and slower fitness gains.

Researchers tracked 38 female collegiate runners throughout a competitive season. Before the season started, they categorized them into two groups based on metabolic health:

  1. Energy deficient (low resting metabolic rate, indicating underfueling)

  2. Energy sufficient (normal metabolic rate, indicating proper fueling)

Then, they measured VO₂max, 5k time trial performance, and key metabolic markers at the start and end of the season.

Runners who started the season underfueled were significantly slower. Their 5k times were 2 minutes slower than their well-fueled teammates and they didn’t improve over the season. VO₂max also stagnated in the energy-deficient group. Well-fueled runners gained aerobic capacity during the season, but those in a deficit didn’t see the same improvement. Lower total triiodothyronine (TT3) was linked to worse performance. TT3 is a key metabolic hormone, and low levels were related to slower times and poor training adaptation.

What this means for runners

Fueling matters before the season starts. If you enter training underfueled, your body will prioritize survival over performance, making it harder to build fitness. Even in the offseason or preseason, prioritizing nutrition is crucial. The best way to train hard is to eat enough to support it. Instead of focusing on weight, focus on fueling for adaptation. It’s a concept that takes a while to learn—and one I wish I’d have realized earlier on in my career as a long-distance runner.

Can “Too Much” Running Increase Your Risk of Heart Disease?

Most runners assume that logging more miles makes them immune to heart disease. After all, exercise improves cholesterol, lowers blood pressure, and strengthens the heart. But what if—paradoxically—long-term endurance training could increase heart disease risk? A new review explores the complex relationship between running and heart health, revealing that some athletes—especially those training at high volumes for decades—may develop more coronary calcification than expected. But there’s a lot of nuance involved.

Researchers conducted a state-of-the-art review of cardiac imaging studies assessing coronary artery calcification (CAC) and atherosclerotic plaque in endurance athletes. They compared highly trained runners and cyclists to less active populations to determine whether years of intense training increase or decrease the risk of heart disease.

The big question is whether endurance exercise protects against or contributes to coronary artery disease, the leading cause of sudden cardiac death in athletes over 35. Some of the findings challenge conventional wisdom.

For one, endurance athletes have a higher prevalence of coronary plaques than non-athletes. Multiple studies find that high-volume endurance athletes have more CAC and a higher prevalence of plaques than non-athletic control groups. However, not all plaques are created equal. While athletes have more total plaque burden, the composition of their plaques is less dangerous than what is typically seen in sedentary individuals. Endurance-trained individuals tend to develop densely calcified, stable plaques, which are less likely to rupture and cause heart attacks than the soft, lipid-rich plaques that are common in sedentary people. For example, in one study, 68% of the plaques found in endurance athletes were calcified, compared to only 47% in non-athletes.

Training intensity and volume seem to influence plaque development. Studies tracking athletes over time found that vigorous exercise was linked to increased CAC scores, but exercise volume alone did not drive plaque progression.

Despite more plaque, athletes have fewer cardiac events. Even in athletes with higher CAC scores, studies show that higher fitness levels (think VO₂max) are associated with a dramatically lower risk of heart attacks and cardiovascular events.

What this means for runners

Running won’t make you immune to heart disease, but higher fitness levels offer significant protection—even for those with coronary plaques.

Should you get a check-up? The authors did not recommend routine coronary artery scans for symptom-free athletes, as high CAC scores in endurance athletes do not necessarily translate to higher event risk. However, athletes with multiple risk factors (e.g., family history, high blood pressure, abnormal cholesterol) or symptoms should consider screening.

Do Super Shoes Really Improve Running Economy? And What About Ketones?

Super shoes—high-tech, carbon-plated racing shoes—have transformed running performance, promising better energy return and improved efficiency. At the same time, ketone supplements have been marketed as a metabolic enhancer that could improve endurance by offering an alternative fuel source. But do either of these actually improve running economy? What about combining them? A new study tested both advanced footwear technology and ketone monoester ingestion to determine their effects on running economy (RE), time-to-exhaustion (TTE), and metabolic efficiency.

Researchers conducted a study with 18 trained middle- and long-distance runners (10 men, 8 women, VO₂max: ~60).

Each participant completed four different trials on a treadmill, each consisting of five 8-minute stages of submaximal running (the men ran at a speed progressing from 10–14 km/h and women ran at a speed progressing from 9–13 km/h followed by a ramp test to exhaustion to measure time-to-exhaustion and peak oxygen uptake.

Each trial involved a combination of footwear and supplementation:

  1. Standard running shoes (Nike Pegasus Turbo) + Carbohydrate (CHO) drink

  2. Standard running shoes + ketone monoester drink

  3. Super shoes (Nike ZoomX Vaporfly Next% 3) + CHO drink

  4. Super shoes + ketone monoester drink

Participants ingested either a ketone supplement or a 10% carbohydrate solution before and during each session.

