Light-to-moderate drinking tied to more colorectal cancer in Koreans, more so with high blood sugar

A drink or two a day has long been treated as the safe middle ground, the amount that does no real harm. For colorectal cancer, the evidence has been less settled than that reputation suggests. Now a study of 4.5 million Korean adults adds a twist: the risk from moderate drinking appears larger when blood sugar is already high.

Researchers at Samsung Medical Center in Seoul, led by Tae-Se Kim with Byung-Hoon Min as senior author, used national health screening records. They followed 4,475,492 adults for a median of 9.3 years. Their nationwide study of alcohol consumption and colorectal cancer risk, published in the Journal of the National Cancer Institute, found more of the cancer in people who drank at levels usually called moderate than in non-drinkers. In those with prediabetes or diabetes, drinking and raised blood sugar together went with more risk than the two would add up to separately. The study is observational, so it describes a link and cannot show that alcohol caused the cancers.

Is light-to-moderate drinking safe for the colon?

Heavier drinking’s tie to colorectal cancer is well established; the open question has been the lower end. The Korean team begins by noting that the effect of light-to-moderate alcohol consumption on colorectal cancer risk remains uncertain, and the largest pooled analysis shows why. In 2011, a team at the International Agency for Research on Cancer combined 61 studies and found strong evidence for an association between alcohol drinking of more than one drink a day and colorectal cancer. Risk was about 21% higher for moderate drinkers, defined there as two to three drinks a day, a heavier band than the Korean study’s light-to-moderate group, and about 52% higher for heavy drinkers. At 10 grams a day, about one drink, the dose-risk analysis estimated a 7% rise, a much fainter signal.

Small risks can add up across a population. A later global analysis from the same agency looked at 2020. It estimated that light to moderate drinking of around one or two alcoholic drinks per day accounted for more than 100,000 new cancers of all types worldwide, out of about 741,300 blamed on alcohol overall. The U.S. Surgeon General’s 2025 advisory goes further, describing a causal link between alcohol consumption and at least seven kinds of cancer, colorectal cancer among them.

What nobody had asked at scale was whether moderate drinking matters more for some people than others. The Korean team’s question was whether the link depends on blood sugar, and their answer is that it seems to.

Why blood sugar might matter for colorectal cancer

Na Wang and colleagues at China-Japan Friendship Hospital in Beijing, in a 2025 review, describe prediabetes as “a transitional phase between normal and diabetic states characterized by elevated blood glucose levels.” One common form is impaired fasting glucose, a blood sugar reading after an overnight fast that is above normal but below the threshold for diabetes. The Korean study used that measure.

Raised blood sugar already has its own tie to colorectal cancer. Wang’s review pooled 11 cohort studies with nearly 5 million people. Those with prediabetes had a higher likelihood of developing colorectal cancer than those with normal blood sugar, by about 18%, and by about 24% in studies that defined prediabetes the way the Korean team did. For diabetes the link is older and stronger. A 2011 meta-analysis led by Hiroki Yuhara at the University of California, Berkeley, concluded that diabetes is an independent risk factor for colon and rectal cancer, with colon cancer risk about 38% higher.

The proposed biology overlaps with alcohol’s. Wang’s group describes chronic inflammation, insulin resistance, and oxidative stress as the likely links between prediabetes and colorectal cancer, with high blood sugar and insulin spurring cells to multiply. Alcohol has cancer-causing effects of its own, so researchers have wondered whether the two might compound each other. The Korean study did not measure any of these processes; it can only show whether the risks combine in the numbers.

How the Korean alcohol and colorectal cancer study was done

South Korea’s national insurer offers regular health checks, and the team took everyone who had one in 2012, 4,475,492 adults undergoing health screening. At that visit each person reported how much and how often they drank, and their fasting blood sugar was measured. The researchers sorted drinking into none, light-to-moderate and heavy, with light-to-moderate set at under 30 grams of alcohol a day for men and under 20 grams for women. Under the definition used in the global cancer analysis, 20 grams a day is the equivalent of up to two alcoholic drinks per day; by the same yardstick, the men’s limit of 30 grams is about three. Blood sugar was sorted into normal, impaired fasting glucose and diabetes.

