Kidneys are fist-sized organs located in the retroperitoneum. Their three primary functions are: 1) excretion and regulation of body water, minerals and organic compounds (products of protein metabolism), 2) synthesis and secretion of the hormones renin, erythropoietin, and 1,25-dihydroxyvitamin D; and, 3) maintenance of electrolyte homeostasis and acid-base balance.
Chronic kidney disease (CKD) is associated with a long-term, normally progressive reduction in excretory, endocrine and metabolic functions of the kidney. Leading causes are diabetic nephropathy, renal vascular disease (including hypertension), and glomerulonephritis (inflammation of nephrons). CKD may be divided into five stages based on estimated or measured glomerular filtration rate (GFR), and the progression of symptoms tends to follow a similar sequence with respect to overall function. By stage 5 CKD, or end-stage renal disease (kidney failure) patients will require dialysis therapy for survival.
Acute renal failure is the rapid reduction of the kidneys' ability to remove waste and maintain fluid and electrolyte balance in the body, occurring over hours to days. It can be caused by a wide variety of factors, including dehydration, obstruction, or direct kidney damage. The patient may go on dialysis temporarily.
Bananas are an important source of potassium - important to
regulate in kidney disease (courtesy of TimothyPilgrim)
When kidney function becomes compromised, nutrition therapy plays a major role in the management of renal disease and in delaying disease progression. Specifically, this article will discuss the interrelationships between kidney function and of the following nutrients:
The article will provide an approach to conducting a thorough nutritional assessment and then highlight how to manage the above key nutrients when kidney function declines.
During the initial visit with a patient with kidney disease, it is important to conduct a thorough assessment to inform the therapeutic diet plan.
To assess the usual energy and nutrient intake of the patient, conduct a Diet History. Assess intake as accurately as possible because anorexia and weight loss are common as CKD progresses, and the avoidance of certain foods may put patients at risk for specific micronutrient deficiencies. Scrutinize the patient’s usual dietary intake for food sources of protein, Na, K, P, protein, iron, Vitamin D and calcium as these are key nutrients affected when kidney function declines. If the patient is Stage 3 or 4, recording fluid balance - Ins and Outs - is recommended.
A nutrient software analysis program is ideal to assess actual intake of key nutrients; however this is costly and time consuming.
Assess for co-morbidities (e.g. diabetes, dyslipidemia, hypertension, anemia) as each requires unique nutrition therapy that will augment the diet therapy for renal disease alone.
Know all medications the patient is on, specifically assess for hypertension treatments that will affect Na or K blood levels.
It is wise to assess the patient’s ability to afford food, supplements and medications, and to inquire how they access food (purchase at markets/stores, family garden, etc.).This will allow nutrition recommendations that can actually be implemented by the patient.
Assess as well if the patient has much control over what foods enter the house and how they are prepared (e.g. who does the procuring and preparation of meals).
Monitor vitals, especially blood pressure.
Monitoring body weight is key to tracking fluid shifts and can give insight to how effective kidneys are exercising their excretory functions.
Body weight measurements, measuring skin folds, and measuring arm and/or leg circumference can help assess for loss of lean muscle mass.
Wet weight (with edema or ascites) and dry weight (without edema or ascites) are important to monitor if patient is undergoing dialysis.
Ongoing monitoring of the following is recommended:
Dietary changes should be progressive following the decline in kidney function. People with kidney disease may also have hypertension, diabetes, or cardiovascular disease, and prescribed diets will need to flex to meet demands of multiple health conditions.
Excretion and regulation of body water, minerals and organic compounds (products of protein metabolism) are the most important functions of kidneys because accumulated wastes are acutely toxic. In a healthy kidney the renal tubules reabsorb over 99% of the filtered sodium. As GFR declines, so does sodium reabsorption. With severely impaired kidney function, excessive dietary intake of sodium can lead to edema, hypertension or congestive heart failure. With decreased ability to excrete sodium, patients are therefore often recommended to restrict fluids and salt intake.
