Heat Training vs Altitude Training: Benefits Explained

You’ve probably heard it before: “Running in the summer heat gives you the same benefits as altitude training.” It’s a claim that’s been circulating widely in running circles lately.

The short answer: the mechanisms behind heat training and altitude training are fundamentally different. That said, the final performance gains can be surprisingly similar.

This article breaks down the science of summer running — what actually happens in your body when you train in the heat, and why it can pay off when conditions cool down.

By the end, you’ll have a clear scientific reason to keep running through the heat.

What Heat Training Does to Your Body: 3 Key Adaptations

Running in hot and humid conditions is known as heat training. With consistent training in a hot environment, your body undergoes heat acclimation — a set of physiological adaptations to the heat. There are three main changes that occur.

Increased Plasma Volume

Heat training increases plasma volume. Plasma is the liquid component of blood, making up more than half of total blood volume.

Several mechanisms drive this plasma volume expansion.

First, when pressure builds outside the blood vessels during exercise, plasma proteins — mainly albumin — shift into the bloodstream. Because albumin attracts water, fluid is drawn from surrounding tissues into the vessels. This is the fastest-acting mechanism behind plasma volume expansion※1.

Second, as sweating and rising core temperature concentrate the blood, the kidneys release antidiuretic hormone (ADH), which increases water reabsorption. At the same time, aldosterone release causes sodium retention, and water follows via osmolality — keeping more fluid in the body※1.

As noted above, the primary reason heat training improves endurance performance is this increase in plasma volume.

Improved Sweating Efficiency

The threshold temperature for sweating drops, meaning you start sweating earlier. This limits core temperature rise and helps you maintain performance even in hot conditions.

Increased Skin Perfusion

Training in heat increases blood flow to the skin’s surface to help dissipate core body heat.

Why Plasma Volume Is the Primary Driver of Endurance Gains

Of all the adaptations from heat training, the one that most significantly drives endurance performance is the increase in cardiac output driven by greater plasma volume.

The other adaptations — improved sweating efficiency and increased skin perfusion — are specific to hot environments. Once the heat is removed, those benefits no longer apply.

Cardiac output is one of the primary determinants of VO2 max. When cardiac output increases, VO2 max improves — and with it, endurance performance.

The following study compared a group that trained in a hot environment (38°C / 100°F) with one that trained in normal conditions (13°C / 55°F), measuring the difference in endurance performance gains. It found that heat training significantly improved endurance performance even in cool conditions.

Twelve trained cyclists were assigned to a heat training group and 8 to a control group. Both groups underwent testing in normal (13°C / 55°F) and hot (38°C / 100°F) environments before and after 10 days of training. The heat group trained at 40°C (104°F) at 50% of VO2 max intensity for two 45-minute sessions per day; the control group performed the same protocol at 13°C.

The heat training group showed significant improvements across all measured parameters.

• VO2 max: +5% in normal conditions, +8% in heat
• Time trial performance: +6% in normal conditions, +8% in heat
• Power output at lactate threshold: +5% in both environments
• Plasma volume: +6.5%
• Peak cardiac output: +9.1% in normal conditions, +4.5% in heat

The control group showed no improvements in any performance or physiological parameter.

Heat Training vs Altitude Training: How the Benefits Compare

Altitude training works through a completely different mechanism: exposure to hypoxic (low-oxygen) conditions. The key benefits include:

These are adaptations unique to hypoxic exposure — they don’t occur with heat training.

The reason heat training is often compared to altitude training is that both produce similar outcomes in the blood system, even though the underlying mechanisms differ. This overlap is likely why the comparison became popular.

For altitude training to meaningfully increase red blood cells and hemoglobin, specific conditions must be met. Research indicates that staying at 2,000–2,200 m (6,600–7,200 ft) or above for at least 3–4 weeks is required for significant red blood cell increases※3. More precisely, a randomized controlled trial (RCT) showed that 2,085 m produces insufficient erythrocyte adaptation, while approximately 2,500 m (8,200 ft) is the optimal altitude※4.

