By: Radha McLean
What’s the weather got to do with type 1 diabetes? A great deal, according to Zvi Laron, MD. Dr. Laron, professor emeritus of the Department of Pediatrics and Pediatric Endocrinology at Tel Aviv University in Israel, is just one of many doctors focusing his diabetes research on seasonality—specifically, the relation of winter weather to children’s risk of developing type 1 diabetes.
The hard work seems to be paying off; researchers have come up with intriguing theories, with results to back them up. Certain evidence points to exposure to viral infections and intolerance of carbohydrates during the winter as factors that could precipitate type 1 diabetes—but only in those who already are genetically susceptible to the disease.
The Would-Be Culprit: Viral Infections During the Winter
Some researchers believe that viral infections, many of which are contracted during the winter, may cause children to develop type 1 diabetes.
Since viral infections weaken the immune system, children at risk for diabetes experience an increase in the rate of beta cell destruction. As a result, they become more susceptible to developing the disease at some point during their childhood, according to leading researchers in the field.
Exposure to viral infections leads to two trends: higher numbers of births during the winter and the summer months (as opposed to the spring or fall) of children who later develop type 1 diabetes.
The explanation for the winter trend is simple: children with type 1 diabetes who were born in winter months may have contracted a viral infection from the environment or from breastfeeding during the winter after their birth. Explaining the summer trend is more complicated: it is possible that children with type 1 diabetes who were born during the summer contracted viral infections from their mothers in the womb during pregnancy the previous winter.
Although these trends might seem contradictory—higher birth rates in the winter and the summer among children who later develop type 1 diabetes—they both point to one problem: viral infections, according to Dr. Laron.
Contracting a Virus After Birth During the Winter
Dr. Laron speculates that children born during the winter who develop type 1 diabetes may have contracted a viral infection, either from breastfeeding or from the winter environment, shortly after birth. In studies that began in Israel, Dr. Laron and Arieh Shamis, MD, of Tel Aviv University, found that children and adolescents who developed type 1 diabetes during childhood were more likely to have been born during the winter than children who did not develop diabetes.
To investigate a larger trend across nations, Dr. Laron and Israel Ashkenazi, MD, also of Tel Aviv University, along with colleagues from several countries, studied other populations in Germany (Bavaria and Berlin), Slovenia, Sardinia (where an exceptionally high incidence was found), Sicily, New Zealand and, most recently, Denver, Colorado. In all these places, the findings were repeated—showing that children who developed type 1 diabetes were more likely to have been born during winter months.
Dr. Laron points out, however, that this phenomenon occurs only in countries with a growing incidence of type 1 diabetes.
Contracting a Virus in the Womb
Peter Rothwell, PhD, consulting neurologist at Oxford University in England and research fellow at Western General Hospital in Edinburgh, Scotland, is the leading researcher to suggest that viral infections contracted during a winter pregnancy are passed from the mother to the fetus. These children, now at risk of developing type 1 diabetes, are born the following summer, thus leading to a high number of summer births recorded among children with type 1, according to Dr. Rothwell.
In two papers published in the British Medical Journal (June 8, 1996, and October 2, 1999), Dr. Rothwell summarized studies in which he and other researchers identified this trend in 16 European countries. The studies showed that a high number of children with type 1 diabetes in Great Britain who had been born during the summer had viral infections at birth. He argues that they had contracted congenital rubella and other viral infections from their mothers while in the womb during the winter months and were then born the following summer with the markers for developing type 1 diabetes.
Dr. Laron has also explored the possibility that children might contract viruses from their mothers during a winter pregnancy—an exposure that would put the children at higher risk for developing diabetes after they were born the following summer.
In a study of 61 children in Munich, Germany, who were at high risk for diabetes, Dr. Laron tested the children of mothers with diabetes for autoimmune antibodies. He then compared the season in which these children were born to the birth dates of children not at risk for diabetes.
The results, published in the November 2001 issue of Diabetes Care, backed up his theory: the children at high risk for diabetes were more likely to have been born during summer months, corresponding to a pregnancy during the previous winter.
“We found that those who have antibodies before the onset of diabetes were born with a different seasonality than their nonrisk siblings,” Dr. Laron tells Diabetes Health.
In his Diabetes Care article, Dr. Laron proposes that “in pregnancies starting in autumn and early winter, periods of the yearly viral epidemics, the mother transmits beta-cell pathogenic viruses to the fetus, which, in genetically susceptible individuals, initiates the autoimmune process leading to diabetes.”
