Heat and Your Health: What Summer Really Does to Your Blood

Heat + Health, Decoded
Summer should feel like a break, but sometimes it feels more like burnout. If you’ve ever felt unusually fatigued, foggy, or irritable in the heat, you’re not imagining it. Hot weather doesn't just make you sweat, it alters your internal balance in subtle but measurable ways.

While most people focus on hydration and sunscreen, few consider the physiological shifts triggered by heat exposure. These changes can show up in your blood values, affecting everything from mood to immunity.

Here, we debunk some common heat-health myths and show how blood tests from Aware can uncover what’s really going on in your body during summer stress.

Myth 1: “As long as I drink water, I’m fine in the heat.”

Reality: While staying hydrated is essential in hot conditions, drinking plain water alone may not be sufficient, especially during prolonged heat exposure or physical activity. Sweating results in the loss of vital electrolytes, including sodium, potassium, and magnesium, which are critical for maintaining nerve function, muscle contraction, and fluid balance.

According to Baker (2017), sweat contains significant concentrations of sodium and chloride, and smaller but physiologically relevant amounts of potassium and magnesium. Rehydrating with plain water, without replacing lost electrolytes, can lead to hyponatremia (low blood sodium), which causes symptoms like fatigue, confusion, and in severe cases, seizures.

What this means:

• Sodium helps maintain blood pressure and nerve function.
• Potassium supports muscle contraction and heartbeat regulation.
• Magnesium is essential for energy metabolism and neuromuscular coordination.

Heat-induced losses of these minerals can affect cardiovascular performance and mood regulation, even without visible signs. Electrolyte replacement with supplements, balanced meals, or tested nutrient levels is essential for maintaining physiological stability in heat.

Aware’s Nutrition Test checks your magnesium, iron and vitamin D levels, each of which plays a critical role in how your body responds to summer stressors like heat and sweat-related losses.

Myth 2: “Your blood values don’t change just because it’s hot.”

Reality: Heat exposure causes real, measurable changes in your blood composition, even in healthy people. One common response is plasma volume expansion, which dilutes blood components such as hemoglobin, hematocrit, and red blood cell concentration. This can give the illusion of anemia or impaired oxygen transport, despite no actual loss in red blood cell mass.

A study published in "Medical Aspects of Harsh Environments" shows that heat acclimation increases plasma volume as a protective mechanism to support blood pressure and cooling during thermal stress. As a result, blood tests during or after heat exposure may temporarily underrepresent red cell markers.

This physiological dilution doesn’t necessarily signal poor health, but if you’re tracking performance, recovery, or chronic fatigue, it’s important to differentiate between temporary fluid shifts and genuine deficiencies.

The Aware Sport Test helps clarify the picture by measuring hemoglobin, red blood cells, hematocrit and more. By understanding your baselines, you can interpret changes with greater accuracy, especially when training or traveling in extreme heat.

Myth 3: “Heat only affects you if you’re exercising outside.”

Reality: Physical activity increases your internal temperature, but even passive heat exposure (just being in hot environments) places stress on your cardiovascular and metabolic systems. This holds true whether you're exercising or simply enduring a heatwave indoors.

Research published in the "Journal of the American College of Cardiology" found that both low and high ambient temperatures significantly raise cardiovascular risk, particularly in vulnerable populations. The body responds to heat by increasing heart rate, vasodilation, and sweating, all of which put additional strain on the heart and blood vessels, even at rest.

Over time, chronic exposure to high temperatures can also raise oxidative stress, a process that damages cells and contributes to aging and inflammation, even without exercise. This effect is especially amplified in urban environments with limited airflow and poor heat dissipation.

The Aware Healthy Heart Test measures markers like hs-CRP (a key indicator of low-grade inflammation) and blood lipids, both of which are sensitive to thermal stress. These insights help you understand how heat is affecting your cardiovascular resilience and where early action may help.

Myth 4: “Summer heat boosts immunity. You only get sick in winter.”

Reality: While it’s true that respiratory infections are more common in colder months, heat stress can actually weaken your immune system, increasing your vulnerability to illness, especially gut-related infections.

Elevated temperatures stimulate the release of stress hormones like cortisol, which suppress immune cell activity and reduce the body’s ability to respond to pathogens. In addition, heat increases oxidative stress, disrupting the protective barrier of the gastrointestinal tract and making it more permeable to harmful bacteria.

One study found that chronic exposure to heat led to immune suppression and oxidative damage, impairing both cellular immunity and barrier function. This can contribute to a higher risk of infections, especially when combined with dehydration or poor recovery.

The Aware Nutrition and Healthy Heart Tests can help identify hidden factors that undermine immune resilience, including low vitamin D, chronic inflammation, and nutrient imbalances that are common in summer.

Myth 5: “I don’t need vitamin D in summer. I get plenty from the sun.”

Reality: Sunlight is crucial for vitamin D production—but many people still show insufficient levels during summer. Why? Because vitamin D synthesis depends on direct exposure to UVB rays, which are often blocked by sunscreen, shade, clothing, and even glass.

Even in sunny climates, vitamin D deficiency can persist. In a clinical study, individuals living in Hawaii who spent more than 11 hours per week outdoors still had suboptimal vitamin D levels. Another review explained that the angle of the sun, latitude, and air pollution also affect how much vitamin D your body can make, even in summer months.

Why it matters:
Vitamin D is essential for supporting immune defenses, maintaining bone strength, and regulating inflammation. Even if your levels decline during summer, the effects can be subtle at first - yet over time, an insufficiency may increase your vulnerability to illness, fatigue, or other health concerns.

The Aware Nutrition Test includes vitamin D, helping you determine whether you’re actually getting enough, rather than just assuming you are.

Myth 6: “These changes go away instantly when the heat ends.”

Reality: While some physiological changes caused by heat, like increased plasma volume, may normalize once temperatures drop, not all effects resolve so quickly. Repeated or prolonged exposure to high temperatures without proper recovery can lead to persistent disruptions in key systems like inflammation regulation, hormone balance, and nutrient metabolism.

A study published in the Journal of Thermal Biology found that chronic heat stress leads to sustained oxidative stress and a weakened immune system, even beyond the immediate period of exposure. This means the body’s defenses may remain compromised, especially if recovery strategies such as rest, hydration, and proper nutrient intake aren’t prioritized.

Why it matters:
If you experience lingering fatigue, low mood, or frequent infections after heatwaves or hot-weather travel, your biology may still be recovering. Understanding how your body responds, by tracking inflammation markers, nutrient reserves, or hormonal shifts, can help you take targeted action.

The Aware All Biomarkers or Long-Term Health panel tracks systemic inflammation, iron metabolism, vitamin D, hormone status, and more - providing a comprehensive picture of whether your body has fully rebounded or needs support.