A low lactate dehydrogenase (LDH) result is uncommon and, in most cases, clinically insignificant. Unlike an elevated LDH level, which doctors routinely flag as a marker of tissue damage or disease activity, a value below the normal range rarely signals an urgent problem. The most well-established cause is a rare inherited enzyme deficiency that primarily affects skeletal muscle during intense exercise. But the story is more nuanced than “nothing to worry about,” because LDH is not one enzyme but a family of related forms, and a shortfall in one particular type can have surprising consequences for everything from energy metabolism to fertility.
What LDH Does in Your Body
LDH is an enzyme that sits at a metabolic crossroads. Its main job is to convert a molecule called pyruvate into lactate while recycling a chemical your cells need to keep burning glucose for energy. This reaction is especially important when cells are working hard without enough oxygen, which is exactly what happens in your muscles during a sprint or a heavy set of squats.1PubMed Central. Critical role for lactate dehydrogenase A in aerobic glycolysis that sustains pulmonary microvascular endothelial cell proliferation The enzyme also works in reverse, converting lactate back to pyruvate so that the heart, liver, and other organs can use it as fuel. Because LDH is present in virtually every tissue, a standard blood test measuring total LDH activity gives a rough snapshot of cellular turnover throughout the body.
What makes interpretation tricky is that LDH comes in five distinct forms, called isoenzymes, numbered LDH-1 through LDH-5. Each form is assembled from combinations of two different protein subunits: one encoded by the LDHA gene (dominant in skeletal muscle) and the other by the LDHB gene (dominant in the heart). LDH-1, for example, is made entirely from LDHB subunits and is concentrated in heart tissue and red blood cells. LDH-5 is made entirely from LDHA subunits and is the main form in skeletal muscle and the liver. A low total LDH reading on your lab report does not tell you which of these five forms is reduced, and the clinical meaning can differ depending on which one is responsible.
What Counts as Low
Normal reference ranges for total serum LDH vary between labs, but they generally fall somewhere between 120 and 246 units per liter (U/L) for adults. Values below about 100 to 120 U/L would typically be flagged as low. The exact cutoff depends on the assay your lab uses, the temperature at which the test was run, and whether the reference range was calibrated for your age and sex. Children normally have higher LDH than adults, so a “low” number in a child may fall within the normal adult range.
Because elevated LDH is far more common in clinical practice and more tightly linked to conditions doctors are actively screening for, many clinicians do not even comment on a mildly low result. If your LDH dips just below the reference range on one blood draw, it may simply reflect natural day-to-day variation, a well-nourished state, or the fact that you were not exercising strenuously in the days before the blood was drawn. A persistently low result across multiple tests is more likely to point to something worth investigating.
The Genetic Cause That Matters Most
The most clearly defined cause of genuinely low LDH is a hereditary deficiency of the LDHA subunit. People with mutations in the LDHA gene produce an abnormal version of the protein that cannot assemble properly with other subunits. Because skeletal muscle relies heavily on the LDHA-containing isoenzymes (especially LDH-4 and LDH-5), the enzyme shortage hits hardest during vigorous physical activity.2PubMed Central. Glycogenosis type XI, a rare association between muscle and skin manifestations – the contribution of proteomics for the understanding of the underlying myopathology
At rest, many people with LDHA deficiency feel perfectly normal. The trouble starts when muscles are pushed hard. Without enough LDH-A, glycogen cannot be broken down efficiently for energy during anaerobic exercise. The result is exercise intolerance, muscle pain, cramping, and in some cases myoglobinuria, a condition where damaged muscle fibers release their contents into the bloodstream and the urine turns dark. Myoglobinuria can strain the kidneys if severe enough, so anyone who experiences repeated episodes of dark urine after heavy exertion should mention it to a doctor.2PubMed Central. Glycogenosis type XI, a rare association between muscle and skin manifestations – the contribution of proteomics for the understanding of the underlying myopathology
LDHA deficiency is classified as glycogen storage disease type XI and is inherited in an autosomal recessive pattern. It has been reported most frequently in the Japanese medical literature, though it occurs in other populations as well. Carriers who have one working copy of the gene usually have slightly lower LDH levels but no symptoms. People with two mutated copies show very low total serum LDH, often with LDH-5 nearly absent on an isoenzyme profile. This is one of the few situations where low LDH is a clear diagnostic clue rather than a statistical curiosity.
