There's now a substantial amount of evidence that eating more meat – particularly red and processed meat – increases the risk of cancer. However, not every study dataset shows an association, and researchers are looking for more clarity, particularly when it comes to specific cancer types.
A new study published in the International Journal of Cancer, put together by an international team of researchers, takes a closer look at meat's relationship to cancers of the stomach (gastric) and esophagus called adenocarcinomas – cancers that start in glandular cells.
Cases of esophageal adenocarcinoma (EAC) in particular have been rising in recent years, and esophageal cancer as a whole is now the 11th most common cancer worldwide. Gastric cancers (GCs) are even more widespread, coming in at 5th in the table.
The team looked at data from 450,112 people involved in a long-running health research project, data which included details on diet and cancer cases. The participants were tracked for an average of 14.1 years, and clear correlations emerged – as well as some unexpected results.
"Higher processed meat intake was associated with an increased risk of gastric cancer, especially intestinal-type gastric cancer, and esophageal adenocarcinoma," write the researchers in their published paper.
"This study adds new evidence for gastric and esophageal cancer prevention related to meat consumption."
An additional 30 grams of processed meat per day was associated with a 9 percent increase in GC risk and a 13 percent increase in EAC risk, the researchers calculated.
However, the study also analyzed unprocessed red meat and unprocessed white meat separately, and that turned up some interesting findings. First, there was no significant connection between unprocessed red meat and these specific cancer types.
Second, a 20 gram daily increase in white meat was associated with a 20 percent increased risk of cancer in the lower and middle parts of the stomach in women – the researchers found no such link in men.
There could be several factors at play here, the researchers suggest, including the carcinogenic effects of cooking chemicals and the byproducts of the body breaking down proteins (which white meats are rich in). However, this study was focused on finding the associations rather than explaining them.
"While white meat is often considered healthier compared to red or processed meats, the potential for increased cancer risk due to protein metabolism, microbiota alterations, and cooking methods warrants further investigation," write the researchers.
There are many more potential pathways from processed meat to cancer that we know about, making it the less surprising finding from the study. Chemicals formed from eating red meat can damage DNA and increase oxidative stress on cells, for example, which increases the chances of cancer developing.
"For processed meat, processing and storage methods are also important factors that can increase the risk of GC and EAC," write the researchers.
"In this context, processed meat consumption contributes to a higher intake of salt, which has been shown to irritate the gastric mucosa and trigger significant gastric damage and inflammation."
While the study's long follow-up period and large number of participants are key strengths, this research alone can't prove cause and effect. Meat consumption habits were only checked once, at the start – and so may have changed over subsequent years.
Despite the limitations, though, and once you add in everything we know from previous studies as well, the case against processed meat is only getting stronger. For the study authors, it's now clear that action needs to be taken – that extra hot dog or slice of bacon may not be worth it for your health.
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"Processed meat may increase the risk of GC and EAC, although further research is needed to clarify the effects of white meat consumption," write the researchers.
"Future studies should elucidate the underlying mechanisms and inform dietary guidelines to reduce cancer risk."
The research has been published in the International Journal of Cancer.
This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.