Estimated read time: 5.2 minutes. (About as long as deciding whether summer heat makes post run ice cream mandatory 🍦).

Hey Performance Nerds! Jonah here. 🤓

Altitude has long been the choice for runners chasing blood adaptations. Now heat is being discussed as the better option?

But are they chasing the same adaptation? And will either one actually make you faster?

Here's what you'll learn today:

The gel I race with and keep recommending because the formula makes sense: BETA FUEL delivers 40g of carbs per gel through a 1:0.8 maltodextrin-to-fructose ratio.

That ratio is the point. It helps you use multiple carb transporters, push higher intake, and keep the gut calmer when fueling gets serious.

Stryd gives runners power, pacing, and mechanical metrics that can make training decisions clearer when pace and heart rate do not tell the whole story.

See this week's full Stryd training tip at the end of this newsletter.

Lundby and Robach (2025) separate heat work into a short block and a long block.

The early changes help you handle hot exercise. They don't recreate an altitude camp.

Sweat cools you only when it evaporates from your skin (Racinais et al., 2023). Heat acclimation can make you start sweating earlier and increase how much you sweat as your body heats.

Your body also gets better at holding on to sodium, making your sweat less salty. Heavy sweaters can still lose plenty overall.

Plasma is the liquid part of blood. Building more is one of the main adaptations during those first 10 to 14 days.

Why does that matter? During hot runs, your blood has two jobs.

It delivers oxygen to working muscles while moving heat toward your skin. More plasma gives your heart more circulating fluid to support both jobs.

That can ease heart strain and improve your tolerance for hot running.

Clock one prepares you for hot exercise. The longer clock asks whether Hbmass rises.

Red blood cells use hemoglobin, an oxygen-carrying protein, to move oxygen from your lungs toward working muscles (Cernaro et al., 2019).

Hbmass is the total amount of that protein in circulation. More Hbmass can help your blood carry more oxygen.

A two-week block can prepare you for heat. It doesn't prove you raised Hbmass.

Clock two adds heat to normal training for about five weeks, usually five or six sessions each week. Studies often used 45 to 60 minutes of cycling at a lower effort in a hot room.

Other studies used heat-retaining clothing. Either way, the heat becomes extra work on top of normal training.

One small study put 10 well-trained runners through both a heat block and a control block. During the heat block, they took 45-minute hot baths after training, five days a week for five weeks.

Compared with their control block, Hbmass rose 33 grams and VO2max rose 2.7 mL/kg/min, or 4.4% (Jenkins et al., 2025).

A separate study reported an Hbmass rise at week three. The study measured a larger rise at week five.

Short acclimation can change plasma volume and heat tolerance, while prolonged heat may later raise Hbmass.

The theory is that more plasma lowers the share of blood made up of red cells. Your kidneys sense that shift and release EPO, a hormone that tells bone marrow, the tissue inside bones, to make red blood cells.

That response may help bring the red-cell share of your blood back toward its normal range.

At altitude, lower air pressure means less oxygen gets into your body, a state called hypoxia. In response to hypoxia, your kidneys again release EPO into the blood.

The signal starts quickly, but making enough new cells to raise Hbmass takes weeks.

More hemoglobin helps your blood carry more oxygen, partly offsetting the lower supply at altitude.

Back at sea level, that may support a higher VO2max and faster running, but neither payoff is guaranteed.

Altitude camps are measured in weeks, not weekends.

Three to four weeks is common when Hbmass is the target. A camp usually means living and sleeping in lower-oxygen air for weeks.

At the same effort, your pace or power can drop. The session can still tax your recovery while giving your legs less race-speed practice.

Responses differ between athletes and even between camps for the same athlete. Any Hbmass gain can fade after returning home, so the return date matters.

After several weeks, prolonged heat and altitude may both raise Hbmass, but their early jobs remain different.

Nybo et al. (2024) compare heat and altitude below.

Seven of 12 controlled altitude studies reported an Hbmass gain. Eight of 17 controlled altitude studies reported a performance gain.

Hbmass gains appeared more often than performance gains.

Research hasn't shown a clear extra Hbmass or performance gain from combining heat and altitude.

My default is to protect the training you can repeat.

Bottom line: Heat and altitude start with different jobs. Neither makes a faster sea-level race certain.

Choose the one that fits your race without weakening the training that already works.

Jonah

Welcome to the prove you're a nerd section. Each week, I ask a question about a common running science myth. Answer correctly, and you'll be entered into a weekly raffle to win a package of Jonah's favorite supplements.

Last week's poll asked: "A runner has a downhill marathon in four weeks. What is the best way to prepare for the extra downhill load?"

Most of you got it. Downhill running creates a specific braking load that flat running and lifting do not fully copy.

The correct answer?

A. Gradually add short downhill exposures now, then reduce them before race week

Gradual downhill exposure can build familiarity with that braking load before the race. Saving one huge downhill session for race week is more likely to create soreness than useful preparation.

Here's how the votes shook out:

A. Gradually add short downhill exposures now, then reduce them before race week: 207 votes, about 88 percent
B. Use flat intervals faster than race pace, because speed creates the same quad load: 8 votes, about 3 percent
C. Save the downhill work for the final long run, so the adaptation is freshest: 2 votes, about 1 percent
D. Rely on heavy squats, because strength work fully replaces downhill-specific exposure: 17 votes, about 7 percent

Total votes: 234

Why D was tempting:

Strength training can prepare your legs for force. It still does not fully recreate the repeated braking demands of downhill running.

Bottom line?

Use small, planned downhill exposures early enough to adapt and recover. Do not make race week your first real test.

Last fall, before my spring marathon block, I saved my marathon-pace LSS at about 8.7 kN/m.

I often get asked: how do I know whether strength work is showing up in my running?

Pace and heart rate help, but struggle to show whether strength changed how I run.

After a 10-minute warm-up, I run 15 minutes at goal marathon pace, around 6:40/mile.

I use the same treadmill when practical, matching shoes, setup, incline, pace, device, pod placement, workout position, fatigue, and recovery.

Then I did a 12-week heavy strength and plyometric block.

I used heavy deadlifts, heavy squats, heavy calf work, calf isometrics, and different plyometrics such as pogo hops.

Then I repeat that test.

I averaged about 9.0 kN/m in that post-block test.

LSS captures how stiff my whole lower body acts as a spring, with joints, muscles, and tendons working together.

Heavy lower-body strength can improve running economy.

High-load calf work and isometrics can build calf strength and Achilles stiffness.

Plyometrics train fast, reactive force.

An appropriately stiff Achilles can stretch less under load.

That lets calf fibers shorten less and more slowly, lowering energy cost.

That is the efficient spring I want to build.

I watch LSS across comparable marathon-pace treadmill tests and ask whether the same pace feels easier.

Because LSS reflects the whole system, this is one personal trend, not proof the strength block caused it.

Why it matters:

This week, save average LSS before your next strength block, then repeat the test under matched conditions afterward.

Try it on your next run