When you exercise, your body produces a remarkable cocktail of chemicals: mood-boosting neurotransmitters, hormones that build and repair tissue, signaling molecules from your muscles, fuel sources, and protective antioxidants. Some of these kick in within minutes, others build up over weeks of consistent training. Together, they explain why exercise affects everything from your mood to your memory to your cardiovascular health.
Neurotransmitters That Shift Your Mood
Physical activity triggers the release of several neurotransmitters that regulate mood and emotions. The main ones are serotonin (the same chemical targeted by antidepressants), dopamine (which drives your brain’s reward system and fosters positive emotions), and norepinephrine (which sharpens alertness). These increases happen during aerobic exercise and contribute to the well-documented antidepressant effect of regular workouts.
Your body also releases endorphins, the chemicals long credited with producing the famous “runner’s high.” But the story of the runner’s high turns out to be more interesting than most people realize.
What Actually Causes the Runner’s High
For decades, endorphins got all the credit. The problem is that endorphins are water-soluble molecules that can’t cross from your bloodstream into your brain. Studies from the 1980s and 1990s failed to find a connection between peripheral endorphin levels during exercise and elevated mood, and blocking the opioid system with drugs didn’t prevent the euphoric feeling.
The real driver appears to be endocannabinoids, particularly a molecule called anandamide. These are lipid-based compounds (chemically similar to the active ingredients in cannabis) that cross into the brain easily. In a double-blind study of 63 participants, researchers found that endocannabinoid levels were twofold higher after 45 minutes of running compared to walking. Euphoria was also nearly twofold higher after running. When researchers blocked the opioid system with a drug called naltrexone, the runner’s high still occurred, and endocannabinoid release was unaffected. The conclusion: the runner’s high does not depend on endorphins. It depends on your body’s own cannabis-like chemicals.
A Growth Factor for Your Brain
One of the most consequential substances your body produces during exercise is brain-derived neurotrophic factor, or BDNF. This protein acts like fertilizer for brain cells. It supports the survival of existing neurons, promotes the growth of new connections between them, and increases the branching and turnover of the tiny spines where neurons communicate. The result is measurable improvements in learning, memory formation, and mood.
BDNF production ramps up most prominently in the hippocampus, the brain region central to memory. The mechanism is surprisingly elegant: prolonged exercise causes a ketone body to accumulate in the hippocampus, where it switches on the genes responsible for BDNF production. When researchers blocked BDNF signaling in animal studies, the cognitive benefits of exercise disappeared, confirming that BDNF is a key link between physical activity and brain health.
Hormones That Build and Regulate
Exercise at sufficient intensity is a potent trigger for growth hormone secretion. Growth hormone supports muscle repair, fat metabolism, and tissue regeneration. After a workout, serum concentrations stay elevated for roughly 105 to 145 minutes, followed by a brief suppression period as levels return to baseline. The magnitude of this response doesn’t change much based on what time of day you exercise.
Cortisol, your primary stress hormone, also rises during exercise, staying elevated for about 150 minutes afterward. Unlike growth hormone, the cortisol response is sensitive to timing. Exercising late at night produces the largest spike relative to baseline, while evening workouts produce the smallest. After a late-night session, cortisol shows a rebound suppression, dropping below normal levels for about 50 minutes. This doesn’t happen with morning or evening exercise.
Signaling Molecules From Your Muscles
Your skeletal muscles aren’t just mechanical engines. When they contract, they release a family of signaling proteins called myokines that communicate with other organs throughout your body. One of the most studied is interleukin-6 (IL-6), which plays several roles at once. Within the muscle itself, it stimulates the breakdown and burning of stored fat. It also improves the muscle’s ability to absorb glucose and respond to insulin. Beyond the muscle, IL-6 signals the pancreas to secrete insulin and prompts the liver to produce new glucose to keep you fueled.
Muscles also release proteins like irisin, myonectin, and interleukin-15, among others. These contribute to fat oxidation and metabolic regulation. The discovery of myokines has reshaped how scientists think about exercise: your muscles function as an endocrine organ, broadcasting chemical messages that influence your metabolism, immune function, and organ health.
Lactate as Fuel, Not Waste
Lactate has a bad reputation as a “waste product” that causes muscle burn, but that’s outdated. Your body treats lactate as a valuable fuel source. During moderate-intensity exercise, the flow of lactate through your blood can actually exceed the flow of glucose, making it one of the most important energy currencies your body produces.
The heart is a particularly eager consumer. As blood lactate levels rise, the heart preferentially switches to burning lactate, which can account for as much as 60% of the fuel it oxidizes. Your brain uses it too. Blood lactate concentrations during steady-state cardio typically sit around 5 to 6 millimoles per liter, climbing to about 8 during high-intensity intervals, and reaching 12 or higher during all-out efforts like Tabata-style training. After a maximal sprint lasting 30 to 120 seconds, levels can peak at 15 to 25 millimoles per liter within a few minutes of stopping.
Nitric Oxide and Blood Vessel Health
The cells lining your blood vessels produce nitric oxide, a gas that relaxes vessel walls and improves blood flow. The trigger is simple: when you exercise, blood moves faster through your vessels, creating a friction-like force called shear stress. Your vessel lining responds by producing more nitric oxide, which dilates the vessels to accommodate the increased flow.
Over time, regular exercise doesn’t just produce temporary bursts of nitric oxide. It increases the amount of the enzyme responsible for making it, and it enhances how effectively that enzyme is activated. This is one of the main reasons consistent exercise lowers blood pressure and improves cardiovascular health. The adaptation happens in both large arteries and small vessels throughout the body.
Antioxidant Defenses
Exercise generates free radicals as a byproduct of increased energy production. In response, your body upregulates its own antioxidant defense system. The key players include superoxide dismutase (SOD), which neutralizes one of the most common free radicals; catalase, which converts hydrogen peroxide into harmless water and oxygen; and glutathione peroxidase, which handles a broader range of damaging molecules using glutathione as a helper.
A systematic review of randomized controlled trials found that exercise significantly enhanced the activity of most antioxidant enzymes and overall antioxidant capacity. This is an important distinction from simply taking antioxidant supplements. Exercise creates a controlled burst of oxidative stress that trains your cells to defend themselves more effectively, building a stronger baseline defense over time.
How Intensity Changes the Mix
The chemical profile your body produces shifts dramatically with exercise intensity. During steady-state cardio at moderate effort, lactate stays around 5 to 6 millimoles per liter and perceived effort registers as “somewhat hard.” High-intensity intervals push lactate to around 8 millimoles per liter with effort feeling “hard plus.” All-out protocols like Tabata training drive lactate to roughly 12 millimoles per liter, with heart rates reaching about 85% of maximum reserve and effort rated as “very hard.”
Higher intensity generally means larger spikes in growth hormone, cortisol, and endocannabinoids. But moderate exercise still produces meaningful increases in serotonin, dopamine, BDNF, and nitric oxide. The practical takeaway is that your body produces beneficial chemicals across a wide range of intensities. Harder sessions amplify certain hormonal and metabolic responses, while moderate sessions reliably deliver the neurochemical and cardiovascular benefits that make exercise so broadly protective.