Physiology Friday #269: Carbohydrates Help Fuel Heat Adaptation

Plus, nutrition for injury and immune health and what Norway gets right about endurance training.

Greetings!

For those who aren’t aware, I’ve been authoring another newsletter for the last several months. It’s called ‘Run Long, Run Healthy’, and I sort of “inherited” it from legendary runner and author Amby Burfoot.

It’s a weekly, short(ish) breakdown of the latest research mostly related to running (but applicable to all types of training and life). Today I’m sharing one of the most recent issues instead of my usual Friday newsletter as a teaser. If you enjoy, consider subscribing (free and paid versions available; what I’m sharing is the full-length paid version).

Thanks for the support!

High-Carb Fuels Heat Adaptation

Hot-weather training can be brutal—especially before we’ve adapted properly. Our heart rate is abnormally high, we sweat profusely, and our legs feel heavier than normal. Heat acclimation helps, but it doesn’t always guarantee a performance bump. Some athletes even get slower immediately after a heat block due to the heightened stress. What’s the explanation?

A new study has a potential answer: carbohydrates. Specifically, it asked whether eating more carbs during a heat acclimation protocol can rescue performance right after the heat stress ends. As someone who’s struggled through summer heat training and wondered if more carbs might help, this study immediately piqued my interest.

Researchers took 14 healthy men and divided them into two diet groups during a 6-day high-intensity heat acclimation protocol. The high-carb group consumed 70% of their total calories from carbs while the control group ate 35% of calories from carbs.

Both groups went through the same heat acclimation protocol and then completed a 3.22 km time trial in the heat, both 1 day and 5 days after the protocol.

Both groups successfully acclimated. Lower heart rates, lower core and skin temps, and higher sweat rates showed the physiological adaptations were there. But performance diverged. The high-carb group ran significantly faster 1 day post-heat acclimation—experiencing a 7% improvement vs. 0.7% in the control group. They also completed the time trial about 1 minute and 48 seconds faster than the control group at this time point. Five days post-heat acclimation, the high-carb group had an 8.4% improvement in performance vs. a 2.8% improvement in the control group.

Interestingly, the difference wasn’t due to heat strain. Both groups had similar heat stress markers and acclimation responses. The carbs seemed to influence performance, not adaptation per se.

What this means for runners

If you're doing short-term heat training to prep for a summer race, don’t skimp on carbs. Even if you feel like your body is adapting (you’re sweating more, your heart rate is dropping), performance can still suffer if your glycogen tanks are low.

Carbs seem to act like a performance insurance policy during heavy heat blocks, likely preserving muscle glycogen, supporting central fatigue resistance, and maybe even helping with recovery from daily heat exposure. Target 60–70% of your daily calories from carbs during any heat-focused training block lasting 5–7 days and remember to fuel during workouts in the heat just like you would for a race—don’t treat these sessions as “just training.” This isn’t the time to try to train low.

Returning from Injury Means Prioritizing Nutrition

We talk a lot about rehab protocols, strength training, and cross-training post-injury. But what about the role of protein timing? Micronutrient status? Energy availability? A new review paper puts nutrition squarely at the center of the injury-recovery-immune function triangle—and every endurance athlete should take note.

This wasn’t a traditional intervention study but rather a review and synthesis of current evidence by experts in clinical nutrition and sports immunology.

Here are the key insights:

Injury and poor nutrition lead to a vicious cycle. Injury disrupts normal training and reduces physical activity, lowering the beneficial effects of exercise on immune and metabolic health. If nutritional intake isn’t adjusted, the risk of prolonged recovery and immune dysfunction increases.

Energy intake and protein are crucial. Low energy availability is a huge red flag because impairs wound healing, increases muscle loss, and delays return to sport. Protein intake should be 1.6–2.5 g/kg body mass/day (during injury as well as training), with ~20–30g per meal, evenly spaced throughout the day. Whole-food proteins (like beef or eggs) may outperform supplements for stimulating muscle protein synthesis.