Non-Genetic Reasons LDH Can Run Low
Outside of inherited enzyme deficiency, genuinely low total LDH is hard to pin to a single cause. The literature on this is thin compared to the mountain of research on high LDH. A few patterns have emerged, though none are as dramatic as the genetic form.
One area researchers have explored is the relationship between vitamin C intake and specific LDH isoenzymes. A study of middle-aged and older adults found that while total serum LDH did not correlate with vitamin C levels, the proportions of LDH-4 and LDH-5 showed a strong inverse relationship with vitamin C across both sexes and multiple age groups.3Journal of the Japanese Association of Rural Medicine. Relationship between the Activity of Serum Lactate Dehydrogenase (LDH) and Serum Vitamin C in Japanese Men and Women in their Middle and Old Age In other words, people with higher vitamin C levels tended to have lower percentages of these muscle- and liver-associated isoenzymes. The mechanism is not fully settled, but one explanation is that vitamin C’s antioxidant properties reduce the background rate of cellular damage that would normally release these enzymes into the blood. This would not make your LDH pathologically low, but it could shift it toward the bottom of the normal range.
Other factors that can contribute to a lower-than-expected LDH include recent periods of inactivity, good overall tissue health (less cellular turnover means less enzyme leaking into the bloodstream), and certain medications. Because physical exercise is one of the strongest short-term drivers of serum enzyme levels, someone who has been sedentary in the days before a blood draw may naturally produce a lower reading.4PubMed Central. Monitoring of serum enzymes in sport
What Happens Inside Cells When LDH Activity Drops
When LDH activity is reduced, the metabolic balance inside cells shifts in a predictable way. Normally, LDH consumes a molecule called NADH each time it converts pyruvate to lactate, regenerating NAD+ in the process. NAD+ is critical because many energy-producing reactions in the cell need it. When LDH is not doing its job at full speed, NADH accumulates and NAD+ becomes relatively scarce. In experimental models where LDH-A was deliberately knocked down, researchers measured roughly a twofold increase in the NADH-to-NAD+ ratio.5Cancer Cell. Attenuation of LDH-A expression uncovers a link between glycolysis, mitochondrial physiology, and tumor maintenance
The consequence is that cells shift their energy strategy. Instead of relying on the fast but oxygen-free pathway that LDH supports, they lean more heavily on mitochondrial respiration, the slower, oxygen-dependent route. This is not inherently dangerous for normal cells. In fact, for most healthy tissues, mitochondrial respiration is the preferred energy source anyway. The trouble arises specifically in skeletal muscle during short bursts of intense effort, where the anaerobic pathway is essential because oxygen delivery cannot keep up with demand. That mismatch is why LDHA-deficient individuals cramp and fatigue quickly during sprints but may do fine on a long, leisurely walk.
Why Doctors Pay More Attention to High LDH
If your LDH is flagged as elevated, your doctor likely takes notice immediately. That is because high LDH is a well-established marker of tissue damage and has been linked to a wide array of conditions: heart attacks, liver disease, hemolytic anemia, certain infections, and many cancers. During the COVID-19 pandemic, LDH and its isoenzymes received intense attention as markers of disease severity. Research showed that total LDH and all five isoenzyme levels were significantly higher in patients with severe COVID-19, with one particular isoenzyme, LDH-3, independently predicting deterioration.6PubMed Central. Diagnostic performance of lactate dehydrogenase (LDH) isoenzymes levels for the severity of COVID-19
Low LDH simply does not have this kind of diagnostic track record. There is no widely recognized set of diseases for which low LDH serves as a red flag the way high LDH does for tissue destruction. This asymmetry means that when your lab report shows a low value, the clinical response is usually one of two things: ignore it if it is mildly below range, or order an isoenzyme profile if it is strikingly low and consistent across multiple draws. An isoenzyme breakdown can clarify whether one specific subunit is missing, which points toward the genetic deficiency described earlier.
