The Significance of Protein in the Urine

To most of us, protein is something that you get from your diet, and it is generally good for you. If your healthcare provider tells you that you have "protein in the urine" you may wonder why is it a problem. The medical term is proteinuria, which means the presence of an abnormally high amount of protein in the urine. A normal person's urine, when tested in the lab with a dipstick, should not show the presence of any protein. However, when you do, it could be a sign of chronic kidney disease (CKD). People with CKD often do not have any symptoms and the only way to diagnose it is through blood and urine tests, or imaging.

Abnormally High Level of Protein in the Urine Could Indicate Kidney Disease

Proteinuria could indicate CKD. Where does protein in the urine come from? The protein is there in the blood. It exists as different types (albumin, globulin, etc.). Your muscles are primarily made of protein. The antibodies that circulate in your system fighting infection are protein. Hence protein is essential, but you normally shouldn't be peeing it out into the urine.

How Does Protein Leak out Into the Urine? 

The kidney's filter, called the glomerulus is usually very selective. It lets the bad toxins out into the urine. But it holds back the good stuff (protein, blood cells, etc). However, damage to the kidneys filter (commonly from diseases like diabetes or hypertension) can make it lose this selectivity (think a sieve whose holes become too large, although it is more complicated than that) making protein escape. Other factors that can cause this is damage to the kidney's drainage system (the tubules) or in rarer cases, an excess amount of protein in the blood where it just overflows into the urine.

Protein in the Urine Is Not Just an Effect of CKD, It Could Be a Cause

Now we know that proteinuria indicates damage to the kidney's filter, and hence CKD. But if protein in the urine is just another downstream effect of kidney disease, then why do nephrologists make so much fuss about treating it? 

Protein in the urine is not just another consequence of CKD, it directly makes kidney disease worse. It leads to further kidney damage, which leads to even more protein in the urine. It might start off as an effect, but it soon becomes (part of) the cause as well. Protein is not just a sign of the problem, it is the problem.

Let us look at the evidence to support the above conclusion. A lot of research has been done on this, and today, minimizing proteinuria is a valid "target" for retarding the progression of CKD.

A key factor in the progression of kidney disease (regardless of the cause) is elevated pressure inside the kidney's filter, the glomerulus. This is called intraglomerular hypertension. This leads to elevated filtration rate (GFR) in the initial stages of kidney disease. Over the short term, this might help the kidney numbers "look pretty." In the long run, it is actually maladaptive and GFR drops and is the reason for scar tissue formation is many kinds of CKD, e.g., diabetic kidney disease.

Protein in the urine makes intraglomerular hypertension worse. And so we have medical evidence that indicates that untreated proteinuria significantly increases the chances of your kidney disease progressing to failure (when dialysis or transplantation become necessary).

Similarly, multiple medical studies have proved that worsening of kidney disease can be prevented if proteinuria is treated using specific drugs like ACE inhibitors or ARBs. Interestingly, the protective effect that comes with these medications is independent of the control in blood pressure induced by the drugs. A greater reduction translates into more kidney protection and a more favorable outcome for your kidneys.

Is a Low Protein Diet the Answer?

Even though it makes intuitive sense, and even though excess protein in the urine is directly associated with hyperfiltration, at this time the effect of protein restriction (especially when it is done in combination with ACE inhibitors or ARBs) on renal function is uncertain. For now, suffice it to say that a moderate protein diet with about 0.8 grams per kilo body weight of protein per day, might be beneficial but going on a low protein diet may not be of benefit.