What are nitrates and how are they processed by the body?
Nitrates ($NO_3$) and nitrites ($NO_2$) are chemical compounds containing nitrogen and oxygen that exist naturally in the environment, soil, and our bodies. The primary source of dietary nitrates for most people comes from vegetables, but they are also found in processed meats and drinking water. In the body, nitrates are relatively stable, but they can be converted into nitrites by bacteria, particularly in saliva. The subsequent fate of these nitrites determines their health impact.
Nitrites have a dual pathway: they can be converted into beneficial nitric oxide (NO) or into harmful N-nitroso compounds (NOCs), such as nitrosamines. Nitric oxide is a crucial molecule for heart health, exercise performance, and regulating blood pressure. However, nitrosamines are potent carcinogens. The key to understanding why nitrates can be 'bad' lies in the conditions under which these harmful nitrosamines form.
The link between processed meat, cooking, and nitrosamines
Processed meats like bacon, ham, and hot dogs contain added nitrites for preservation, color enhancement, and flavor. These meats are also high in protein, which is made of amino acids. When cooked at high heat, the nitrites can react with the amines in the meat protein, forming carcinogenic nitrosamines. The International Agency for Research on Cancer (IARC) classifies processed meat as a carcinogen, partly due to the formation of nitrosamines during processing and digestion. In contrast, the nitrates found naturally in vegetables are less likely to form these harmful compounds. This is because vegetables contain protective antioxidants, such as vitamin C, which actively inhibit the nitrosation process.
The danger of methemoglobinemia, especially in infants
Excessive nitrate levels in drinking water are linked to a severe condition known as methemoglobinemia, or 'blue baby syndrome'. This condition is particularly dangerous for infants under six months of age. Here’s why:
- Higher conversion rate: Infants have a higher conversion rate of nitrate to nitrite in their digestive systems.
- Different hemoglobin: Their fetal hemoglobin is more susceptible to oxidation by nitrite compared to adult hemoglobin.
- Enzyme deficiency: Infants lack sufficient quantities of the enzyme that converts methemoglobin back into oxygen-carrying hemoglobin.
When nitrite oxidizes hemoglobin into methemoglobin, the blood loses its ability to transport oxygen effectively. This can lead to a bluish-gray skin discoloration (cyanosis), shortness of breath, and in severe cases, death. Boiling water does not remove nitrates; in fact, it can concentrate them further. This is why public health authorities issue clear warnings against using well water with high nitrate levels for infant formula preparation.
Other potential health risks associated with nitrates
Beyond the well-established risk of methemoglobinemia, other potential long-term health concerns have been investigated, though evidence is often mixed or still developing. Some studies suggest links between high nitrate exposure and adverse pregnancy outcomes, thyroid problems, and certain types of cancer.
However, it is crucial to differentiate between natural and additive sources. As previously mentioned, the protective compounds in vegetables are thought to mitigate risks associated with their high nitrate content. The primary concern for cancer and other chronic diseases remains linked to processed meat consumption rather than vegetable intake. Ongoing research continues to clarify the precise mechanisms and risks involved. For example, recent studies on the gut microbiome are exploring how nitrates affect microbial populations and, in turn, human health.
Natural vs. Additive Nitrates: A comparison
To fully understand why nitrates can be considered 'bad' in some contexts and beneficial in others, it's helpful to compare their sources.
| Aspect | Nitrates in Vegetables (Natural) | Nitrates in Processed Meats (Additive) |
|---|---|---|
| Source | Naturally absorbed from soil, especially high in leafy greens, root vegetables. | Added artificially as sodium or potassium salts during curing. |
| Health Impact | Generally considered beneficial. Converted into heart-healthy nitric oxide in the body. | Associated with cancer risk, particularly colorectal, when converted to nitrosamines. |
| Cooking Reaction | Not typically cooked at high heat with amines, and contains protective antioxidants like vitamin C. | Reacts with high-protein meat and high heat to form carcinogenic nitrosamines. |
| Preservative Role | None. The health benefits are linked to fresh consumption, often raw. | Prevents bacterial growth, such as Clostridium botulinum, and enhances color. |
| Overall Risk | Very low to none, as protective compounds mitigate any potential harm. | Increased risk with higher, regular consumption, especially due to processed meat intake. |
How to reduce your risk from nitrates
For most healthy adults, dietary nitrates from vegetables are not a concern and are likely beneficial. For processed meats and contaminated drinking water, however, taking steps to reduce exposure is prudent.
- Limit processed meat: The World Health Organization recommends treating processed meats as an occasional treat rather than a daily staple.
- Choose carefully: When buying processed meat, check labels for added sodium or potassium nitrates and nitrites. While 'uncured' products use vegetable sources, they still contain nitrates that can form nitrosamines.
- Cook properly: Cooking cured meats at lower temperatures for longer, and avoiding charring, can reduce the formation of nitrosamines.
- Test drinking water: If you rely on well water, especially in agricultural areas, have it tested regularly for nitrate levels.
- Protect infants: For infants under six months, do not use water with a nitrate level above 10 mg/L for formula. Use a safe alternative like bottled water.
Conclusion
The question of whether nitrates are 'bad' for you is complex, depending largely on their source and context. Natural nitrates in vegetables, which constitute the majority of our intake, are generally considered healthy, contributing to beneficial nitric oxide production that supports cardiovascular health and exercise performance. The real health risk comes from the specific conditions under which nitrates and nitrites in processed meats can react to form carcinogenic nitrosamines, a risk amplified by high-heat cooking. This is compounded by the acute and severe threat of methemoglobinemia from nitrate-contaminated drinking water, which is a critical concern for infants. By understanding these differences and making informed dietary choices, such as favoring fresh vegetables and limiting processed meat, you can mitigate the risks associated with harmful nitrate exposure while still reaping the benefits of healthy, nitrate-rich foods.
An excellent resource for understanding food additive safety is the Food Standards Agency. Food Standards Agency