They then followed everyone through insurance records until a colorectal cancer diagnosis, death or the end of 2022. New cancers were identified from diagnosis codes and the cancer-specific reimbursement codes that Korean hospitals use to bill for cancer care. Over a median of 9.3 years, 51,592 participants developed colorectal cancer, about one person in 87.

A study this size can detect small differences with great precision, which is its strength. Its weakness comes with any observational study. Drinkers differ from non-drinkers in many ways, and the statistics can only account for the differences that were recorded.

How much risk drinking added with prediabetes and diabetes

Blood sugar made a difference across the whole cohort, drinkers and non-drinkers together. Compared with people whose fasting blood sugar was normal, those with impaired fasting glucose had about 11% higher risk of colorectal cancer, and those with diabetes about 26% higher. In the authors’ words, worse glycemic status was significantly associated with increased risk, meaning it went with more cancer by a margin too consistent to be chance.

Adding alcohol widened the gap. For light-to-moderate drinkers, risk ran 23% higher in individuals with IFG (impaired fasting glucose) and 48% higher in those with diabetes. By the authors’ measure, those combinations carried more risk than drinking and high blood sugar would if their effects simply stacked. The team measured that with a synergy index, which compares the risk of the pair with the sum of the separate extra risks. A value of 1 means they simply add; above 1, the combination carries more. The index was 1.72 for drinking with impaired fasting glucose and 2.04 for drinking with diabetes, so in people with diabetes the combined excess risk was about double what the parts would suggest.

The pattern held as drinking rose. Greater alcohol intake amount and frequency showed a linear dose-response link with almost every part of the colon and rectum in every blood sugar group, the exception being cancer of the first stretch of the colon in people with normal blood sugar. The slope was steepest in people with diabetes. A rise in risk that tracks the dose step by step is one of the signs epidemiologists look for when judging whether an association might be causal, although it cannot settle the question by itself.

The authors draw a firm conclusion from this. “Even light-to-moderate alcohol consumption was associated with increased CRC risk; therefore, strict restriction of alcohol intake may help reduce this risk, particularly among individuals with IFG or diabetes,” they write, using CRC for colorectal cancer and IFG for impaired fasting glucose.

What the Korean drinking data cannot settle

The study recorded drinking once, at a single health check in 2012, from what people said about themselves. Self-reports of drinking can be inaccurate, and habits change over a decade, so the categories are a snapshot rather than a lifetime history. Among the non-drinkers there may also be former drinkers who stopped because they were already ill. Their higher risk would raise the non-drinkers’ cancer rate and make drinking look less harmful than it is.

Drinking and high blood sugar both travel with other habits and traits, among them body weight, smoking, diet, exercise and cancer screening. Any of these that the analysis did not fully account for could explain part of the link. The cancers were counted from insurance billing codes rather than confirmed one by one in medical records.

Who was studied matters too. Everyone was Korean, and the 2011 pooled analysis from the International Agency for Research on Cancer found the link with heavy drinking was stronger in Asian studies, so the size of the risk may not carry over to other populations. The figures are relative increases. How many extra cancers they represent depends on how common colorectal cancer is in each group, which rises steeply with age.

What the colorectal cancer results mean for people with high blood sugar

For people with prediabetes or diabetes, the Korean data suggest that the amount of drinking usually described as moderate is not neutral for colorectal cancer, and may matter more for them than for people with normal blood sugar. The study cannot say whether cutting back lowers the risk, because it did not follow people who changed their drinking.

The finding sits alongside guidance that already exists. The Surgeon General’s 2025 advisory treats alcohol as a cause of colorectal cancer. The Korean authors argue for limiting alcohol most strongly in people with prediabetes or diabetes. Decisions about drinking, blood sugar and cancer screening for any one person are discussed with their doctor.

Over nine years in 4.5 million Korean adults, moderate drinking went with more colorectal cancer, and with raised blood sugar the link was stronger, an observational result that still needs testing in other populations.