If type 2 diabetes was an infectious disease, passed from one person to another, public health officials would say we're in the midst of an epidemic. This difficult disease, once called adult-onset diabetes, is striking an ever-growing number of adults. Even more alarming, it's now beginning to show up in teenagers and children.

More than 18 million Americans have diabetes; about 5 million don't know they have the disease. If the spread of type 2 diabetes continues at its present rate, the number of people affected in the United States will increase from about 14 million in 1995 to 22 million in 2025. Worldwide, the number of adults with diabetes will rise from 135 million in 1995 to 300 million in the year 2025.

The problems behind the numbers are even more alarming. Diabetes is the leading cause of blindness and kidney failure among adults. It causes mild to severe nerve damage that, coupled with diabetes-related circulation problems, often leads to the loss of a leg or foot. Diabetes significantly increases the risk of heart disease. And it's the sixth leading cause of death in the U.S., directly causing almost 60,000 deaths each year and contributing to thousands more.

The good news is that type 2 diabetes is largely preventable. About 9 cases in 10 could be avoided by taking several simple steps: keeping weight under control, exercising more, eating a healthy diet, and not smoking.

What Is Type 2 Diabetes?

Our cells depend on a single simple sugar, glucose, for most of their energy needs. That's why the body has intricate mechanisms in place to make sure glucose levels in the bloodstream don't go too low or soar too high.

When you eat, most digestible carbohydrates are converted into glucose and rapidly absorbed into the bloodstream. Any rise in blood sugar signals the pancreas to make and release insulin. This hormone instructs cells to sponge up glucose. Without it, glucose floats around the bloodstream, unable to slip inside the cells that need it.

Diabetes occurs when the body can't make enough insulin or can't properly use the insulin it makes.

One form of diabetes occurs when the immune system attacks and permanently disables the insulin-making cells in the pancreas. This is type 1 diabetes, once called juvenile-onset or insulin-dependent diabetes. It affects about one million Americans.

The other form tends to creep up on people, taking years to develop into full-blown diabetes. It begins when muscle and other cells stop responding to insulin's open-up-for-glucose signal. The body responds by making more and more insulin, essentially trying to ram blood sugar into cells. Eventually, the insulin-making cells get exhausted and begin to fail. This is type 2 diabetes.

In addition to the 18 million adults with diabetes, another 41 million have "pre-diabetes." This early warning sign is characterized by high blood sugar levels on a glucose tolerance test or a fasting glucose test. Whether pre-diabetes expands into full-blown type 2 diabetes is largely up to the individual-making changes in weight, exercise, and diet can not only prevent pre-diabetes from becoming diabetes, but can also return blood glucose levels to the normal range.

Type 2 Diabetes Can Be Prevented

Although the genes you inherit may influence the development of type 2 diabetes, they take a back seat to behavioral and lifestyle factors. Data from the Nurses' Health Study suggest that 90% of type 2 diabetes in women can be attributed to five such factors: excess weight, lack of exercise, a less-than-healthy diet, smoking, and abstaining from alcohol.

Among 85,000 married female nurses, 3,300 developed type 2 diabetes over a 16-year period. Women in the low-risk group were 90% less likely to have developed diabetes than the rest of the women. Low-risk meant a healthy weight (body-mass index [BMI] less than 25), a healthy diet, 30 minutes or more of exercise daily, no smoking, and having about three alcoholic drinks per week.

Similar factors are at work in men. Data from the Health Professionals Follow-up Study indicate that a "western" diet combined with lack of physical activity and excess weight dramatically increases the risk of type 2 diabetes in men.

Information from several clinical trials strongly support the idea that type 2 diabetes is preventable. The Diabetes Prevention Program examined the effect of weight loss and increased exercise on the development of type 2 diabetes among men and women with high blood sugar readings that hadn't yet crossed the line to diabetes. In the group assigned to weight loss and exercise, there were 58% fewer cases of diabetes after almost three years than in the group assigned to usual care. Similar results were seen in a Finnish study of weight loss, exercise, and dietary change.

Simple Steps

Making a few changes can dramatically lower the chances of developing type 2 diabetes. The same changes can also lower the chances of developing heart disease and some cancers.

Control your weight. Excess weight is the single most important cause of type 2 diabetes. Being overweight increases the chances of developing type 2 diabetes seven-fold. Being obese makes you 20 to 40 times more likely to develop diabetes than someone with a healthy weight.

Losing weight can help if your weight is above the healthy-weight range. Losing 7-10% of your current weight can cut in half your chances of developing type 2 diabetes.

Get moving. Inactivity promotes type 2 diabetes. Every two hours you spend watching TV instead of pursuing something more active increases the changes of developing diabetes by 14%. Working your muscles more often and making them work harder improves their ability to use insulin and absorb glucose. This puts less stress on your insulin-making cells.

Long bouts of hot, sweaty exercise aren't necessary to reap this benefit. Findings from the Nurses' Health Study and Health Professionals Follow-up Study suggest that walking briskly for a half hour every day reduces the risk of developing type 2 diabetes by 30%.

This amount of exercise has a variety of other benefits as well. And even greater cardiovascular and other benefits can be attained by more, and more intense, exercise.

Tune-up your diet. Two dietary changes can have a big impact on the risk of type 2 diabetes.

• Choose whole grains and whole-grain products over highly processed carbohydrates. White bread, white rice, mashed potatoes, donuts, bagels, and many breakfast cereals have what's called a high glycemic index. That means they cause sustained spikes in blood sugar and insulin levels. Carbohydrates that aren't as easily digested cause lower, slower increases in blood sugar and insulin. As a result, they stress the body's insulin-making machinery less, and so help prevent type 2 diabetes. Such foods have a low glycemic index. Examples include whole wheat, brown rice, other whole grains, most beans and nuts, and whole grain breakfast cereals.