Micronutrients matter too. Deficiencies in vitamin D, iron, zinc, and omega-3s are common culprits in immune dysfunction and impaired recovery. Supplementation is typically only advised when deficiencies are present, and excessive antioxidant supplementation (like megadoses of vitamin C or E) can blunt training adaptations, so be wary of using these, especially during fitness-building periods.

Our immune system needs fuel too. During illness or recovery, immune cells require more glucose, amino acids, and antioxidants. Proper nutrition not only boosts immune resistance but also enhances tolerance (i.e., reducing excessive inflammation that can damage tissues). So if you feel like you’re coming down with an illness or doing a lot of traveling or mingling, make sure to eat enough.

What this means for runners

This review is a wake-up call for endurance athletes who pride themselves on pushing through injuries and skimping on calories in pursuit of race weight. If you’re injured or recovering, don’t skimp on nutrition. And even if you’re not injured, regular nutrition assessments—especially in high-load training periods—can prevent issues before they arise.

Train The Norwegian Way

When it comes to endurance performance, Norway punches way above its weight. A small country with a small talent pool, and yet, in endurance sports from cross-country skiing to triathlon to distance running, they’re consistently on the podium. What are they doing differently?

A new study gives us a rare behind-the-scenes look. Researchers interviewed 21 top Norwegian coaches working with Olympic-level endurance athletes across eight endurance sports (cross-country skiing, biathlon, cycling, orienteering, triathlon, athletics, rowing, and swimming) to understand their approach to training, periodization, and performance development (yes, the Norwegian “double threshold” model makes an appearance).

They distilled 20 best-practice themes across these categories. Here's a breakdown of what they found.

Coaches emphasized long-term development over short-term results. They used detailed performance models to guide both daily training and season planning. Training volumes were high and progressively increased, especially during the athlete’s junior-to-senior transition.

Across all sports, coaches structured training around intensity zones, using zone 1 (low-intensity) for ~80% of training. Threshold training (zone 2) and high-intensity work (zone 3) were prescribed with precision. While the now-famous double threshold model was used in running and cross-country skiing, it wasn’t universally applied.

All coaches incorporated year-round strength training—not just for injury prevention but also for performance. Strength work was sport-specific, periodized, and adapted around competitions.

Coaches actively managed training load, sleep, and life stress, not just workout metrics. Open communication was a cornerstone for adjusting training in real time, and coaches promoted athlete autonomy and self-regulation—many athletes had input into how sessions were executed. Coaching wasn’t just about prescribing sessions, but instead, long-term, trust-based relationships. Several coaches emphasized consistency, patience, and emotional intelligence over novel training fads.

What this means for runners

Even if you’re not chasing Olympic medals, there’s a lot to learn from how elite Norwegian coaches operate: long-term thinking, low-intensity training as a cornerstone, an emphasis on strength training for whole-body development, and (perhaps most importantly), an open relationship with yourself and your coach about how training and racing are going.

You’re Not Burned-Out—Just Bored

Running is supposed to be fun. But we’ve all experienced a time when training isn’t as “exciting” as it should be. A new study takes a serious look at boredom in endurance sports. It stood out because it explores something I’ve personally felt during countless runs: that mental question of “Why am I still doing this?”

Researchers surveyed 667 mostly recreational endurance athletes (runners and cyclists), aiming to understand what drives boredom in these sports and what outcomes it leads to. They used control-value theory (CVT), a well-supported framework in educational psychology, to guide their investigation. Key concepts include how much athletes feel in control of their performance (self-concept), how worthwhile or meaningful they perceive their sport (value), and boredom (for example, mind wandering or lack of a desire to train).

Low control (poor self-concept, under- or over-challenge) and low value were strongly linked to boredom, and boredom was negatively associated with key outcomes including lower satisfaction, reduced training frequency, less experience in the sport, less cognitive effort, and a greater sense of exertion. Interestingly, runners reported more boredom than cyclists, and their boredom was more strongly correlated with reduced satisfaction and cognitive effort. High self-concept buffered boredom, especially for runners, whereas cyclists’ boredom seemed more influenced by perceived value alone.

