Quitting smoking tied to less nerve damage in diabetes after six years, less kidney and eye damage after eight

People with type 2 diabetes who stop smoking are told it will help protect their heart. What it does for the kidneys, eyes and nerves, the organs that diabetes damages through its effect on small blood vessels, has been much less clear. A study of more than 200,000 Korean adults now suggests that a lower risk does appear, and that it takes years.

The study, published on October 2 in JAMA Network Open, found no lower risk in the first five years after quitting. A lower risk of the three complications taken together, and of nerve damage, came into view after six to seven years, while for kidney damage and eye damage it appeared only after eight or more. The link was seen mainly in men, the researchers had no record of blood sugar control, and people who quit recently looked slightly worse for kidney disease than those who carried on.

Why smoking and small-vessel diabetes complications are studied

Long-term high blood sugar harms the body’s smallest blood vessels, and the conditions that result are called microvascular complications, after the tiny vessels involved. There are three main kinds: nephropathy, which is kidney damage; retinopathy, which is damage to the retina at the back of the eye; and neuropathy, which is nerve damage, most often in the feet and legs.

Together they are common, affecting nearly half of patients with diabetes by the paper’s count, and each can end badly. Retinopathy ranks among the major causes of vision loss worldwide, kidney damage from diabetes is a leading cause of kidney failure in many populations, and in most high-income countries nerve damage is the most common reason for amputation of a foot or leg that does not follow an injury. Smoking is thought to add to the harm through systemic inflammation and endothelial dysfunction, meaning the thin lining of the blood vessels stops working properly.

For the heart and large arteries, the case is long settled. A pooled analysis from 2015, covering a total of 89 cohort studies in people with diabetes specifically, found that smokers with diabetes had a 55% higher risk of death than those who did not smoke. Its authors also concluded that quitting went with reduced risks in comparison with current smoking.

For the small vessels the picture has been patchier. A scoping review published in 2024 searched for every human study of quitting and diabetes complications, and of 1,023 studies it turned up, only 26 met the criteria for eligibility. Among the small-vessel complications, the reviewers found the strongest evidence for the beneficial effects of quitting in diabetic nephropathy, the kidney disease. The relationship between smoking cessation and retinopathy, neuropathy and other complications had barely been examined, they wrote, and none of that work could say how long a person has to stay quit before risk begins to fall.

How the study tracked quitting in people with diabetes

The research was led by Soonsu Shin, with Jae-Hong Ryoo as senior author, and was approved by the ethics board of Kyung Hee University Hospital in Korea. It drew on the National Health Insurance Service database, a single-payer system covering almost everyone in Korea. Alongside medical claims, the database holds the results of health checkups offered every two years to adults aged 40 and over, where people are asked about smoking.

The team selected adults whose type 2 diabetes was first diagnosed between 2003 and 2013 and who had at least four checkups, no more than about three years apart, with the last one falling between 2009 and 2013. Anyone who already had one of the three complications was excluded.

Smoking status also had to be consistent across every checkup. People who had never smoked said so each time and continuous smokers reported smoking each time, while quitters reported current smoking at their first checkup and then having stopped at every later one, with no relapse. People who took up smoking during the period, or whose answers changed more than once, were left out.

From 1,599,870 people with type 2 diabetes who had a checkup in those years, the criteria left 209,698: 162,861 who had never smoked, 34,989 who smoked throughout and 11,848 sustained quitters. Their average age was 57.2 years and 61.0% were women. Those who had never smoked were older, at an average of 58.8 years, than quitters (53.0) or continuous smokers (51.1).

For each quitter the team estimated a quit date, taking the midpoint between the health checkup where stopping was first reported and the one before it. Quitters were then grouped by how long they had been off cigarettes: less than 2 years, 2 to 3, 4 to 5, 6 to 7, or 8 or more.

Everyone was followed from their last checkup until a first diagnosis of kidney, eye or nerve damage appeared in their insurance records, or until death or the end of 2023. The comparisons were adjusted for age, sex, body mass index, blood pressure, cholesterol, a liver enzyme, alcohol, physical activity, high blood pressure, lifetime amount smoked and years since the diabetes diagnosis.

What the study found about years since quitting smoking

Before any adjustment, continuous smokers had 301.9 new cases of any of the three complications for every 10,000 person-years (a person-year is one person followed for one year), against 287.6 among never smokers. The highest rate of all was among people who had quit less than two years earlier, at 312.8, and it fell with each longer band, reaching 237.6 among those who had quit eight or more years before.

After adjustment, and with continuous smokers as the comparison:

• Under 6 years. No band showed a lower risk of the combined outcome.
• 6 to 7 years. Risk of any of the three complications was 12% lower, with a range of uncertainty from 1% to 22%. Risk of nerve damage was 24% lower, with a range from 11% to 36%. Kidney damage and eye damage were not lower in this band.
• 8 or more years. Risk of any complication was 19% lower, with a range from 5% to 30%. Kidney damage was 21% lower (6% to 36%), nerve damage 20% lower (2% to 34%) and eye damage 41% lower (3% to 64%).

