Low-level exposure to arsenic may cause diabetes

Inherited risks, high fat diets, lack of exercise and aging are only some of the well-known factors that can lead to type 2 or adult-onset diabetes.

New research from the Johns Hopkins Bloomberg School of Public Health adds another possible cause to the list: long-term exposure to arsenic in drinking water and food.

High concentrations of arsenic have previously been linked to the development of type 2 diabetes in Bangladesh, Taiwan and Mexico, where people are exposed to about 10 times as much arsenic as in the United States. But the Hopkins research team, lead by Ana Navas-Acien, uncovered a strong connection between low arsenic concentrations and the disease.

This is the first study to consider the effects of low levels of the toxin on diabetes. They speculated that arsenic inhibits the body's ability to metabolize glucose, the most basic sugar used by the body, and contributes to inflammation and cell death.

The researchers measured inorganic arsenic levels from urine samples of 788 adults. Those participants with pre-existing type 2 diabetes had 26 percent greater arsenic levels than those without diabetes.

Additionally, among the non-diabetic participants, those who had the highest arsenic levels were between three to four times more likely to develop the disease than those who had the lowest arsenic levels.

However, the study does not necessarily show that arsenic causes diabetes. It is possible, for example, that diabetes causes the body to metabolize arsenic differently so that more arsenic is excreted regardless of the level of exposure.

One way that arsenic - a colorless, odorless and highly soluble metal - could actually contribute to diabetes is by damaging or killing cells in people already at high risk for developing the disease.

Separate studies have found that pancreatic cells secrete less insulin in the presence of arsenic and that arsenic also inhibits other cells' ability to take up glucose.

The hormone insulin is made only in the pancreas, and its presence allows cells in the body to absorb glucose from the bloodstream to convert into energy. If insulin is not produced or is unable to influence the activities of other cells, diabetes develops, causing fluctations in blood glucose levels.

In type 2 diabetes, which accounts for over 90 percent of all cases, the pancreas makes less insulin, and cells are less sensitive to the insulin, leading to decreased glucose uptake and high concentrations of glucose in the blood. Type 1 diabetes, which is found predominantly in children, is an autoimmune disease caused by the body's destruction of its own insulin-producing cells.

While the risk for type 2 diabetes can be lowered with proper exercise and diet, there is no known way to prevent type 1 diabetes. Eight percent of the American population suffers from one form of the disease, and over half of these cases are found in people older than 60.

Inorganic arsenic, commonly used as a rat poison, can seep into water sources when minerals naturally dissolve or as a byproduct from industrial processes like coal burning and copper smelting. Seafood contains organic arsenic, which is nontoxic and which was accounted for in the study.

Unlike many other factors involved in diabetes, such as poor diet or exercise, which are rooted in lifestyle habits and are difficult to tackle, the arsenic problem is easier to solve.

Filtering tap water and decreasing the legally allowed concentrations of arsenic in public water supplies are two ways to greatly lower an individual's exposure to the toxin. This may be a challenge in small rural communities that get drinking water from private wells instead of the public water supply, which is regulated by government standards.

While the data was being collected in 2003 and 2004, water treatment facilities nationwide were on their way to meeting a lower federally mandated arsenic standard of 10 parts per billion.

The stricter rule was prompted by separate earlier concerns that arsenic contributes to skin, liver, lung, kidney and bladder cancer.

Navas-Acien's team hopes to reproduce the study on a larger scale with up to 4,000 patients in an effort to pin down the exact link between diabetes and arsenic.

The study was published in the Journal of the American Medical Association in August.