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 runnersRunning 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 |
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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 runnersIf 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 runnersFor 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 |
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