Evidence Shows That Alcohol Is Likely Cause of Cancer At Multiple
Sites, Says Reviewer
Alcohol is one of the
most widely consumed psychoactive substances in the world. The US 2014
National Survey on Drug Use and Health found that almost 88% of people
over the age of 18 report having consumed alcohol at some point in
their life and more than half have drank alcohol in the past month.
Although it offers many social and mood benefits, alcohol consumption
is estimated to contribute to a significant proportion of the global
economic, social, and health burden.
A recent report by the World Health Organization (WHO)
in 2014 attributed 5.9% of all deaths worldwide each year (roughly 3.3
million) to harmful alcohol use. The WHO report also states that
harmful alcohol use has been shown to have a causal relationship with
200 adverse health conditions (e.g. cirrhosis and alcohol dependence)
and even influences the clinical outcomes of infectious diseases
including tuberculosis and pneumonia.
Cancer and Alcohol
One association that has been less clear however is the
relationship between alcohol consumption and cancer. Decades of
research and reporting on the association between the two has often
only resulted in equivocal statements that do not fully commit to a
conclusion of causality. Some of this ambiguity is for good reason,
and stems from the underlying complexity of these cancers or is
attributable to the strength of the evidence (or lack thereof) given
the designs and limitations of the available studies. Over time
though, more evidence has been mounting, particularly from larger
observational studies, demonstrating that the ‘link’ between alcohol
and some types of cancer may indeed be causal.
Alcohol As Cause
Now, a recent review
in the journal Addiction authored by Jennie Connor, from
the Department of Preventive and Social Medicine at the University of
Otago in New Zealand, takes aim at summarizing the most recent data
that support a causal association between alcohol consumption and
cancer. Dr. Connor believes that there is enough evidence to
conclusively state that alcohol is a cause of the disease.
Specifically, she reviews the direct evidence that alcohol can cause
cancer in seven sites throughout the body in the oropharynx, larynx,
oesophagus, liver, colon, rectum and female breast.
Connor’s conclusion is based on comprehensive reviews published over
the past decade, some from a number of organizations including the
World Cancer Research Fund and American Institute for Cancer Research,
the International Agency for Research on Cancer, and the Global Burden
of Disease Alcohol Group, that indicate there is a dose-response
relationship between alcohol and these cancers. Evidence appears to
be most strong for cancers of the mouth, pharynx, and oesophagus where
consumption of at least 50 grams of alcohol per day (equivalent to
about 4 beers) was associated with a relative risk of cancer ranging
from 4-7 compared to no drinking. Furthermore, this risk may be
reversible for the latter two cancer types, whereby cessation of
drinking can attenuate risk over time. Weaker was the support for
colorectal, liver and breast cancers, in which relative risk was about
1.5 for similar daily alcohol intake.
Lower Doses
Even light to moderate
drinking may put one at risk according to Connor’s review. She cites
newer research by Cao et al. (2015), who used data from two
prospective US cohort studies, and found that women who drank just one
alcoholic beverage a day were already at a higher risk for breast
cancer and that their total risk of cancer was also increased. Similar
conclusions are supported by data from the United Kingdom’s Million
Women cohort study. Furthermore, smoking has been shown to compound
these risks, particularly for cancers of the mouth, pharynx, larynx
and esophagus.
Mechanisms
Besides the the evidence from observational studies,
Connor provides a brief discussion of the mechanisms by which alcohol
might cause cancer at some sites but not others. One reason could be
from DNA damage due to acetaldehyde, a carcinogenic metabolite
produced during the metabolism of ethanol. But genetic factors are
also thought to play a role as they do in many cancers. Breast tissue
seems to be particularly sensitive to alcohol, which may interfere
with estrogen metabolism, and appears to be separate from other
cancer-causing pathways.
Overall the interplay
between these mechanisms and factors are complex and not entirely
understood, so Connor cautions that the biological mechanisms must be
supported by sound, consistent and robust epidemiological research to
ultimately provide a strong conclusion.
Limitations
Much of the skepticism
and criticism towards the evidence that alcohol causes cancer, or the
levels at which it may do so, is based on the limitations of the
studies which have addressed the issue. Chief among these problems is
estimating consumption via self-report. Consumers commonly
underestimate and/or underreport their consumption but this effect
varies by sex, socioeconomic status, and country, making it difficult
to statistically correct for. There is also a lack of data regarding
the effect that patterns of drinking (binge vs average) have on these
associations. Better attention must be paid, not just to the average
amount of alcohol consumed over a given time period, but also to the
frequency and ‘other dimensions’ of consumption. Another problem
stems from the underestimation of the cancer effect caused by
including former but now non- drinkers in the abstainer reference
group. Lastly, there is the potential for residual confounders to
affect the outcome by cancer-type.
Despite these
criticisms of the research, Connor firmly concludes that ‘there is
strong evidence that alcohol causes cancer at seven sites, and
probably others’ and that population-wide reduction in alcohol use has
the potential to greatly impact the incidence of many of these
diseases. She points out that some public health officials around the
world have begun to consider cancer risk in the development of alcohol
consumption recommendations. The UK specifically has initiated a
campaign to educate the public on cancer risks associated with alcohol
intake. Additionally, she takes aim at the alcohol industry and its
promotion of health benefits from drinking, particularly as the
current scientific evidence cannot support a safe level of alcohol
consumption with respect to cancer risk.
Reactions To The
Review
In an article
published in a New Zealand news site, Robert Brewer of the New
Zealand Spirits industry voiced some protest, calling the real story
“more complicated” and emphasizing that many other factors must be
considered. Dr Samir Zakhari, who works for the Distilled
Spirits Council of the United States, also thinks the story is more
complex. In an editorial piece written by Professor Doug Sellman,
Dr. Zakhari is quoted as saying that not enough consideration is given
to “interactions between lifestyle, sociocultural and genetic
issues”. Zakhari even goes one step farther and takes aim at Connor
directly, accusing her of “cherry-picking data”.
However, Professor
Sellman, who is director of the National Addiction Centre at the
University of Otago thinks that these critiques amount to industry
obfuscation. Sellman emphasizes the enormous burden that alcohol has
on society in New Zealand and the fact that 30 per cent of all
alcohol-related deaths are due to cancer. So the battle will continue
to play out between the industry and academic research, but it seems
clear that the evidence is stacking up in support of a direct link for
alcohol causing cancer.
References:
Connor 2016 :
https://tinyurl.com/h2ubun2
Cao et al, 2015:
https://tinyurl.com/qgrrjuw
https://tinyurl.com/jz68h25
https://tinyurl.com/zjo7b9j
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