Polycyclic Aromatic Hydrocarbons

Sporadic colorectal cancer can arise as a result of exposure to carcinogens. Compared to genetic or hereditary colorectal cancers, about 75% of all colorectal cancers occur sporadically. In such cases, cancerous growth does not occur because of preexisting mutations that are present in the cells from birth. Rather, these mutations occur as a result of external factors that cause damage to DNA.

Exposure to carcinogens may happen as part of occupational hazards, but more commonly occurs because of diet and lifestyle. In fact, it’s estimated that diet contributes to about 80% of colorectal cancer cases. Polycyclic aromatic hydrocarbons (PAHs) are carcinogens that are easily ingested either through diets high in red meat or lifestyle vices such as smoking tobacco, but how do these compounds cause cancerous growth in the colon and rectum?

PAHs are compounds formed from hydrogen and carbon, and these compounds contain at least one benzene ring and more than one ring structure. PAHs result from the incomplete combustion of organic hydrocarbons. PAHs are organic compounds, and these pollutants can also enter the human body as they are easily found in the environment — in the air, soil, water and food products besides red meat.

For food products and plants, including leafy vegetables and tubers can become contaminated with PAHs when the pollutant is taken up through the soil and absorbed by the roots. This can also subsequently lead to PAHs accumulating in bodies of livestock that feed on vegetation. Similarly, PAHs can also be found in fish and other marine catch when the waters they are found in is contaminated. Though the accumulation occurs, the impact is minor compared to the PAHs produced during cooking.

While in the context of colorectal cancer it is commonly known that PAHs form when red meats are cooked at high temperatures, this does not mean that other foods are excluded. As food products typically contain organic compounds, it is all possible for PAHs to enter the human body if even vegetables or even processed food are cooked at high temperatures.

In addition, cooking methods such as stir frying or deep-frying require the use of cooking oil. Unsurprisingly, PAHs are also found in high quantities in cooking oils, and as such, heating cooking oil to high temperatures for frying can also introduce PAHs into the human body when fried food is ingested.

PAHs are lipophilic compounds; this means that they dissolve easily in fat, and as such are transported throughout the body the same way fatty acids are during digestion. However, PAHs are xenobiotic and are not naturally found or produced in humans, and because they are carcinogenic, the body needs to break the PAHs down before it can be excreted.

This, however, is where it gets problematic. The act of xenobiotic metabolism (a process where non-naturally occurring substances in the body are broken down) can also introduce harmful compounds into the body. It’s often referred to as double-edged — while the end-goal is to remove PAHs from the body, the intermediate substrates formed are often reactive compounds that can lead to further DNA and cellular instability. The reactive compounds produced are summarized in the image below.

Given that PAHs are such widely occurring chemical compounds and easily found in our food, it is difficult to eliminate PAHs from our diet and prevent ingestion. However, as the production of PAHs greatly changes with the method of cooking, special care should be taken in how we prepare our food. Employing cooking methods that use lower temperatures such as sous vide can minimize the production of PAHs and thus reduce the risk of colorectal cancer.