MMA Test · Blood

Your serum vitamin B12 level can look perfectly normal while your cells are quietly starving for it. That is the gap MMA (methylmalonic acid) fills. When your cells do not have enough active B12 to run a specific chemical reaction inside their energy-producing compartments (called mitochondria), MMA accumulates in your blood. A rising MMA level is one of the earliest signs that B12 is functionally insufficient, often appearing before your B12 number itself drops below the lab's flagged range.

But MMA is more than a B12 test. Large studies now link higher MMA to increased risk of heart attack, cardiovascular death, cognitive decline, and overall mortality, even after accounting for B12 levels. This makes it a dual-purpose marker: it catches hidden B12 deficiency that a standard blood panel misses, and it may reflect broader stress on your cells' energy systems that accumulates with age, kidney decline, and chronic disease.

What MMA Actually Measures

MMA is a small molecule your body produces when it breaks down certain amino acids (the building blocks of protein) and certain fats called odd-chain fatty acids. Normally, an enzyme called methylmalonyl-CoA mutase converts MMA's precursor into a fuel your mitochondria can burn. That enzyme requires a specific form of vitamin B12 (adenosylcobalamin) to work.

When B12 is scarce inside your cells, that conversion slows down. MMA builds up and leaks into your bloodstream, where a blood test can detect it. This is why MMA is considered a "functional" marker of B12 status. It does not measure how much B12 is floating in your blood. It measures whether your cells have enough B12 to do the job.

MMA can also rise for reasons unrelated to B12. Reduced kidney function slows MMA clearance from the blood. Aging itself raises MMA even after adjusting for B12 and kidney function. Common genetic variants in the HIBCH gene can shift MMA up by roughly 46% independently of B12 status. These confounders matter when interpreting your result.

Heart Disease and Cardiovascular Death

Two large Norwegian cohorts followed over 7,600 people with known or suspected coronary heart disease for 3 to 11 years. Each standard-deviation increase in MMA was linked to about 18% to 19% higher risk of heart attack and 28% to 30% higher risk of cardiovascular death. These associations were only slightly weakened after adjusting for standard heart disease risk factors and B vitamin treatment.

A separate U.S. study of about 1,755 adults with pre-existing heart disease found that those in the top third of MMA levels had roughly 70% higher risk of dying from any cause and about double the risk of dying from cardiovascular disease compared to the bottom third, even after controlling for B12 levels, kidney function, cholesterol, blood pressure, and medications.

If you already know you have heart disease or significant risk factors, an elevated MMA adds information that standard lipid and metabolic panels do not capture. It may reflect stress on the energy-producing machinery in the heart and blood vessels, a dimension of cardiovascular risk that conventional markers overlook.

Overall Mortality

A study of over 14,500 U.S. adults with cardiovascular, kidney, or metabolic conditions found that those in the highest quarter of MMA had roughly double the risk of dying from any cause and about 2.3 times the risk of cardiovascular death compared to the lowest quarter. These associations held across different stages of disease severity.

In a general population cohort of about 1,500 older adults followed for 8.5 years, higher MMA predicted increased mortality independently of B12, kidney function, and sex. The link was strongest in people with impaired kidney function.

Among nearly 2,000 cancer survivors tracked for up to 10 years, those in the top third of MMA had about 37% higher risk of dying compared to the bottom third. The risk was especially pronounced when MMA was high despite normal B12 levels, with roughly double the mortality risk. This pattern suggests that what MMA captures in these populations goes beyond simple B12 deficiency.

Cognitive Decline and Depression

In a study of about 2,760 older adults, higher MMA was associated with worse scores on tests of learning, memory, and processing speed, even after adjusting for age, education, B12, and kidney function. The association appeared strongest in men and in those over 75.

A population study of over 8,300 adults found that elevated MMA was associated with depressive symptoms and increased mortality risk, though the depressive symptoms themselves did not appear to mediate the link between MMA and death.

If you are tracking brain health or experiencing subtle cognitive changes, MMA can help determine whether a functional B12 shortfall might be contributing, something a standard B12 level might miss entirely.

Diabetic Eye Disease

Among roughly 2,370 adults with diabetes, MMA levels at or above 250 nmol/L were associated with increased risk of diabetic retinopathy (damage to the blood vessels in the back of the eye). Low dietary antioxidant intake appeared to amplify this risk, suggesting that the combination of impaired cellular energy production and inadequate antioxidant defense may be particularly harmful to the delicate blood vessels of the retina.

Accelerated Biological Aging

A study of over 13,000 adults found that higher circulating MMA was associated with faster biological aging as measured by a composite of organ-function markers, independent of B12, creatinine, and homocysteine. This suggests MMA may reflect cumulative wear on cells' energy systems that accelerates the aging of multiple organ systems simultaneously.

Reference Ranges

MMA reference ranges shift meaningfully with age and kidney function, so a number that is normal for a 30-year-old may not be normal for a 70-year-old. The ranges below come from NHANES data on U.S. adults who had adequate B12 levels and normal kidney function, measured by a standardized laboratory method. They are useful orientation, but your own lab may report slightly different numbers depending on the assay used.

In large outcome studies, MMA levels at or above roughly 240 to 250 nmol/L consistently mark higher risk strata for cardiovascular death and overall mortality, particularly in people with kidney or heart disease. A commonly used clinical threshold for suspecting B12 deficiency is MMA above 260 to 300 nmol/L, though some labs set the cutoff as high as 400 nmol/L. Always compare your results within the same lab over time for the most meaningful trend.

When Results Can Be Misleading

MMA has meaningful biological noise. In studies of repeated measurements in the same person, within-person variation of roughly 15% to 20% has been documented, meaning a single borderline reading could easily fall on either side of a threshold just by chance. That alone is reason to retest before drawing conclusions from a borderline result.

• Kidney function: This is the biggest confounder. Impaired kidneys slow MMA clearance, raising your level even when B12 is adequate. In one large analysis, correcting MMA for kidney filtration rate reclassified about 33% to 45% of people with reduced kidney function from "B12 deficient" to "not deficient." Always interpret MMA alongside creatinine or eGFR (a measure of kidney filtration).
• Age: MMA rises with age independently of B12 and kidney function. A value of 280 nmol/L means something different at 35 than at 75. Use age-adjusted reference intervals when available.
• Genetic variants: A common variation in the HIBCH gene raises MMA by roughly 46% without any B12 deficiency. If your MMA is persistently mildly elevated despite normal B12 and kidney function, genetics may be the explanation.
• Metformin and acid-blocking medications: Long-term metformin use can impair B12 absorption and may gradually raise MMA over months to years. Proton pump inhibitors (PPIs) and H2 blockers reduce stomach acid and can similarly reduce B12 absorption. If you take either class of medication, an isolated MMA elevation may reflect medication-induced B12 malabsorption rather than dietary deficiency or cellular energy dysfunction.