Winter Weather Doesn’t Seem to Matter Among Low-Risk Groups
According to Dr. Laron, there is no connection between seasonality and diabetes in countries with a low incidence of type 1 diabetes—namely, China, Japan and Cuba.
“In countries where incidence of diabetes is very low, we couldn’t find seasonality that was different from the general population,” Dr. Laron says.
It’s only in areas where people have a medium to a high chance of developing diabetes that seasonality becomes a factor, he explains.
He attributes this phenomenon to the theory that children at low risk for diabetes have an immunity to viruses. He says that they may be exposed to the immunity from their mothers during pregnancy or from the environment after birth.
“It could be that in certain populations children develop an immunity to infections at an early age or that the immune bodies are transmitted by pregnant mothers with a history of infections.”
More Evidence of Viral Infections as the Culprit
In an article published in the October 2001 issue of the Journal of Clinical Endocrinology and Metabolism, researchers in Finland presented more evidence linking viral infections with type 1 diabetes.
In this study, 2,448 Finnish children who were genetically at risk to develop type 1 diabetes were screened for islet cell antibodies (ICA), a marker for developing diabetes, every three to six months for the first two years of life. Those who tested positive for ICA were also tested for a second type of antibodies, called insulin antibodies.
Of the children who tested positive for at least one antibody, the first one appeared during the fall or winter, “suggesting that infectious agents may play a role in inducing beta-cell autoimmunity,” the researchers state.
Dr. Ludvigsson of Linkoping University and colleagues, who conducted two studies in Sweden, found similar results (published in Pediatric Research, November 1996, and Diabetes Research, January 1991).
Intolerance of Carbohydrates During the Winter
Michael J. MacDonald, MD, professor of pediatrics at the University of Wisconsin, suggests that viral infections might not be the only season-influenced cause of type 1 diabetes. Rather, he proposes that cold weather makes people who are already at risk for diabetes less tolerant of carbohydrates, which precipitates the onset of the illness.
“This is probably because in winter counter-insulin hormones are higher and raise the insulin requirement,” he says. “This may be because of cold and/or shorter length of daylight.”
In one study (Diabetes Research, December 1989), Dr. MacDonald and colleagues tracked the incidence of diabetes in Wisconsin from July 1982 to June 1984. Then they compared the antibodies for the coxsackie virus, pneumonia and immune response genes (all associated with autoimmune diseases) in children and young adults who had type 1 diabetes against control subjects without diabetes.
The researchers found that two-thirds of all cases of type 1 diabetes appeared during cold months in most years, but they found no difference between the viral antibody or immune response gene levels of those with and without diabetes.
“Since evidence of insulin cell destruction is evident years before the clinical diagnosis of diabetes (e.g., islet cell antibodies), winter is merely a precipitating factor in the overt clinical appearance of insulin deficiency,” Dr. MacDonald explained to Diabetes Health.
“It is the proverbial straw that breaks the camel’s back. Since people with diabetes have higher blood sugars in winter than in summer, it means winter is more stressful than summer on carbohydrate tolerance.”
Blaming Winter Weather All Around
One study from Scotland proposes that viral infections combined with intolerance of carbohydrates during the winter could explain why seasonality seems linked to type 1 diabetes. Researchers from the Royal Aberdeen Children’s Hospital found an “excess” of diabetes diagnoses during winter months in 2,000 children with diabetes in Scotland. In a larger analysis of 21 previous studies with more than 13,000 participants, “the same results at a much higher level of significance” were found, the researchers write in the April 1999 issue of the Scottish Medical Journal.
As an explanation, they cite islet cell damage from viral infections combined with the progress of an autoimmune condition.
“There may be physiological seasonal changes with winter stress on carbohydrate and lipid metabolism,” they state, adding that some hormones have increased production during the winter and are not offset by higher levels of insulin.
Dr. Laron tells Diabetes Health, “I cite evidence that viral infections initiate the development of diabetes, but an overdose of carbohydrates may be an additional factor contributing to the progressive destruction of pancreatic beta cells.”
Where Do We Go From Here?
Knowing that winter weather may be contributing to the incidence of type 1 diabetes in certain locations is only the first step—we need to figure out how to use this information to help prevent the illness, says Dr. Laron.
To do this, the medical community would need to employ preventive efforts such as giving at-risk children some type of medications to stop viral infections from precipitating the disease.
The problem with making this happen, Laron believes, is lack of funding.
“More money and effort should be put into primary prevention,” he argues.
“Very little money is put into it now.”
“We should try to use primary prevention to treat type 1 diabetes, at least in those instances where it is of viral origin,” he says.