"It doesn't really surprise me because we physicians have been telling patients to stop smoking, exercise regularly and be on a sensible diet for a long time," said internist Dr. Pouya Bahrami.
Dr. Bahrami says the other three risk factors are smoking, high blood pressure and obesity.
"Obesity itself is associated with bringing up the patient's blood pressure, and so these are interrelated issues that affect someone's health," said Dr. Bahrami.
Researchers used National Center for Health Statistics to determine how these four risk factors affected mortality rates.
Asians were affected the least and southern rural blacks were affected the most. But on average, scientists determined the four risk factors reduce life expectancy by nearly five years in men and four years in women.
Dr. Bahrami says this is one of the few studies that looks directly at things people can do improve their quality and length of life.
"You have to battle obesity," said Dr. Bahrami. "You have to battle against the desire to smoke. If everyone just pays attention to those issues, just remain focused on them everyone can benefit from it."
Researchers say of the four risk factors, smoking and high blood pressure had the largest effect on life expectancy.