Cynthia Baldwin, age 14, of Carson City, Nov., for her question:

WHAT CAUSES DIABETES?

People with diabetes tend to build up a higher than normal level of sugar, or glucose, in the blood because they lack a hormone called insulin, the chemical messenger responsible for moving glucose out of the blood to other parts of the body. Causes of diabetes are not fully understood but both juvenile and mature onset diabetes tend to run in families.

But heredity is not the only factor and diabetes is definitely not passed from parent to child in any easily predictable way, like a trait carried on a single gene. A person’s environment and life style interact with heredity to determine whether he or she will become diabetic.

Juvenile and mature onset diabetes behave as if they were separate diseases and seldom occur in the same family.In mature onset diabetes, being overweight is the major risk factor. The mature onset diabetic has an insulin deficiency partly because he or she has large fat cells, so large that they create a greater demand for insulin than the beta cells can keep up with.

Obesity is a problem that fat parents tend to pass on to their children, probably both through genes and lifestyle, so they also pass on their increased risk of diabetes. In the United States, the increasing incidence of diabetes may be explained by our tendency to overeat. Forty percent of the American population is obese.

Juvenile diabetes is a problem of beta cells’ absence rather than increased demand by fat cells. Heredity plays a role. For instance, the children of two juvenile diabetic parents have about 30 percent chance of becoming diabetic.

Viruses have also been implicated in diabetes. About 20 viruses or virus particles have been connected with diabetes, so it seems likely that no single virus sets off the disease. Juvenile diabetes, or ketosis prone diabetes, makes up 15 percent of all diabetics.

Diabetes occurs in children more often at ages when they are likely to be exposed to certain virus strains for the first time. Some researchers have suggested a possible explanation that the body, in fighting off a virus infection by making protein antibodies and special cells to attack the invading virus, inadvertently makes antibodies that attack some of the antigens or protein markers on its own cells. This backfiring of the normal immune mechanism has been shown to also occur in other diseases such as rheumatic fever and lupus erythematosus.

Another possible mechanism is that an altered protein is produced by the beta cells or an altered cell surface occurs that is perceived by the body as foreign. Thus, a person with the cell antigens associated with diabetes develops the disease because when he or she produces antibodies to fight off a routine virus infection, the antibodies attack not only the virus, but the person’s own beta cells. This autoimmune response destroys the beta cells and produces diabetes.

Another factor may come with normal aging as the pancreas “wears out” and the body develops higher levels of blood glucose.