What this means for runners

Boredom isn’t just a mood—it’s a psychological signal that your training lacks either meaning (value) or a sense of progress and control. To reduce training boredom, remind yourself why you train; avoid monotony by changing routes, shoes, gear, and even training partners; make workouts challenging (but not too challenging); and use mental tools like self-talk, music, or mentally engaging tasks during the run to enhance cognitive effort and reduce boredom.

Could a Colostrum Supplement Help Your Gut?

Colostrum—the antibody-rich fluid produced by cows in the first days after giving birth—has been hyped for immune and gut benefits. But could it actually help endurance athletes? A new randomized, placebo-controlled study tested this idea head-on in one of the most GI-distress-prone sports out there: rowing. The results were impressive (and apply to runners too!)

Researchers recruited 28 elite male and female rowers and randomly assigned them to either a colostrum group (5g/day) or a placebo group for 12 weeks. The athletes continued their regular training and were tested both before and after the intervention for their endurance capacity, gastrointestinal permeability, inflammatory markers, body composition, and muscle damage

The colostrum group saw two key benefits. For one, they improved their performance—with a significant increase in mean power output and a decrease in 2,000-meter rowing time. They also had better gut barrier function. Findings revealed a reduction in exercise-induced GI permeability alongside decreased intestinal cell damage. Interestingly, these improvements occurred without any significant changes in body mass or composition, meaning the erg gains weren’t simply from getting leaner or heavier. Meanwhile, the placebo group saw no significant changes in either performance or gut health markers.

What this means for runners

While this study looked at rowers, the findings are highly relevant to runners—especially those who compete in the heat or go long enough to flirt with GI issues. Running, like rowing, puts major mechanical and thermal stress on the gut lining. Over time or during hard efforts, this can lead to increased intestinal permeability (a.k.a. “leaky gut”), systemic inflammation, and the dreaded mid-race bathroom break. Colostrum may be a gut-protective supplement during periods of heavy training or competition. This isn’t just about avoiding distress; there may be a legitimate link between reduced GI stress and improved output. When your gut is functioning properly, you’re better able to absorb nutrients, tolerate higher carb intakes, and avoid systemic inflammation.

Men or Women: Who’s Got Better Endurance?

Do women perform better as the race distance gets longer? Or do they experience larger pace drop-offs compared to men? A new study introduces a powerful, underused metric for evaluating endurance performance: the Coefficient of Special Endurance (KsA). Think of it as a pace-loss fingerprint across events, now applied specifically to female runners.

This study helps explain where women may face greater performance drops and why. And if you’re a coach or athlete looking to better understand strengths, weaknesses, and event suitability, KsA could become your new favorite stat.

KsA quantifies the relative pace loss between two neighboring race distances (like 400m to 800m, or 5km to 10km). Previously used in male runners, this study applied KsA to over 20,000 performances by female runners from 1980 to 2022.

Results showed that KsA values for female runners have remained remarkably stable for over 40 years. Women experience more pace loss than men from 100m to 1500m. For example, from 100–200m there’s a 1.11% greater drop, and from 800–1500m there’s a 0.63% greater drop. However, there were no major sex differences beyond 1500m.

What this means for runners

KsA is a diagnostic tool. Think of it like a blood test for your running profile. Are you better suited to short bursts or long grinding intervals? This can help you decide. If your 800m performance drops off more than expected compared to your 400m, you might be better suited to longer events or need targeted high-intensity short intervals. Sprint-to-mid distance is where female pace loss is most pronounced. Training in this range may benefit from strength, anaerobic, and fast-twitch fiber-targeted work.

Of course, you don’t need to calculate your KsA to determine which events you’re best suited for or where your training could use some TLC. Most of us know intuitively whether we are better suited for short distances or whether our performance relative to our peers gets better as the miles (or kilometers) add up.