Super shoes (not surprisingly) improved running economy—ketones didn’t. Wearing the Vaporfly Next% 3 reduced oxygen consumption by 2.5-4.0% at submaximal speeds compared to the standard running shoes. This means runners used less energy to maintain the same pace, making them more efficient. The improvement was most pronounced at faster running speeds (third, fourth, and fifth stages). Super shoes also improved time-to-exhaustion—runners lasted 7% longer in the Vaporfly+CHO condition (381 sec) compared to Pegasus+CHO (356 sec). With ketones, the Vaporfly group improved time-to-exhaustion by 14% over Pegasus+KME.

Ketones alone did NOT improve running economy or endurance. There were no significant differences in economy between the carbohydrate and ketone trials despite elevated blood ketone levels. Time-to-exhaustion was actually lower in the ketone condition when runners wore the standard shoes.

What this means for runners

Super shoes genuinely improve running economy and extend time-to-exhaustion, making them one of the few equipment-based performance enhancers that live up to the hype. Ketone supplements, on the other hand, do not deliver on their performance promises and may be more useful for promoting post-exercise recovery or cognitive benefits.

HERE’S WHAT ELSE YOU WOULD HAVE RECEIVED this week if you were a subscriber to the complete, full-text edition of “Run Long, Run Healthy.”

SUBSCRIBE HERE.

What is "zone 2" training? Experts weigh in.

Do supershoes and ketones improve running performance?

How to break world records at 76 years old

That’s all for now. Thanks for reading. As always—Run Long, Run Healthy.

~Brady~

Run Long, Run Healthy--February 7, 2025

 Greetings! Brady here. I wanted to (briefly) introduce myself as the new author of the Run Long, Run Healthy newsletter.

First of all, I want to express what an incredible honor it is to have been asked by Amby to take the reins at RLRH. I have huge (and fast!) shoes to fill.

Amby’s confidence in my writing and research abilities gives me assurance that I can continue to provide you with the latest and greatest research on how you can be a better runner. It’s my goal to translate the science of running into something that’s fun to read and, most importantly, applicable.

About me: I’m a former collegiate endurance runner (I ran during my undergraduate years at Northern Kentucky University) and current “wannabe elite”—I’ve run 1:09:34 for the half marathon, 2:26:28 for the full marathon, and 32:07 for 10k all in the last year (at the ripe old age of 31). I’ve been living in Austin, Texas for the last 3 years with my wife and our soon-to-be 2-year-old son and dog.

Please don’t hesitate to reach out to say hello, comment on a newsletter, or critique something you find off-putting (I have thick skin).

~Brady~


Does Compression Gear Improve Running Performance? A Deep Dive into the Science of Squeeze

You've probably heard people swear by compression socks, tights, and sleeves to improve performance. I’ve never been a fan of them, but I have worn a few pairs during my day. They’re supposed to increase blood flow, reduce muscle vibration, and help you run faster and longer. But does the science back this up?


A recent meta-analysis of 51 randomized controlled trials (RCTs) sought to answer that question. The research looked at 899 runners across multiple studies, analyzing whether compression garments make runners faster or help them run longer (in scientific speak—do they improve time to exhaustion?)


The answer was a resounding “no.”

Wearing compression garments did not significantly improve race times or extend time to exhaustion. Compression gear also failed to improve running speed, oxygen uptake, or tissue oxygenation—key factors for endurance performance. The one clear benefit? Compression garments significantly reduced soft tissue vibration, which could aid in muscle recovery and reduce fatigue perception—not an unimportant finding but perhaps meaningless because this didn’t appear to translate into race-day performance (that’s all we really care about after all).


The study also found no difference in effectiveness based on garment type, race type, or running surface—meaning those pricey compression tights probably won’t make a difference whether you’re racing a 5K or a marathon.


Key takeaways for runners (and their wallets): Compression won’t make you run faster—but it might help with recovery. If you like wearing compression gear, keep using it—but don’t expect a performance edge.

You can find the full text of this study in the Journal of Sport and Health Science.

Most Runners Can’t Identify Their Foot Strike Pattern—Here’s Why It Matters

Think you know how your foot hits the ground when you run? You might want to think again. A recent study reveals that over half of endurance runners incorrectly identify their foot strike pattern—and it could be impacting their performance and injury risk.


Researchers from the University of Florida (my alma mater) analyzed data from 710 endurance runners who had undergone biomechanical gait analysis. Runners were asked to self-report their foot strike as rearfoot (heel-first), non-rearfoot (midfoot or forefoot), or "don’t know." Their actual foot strike was determined using high-speed video and 3D motion capture during treadmill running.


Most runners were spectacularly poor at identifying their foot strike. Only 42.7% of runners accurately identified their foot strike pattern. Rearfoot strikers were particularly bad at this—just 34% got it right, compared to 70% of non-rearfoot strikers. Furthermore, a whopping 81% of runners who said they didn’t know their foot strike pattern were rearfoot strikers. These runners—the “don’t know” group—had the highest prevalence of running-related injuries.


Shoe characteristics were also important for determining injury risk. Shoes with a high heel-to-toe drop were linked to lower accuracy in identifying foot strike patterns and a higher risk of sustaining a running-related injury. Non-rearfoot runners tended to wear lighter shoes with lower heel heights and heel-to-toe drops, which may aid in their more accurate self-assessment.

