Editor's note: In Jan. 2005, the CDC issued a correction to the statistics cited below. Rather than causing 400,000 deaths in 2000, the agency said poor diet and physical inactivity actually caused closer to 365,000 deaths. It blamed the error on an oversight when updating statistical data. Despite the change, the CDC said its main conclusions still hold: "Tobacco use and poor diet and physical inactivity contributed to the largest number of deaths, and the number of deaths related to poor diet and physical inactivity is increasing," the correction said. It was published in the Journal of the American Medical Association (Vol. 293, No. 3: 293).

If you've broken your New Year's resolution to eat better and exercise more, here's a reason to hop back on the healthy-living bandwagon. A new study from the Centers for Disease Control and Prevention (CDC) finds that diet and inactivity may soon overtake smoking as the leading cause of death in the United States.

Some 2.4 million people died in the US in the year 2000, most of them from heart disease, cancer, and stroke. The CDC researchers analyzed what factors led to the deadly diseases in the first place. Although most diseases have multiple causes, the scientists were able to estimate how much things like smoking, diet, infections, or drinking contributed to the deaths. Their findings were published in the Journal of the American Medical Association (Vol. 291, No. 10: 1238-1245).

Tobacco use was responsible for about 435,000 of the deaths in 2000, while bad diet and physical inactivity accounted for about 400,000. That ratio is likely to change, though, because the number of Americans who smoke is slowly dropping, while the number of overweight and obese Americans is growing rapidly.

"The impact of diet and inactivity is substantial, has increased, and is a very important challenge to the health of Americans," said Michael McGinnis, MD, MPP, who published a similar analysis in 1993 and wrote an editorial accompanying the current report. His study, based on data from 1990, also found that smoking and diet/inactivity were the leading killers in the US.

Trends of smoking and overweight among youth are of particular concern because of the impact these trends will have on future rates of cancer, heart disease, and diabetes, said Colleen Doyle, MS, RD, Director of Nutrition and Physical Activity for the American Cancer Society.

The CDC researchers estimate that one-third of all US deaths are due to diseases related to diet, physical inactivity, and smoking. However, the specific links to cancer may be even stronger.

It is estimated that about one-third of all cancer deaths in the US are related to smoking, while about another third are linked to diet, excess weight, and physical inactivity. For the majority of Americans who do not smoke, dietary choices and physical activity are the most important factors we can control in terms of cancer risk.

The numbers point to a critical need for action, both from individuals and society as a whole, said McGinnis, of the Robert Wood Johnson Foundation.

"Behavior is only the end product of a long line of influences and we need to make sure we address our attention to all elements along the spectrum," he said.

From a policy perspective that may include things like encouraging food manufacturers to offer more nutritional value in each calorie that's consumed, or improving food labeling so people can make better choices, or offering smaller portion sizes. McGinnis called the recent decision by McDonald's restaurants to eliminate supersizing "encouraging."

Designing communities with physical activity in mind is also important. The availability of parks, sidewalks and safe places to play can impact physical activity levels.

"Investments in programs to increase physical activity, improve diet, and increase smoking cessation are more important than ever before and must continue to be high priorities," said CDC director Julie Gerberding, MD, a co-author of the JAMA report.

But people have to take responsibility for their own behavior, too. They must be aware of what they eat and make a concerted effort to get exercise. "Calories out need to equal calories in," McGinnis observed. For the 64% of Americans who are currently overweight, that balance needs to be tipped even further, Doyle said: Calories out must be greater than calories in.

For many people, cutting back those calories and getting more exercise is a real challenge. A relatively easy first step, Doyle said, is to keep an eye on portion sizes.

"We seem to have lost sight of what a 'normal' portion size is," she said. "We tend to be served portions that frequently are double the amount of the standard portion, and when we're served large portions, we eat more calories."

To get a better sense of proper serving sizes, Doyle suggested doing some test runs at home. Measure a half cup of pasta or a 3- to 4-ounce portion of meat, and compare this amount to what you usually eat. ACS nutrition guidelines also recommend limiting red meat -- fish, poultry, or beans are a better choice -- and eating at least 5 servings of fruits and vegetables a day.

As for exercise, start looking for ways to incorporate activity into your day.

"Commit to taking the stairs at the office instead of the elevator," Doyle suggested. "Get off the subway a few stops early and walk to your destination, play ball with your kids, or go for a family walk after dinner." According to ACS physical activity guidelines, adults should get at least 30 minutes of moderate exercise 5 or more days each week; more vigorous or frequent activity may be even better. Children need at least an hour of activity 5 or more days a week.

McGinnis praised the CDC for collecting and analyzing the type of data included in the latest report. For a number of years, he explained, bad diet and lack of exercise weren't recognized as significant problems. Now that they are, the health agency can keep public attention focused on reversing these deadly trends.

The new findings have also prompted the Department of Health and Human Services to launch a public education campaign, dubbed the Healthy Lifestyles and Disease Prevention Initiative, urging Americans to make changes in their diet and exercise habits. The National Institutes of Health also plans to fund more research into obesity prevention and treatment.

"We need to tackle America's weight issues as aggressively as we are addressing smoking and tobacco," said Health and Human Services Secretary Tommy Thompson, in announcing the new initiatives.

But progress is likely to take time.

"It took 50 years to get us into this predicament," McGinnis said. "It's probably going to take a few decades to get us out of it."