Hearts and kidneys: If one's diseased, better keep a close eye on the other. Surprising new research shows kidney disease somehow speeds up heart disease well before it has ravaged the kidneys. And perhaps not so surprising, doctors have finally proven that heart disease can trigger kidney destruction, too.

The work, from two studies involving over 50,000 patients, promises to boost efforts to diagnose simmering kidney disease earlier. All it takes are urine and blood tests that cost less than $25, something proponents want to become as routine as cholesterol checks.

"The average patient knows their cholesterol," says Dr. Peter McCullough, preventive medicine chief at Michigan's William Beaumont Hospital. "The average patient has no idea of their kidney function."

Chronic kidney disease, or CKD, is a quiet epidemic: Many of the 19 million Americans estimated to have it don't know they do. The kidneys lose their ability to filter waste out of the bloodstream so slowly that symptoms aren't obvious until the organs are very damaged. End-stage kidney failure is rising fast, with 400,000 people requiring dialysis or a transplant to survive, a toll that has doubled in each of the last two decades.

And while CKD patients often are terrified of having to go on dialysis, the hard truth is that most will die of heart disease before their kidneys disintegrate to that point, something kidney specialists have recognized for several years but isn't widely known.

Indeed, the new research is highlighted in this month's Archives of Internal Medicine with a call for doctors who care for heart patients to start rigorously checking out the kidneys — and for better care of early kidney disease.

The link sounds logical. After all, high blood pressure and diabetes are chief risk factors for both chronic kidney disease and heart attacks.

But the link goes beyond those risk factors, stresses McCullough: Once the kidneys begin to fail, something in turn accelerates heart disease, not just in the obviously sick or very old, but at what he calls "a shockingly early age."

McCullough and colleagues tracked more than 37,000 relatively young people — average age 53 — who volunteered for a kidney screening. Three markers of kidney function were checked: The rate at which kidneys filter blood, called the GFR or glomerular filtration rate; levels of the protein albumin in the urine; and if they were anemic. They also were asked about previously diagnosed heart disease.

The odds of having heart disease rose steadily as each of the kidney markers worsened. More striking was the death data. At this age, few deaths are expected, and indeed just 191 people died during the study period. But those who had both CKD and known heart disease had a threefold increased risk of death in a mere 2 1/2 years, mostly from heart problems.

"This study is very much a wake-up call," McCullough says.

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 the heart 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, say researchers. 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.