Here’s how the hematological effects of heat training and altitude training compare:

Factor	Heat Training (Summer Running)	Altitude Training (Hypoxic)
Stimulus	High temperature & humidity	Low oxygen (hypoxia)
Onset of adaptation	1–2 weeks	3+ weeks
Duration of effect	Fades in ~2–4 weeks※5※6	Several weeks to months
Blood adaptation	Plasma volume increases	Red blood cell mass increases, Hb concentration rises
Circulatory effect	Higher cardiac output → lower heart rate	Lower heart rate
Primary performance driver	Increased cardiac output	Improved blood oxygen-carrying capacity

How long heat acclimation benefits last depends on the type of adaptation. Heart rate and core temperature adaptations decay at roughly 2.5% per day, disappearing completely within 3–5 weeks※5. Improvements in total hemoglobin mass and exercise performance reverse more quickly — within 2 weeks※6. To carry heat acclimation gains into a fall race, aim to complete your training buildup at least 2–3 weeks before race day.

Both heat training and altitude training produce a decrease in resting heart rate — but for different reasons.

With heat training, increased plasma volume raises cardiac output, reducing the heart rate needed to deliver oxygen. With altitude training, more red blood cells and higher hemoglobin concentrations improve oxygen-carrying capacity — and that’s what drives the heart rate down.

Can Heat Training Also Boost Red Blood Cells and Hemoglobin?

Heat training may also increase red blood cells and hemoglobin — though not to the same degree as altitude training.

The Immediate Response After Heat Training

In the early phase of heat acclimation (days 1–7), plasma volume expands rapidly. This dilutes the blood, causing hemoglobin concentration to temporarily drop.

Since total red blood cell mass hasn’t decreased, oxygen-carrying capacity remains largely intact.

What Happens Over Weeks of Training

After several weeks of consistent heat training, the hematopoietic system begins to respond — through a mechanism different from altitude training, but real nonetheless.

As plasma volume expansion dilutes hemoglobin concentration, the kidneys sense the relative drop and slightly increase erythropoietin (EPO) production. This mildly stimulates red blood cell production.

As a result, hemoglobin concentration returns toward baseline while plasma volume remains elevated — meaning total red blood cell mass can increase.

But Far Less Than Altitude Training

With altitude training, the hypoxic stimulus drives a much stronger EPO response, producing significant increases in total red blood cell mass — often several percent to over 10%. Heat training’s hemoglobin gains are modest, typically just a few percent, and some studies find no statistically significant change.

In short, some research confirms that a modest increase in red blood cells can follow plasma volume expansion from heat training — but the effect is small, and it’s virtually impossible to replicate altitude-level erythrocyte gains through heat training alone.

The graphs below illustrate how plasma volume and hemoglobin change over time under each training condition.

Heat training is best understood not as a substitute for altitude training, but as a tool for building a circulatory advantage through plasma volume expansion.

What Running Should You Do in Summer?

The key mechanism behind heat training’s performance benefits is the increase in cardiac output driven by plasma volume expansion. In the Lorenzo et al. study※2 mentioned earlier, the training intensity was just 50% of VO2 max — an easy jog. Yet plasma volume still increased by 6.5%. You don’t need high-intensity sessions to trigger summer’s unique adaptations.

What matters more is simply staying consistent in the heat. If you build plasma volume and trigger even modest hematopoietic adaptation, you’ll be physiologically primed when cooler fall and winter conditions arrive.

The best summer training options are easy jogging and sprint-based sessions with extended rest intervals. See the related article below for a practical breakdown of summer training strategies.

References

※1 Fellmann N (1992) “Hormonal and plasma volume alterations following endurance exercise. A brief review.” Sports Medicine

※2 Lorenzo S, Halliwill JR, Sawka MN, Minson CT (2010) “Heat acclimation improves exercise performance.” Journal of Applied Physiology

※3 Rusko HK, Tikkanen HO, Peltonen JE (2004) “Altitude and endurance training.” Journal of Sports Sciences

※4 Chapman RF, Karlsen T, Resaland GK, Ge R-L et al. (2014) “Defining the ‘dose’ of altitude training: how high to live for optimal sea level performance enhancement.” Journal of Applied Physiology

※5 Daanen HAM, Racinais S, Périard JD (2018) “Heat Acclimation Decay and Re-Induction: A Systematic Review and Meta-Analysis.” Sports Medicine

※6 Cubel C, Fischer M, Stampe D et al. (2024) “Time-course for onset and decay of physiological adaptations in endurance trained athletes undertaking prolonged heat acclimation training.” Temperature (Austin)