What are Pyrrolizidine Alkaloids (PAs)?
Pyrrolizidine alkaloids (PAs) are a large group of natural compounds found in over 6,000 plant species worldwide, serving as a chemical defense mechanism against herbivores. While many PAs are harmless, some are considered hepatotoxic, genotoxic, and potentially carcinogenic to humans and animals. Concerns surround the 1,2-unsaturated PAs, which are converted into toxic metabolites in the liver. The presence of PAs in the food supply is typically due to unintentional contamination rather than the intentional use of PA-producing plants in food production, with a few exceptions.
Primary food sources of pyrrolizidine alkaloids
Contamination of food by PAs happens primarily through three routes: the accidental co-harvesting of PA-producing weeds with crops, the transfer of PAs into products made by bees, and the carry-over from contaminated animal feed.
Herbal Teas and Infusions
Herbal teas are one of the most significant sources of PA exposure, especially for high consumers. PAs can get into herbal teas when PA-containing weeds, such as ragwort (Senecio) or viper's bugloss (Echium vulgare), are harvested alongside tea plants. The PA concentration can vary dramatically between different brands and even batches of the same tea. Borage leaves (Borago officinalis) and comfrey (Symphytum officinale) are also known PA-producing plants that are used in herbal infusions and can contain high levels of the alkaloids.
Honey and Bee Products
Bees that forage on PA-containing flowers, such as Paterson's Curse (Echium plantagineum), transfer the alkaloids into the honey and pollen they produce. While the risk from honey varies depending on its floral source, children who are high consumers can be particularly vulnerable. Studies have shown that PA levels in honey can fluctuate significantly. Blending honeys from different sources is a common practice that can reduce the overall PA concentration.
Spices and Dried Herbs
Spices and dried herbs, such as oregano, marjoram, and cumin, are also susceptible to contamination from co-harvesting with PA-producing weeds. High levels of PAs have been detected in various spice mixtures, and even small quantities can contribute significantly to a person's total PA intake over time. Quality control is critical for these products, as contamination can be localized and unevenly distributed.
Grains and Cereal Products
Contamination of grains and cereals has led to large-scale poisoning incidents, particularly in regions where PA-containing weeds, such as Heliotropium, grow alongside crops. When harvested together, the seeds of these weeds can end up in the finished cereal products, posing a significant risk. These types of incidents are more common in certain parts of the world but underscore the importance of robust agricultural practices.
Milk and Eggs
PAs can be transferred into animal products, like milk and eggs, when livestock consume PA-containing plants. Research suggests that PA levels in milk and eggs are generally much lower than in plant-based products, and they are not considered a major source of exposure in most Western diets. However, the risk depends heavily on the animals' diet and the presence of PA weeds in their grazing areas or feed.
Comparison of PA risk by food group
| Food Group | Primary Source of Contamination | Risk Level (Based on Potential Exposure) | How to Reduce Risk |
|---|---|---|---|
| Herbal Teas | Co-harvested PA-weeds (e.g., Ragwort, Comfrey) | High (especially for frequent consumers) | Rotate brands/types, limit intake, avoid known PA plants |
| Honey | Bees foraging on PA-plants (e.g., Echium, Ragwort) | Variable (depends on floral source and consumption level) | Don't consume high quantities of specific single-origin honey |
| Spices & Herbs | Co-harvested PA-weeds during processing | Significant (even small amounts contribute) | Choose reputable brands with good quality control, vary products |
| Grains | Weed seeds contaminating grain harvest | Low (for most Western diets), but can be high regionally | Adhere to good agricultural practices to prevent contamination |
| Milk & Eggs | Livestock consuming PA-containing feed | Low (generally considered safe at normal consumption) | Not a primary concern for most consumers |
| Food Supplements | PA-producing plant extracts or pollen | High (can be highly concentrated) | Avoid supplements containing PA-producing plants like comfrey |
How to reduce your exposure to pyrrolizidine alkaloids
Given that PAs are natural but undesirable contaminants, proactive steps can help minimize exposure. Good Agricultural Practices (GAPs) are fundamental, but consumer-level actions are also beneficial.
Here are some actionable tips:
- Diversify your diet: Consuming a wide variety of foods helps prevent one-sided exposure to any single contaminant, including PAs.
- Vary herbal tea choices: Avoid drinking the same brand or type of herbal tea exclusively for extended periods. Alternate with other beverages like water or fruit tea.
- Exercise caution with honey: If consuming large quantities of honey, especially for specific therapeutic purposes, vary the source or brand. This is particularly important for children.
- Be selective with supplements: Avoid food supplements containing known PA-producing plants like comfrey and borage.
- Choose fresh herbs over dried: When possible, use fresh, clearly identifiable herbs instead of dried ones, as fresh products are less likely to be contaminated.
- Read labels and source products carefully: For certain high-risk products like spices and herbs, purchase from trusted brands known for their strict quality control measures.
Conclusion
Pyrrolizidine alkaloids are naturally occurring plant toxins that can inadvertently find their way into the human food chain through various pathways, primarily through the accidental co-harvesting of PA-containing weeds. Major sources include herbal teas, honey, spices, and some grains. While acute poisoning is rare in Western diets, chronic exposure is a public health concern due to the genotoxic and carcinogenic potential of some PAs. By adopting good agricultural practices and simple consumer-level strategies like diversifying intake and being aware of product sourcing, consumers can effectively minimize their dietary exposure to these undesirable compounds. The European Food Safety Authority (EFSA) continues to monitor and assess the risks of PAs in food, prompting ongoing efforts to reduce contamination.
Important considerations regarding pyrrolizidine alkaloids
- Chronic vs. Acute Risk: While high-dose, acute PA poisoning is possible, particularly from contaminated herbal medicines, the primary concern for modern diets is long-term, low-dose chronic exposure and its potential carcinogenic effects.
- Regulation and Monitoring: Regulatory bodies like the EU have set maximum levels for PAs in certain foods to help ensure consumer safety and drive industry efforts to reduce contamination.
- Varying Contamination Levels: The concentration of PAs in food can be highly variable and dependent on factors like geography, climate, and harvesting methods.
- Vulnerable Groups: Children and pregnant or breastfeeding women may be at higher risk due to their body size and vulnerability, and should be particularly mindful of intake.
- Product Dilution: In products like honey, PA levels can be reduced by blending, making sourcing a key factor in assessing risk.
