Mercury is a naturally occurring element that accumulates in living organisms through the food chain. For breastfeeding mothers, consuming certain types of fish is the main source of mercury exposure for the infant. Understanding the specific form of mercury, its clearance rate, and the levels that transfer into breastmilk is essential for making informed dietary choices.
Methylmercury Versus Other Forms
Mercury exists in several forms, including elemental, inorganic, and organic, each with different toxicological properties. The form of greatest concern for breastfeeding women is methylmercury, an organic compound. Methylmercury accumulates in the tissues of fish and shellfish, making the maternal diet the primary route of exposure.
This organic form is highly lipophilic, meaning it is fat-soluble, which allows it to cross biological barriers like the intestinal lining and the blood-brain barrier more easily than inorganic mercury compounds. Approximately 90% of consumed methylmercury is absorbed by the body, where it binds to proteins and persists for an extended period. In contrast, inorganic mercury, often found in dental amalgam or industrial settings, is less efficiently absorbed orally.
The Clearance Rate in Breastmilk
The length of time mercury stays in the body is described by its biological half-life, the time required for the concentration to decrease by half. For methylmercury in the human body, the half-life is long, typically ranging from 40 to 90 days. This indicates that a change in mercury intake will take many weeks or months to result in a lower concentration in the mother’s system.
The concentration of mercury in breastmilk generally decreases proportionally with the mother’s systemic clearance. Methylmercury levels in breastmilk are often reported to be about one-third of the concentration found in the mother’s blood. Total mercury levels decrease during the first few weeks postpartum, likely due to changes as milk transitions from colostrum to mature milk.
Understanding Infant Safety Thresholds
Assessing infant risk requires differentiating between exposure during pregnancy and exposure through breastmilk. Exposure to methylmercury in utero, via the placenta, results in higher levels in the infant’s blood than exposure through nursing after birth. Since the central nervous system is vulnerable, the prenatal period is the time of greatest risk.
Scientific bodies like the U.S. Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) establish reference doses to protect against neurodevelopmental effects. The amount of mercury transferred via breastmilk is low and is not considered a health concern under normal circumstances. The nutritional, immunological, and developmental benefits of breast milk outweigh the risk associated with low-level mercury transfer.
Reducing Maternal Mercury Levels
Mothers seeking to lower their mercury exposure and the levels in their breastmilk can do so through dietary modifications. Since fish consumption is the primary source of methylmercury, focusing on the types and quantities of seafood consumed is the most effective strategy. The FDA and EPA encourage consuming 8 to 12 ounces of low-mercury fish per week to gain the neurological benefits of Omega-3 fatty acids without undue risk.
Limit or avoid fish species known to accumulate high levels of mercury, which are often larger, longer-lived predator fish. These include:
- Shark
- Swordfish
- King mackerel
- Tilefish from the Gulf of Mexico
Many popular and nutritious fish are low in mercury, making them excellent choices for regular consumption. Low-mercury options include:
- Salmon
- Anchovies
- Shrimp
- Cod
- Catfish
- Canned light tuna