Kidney disease and Protein metabolism in the body

Chronic kidney disease (CKD) is characterized by decreased excretory functioning of kidneys because of which unexcreted toxins get accumulated in the body. These toxins cause metabolic abnormalities in the body. As these toxins get accumulated more, patients develop uremic syndrome which is characterized by loss of body mass, weakness, fatigue etc.

Toxins accumulated in kidney failure are called uremic toxins and are produced because of breakdown of proteins and amino acids. Uremic toxins accumulated can cause problems with wound healing, heart disease, kidney disease worsening and inflammation.

Muscle is the largest store of protein and amino acids in the body. In normal adults, roughly 3%-5% of body proteins are broken down and made daily. In CKD patients there is imbalance between breakdown and making of protein in the body which leads to muscle loss. Muscle protein breakdown in CKD patients will produce amino acids and breakdown products, which are the uremic toxins. High dietary protein intake causes excess load on kidneys, which lead to overworking of kidneys and gradual damage.

In patients with CKD who are not on dialysis, protein restriction in the diet is advised to slow the kidney disease worsening. Protein restriction helps in reducing the toxicity from uremic toxins in the following ways:

• As uremic toxins are produced by breakdown of protein in the diet and body (muscle), less protein in the diet leads to less availability of protein for breakdown, so less uremic toxins.
• Bacteria are altered in the intestines in patients with CKD due to change in dietary habits, iron intake, frequent antibiotic intake and phosphate binder intake. This altered bacteria (dysbiotic bacteria) in the intestines of CKD patients also covert protein in the diet to uremic toxins which are absorbed into the blood from intestines. So, less protein in the diet leads to less production of uremic toxins by bacteria.
• Diets rich in protein have excess of inorganic ions like sodium, potassium, phosphorus and hydrogen. These are toxic to the body is excess amounts, which can be prevented by taking less dietary protein.
• Protein breakdown in the body produces acid. This acid is excreted out in urine in normal conditions. In patients with kidney disease, acid accumulates in the blood (this is called metabolic acidosis) and it can damage the bones and make them weak and also stimulates muscle breakdown. High protein intake in the diet leads to excess production of acid in the body, which can be decreased by restricting the protein intake in the diet. Taking more fruits, vegetables and medications (tablets containing sodium bicarbonate) can also decrease the acid levels in the blood.
• Excess protein intake is associated with excess salt intake. Excess salt intake causes high blood pressure, worsening of kidney disease and protein loss in urine. It also worsens the heart disease.
• Phosphate accumulates in patients with CKD, and it is toxic to the body at high levels. Phosphates are found in high quantities in protein sources (meat, dairy products, beans, nuts etc.) and packaged foods. High phosphorus levels can worsen the kidney disease, antagonize the kidney protective effects of few medications (Angiotensin converting enzyme inhibitors and receptor blockers). It can lead to calcification of blood vessels leading to heart disease and brain stroke.

Reduced protein stores due to protein breakdown in patients with CKD increases risk of death and disability. Low levels of albumin, cholesterol, low body weight and body measurements can identify patients who have lost protein stores. Protein-energy wasting is the term used for such condition in patients with CKD. Protein restriction in the diet is advised in consultation with dietician and nephrologist to prevent the development of protein energy wasting in patients with non-dialysis requiring CKD.

Protein energy wasting is common in patients on dialysis. It increases the risk of death. Around 10-12 grams of amino acids are lost during each hemodialysis session, and 5 -15 grams of albumin is lost daily in peritoneal dialysis patients during the procedure. Hemodialysis procedure as such can trigger the breakdown and loss of protein stores in the body. Additional oral nutritional supplements are advised in patients on dialysis to maintain the muscle protein stores. More than 1.2gram/kg body weight of protein intake per day is advised in patients on dialysis, which is different compared to patients who are not on dialysis in whom protein restriction is advised.

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