What about the heart's effect on the kidneys? Researchers at Tufts-New England Medical Center in Boston evaluated more than 13,000 people who had participated in two large heart-health studies. People diagnosed with heart disease at the studies' start had twice the risk of declining kidney function in the next nine years.

That makes sense. Heart disease narrows arteries all over the body, kidneys included. Also, some heart imaging tests use compounds that may harm kidneys.

But McCullough suspects a more complex culprit: Both theheart and kidneys send various signals to the bone marrow, which produces a type of stem cell that keeps those organs in good repair. When either starts to fail, this key repair mechanism falters, too, he explains.

Cause aside, what does the research mean for patients today?

-Cardiologists and primary care physicians should carefully monitor heart disease patients for declining kidney function, Wake Forest University physicians urge in an editorial accompanying the two studies.

-Anyone with kidney risk factors - a relative with kidney disease, or someone with diabetes, high blood pressure or a relative with either - should get their GFR tested, advises Dr. Leslie Spry of the National Kidney Foundation. McCullough goes further, and says every adult should know that number. Surveys suggest such routine screening even of the high-risk is rare today.

A normal GFR is 120. If it slips below 60, they have kidney disease.

-Treat kidney disease aggressively. Fewer than a third of CKD patients are prescribed pills proven to protect their damaged kidneys from getting worse - pills such as ACE inhibitors or "angiotensin II receptor blockers" that also lower blood pressure and thus are good for the heart, too.

McCullough adds that CKD patients should have other heart-related risk factors controlled even more tightly than doctors often recommend. Systolic blood pressure - the top number - should never be above 130, and the so-called "bad" or LDL cholesterol should be below 70.

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