The eye result has by far the widest range, and the authors caution that it rests on a small number of events.

Treated as a sliding scale and averaged across all quitters, each additional year without smoking went with about 1% lower risk of the combined outcome, with a range from 1% to 2%.

Two other results complicate the story. Smoking itself carried only a small penalty in these data: continuous smokers had a 3% higher risk of any complication than never smokers, with a range from 1% to 5%, and a 9% higher risk of nerve damage. And the most recent quitters looked worse, not better, since those who had stopped less than two years earlier had a 12% higher risk of kidney damage than people still smoking, with a range from 3% to 23%.

The authors conclude that lowering the risk required sustained, long-term cessation, with no benefit appearing soon after quitting.

Why the smoking results were clear only in men

When men and women were analyzed separately, the pattern held in men. Among men who had quit for eight or more years, risk of any complication was 18% lower than in men who kept smoking, with a range from 5% to 30%. In women it could not be assessed, because female-specific analyses were limited by small sample size. Women made up most of the study, but the great majority of them had never smoked, which left few female quitters and smokers to compare, and the authors draw the cautious conclusion that the results may not be generalizable to female patients.

How this fits with other research on quitting smoking in diabetes

The same research group had looked at the question before, in men only, with a 2023 study of 26,673 men with diabetes who had health checkups in both 2003-2004 and 2009. Lasting and heavy smoking increases the risk of these complications, its authors concluded, kidney and nerve damage among them. That study also found a higher risk among men who had moved from smoking to not smoking between the two checkups, the same early bump seen in the new data.

Why would recent quitters do worse? Illness may prompt some people to quit, and blood sugar offers a second explanation. A UK study of 10,692 smokers with type 2 diabetes in primary care records found that HbA1c, a blood test reflecting average blood sugar over about three months, rose after people quit and stayed worse for 3 years, a change the authors found is unrelated to weight gain. Across a population, they warned, this temporary rise could increase microvascular complications. A rise in blood sugar in the first years after quitting would fit a benefit that takes time to show.

Shorter-term studies have pointed in a hopeful direction for the kidneys. In Greece, researchers followed 193 smokers newly diagnosed with type 2 diabetes who already had microalbuminuria, small amounts of protein in their urine that are an early sign of kidney damage. Over the year, 120 of them (62.2%) quit smoking, and of the treatments and lifestyle changes the authors examined, quitting made the largest independent contribution to the reduction of microalbuminuria.

On the heart, the benefit shows sooner. An analysis of people with type 2 diabetes drawn from two prospective cohorts in the USA found that those who had quit two to six years earlier had a 17% lower risk of cardiovascular disease than continuing smokers, and long-term quitters a 28% lower risk.

Guidance does not hinge on the small-vessel question. This year a multidisciplinary panel of 35 experts from 13 countries issued consensus recommendations, which say smokers with type 2 diabetes need integrated, high-intensity cessation support throughout the quitting process. The new paper notes that about one in five people with type 2 diabetes worldwide still smokes.

What the study of smoking and diabetes complications cannot show

Cause and effect. This was an observational study. It compared people who made different choices and cannot prove that quitting produced the lower risk.

No blood sugar data. The database did not hold HbA1c results, even though blood sugar is the main driver of these complications. The authors adjusted for how long people had had diabetes as a stand-in, but concede that residual confounding by glycemic control, meaning differences in blood sugar control between the groups, cannot be fully excluded.

Quitting because of illness. The authors cannot exclude the possibility of sick quitter bias, in which people give up cigarettes because they are already becoming ill. Some may have quit as their health worsened or after early manifestations of diabetic complications, which would make recent quitters look worse than they otherwise would, and the raised kidney risk in the first two years fits that explanation.

Healthier in other ways. The opposite bias may flatter long-term quitters. Compared with continuous smokers, quitters here were more likely to exercise regularly and less likely to be frequent drinkers, and habits that were not measured may differ too. One result points the same way: people who had quit for eight or more years had a 19% lower risk of any complication than people who had never smoked, with a range from 6% to 30%, which is hard to explain by quitting alone.

A selected group. Only people with at least four checkups and a perfectly consistent smoking history were kept, about one in eight of those screened. They may be more health-conscious than most people with diabetes.

Smoking was self-reported and then frozen. Status came from questionnaires, and it was fixed at the index date, the last checkup, and not updated during follow-up. Anyone who quit or relapsed later was still counted in their original group.

Diagnoses from insurance codes. Complications were identified from billing records, which depend on a person being tested and the result being coded.

Women, and other countries. The study could not assess women separately with any confidence, and all participants were Korean.

Whether and how a particular person with diabetes is helped to stop smoking is assessed by their doctor.

In these records the lower risk of small-vessel damage showed up only in people who had stayed off cigarettes for most of a decade.