Rethinking your gait now? Here’s what you can do to apply these findings:


  • Get a gait analysis: If you’re unsure about your foot strike—or think you know—it’s worth having it professionally assessed. Many runners are surprised to learn they’ve been misinterpreting their form. I’ve had several of these done throughout my career (a few by the same researchers who conducted this study!) and each has given me valuable takeaways that have improved my running and reduced my injury risk.


  • Pay attention to the heel-to-toe drop, weight, and height of your running shoes: A lower drop might help you better sense your strike pattern and reduce injury risk. This doesn’t mean you need to go minimal…but something to consider.


  • Don’t change your gait blindly (or too quickly): The study showed that trying to alter your foot strike without proper understanding can backfire, increasing injury risk. Work with a coach or specialist if you’re planning to modify your gait.

More on this study in the journal Frontiers in Sport and Active Living.


Fueling for the Long Haul—How Carbs Influence Your Durability

Endurance events are generally run at a consistent moderate intensity, but races can be stochastic—with speed and intensity increasing at certain points and decreasing at others in an unpredictable fashion. 


Tour de France cyclists experience this oscillatory nature of endurance events often—they need to respond to quick and large changes in power output to match a surge or stay with the Peloton on a steep climb. After hours of pounding the pavement or in the saddle battling fatigue, these surges can be tricky. But fueling properly—with carbohydrates—could allow us to handle intensity transitions better.


A recent study explored how consuming carbs during prolonged cycling impacts performance, particularly at key intensity thresholds. From steady efforts to all-out sprints, the findings reveal how carbs can help keep you in the race.


To appreciate the study’s findings, it’s helpful to understand the exercise intensity zones analyzed:


  • VT1 (first ventilatory threshold): This marks the moderate-to-heavy intensity transition—the breakpoint in the relationship between oxygen uptake and ventilation (breathing rate). Power output at VT1 was used as a marker of metabolic efficiency, with moderate intensity below VT1 characterized by minimal metabolic disturbances and heavy intensity (above VT1) marked by rising blood lactate and oxygen consumption.


  • Severe-intensity domain: Defined by its unsustainable nature, this domain involves maximal effort where physiological variables like lactate and oxygen consumption spike to their limits.

Researchers recruited 12 trained cyclists and triathletes (10 males, 2 females, average age 31) who completed a test of their power output at VT1 and a 5-minute time trial on fresh legs to determine their baseline performance. Then they did the same two tests after 2.5 hours of moderate-intensity cycling, during which they consumed 60 grams of carbohydrates per hour on one occasion and no carbs (the placebo) on another.


Carbohydrate ingestion partially mitigated the decline in power output at VT1, the moderate-to-heavy intensity transition: Power output dropped by 3% in the carb condition and by 6% in the placebo condition compared to the “fresh” condition.


In the 5-minute time trial, power output also declined in both conditions compared to baseline: Carb consumers saw a 4% decrease vs. baseline while the placebo consumers saw a 10% decrease.


These findings provide a couple of practical takeaways that can help you fuel smarter for better durability:


  • Consuming 60 grams of carbs or more per hour during prolonged exercise could help you when you need to kick-start the engines in the middle of or at the end of a race or training session, especially at the transition from moderate to heavy exercise—a critical zone for pacing in endurance events like the marathon which is run somewhere just below this intensity.

  • Understanding where you are on the intensity spectrum can help you optimize your fueling strategy for performance. If you’re going out on a long run that’s nowhere near the moderate-to-heavy transition, you might not need an aggressive fueling strategy. If you’re planning a 20-mile marathon long run workout with higher-intensity efforts sprinkled throughout, bring along some fuel!

This study was published in the European Journal of Applied Physiology.

Short stuff you don’t want to miss

A Deep Dive into the World of Peak Endurance Performance with Olav Aleksander Bu


If you’re an endurance athlete looking to level up your performance—and don’t mind going deep into the weeds of physiology—this episode of The Peter Attia Drive with sports scientist Olav Aleksander Bu is packed with insights. Bu breaks down the key metrics that matter most—like VO2 max, lactate threshold, and functional threshold power (FTP)—and why testing these consistently (not just once in a while) is critical for making real progress. He also dives into the nuances of training across different sports and the evolving science of fueling, especially when it comes to using carbs efficiently during long races. One of my favorite parts of the discussion was the talk of the “nutrition technology” that’s changing the endurance fueling game.


HERE’S WHAT ELSE YOU WOULD HAVE RECEIVED this week if you were a subscriber to the complete, full-text edition of “Run Long, Run Healthy.” SUBSCRIBE HERE.


  • Fueling for the Long Haul – How Carbs Influence Your Durability

  • Training Load Metrics Predict Carb Use and Energy Burn

  • Strength Training for Runners—Does It Really Make You Faster and Injury-Free?

  • Taking Time Off From Training Doesn’t Destroy Your Fitness. You Might Come Back Stronger

  • Strava PR vs. Real PR: The Great Running Debate

  • How Much Protein Do Runners Really Need?

  • Steve Prefontaine on the “art” of running.

That’s all for now. Thanks for reading. As always—Run Long, Run Healthy.

~Brady~