Your body makes lactic acid whenever cells break down glucose without enough oxygen to complete the process. This happens constantly, not just during exercise. Healthy adults produce roughly 15 to 20 mmol per kilogram of body weight each day through normal metabolism. The muscles, skin, red blood cells, brain, and gut are all steady producers.
How Your Cells Produce Lactic Acid
Every cell that burns glucose for energy starts with the same process: splitting a glucose molecule into two smaller molecules called pyruvate. When oxygen is plentiful, pyruvate moves into the cell’s mitochondria (its power plants) and gets converted into a large amount of energy. But when oxygen is scarce, or when energy demand spikes faster than the mitochondria can keep up, pyruvate stays in the main body of the cell and gets converted into lactate instead.
An enzyme called lactate dehydrogenase drives this conversion. The reaction also recycles a helper molecule (NAD+) that the cell needs to keep breaking down glucose. Without this recycling step, glucose processing would stall entirely. So lactic acid production is actually a workaround that lets cells keep generating energy, even if it’s a smaller amount: just 2 units of energy per glucose molecule, compared to roughly 36 when oxygen is available.
Why Your Body Makes It Even at Rest
Red blood cells are one of the body’s most prolific lactic acid producers, and they make it all the time, not just during a sprint. The reason is simple: red blood cells have no mitochondria. They physically cannot use oxygen to process glucose, so every bit of energy they produce comes through the oxygen-free pathway, generating lactate as a constant byproduct.
The skin, gut lining, and parts of the brain also produce lactic acid under normal conditions. At rest, your blood lactate level typically sits between 0.5 and 2.2 mmol/L. This baseline level reflects the ongoing production from these tissues balanced against your body’s ability to clear it.
Exercise and the Surge in Production
During intense exercise, your muscles demand energy far faster than oxygen delivery can support. Muscle cells shift heavily toward the oxygen-free pathway, and lactate production rises sharply. This is why your legs burn during an all-out effort: the local concentration of lactate and associated acid byproducts temporarily outpaces what your blood can shuttle away.
But lactic acid doesn’t just pile up. Your liver runs a recycling loop called the Cori cycle. Lactate travels through the bloodstream to the liver, which converts it back into glucose through a process called gluconeogenesis, essentially reversing the original breakdown. That freshly made glucose then returns to the muscles through the blood, ready to be used again. This cycle keeps working during and after exercise, and blood lactate levels typically return to their resting range within 30 to 60 minutes after you stop.
Lactic Acid Does Not Cause Next-Day Soreness
One of the most persistent fitness myths is that lactic acid causes the muscle soreness you feel a day or two after a hard workout. Research has directly tested this idea and found no connection. In one well-known experiment, runners on a flat treadmill produced significantly elevated blood lactate but experienced no soreness afterward. Runners on a downhill treadmill never showed elevated lactate at all, yet they developed significant delayed-onset soreness. That soreness comes from microscopic damage to muscle fibers during the exercise itself, particularly during movements where muscles lengthen under load (like running downhill). Lactic acid is long gone from the tissue by the time soreness peaks.
When Lactic Acid Becomes a Problem
Because the body clears lactate efficiently under normal circumstances, elevated levels in a blood test are a red flag. When blood lactate climbs well above the normal 2.2 mmol/L ceiling and stays there, it signals that tissues are not getting enough oxygen, or that the liver cannot keep up with clearance. This condition, called lactic acidosis, is not something caused by exercise. It’s associated with serious medical situations: severe infections (sepsis), heart failure, liver disease, significant blood loss, or anything that disrupts oxygen delivery to tissues on a systemic level.
How Bacteria Make Lactic Acid
Your body isn’t the only source. Bacteria produce lactic acid through fermentation, and this process is behind some of the most familiar foods in your kitchen. Yogurt, sauerkraut, kimchi, sourdough bread, and pickles all get their tangy flavor from bacterial lactic acid production. The bacteria responsible, broadly called lactic acid bacteria, feed on sugars in milk, vegetables, or flour and excrete lactic acid as their primary waste product.
On an industrial scale, specific strains of Lactobacillus are the workhorses of commercial lactic acid production. These bacteria can ferment sugars from a wide range of sources: pure glucose, molasses, dairy whey, and even agricultural waste like wood pulp byproducts. The strains used commercially are “homofermentative,” meaning nearly all the sugar they consume gets converted into lactic acid rather than a mix of byproducts. This gives yields above 99% purity. Industrial lactic acid goes into biodegradable plastics, food preservatives, cosmetics, and pharmaceutical products.
Genetically engineered strains of other bacteria, including E. coli, have also been developed to produce lactic acid from cheaper feedstocks like glycerol, reducing manufacturing costs. Certain fungi can do it too. But traditional lactic acid bacteria remain the dominant producers worldwide.
Lactate vs. Lactic Acid
You’ll sometimes see “lactate” and “lactic acid” used interchangeably, and for practical purposes they refer to the same thing. The technical distinction is about chemistry: lactic acid is the full molecule with its hydrogen ion (proton) attached, while lactate is the form that exists after that proton has been released. At the pH inside your body (around 7.4), virtually all of it exists as lactate, not intact lactic acid. This is why blood tests measure “lactate” rather than “lactic acid.” In everyday conversation and most fitness contexts, the terms mean the same thing.