The Fertility Connection
Beyond LDHA and LDHB, mammals carry a third LDH gene called LDHC, which encodes a form of the enzyme found almost exclusively in the testes. This form, sometimes called LDH-C4, plays a unique role in sperm energy metabolism. Animal research has shown that disrupting the LDHC gene leads to severe male infertility, not because sperm production drops, but because the sperm that are produced cannot sustain their motility long enough to reach and penetrate an egg.7PubMed Central. Expression of the gene for mouse lactate dehydrogenase C (Ldhc) is required for male fertility Without LDH-C4, sperm ATP levels fall rapidly, progressive motility declines, and the burst of energy needed for the final approach to the egg never materializes.8PubMed Central. Lactate dehydrogenase C and energy metabolism in mouse sperm
Human studies have backed up this picture. When researchers compared LDH-C4 activity in fertile men versus men with low sperm counts or absent sperm, the differences were stark. Fertile men had enzyme activity roughly four times higher than men with reduced sperm counts and about ten times higher than men with no measurable sperm.9Baghdad Journal of Biochemistry and Applied Biological Sciences. Lactate dehydrogenase C4 (LDH-C4) is essential for the sperm count and motility: A case-control study Even among men with low sperm counts, those with somewhat better counts and more motile sperm had higher LDH-C4 activity than those with the lowest counts.
LDH-C4 does not show up on a standard total LDH blood test because it is not circulating in the bloodstream in meaningful amounts. It would only be measured in a specialized semen analysis. But it illustrates an important broader point: the LDH enzyme family is not a single entity, and low activity of a specific form can have very targeted consequences that would never be caught by a routine metabolic panel.
Exercise, Training, and How They Move the Needle
One of the strongest short-term influences on serum LDH levels is physical activity. Intense exercise, especially the kind that involves eccentric muscle contractions (think downhill running, heavy lowering of weights, or plyometrics), causes temporary muscle fiber damage that releases enzymes into the bloodstream. LDH, creatine kinase, and certain aminotransferases all spike after a hard workout, sometimes dramatically.4PubMed Central. Monitoring of serum enzymes in sport How much they spike depends on the intensity and duration of the exercise and on how well trained the person is. A trained athlete’s muscles are better adapted to handle mechanical stress, so their enzyme release after a given workout tends to be smaller than that of a beginner doing the same session.
This creates an ironic situation for interpreting low LDH. A person who has been quite sedentary, or who was specifically told to rest before a blood draw, might produce a lower LDH result simply because there has been minimal muscle turnover. Conversely, someone who ran a marathon the day before their blood work could easily show LDH well above the normal range, entirely because of exercise-induced muscle microdamage rather than any disease. If you received a low LDH result and you had not been particularly active in the preceding days, the result may simply reflect your resting baseline.
