Editor's note: This popular story from the Daily Briefing's archives was republished on Oct. 12, 2018.
By Jackie Kimmell, Senior Analyst
Cancer is the second leading cause of death in the United States, accounting for nearly 600,000 deaths in 2015. This year, an estimated 1.7 million new cases of cancer are expected to be diagnosed in the United States. While researchers are still grappling with finding better ways to treat, or even cure, the disease—they've conducted extensive research into the everyday behaviors that account for nearly half of all cancer deaths.
Below we round up the evidence on five of the potentially modifiable lifestyle factors that have been linked to an increased risk.
A number of large studies have consistently tied alcohol use to an increased risk of cancer, particularly those of the mouth, throat, esophagus, larynx (voice box), liver, and breast. One 2013 study estimated that 19,500 cancer deaths annually (about 3.5% of the total) are alcohol consumption-related.
Research has shown that the more alcohol a person regularly drinks over time, the higher their risk of developing an alcohol-associated cancer. In particular, studies have shown that those who consume 3.5 or more drinks per day have at least a two to three time greater risk of developing head and neck cancers. A meta-analysis of 53 studies (involving 58,000 women) found that women who drank more than three drinks per day had 1.5x greater risk of developing breast cancer (as opposed to non-drinkers). The trend also holds true with lower consumption levels. One study found for every 10 grams of alcohol consumed per day (slightly less than one drink), one's total risk of breast cancer rose by 7%, while another study put that figure at 13%.
The impact is even greater for tobacco users. Many studies have shown that the risks associated with using both alcohol and tobacco are multiplicative, placing individuals at a much greater risk of developing cancer than those who only drink or only smoke.
So if you stop drinking more, you should be okay, right? Not so fast. Many studies have tried to determine how quickly cancer risk declines after a person stops drinking. The largest one found that it took more than a decade after the person stopped drinking for alcohol-associated cancer risk to even begin to decrease. Even after 16 years, the risk of cancer was still higher for ex-drinkers than never-drinkers.
Most of the evidence on obesity and cancer risk comes from large, observational cohort studies (ex. comparing the overall rate of cancer among those who are obese versus those who are not). While such studies cannot establish a causal relationship (it's impossible to know if differences in overweight people, aside from their body fat, account for the difference in cancer risk) they do indicate that higher amounts of body fat are associated with an increased risk for a number of cancers.
For instance, obese and overweight women are about two to four times more likely to develop endometrial (the lining of the uterus) cancer. Research has also found people who are overweight or obese are up to twice as likely to develop liver cancer (the association is stronger in men); twice as likely to develop renal cancer (the most common form of kidney cancer); 1.5 times as likely to develop pancreatic cancer; 30% more likely to develop colorectal cancer; and around 60% more likely to develop gallbladder cancer.
One study, which relied on calculations from BMI and cancer incidence data, estimated that, in 2012, about 28,000 new cases of cancer in men (3.5% of the total) and 72,000 for women (9.5% of the total) were related to a person being overweight or obese.
Unfortunately, the reasoning behind the cancer-obesity link is still not completely known. However, some researchers have said it's possible that extra body fat leads to chronic low-level inflammation which, over time, can cause DNA damage. Fat tissue also produces excess amounts of estrogen, high levels of which are associated with a number of cancers. Finally, researchers have found certain fat cells produce hormones that can impact cell growth and regulation in a way that can increase the risk of mutation.
Research out of the UK has shown that about 86% of melanoma skin cancers are due to excess exposure to natural sunlight or artificial sunbed use. While sun exposure is linked to several benefits (such as increased vitamin D), it also contains harmful ultraviolet radiation (UV). UV radiation consists of UVB rays, which are responsible for most sunburns, and UVA rays, which penetrate below the skin's surface and can damage skin cells.
Surprisingly, the risk of cancer appears to be higher for those who get bad burns during intermittent sun exposure, rather than those who have consistently high sun exposure.
Risk of melanoma is also greatest for those with fair skin, skin that burns easily, those with many moles or freckles, red or fair hair, light-colored eyes and a family history of cancer. Conversely, the risk for those with naturally darker skin tends to be lower.
Most cumulative sun exposure causes less serious types of skin cancer, basal cell and squamous cell skin cancer, but episodes of severe blistering sunburns, especially those before age 18, can cause melanoma later in life.
Certain infectious agents, like viruses, bacteria, and parasites can either cause cancer or increase the risk of cancer. Some infections can also weaken the immune system, which makes the body less able to fight off other cancer-causing infections, or leads to chronic inflammation which can increase one's risk for cancer.
The most common viruses that cause cancer are Hepatitis B or C (which can cause liver cancer); HIV (which increases the risk of a number of cancers including Kaposi sarcoma and cancers of the cervix, lung, liver, and throat); and HPV (which causes nearly all cervical cancers).
Most of these viruses are transferred from person to person via blood and other bodily fluids. One can increase their risk of these infections by engaging in unprotected sex, not getting vaccinated, or by sharing needles.
A number of environmental exposures are responsible for an increased risk of cancer. Environmental carcinogens (the name of a substance that causes cancer) are responsible for anywhere from 4 to 19% of total cancer cases, depending on the calculation methodology.
Most of the dangerous carcinogens are well-known to the public, such as asbestos (a group of naturally-occurring fibrous minerals that are often used in commercial products and can increase the risk of lung cancer); arsenic (which is often found in contaminated water and can cause an increased risk of bladder cancer); formaldehyde (a flammable chemical that increases the risk of cancers of the nose for those who smell it regularly); and coal (which, especially when burned indoors, is linked with an increased risk of lung cancer).
Exposure to all of these elements is often linked to people's occupations or living environments and overall risk is generally tied to the amount of the exposure, the duration of exposure and the individual's genetic background.
It's also worth noting that not all cancer risk factors are modifiable. Age, for example, is the most important risk factor for cancer overall and for many individual types of cancer. The reason is fairly intuitive—the longer a person is alive, the more times their cells replicate and the higher the risk that a cancer-causing mutation will occur during replication.
While some cancers—such as bone cancer and leukemia—appear more often in younger age groups, recent data show that the median age of a person's first cancer diagnosis is 66 years—although there is some variation among different types (it is slightly lower, 61 years-old, for breast cancer; and slightly higher for both colorectal cancer at age 68 and lung cancer at age 70.)
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