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Diet and lifestyle

Surprise: smoking reduces blood sugar levels but still leads to diabetes

Many smokers believe that smoking reduces appetite and keeps them slim, but not much is known about how cigarettes affect glucose metabolism. Now researchers have mapped more precisely how heavy smokers metabolize carbohydrates – with surprising results. Smoking blunts the blood glucose levels of heavy smokers, but for different and unexpected reasons, and the lower blood sugar does not help: smoking increases the risk of type 2 diabetes.

Scientific studies have indicated that smokers weigh on average 1.2 kg less than non-smokers. Other studies have shown that smoking increases the risk of type 2 diabetes by as much as 30–50%. These seemingly conflicting results clearly show that smoking affects the metabolism in many ways. Now researchers have tried to examine the hormonal mechanisms resulting from smoking to clarify whether it directly affects the glucose metabolism.

“Our results confirm that smoking affects blood glucose levels, but we had expected that the nicotine in cigarettes would cause the smokers’ blood glucose levels to rise. We were therefore surprised that our study showed the opposite: smoking reduced the normal rise in blood glucose after a meal. Furthermore, smokers had higher fasting circulating concentrations of glucagon, a hormone that increases blood glucose levels. We believe that this imbalance may help to explain why smokers more often develop type 2 diabetes,” explains Magnus F. Grøndahl, who together with Filip K. Knop, Steno Diabetes Center Copenhagen, are the main authors of the study.

Food moves more slowly

In the study, heavy smokers underwent two nearly identical meal tests in which they consumed a 400-ml chocolate drink with nutrients equivalent to eating a normal meal. The only difference between the two tests was that, during one of the test visits, the smokers were asked to smoke a cigarette both before and after consuming the drink while abstaining during the other test visit. The researchers measured blood glucose, blood pressure and several hormones that affect the glucose metabolism and appetite regulation.

“We had expected that smoking before and after the meal would cause blood glucose levels to rise further, because previous studies showed that nicotine alters chemical processes in cells so that people respond less strongly to insulin, a hormone from the pancreas that lowers blood glucose. We were therefore quite surprised to actually observe a smaller increase in blood glucose levels when the smokers smoked a cigarette before and after the meal compared with not smoking together with the meal,” says Magnus F. Grøndahl.

The researchers found the explanation in another area. To discover how quickly the body absorbs nutrients, the trial participants ingested 1.5 grams of paracetamol mixed into the chocolate drink, and this revealed an important difference caused by the smoking.

“Smoking made the nutrients from the meal, including glucose, travel more slowly from the gastrointestinal system into the bloodstream,” explains Magnus F. Grøndahl.

Smoking as an independent risk factor

To measure the difference between smokers and nonsmokers in this experiment, the researchers also measured how nonsmokers reacted to the same meal test and observed important differences.

“Smokers and nonsmokers had very similar insulin levels, but before meals, smokers had a higher level of glucagon: a hormone that causes blood glucose to increase. We therefore think that this may represent a problem in smokers’ metabolism of carbohydrate, and this could be one reason why smokers have a greater risk of developing type 2 diabetes,” says Magnus F. Grøndahl.

The researchers cannot yet determine whether the nicotine or the other chemicals in cigarettes cause this effect among smokers. However, previous studies have shown that nicotine products further aggravate the insulin resistance characterizing people with type 2 diabetes.

“It is well-known that smokers develop type 2 diabetes more often than nonsmokers, and epidemiological studies suggest that smoking constitutes a risk factor for developing type 2 diabetes, independent of a generally unhealthier lifestyle. Our results help to explain the mechanisms behind smoking as an independent risk factor that, together with a range of other factors, results in type 2 diabetes. Smoking greatly influences the glucose metabolism, but these are not hidden benefits that compensate for all the damaging effects,” concludes Magnus F. Grøndahl.

Effects of smoking versus nonsmoking on postprandial glucose metabolism in heavy smokers compared with nonsmokers” has been published in Diabetes Care. The senior author, Filip K. Knop, Professor and Consultant, Clinical Metabolic Physiology, Steno Diabetes Center Copenhagen, has received research funding from several public and private foundations, including the Novo Nordisk Foundation, in 2007–2018.

Magnus Frederik Grøndahl