LDH Inhibition in Cancer Research
An entirely separate area of research has been exploring what happens when you deliberately lower LDH-A activity in tumors. Many cancers hijack the same metabolic shortcut that muscles use during sprints: they convert large quantities of glucose to lactate even when oxygen is available, a phenomenon called the Warburg effect. LDH-A is central to this process, and tumors with high LDH-A activity tend to grow more aggressively.10PubMed Central. LDH-A Inhibitor as a Remedy to Potentiate the Anticancer Effect of Docetaxel in Prostate Cancer
This has made LDH-A an attractive drug target. In animal models of prostate cancer, combining an LDH-A inhibitor with a standard chemotherapy drug reversed the Warburg effect, increased tumor cell death, reduced tumor size, and slowed markers of metastasis.10PubMed Central. LDH-A Inhibitor as a Remedy to Potentiate the Anticancer Effect of Docetaxel in Prostate Cancer Similar results have appeared in breast cancer research, where selective LDH-A inhibition sensitized cancer cells to chemotherapy and increased oxidative stress inside tumors, and the effect held up in three-dimensional tumor models designed to mimic real tissue conditions.11Clinical Cancer Research. Abstract B080: Targeting tumor metabolism in breast cancer using LDHA inhibition to enhance chemotherapy response Researchers are also investigating natural compounds, including polyphenols and alkaloids found in plants, that act as LDH inhibitors.12PubMed Central. Natural compounds as lactate dehydrogenase inhibitors: potential therapeutics for lactate dehydrogenase inhibitors-related diseases
This does not mean that having naturally low LDH protects you from cancer, and it would be a mistake to draw that conclusion from these laboratory studies. The research involves deliberate, targeted suppression of LDH-A inside tumor cells using drugs at concentrations that would not occur from a person simply having a lower-than-average enzyme level on a blood test. Still, the work highlights just how metabolically important this enzyme family is and why its activity level in different tissues matters.
An Evolutionary Perspective on LDH Diversity
The fact that mammals carry multiple LDH genes is itself a product of evolution. The LDHA and LDHB genes diverged from a common ancestor hundreds of millions of years ago, and the testis-specific LDHC gene arose from a separate duplication event. Interestingly, fish also have a tissue-specific LDH-C, but theirs arose independently from their LDHB gene rather than from LDHA, meaning the mammalian and fish versions are not directly descended from the same ancestral gene.13PubMed. Sequence analysis of teleost retina-specific lactate dehydrogenase C: evolutionary implications for the vertebrate lactate dehydrogenase gene family In fish, LDH-C is concentrated in the retina rather than the testes, suggesting that the pressure to have a specialized LDH in a particular tissue has driven nature to independently invent the same basic solution more than once.
Researchers studying how LDH enzymes have changed over evolutionary time have found that even single amino acid mutations can dramatically alter the enzyme’s behavior, shifting it between active and inactive conformations.14PubMed Central. Deciphering Evolutionary Trajectories of Lactate Dehydrogenases Provides New Insights into Allostery This helps explain why the handful of known human LDHA mutations can so thoroughly knock out enzyme function: the protein’s architecture is finely tuned, and even small disruptions can prevent it from folding or assembling correctly. From the perspective of someone with a low LDH result, the evolutionary fragility of these enzymes is a reminder that when the genetic form of deficiency does occur, it tends to be all-or-nothing for the affected isoenzyme rather than a mild reduction.
What to Do With a Low LDH Result
If your total LDH comes back slightly below the reference range on a single test, the honest answer is that it probably does not mean much. Factors as mundane as how active you were before the blood draw, your vitamin C intake, and normal biological variation can all push the number around. A mildly low result in the absence of symptoms is not generally a cause for concern or follow-up.
A result that is substantially below range, especially if repeated, warrants a conversation with your doctor. The most informative next step is an LDH isoenzyme panel, which separates the five forms and can reveal whether one is conspicuously absent. If LDH-5 is nearly undetectable, that points toward LDHA deficiency. If you also experience exercise-related muscle pain, cramping, or dark urine, bringing those symptoms to your doctor’s attention becomes important because the combination paints a much clearer clinical picture than the blood test alone.
For the rare individual diagnosed with hereditary LDHA deficiency, management is practical rather than pharmaceutical. There is no drug that replaces the missing enzyme. The main strategy is to avoid the kind of short, explosive exercise that overwhelms the anaerobic system, while moderate-intensity endurance activities that rely more on oxidative metabolism tend to be well tolerated. Staying well hydrated during exercise helps protect the kidneys in the event of muscle breakdown. Genetic counseling may also be relevant for family planning, since the condition is inherited in a recessive pattern and siblings and children may carry the mutation without knowing it.