Lactate is a substance your body produces constantly as part of normal energy metabolism. A normal resting blood lactate level falls between 0.5 and 2.2 mmol/L (4.5 to 19.8 mg/dL). When levels climb significantly above that range, it usually signals that your body is producing energy faster than it can keep up with oxygen supply, or that something is interfering with lactate clearance. Doctors order a lactate test to help assess how well your tissues are getting oxygen and whether a serious condition like sepsis or shock may be developing.
How Your Body Produces Lactate
Every cell in your body breaks down glucose for energy through a process called glycolysis. One of the end products of that process is a molecule called pyruvate. When plenty of oxygen is available, pyruvate moves into the cell’s energy-producing machinery for a highly efficient fuel burn. But when oxygen is limited, or when cells are producing energy very quickly, pyruvate gets converted into lactate instead.
A common misconception is that lactate only appears when you’re oxygen-deprived. In reality, your body produces lactate continuously, even at rest and under fully aerobic conditions. Red blood cells, for instance, lack the internal structures needed for oxygen-based energy production, so they always generate lactate. Your muscles, brain, and other tissues produce it routinely as well. The amount simply increases when energy demands outpace oxygen delivery.
Interestingly, lactate production actually helps buffer acid buildup. The chemical reaction that converts pyruvate to lactate consumes a hydrogen ion, which slightly counteracts the acidity generated during rapid energy production. Lactate itself is not the acid people often assume it is. At the pH of your blood, lactic acid is almost completely split into lactate (which is neutral) and a separate hydrogen ion. The burning sensation during intense exercise comes from overall acid accumulation, not from lactate directly.
How Your Body Clears Lactate
Your liver does the heavy lifting when it comes to recycling lactate. Once lactate enters the bloodstream, the liver picks it up and converts it back into glucose through a process sometimes called the Cori cycle, named after Nobel laureates Gerty and Carl Cori. That freshly made glucose then travels back through the blood to muscles and other tissues, where it can be broken down for energy again. It’s a continuous loop: muscles produce lactate, the liver turns it back into fuel.
Your kidneys also contribute to this recycling. They take up lactate from the blood and use it as raw material for making glucose, working alongside the liver to keep lactate levels in check. This system is remarkably efficient under normal conditions. Problems arise when lactate production overwhelms the body’s ability to clear it, or when liver or kidney function is impaired.
What Causes High Lactate Levels
Elevated blood lactate generally falls into two categories based on whether oxygen deprivation is the root cause.
The most serious scenario involves tissue oxygen deprivation. When organs aren’t getting enough blood flow or oxygen, cells shift heavily toward lactate-producing energy pathways. This happens in conditions like septic shock (a dangerous immune response to infection), cardiogenic shock (when the heart can’t pump enough blood), and hypovolemic shock (severe blood or fluid loss). Cardiac or respiratory failure can also drive lactate up by starving tissues of oxygen. In these situations, a rising lactate level is a red flag that organs are in distress.
The second category covers situations where oxygen delivery is adequate overall, but something else is driving lactate up. Vigorous exercise is the most familiar example. Seizures and severe shivering can do the same thing, because muscles are contracting intensely. Certain medications, liver disease, and some cancers can also raise lactate by either increasing production or slowing clearance. A deficiency in thiamin (vitamin B1) can impair the body’s ability to process pyruvate through its normal oxygen-dependent pathway, pushing more of it toward lactate instead.
Lactate During Exercise
If you’ve ever pushed hard during a run or a cycling session and felt your legs start to burn and fatigue, you’ve crossed what exercise physiologists call the lactate threshold. This is the exercise intensity at which lactate starts accumulating in your blood faster than your body can clear it. Below this threshold, your aerobic energy systems handle the workload and lactate stays relatively stable. Above it, anaerobic pathways kick in heavily, lactate builds up rapidly, and fatigue sets in.
Athletes and coaches use lactate threshold testing to design training programs. Knowing the heart rate or power output at which your body tips into rapid lactate accumulation helps you train at the right intensity to push that threshold higher over time. A higher lactate threshold means you can sustain faster speeds or greater effort before fatigue forces you to slow down.
How Lactate Is Tested
A lactate test is a simple blood draw, but where the blood comes from matters. Arterial blood (drawn from an artery, usually in the wrist) is considered the gold standard because it reflects what’s being delivered to tissues. Venous blood, drawn from a vein in the arm, is easier to collect and often used as a screening tool.
Venous lactate levels tend to run slightly higher than arterial levels, with differences ranging from about 0.18 to 1.06 mmol/L. At lower levels, the two measurements agree well enough that a venous reading of 2 mmol/L or below can reliably rule out elevated arterial lactate, with sensitivity between 97% and 100%. At higher levels, though, the agreement between venous and arterial samples gets worse. When a venous sample comes back above 2 mmol/L, an arterial draw is typically needed to confirm the result and guide treatment decisions.
When Elevated Lactate Becomes Dangerous
Mild elevations in blood lactate are common and often harmless. After a hard workout, your levels might spike well above resting range before settling back down within an hour or so. That’s normal physiology at work.
Lactic acidosis is the clinical concern. It’s formally diagnosed when blood pH drops below 7.35 and lactate exceeds roughly 5 to 6 mmol/L. At that point, the acid-base balance of the blood has shifted enough to affect how organs function. Symptoms of the underlying conditions driving lactic acidosis, such as rapid breathing, confusion, low blood pressure, and signs of poor circulation, are what typically bring someone to medical attention. Lactic acidosis tied to shock or sepsis is the most critical form, because it signals that vital organs may not be receiving enough oxygen to survive.
Lactic acidosis from non-oxygen-related causes is generally less immediately dangerous, but still requires treatment. Identifying and addressing the underlying trigger, whether it’s a medication side effect, liver dysfunction, or a metabolic disorder, is the primary approach. The lactate level itself serves as a marker, giving medical teams a way to gauge severity and track whether a patient is improving or deteriorating.