Heart disease. Stroke. Cancer. Osteoporosis. Diabetes. It's been 40-odd years since researchers first started to link these diseases to the average American's diet.
Since then, they have hunted for connections between food and Alzheimer's, arthritis, macular degeneration, gallstones, and other illnesses. * For no disease are all the questions answered.
But for some, we've learned how to reduce our risk, while for others, the causes remain largely elusive.
As the year, decade, and millennium come to a close, it's time to take stock of the evidence.
DISEASES WE CAN PREVENT
It's the number-one killer of both men and women in the U.S. Each year, more than a million Americans suffer a heart attack. Nearly half haven't yet turned 65.
Two out of three heart attack victims never make a complete recovery. Roughly half a million die. The death rate has dropped dramatically over the last few decades, in part because of coronary care units, clot-busting drugs, and other treatments. But in theory, the death rate should have fallen much further.
"The essentials of preventing heart disease have been before us since the late 1950s," says pioneering heart disease researcher Jeremiah Stamler, professor emeritus at Northwestern University Medical School in Chicago. "We've known that severe atherosclerosis--the underlying disease--is a reflection of the Western lifestyle in the 20th century."
Diet's role has become clearer over the last 40 years:
* Too much saturated and trans fat and cholesterol, largely from meat, dairy products, pastries, and eggs--raise blood cholesterol.
* Too many calories lead to obesity.
* Too little fiber, folate, and possibly omega-3 fatty acids (like the fat in fish) and antioxidants (like vitamin E) leave the heart unprotected.
* Too much sodium and too little potassium, magnesium; and protein (in foods like fruits, vegetables, and low-fat dairy products) raise blood pressure. So do obesity, lack of exercise, and excess alcohol.
"Sedentary living compounds our nutritional problems and cigarette smoking raises coronary risk," says Stamler. "That triad of an unhealthy lifestyle--adverse dietary patterns, inactivity, and smoking--is unprecedented in human evolution."
Over the decades, scientists have also come to recognize that preventing heart disease isn't just a matter of avoiding high blood cholesterol or high blood pressure. It's a matter of moving most of the population down below so-called normal levels, to optimal levels--that is, to blood cholesterol below 200 and blood pressure below 120 (systolic) and below 80 (diastolic). Right now, less than ten percent of the population is in that optimal group.
"If we want to break the back of this disease, we've got to end the situation where optimal levels are a rare event," says Stamler. "That won't eliminate heart disease, but it would end the epidemic."
 Circulation 94:1795, 1996.
 Cardiology 82:191, 1993.
It's a classic success story. Since 1970, the death rate due to strokes has dropped by 55 percent. But strokes still kill more than 150,000 Americans a year, making it the number-three killer (after heart disease and cancer). And that number will rise as the population ages. Still, the 30-year decline is a remarkable achievement.
The key: keeping a lid on blood pressure. High blood pressure raises the risk of a heart attack, but "the impact on stroke is much greater," says Norman Kaplan of the University of Texas Southwestern Medical Center in Dallas.
And it doesn't matter how old the patient is. "We used to think that older people needed higher pressure to push blood through the blood vessels," says Kaplan.
But in a recent paper pooling …