Exercise triggers a cascade of chemical signals that reaches virtually every organ in your body. The list is long and still growing: endocannabinoids, endorphins, adrenaline, noradrenaline, cortisol, growth hormone, dopamine, serotonin, lactate, nitric oxide, and dozens of muscle-derived signaling proteins called myokines, among others. Some of these chemicals spike within seconds of your first stride; others build over minutes or hours. What makes the picture genuinely interesting is that many of these molecules do not just help you exercise but reshape your brain, your metabolism, and your mood long after you cool down.
Endocannabinoids and the Runner’s High
For decades, the euphoric feeling after a long run was credited to endorphins, the body’s own opioids. That story turns out to be incomplete, and possibly wrong in its emphasis. A double-blind study of 63 recreational exercisers found that blocking opioid receptors with a drug called naltrexone did not prevent the euphoria or anxiety reduction that followed 45 minutes of running. Endocannabinoid levels, however, roughly doubled after the run and tracked closely with the mood boost.1PubMed Central. Do Endocannabinoids Cause the Runner’s High? Evidence and Open Questions Mouse studies reinforced this: when researchers knocked out cannabinoid receptors in the brain, the animals lost the anxiety-reducing and pain-relieving effects of running, even though their endorphin systems were intact.2PubMed Central. A runner’s high depends on cannabinoid receptors in mice
Endocannabinoids are lipid molecules your body makes that bind to the same receptors as the active compounds in cannabis. The two main ones, anandamide and 2-AG, rise reliably during moderate-intensity endurance exercise. They can cross into the brain more easily than endorphins can, which is one reason researchers now think they play the bigger role in that calm, slightly dreamy post-run feeling. A recent narrative review argues the runner’s high is best understood not as a single chemical event but as an orchestra, with endocannabinoids as the conductor and endorphins, serotonin, dopamine, and noradrenaline each contributing a section.3PubMed Central. The Neurochemical Orchestra of the Runner’s High: A Narrative Review of Neuromodulatory Mechanisms with a Focus on Endocannabinoids
Adrenaline and Noradrenaline
If endocannabinoids are the slow reward, adrenaline and noradrenaline are the fast ignition. These catecholamines are the chemicals most dramatically elevated by exercise, rising anywhere from about one-and-a-half to more than twenty times their resting levels depending on how hard and how long you work out.4PubMed. Catecholamines and the effects of exercise, training and gender They surge within seconds, dialing up heart rate, opening airways, and mobilizing stored fuel from glycogen and fat deposits so your muscles have something to burn.5PubMed. Physical stress and catecholamine release
The noradrenaline response is shaped by both absolute and relative intensity. After you become fitter through training, your body actually releases more noradrenaline at the same relative effort level, even though your heart rate stays roughly the same. This seems to reflect a more efficient sympathetic nervous system rather than greater stress.6PubMed. Norepinephrine response to exercise at the same relative intensity before and after endurance exercise training For the exerciser, this is largely invisible; you just feel more alert, focused, and energized.
Cortisol and the Intensity Threshold
Cortisol gets a bad reputation as “the stress hormone,” but its role during exercise is more nuanced than that label suggests. At low intensity, around 40 percent of maximum capacity, cortisol levels actually drop once you correct for changes in blood volume and the body’s natural daily rhythm. At moderate intensity (around 60 percent of max), cortisol rises about 40 percent above baseline. Push to high intensity (80 percent of max) and it jumps roughly 83 percent.7PubMed. Exercise and circulating cortisol levels: the intensity threshold effect In other words, there is a clear threshold: easy movement calms the stress axis, while hard training activates it.
That activation is not inherently bad. Cortisol helps free up glucose and fatty acids when your muscles need them most. The concern arises when prolonged high-intensity training occurs without adequate recovery, pushing cortisol chronically high and potentially tipping the balance toward muscle breakdown.8Journal of Exercise and Nutrition. The Effects of Different Exercise Intensities and Modalities on Cortisol Production in Healthy Individuals: A Review For most people doing moderate-duration workouts, the cortisol bump is temporary and useful.
Growth Hormone, IGF-1, and Testosterone
Resistance exercise in particular provokes a release of anabolic hormones. Growth hormone rises during and after a hard lifting session, with the magnitude depending on which form of the hormone you measure and what kind of workout you performed. Insulin-like growth factor 1 (IGF-1), which mediates many of growth hormone’s tissue-building effects, shows variable increases after resistance training and is tied to an array of binding proteins that fine-tune where and how it acts.9PubMed. Recovery responses of testosterone, growth hormone, and IGF-1 after resistance exercise Testosterone also rises acutely after heavy lifting, though the magnitude is modest and short-lived. Together, these hormones contribute to the repair and growth of muscle tissue in the hours and days following exercise, though the acute post-workout spike alone is not what drives long-term muscle gain. Consistent training over weeks and months is what changes the hormonal environment enough to matter.
