Coffee and Cancer
Historical Classification and Methodological Shift
Three decades ago, scientists initially suspected that coffee might cause cancer. In 1991, the International Agency for Research on Cancer (IARC) classified coffee as possibly carcinogenic to humans.
- This early classification was based on animal data (rodents) and early human case-control studies, which suffered from biases because participants already had cancer when the study began.
- Since 1991, there has been an accumulation of prospective studies (following large, healthy groups over decades), which is the preferable and much stronger methodology.
Current Overall Findings and IARC Reclassification
The newer, accumulated data suggests that overall, coffee consumption is associated with a lower risk of cancer, particularly among high coffee drinkers.
- On average, there is an 18% reduction in all types of cancer combined among coffee drinkers.
- Due to this strong evidence, the IARC reviewed the data again in 2016 and changed its classification, no longer considering coffee possibly carcinogenic. The data now suggests coffee might actually be protective for some cancer types.
Specific Cancer Links: Where Protection is Strongest
The protective link is strongest for two specific types of cancer, with the benefit appearing linear up to about five or six cups per day:
- Liver Cancer: Consistently lower risk. High consumers may see about a 50% lower risk compared to low or non-consumers, equating to about a 15% lower risk per daily cup of coffee. Coffee is thought to be generally good for liver health.
- Endometrial Cancer: Consistently seen at lower levels in coffee drinkers. The risk reduction appears linear up to about five or six cups a day.
Other Common Cancers
- Prostate Cancer: Most studies suggest a protective effect, with a roughly 9% to 10% lower risk of both incidence and mortality in coffee drinkers.
- Colorectal Cancer: Studies show either no link or a modest protective effect. A proposed mechanism is that coffee may accelerate intestinal transit time, reducing contact with potential carcinogens.
- Skin Cancer (e.g., Melanoma): Some studies suggest a protective effect.
- Breast Cancer: Overall, there is no significant effect either way, but there is a suggestion of protection for specific demographics, such as post-menopausal women, possibly linked to phytoestrogens.
- Pancreatic Cancer: There is no significant effect either way.
Revisiting Earlier Suspicions (1991 Concerns)
Recent prospective data has addressed the initial concerns regarding bladder and lung cancer:
- Bladder Cancer: The initial suggested link from 1991 has been challenged by prospective studies, which generally find no link either way.
- Lung Cancer: An early suggested link completely disappeared when scientists adjusted for factors commonly associated with coffee consumption, such as smoking and BMI.
Potential Mechanisms for Protection
The protective effects of coffee are not fully attributed to one compound:
- Antioxidants: Coffee is very high in polyphenols (plant compounds) with a strong antioxidant effect. Coffee is reportedly the number one source of antioxidants in the American diet.
- Caffeine vs. Decaf: The role of caffeine is unclear. Some studies show a protective effect of decaf coffee, suggesting benefits come from other components, though some studies suggest caffeinated coffee has a stronger overall effect.
- Hormonal Effects: For breast cancer, the suggested protection is hypothesized to involve phytoestrogens, plant-derived, estrogen-like molecules found in coffee.
Risks Associated with Preparation and Other Considerations
While coffee itself seems protective, the sources point out one significant preparation risk:
- Very Hot Temperature: Drinking coffee above 65 degrees Celsius is thought to be linked to a higher risk of esophageal cancer and possibly oral cancer. This risk is due to the physical damage the heat causes to the cells.
- Acrylamide: Although formed during roasting, measured levels in coffee are "tiny" and far below thresholds of concern.
- Not for Everyone: Coffee is not recommended for individuals hypersensitive to caffeine or women during pregnancy, due to a link to low birth weight and pre-term birth.