WASHINGTON (Reuters) – People who drink the most coffee and tea, even decaffeinated versions, can dramatically lower their risk of diabetes, researchers reported this week.

Their study does not answer why this might be but strengthens the findings of earlier studies showing the beverages may prevent type-2 diabetes.

"Every additional cup of coffee consumed in a day was associated with a seven percent reduction in the excess risk of diabetes," Rachel Huxley of The University of Sydney in Australia and an international team of colleagues wrote in the Archives of Internal Medicine.

This meant that people who drank three to four cups a day had a 25 percent lower risk of developing diabetes than those who drank little or no tea or coffee.

Huxley's team did what is known as a meta-analysis, taking a look at smaller published studies to add up greater numbers of people to show patterns more clearly.

They looked at 18 different studies covering more than 450,000 people.

The trend was clear, but the reasons were not.

"Because most of the studies included in this review did not provide data on the effects of these beverages or their components on measures of hyperglycemia (high blood sugar) and insulin sensitivity, we cannot provide further evidence on the mechanisms involved," they wrote.

For instance, the research did not show whether people who drank more coffee were eating or drinking less unhealthy stuff.

"Our findings suggest that any protective effects of coffee and tea are unlikely to be solely effects of caffeine, but rather, as has been speculated previously, they likely involve a broader range of chemical constituents present in these beverages, such as magnesium, lignans, and chlorogenic acids," the researchers wrote, citing other studies.

Coffee has been shown to be a major source of antioxidants in the US diet. These compounds can prevent damage to cells.

The findings could be significant – the International Diabetes Federation projects that by 2025, 380 million people worldwide could have type-2 diabetes, which is associated with overeating and a lack of exercise.

In North America, 10.2 percent of the adult population has diabetes and India alone has 50.8 million diabetics.