Lactate as a Chemical Messenger
Lactate spent decades as the villain of exercise physiology, blamed for muscle fatigue and that burning sensation during hard effort. The science has moved on considerably. Lactate is now recognized as a fuel source, a metabolic buffer, and a signaling molecule with effects that extend throughout the body.10PubMed Central. Lactate as a Signaling Molecule That Regulates Exercise-Induced Adaptations
When muscles contract hard, they produce lactate and release it into the blood, where it is taken up by the heart, liver, brain, and other organs as a ready-to-use energy source. Beyond fueling other tissues, lactate acts on receptors expressed across a wide range of cell types, influencing fat metabolism, immune regulation, inflammation, and wound healing.11Journal of Yeungnam Medical Science. Lactate: a multifunctional signaling molecule One of its more striking effects is in the brain: lactate produced by exercising muscles crosses the blood-brain barrier and stimulates production of brain-derived neurotrophic factor (BDNF) in the hippocampus, a region central to learning and memory.12PubMed Central. Lactate Mediates the Effects of Exercise on Learning and Memory through SIRT1-Dependent Activation of Hippocampal Brain-Derived Neurotrophic Factor (BDNF) BDNF acts like fertilizer for nerve cells, promoting new connections and supporting the growth of neurons. This is one of the clearest molecular links between physical exercise and improved cognitive function.
Myokines From Contracting Muscles
Skeletal muscle is now understood to be a full-fledged endocrine organ. When muscles contract during exercise, they secrete hundreds of signaling proteins collectively called myokines. Two of the most studied are interleukin-6 (IL-6) and irisin.
IL-6 is the most abundantly produced myokine during exercise.13PubMed Central. Anti-Inflammatory Effect of Muscle-Derived Interleukin-6 and Its Involvement in Lipid Metabolism This is confusing at first, because IL-6 is also a well-known inflammatory signal produced by the immune system during infections. The paradox is that muscle-derived IL-6 behaves differently: it triggers anti-inflammatory pathways and helps regulate fat metabolism. It appears that context matters enormously. Chronic, low-grade IL-6 from immune cells in an unhealthy body drives inflammation; acute, pulsed IL-6 from contracting muscles drives the opposite response.14PubMed Central. Interleukin-6 myokine signaling in skeletal muscle: a double-edged sword?
Irisin, the other headline myokine, is produced when muscles cleave a membrane protein during contraction. Its primary claim to fame is “browning” white fat, converting energy-storing fat cells into energy-burning ones that generate heat.15PubMed Central. Irisin: A Hope in Understanding and Managing Obesity and Metabolic Syndrome Research has shown that irisin secretion during exercise mirrors the pattern seen during shivering, suggesting this mechanism may have originally evolved from cold-exposure responses and was later repurposed by physical activity.16PubMed Central. Irisin and FGF21 are cold-induced endocrine activators of brown fat function in humans
How Exercise Changes Appetite Hormones
You might expect that burning a lot of calories would make you ravenously hungry, but exercise often does the opposite, at least temporarily. A study of healthy young adults found that a bout of vigorous exercise reduced levels of ghrelin, the main hunger-stimulating hormone, by about 17 percent while increasing GLP-1, a satiety hormone, by about 13 percent.17PubMed Central. The Effects of Exercise on Appetite-Regulating Hormone Concentrations over a 36-h Fast in Healthy Young Adults: A Randomized Crossover Study Another appetite-suppressing hormone, peptide YY, also tends to rise after moderate-to-vigorous exercise.18PubMed. The impact of acute exercise on appetite control: Current insights and future perspectives
These shifts are transient. Hormone levels typically return to baseline within a few hours, and people do not usually overcompensate by eating more later that day. This helps explain why exercise can support weight management without triggering runaway hunger, even if it is not a magic appetite eraser either.
Nitric Oxide and Blood Vessel Health
During exercise, blood flows faster through your arteries, creating a physical force called shear stress against the vessel walls. This mechanical stimulus triggers the inner lining of blood vessels to produce nitric oxide, a gas that relaxes arterial walls and lowers blood pressure.19PubMed. The effect of physical exercise on endothelial function The effect is immediate during a workout, but regular exercise also increases the body’s baseline capacity to produce and maintain nitric oxide over the long term. This is one of the most important mechanisms behind the cardiovascular benefits of being physically active.
Adenosine, a molecule released when muscle cells break down their energy currency during contraction, plays a complementary role. In exercising muscle, adenosine accounts for roughly 20 to 40 percent of the blood vessel dilation that sustains blood flow during work.20PubMed Central. The roles of adenosine and related substances in exercise hyperaemia Interstitial adenosine concentrations in human skeletal muscle rise in proportion to how hard the muscle is working.21PubMed. Adenosine concentrations in the interstitium of resting and contracting human skeletal muscle Adenosine is also the molecule that caffeine blocks to keep you feeling alert, which is why a cup of coffee and a good workout can feel like they are pulling some of the same levers.
The Kynurenine Pathway and Protection Against Depression
One of the more remarkable discoveries of the past decade connects trained skeletal muscle to mental health through a biochemical detour. Kynurenine is a substance produced from the amino acid tryptophan. When kynurenine accumulates in the blood and enters the brain, it can be converted into toxic metabolites that are associated with depression and neuroinflammation. Exercise-trained muscles express higher levels of enzymes that convert kynurenine into kynurenic acid, a form that cannot cross the blood-brain barrier. This effectively diverts a potentially harmful substance away from the brain before it can do damage.22Cell. Skeletal Muscle PGC-1α1 Modulates Kynurenine Metabolism and Mediates Resilience to Stress-Induced Depression
In mouse studies, animals with elevated levels of the relevant muscle protein were resistant to depression induced by chronic stress or direct kynurenine injections. This finding has broader implications: it suggests that skeletal muscle, by clearing kynurenine from the bloodstream, acts as a kind of metabolic shield for the brain.23PubMed. Kynurenines: Tryptophan’s metabolites in exercise, inflammation, and mental health It is a compelling example of how the chemicals released during exercise can protect organs far removed from the muscles doing the work.
Natriuretic Peptides and the Heart
The heart itself gets in on the chemical signaling during exercise. Cardiac natriuretic peptides, produced when the heart walls stretch under increased blood flow, help regulate blood pressure by promoting sodium excretion through the kidneys. Recent research has added new dimensions to these molecules: they appear to influence energy expenditure in both skeletal muscle and fat tissue, contributing to thermogenesis in fat and oxidative capacity in muscle.24PubMed Central. The Effects of Exercise on Natriuretic Peptides in Individuals without Heart Failure Exercise acutely increases their release, particularly atrial natriuretic peptide (ANP).25PubMed. Differential response of the natriuretic peptide system to weight loss and exercise in overweight or obese patients This means the heart is not just pumping blood during a workout; it is broadcasting hormonal signals that influence how fat and muscle handle energy.
Exerkines and Whole-Body Crosstalk
The term “exerkines” has emerged as an umbrella for any molecule released into circulation during exercise that affects another tissue. Muscles are the biggest contributors, but the liver, fat, bone, and even the gut release signaling molecules during physical activity. Some of these travel as free proteins in the blood, but others are packaged inside tiny membrane-bound bubbles called exosomes, which can deliver cargo including small RNA molecules and proteins to distant cells.26PubMed Central. Exerkine-loaded exosomes in muscle aging: a nexus of exercise, regeneration, and crosstalk This exosome-mediated signaling is a relatively new frontier, but early evidence suggests it plays a role in counteracting age-related muscle loss, modulating inflammation, and coordinating metabolic changes between muscle and fat tissue.27PubMed. Factors mediating exercise-induced organ crosstalk
Osteocalcin, a hormone released by bone cells, is another example of an organ you would not normally think of as endocrine stepping into a signaling role during exercise. It affects an unexpectedly wide range of physiological processes, including the acute stress response. Heat shock proteins, produced when cells experience thermal or mechanical stress during vigorous exercise, serve as danger signals that prime the immune system.28PubMed Central. Human resting extracellular heat shock protein 72 concentration decreases during the initial adaptation to exercise in a hot, humid environment Even ketone bodies shift during exercise: at moderate fasting levels, physical activity enhances their production by the liver while muscles adjust their uptake to spare ketones for the brain.29PubMed. Response of ketone body metabolism to exercise during transition from postabsorptive to fasted state
When You Exercise Changes What Gets Released
The chemical response to exercise is not identical at every hour of the day. Research measuring hormonal responses to the same workout performed at different times found that the glucose drop during exercise was about 50 percent larger around midnight compared to the afternoon or early morning. Cortisol, which peaks naturally in the morning, was at its lowest and least responsive to exercise late at night. Growth hormone and thyroid-stimulating hormone also showed distinct circadian patterns in their exercise responses. These findings point to a genuine interaction between your body’s internal clock and the chemical signals triggered by a workout. For most people, the practical takeaway is modest: exercising at any time of day produces meaningful chemical benefits, but the hormonal profile of a morning session is not a carbon copy of a late-night one.