To complicate matters, some patients with salt-wasting nephropathy may be unable to conserve sodium, and a low sodium intake may not be sufficient to replace urinary and extrarenal sodium losses (e.g. feces, sweat). The patient may therefore develop sodium depletion and decreased extracellular volume, blood volume and renal blood flow. Signs of this may be unexplained weight loss, low blood pressure, and low skin turgor. Plasma levels of sodium will also be decreased. These patients may be cautiously given more dietary sodium to determine if GFR can be improved.
Individuals with early CKD (Stage 1 or 2): Fluid restrictions are not routinely recommended, but may be needed to manage severe edema or hypertension if diuretics are not being used (Dietitians of Canada, 2009a).
Individuals with more advanced CKD (Stage 3 or 4): The amount of fluid intake allowed is based on the amount of urine the patient can produce - so a careful monitoring of Ins and Outs is required.
Be sure to recognize the following as sources of dietary fluid: water, juice, coffee, tea, soda, soup, jello, ice, pudding, yogurt, applesauce, and popsicles.
Individuals with CKD at any stage: Restrict sodium (salt) intake to prevent or manage hypertension (Dietitians of Canada 2009a). The Dietary Reference Intakes (DRIs) set the Adequate Intake (AI) for sodium at 1500 mg (Health Canada, 2010), this is equal to the amount of sodium that you would find in 2/3 teaspoon of table salt. All patients with CKD should aim for sodium intake =/< the AI. Counsel patients to flavour foods with herbs and spices instead of salt and to limit consumption of foods that are high in sodium, such as processed meats, some canned foods (e.g. canned soups), cheeses, breads, sauces, pickled foods, and condiments (Dietitians of Canada, 2014a).
If the patient is taking ACE inhibitors or Calcium Channel Blockers to manage hypertension, it is especially recommended to follow a low sodium diet.
In the past it was common for protein to be monitored and restricted as a means of nutrition therapy; however, it has not been established that a protein restricted diet (<0.7 g/kg/day) delays the progression of CKD and a high protein intake (>1.0 g/kg/day) may compromise kidney function (Dietitians of Canada, 2009a). Severe restriction of protein intake may compromise overall nutritional and health status.
Serum creatinine can transiently increase after ingestion of a large amount of dietary meat; thus, nutrition can sometimes play a role in creatinine measurement, therefore effecting GFR (MedicineNet, 2014).
Individuals with CKD at any Stage: Patients should consume a moderate protein intake of about 0.8 g/kg/day (Dietitians of Canada, 2009a). This is the same amount recommended for any healthy individual to consume as set out in the joint American/Canadian Dietary References Intake’s Recommended Dietary Allowance (Health Canada, 2010).
Animal products (e.g. meat, fish, milk, milk products, eggs) provide high quality protein, supplying ample amounts of all the essential amino acids. Most plants provide low quality, or incomplete proteins, in that they are low or deficient in one or more of the essential amino acids. There are some exceptions, one being the soybean which provides high quality protein. Good plant protein sources include legumes, nuts, seeds, and to a lesser amount whole grains (EatRight Ontario, 2014b).
The maintenance of intracellular fluid volume depends on potassium (K) and phosphate ions. Approximately 98% of total body potassium is intracellular. Normally the kidneys provide the major route for K excretion - declining kidney function increases risk for fatal hyperkalemia.
Potassium mainly becomes a problem as urinary input drops; however dietary intake should be monitored and conservative.
Individuals with more advanced CKD (Stage 3 or 4): A potassium restricted diet is recommended for individuals with advanced CKD and elevated serum K to reduce their risk of hyperkalemia (Dietitians of Canada, 2009a). A healthy adult is recommended to consume 4700mg/day (Health Canada, 2010).
If the patient is being treated for hypertension, different medications can have opposite effects: ACE inhibitors and potassium-sparing diuretics increase serum K; thiazides and loop diruetics decrease serum K. Thus blood levels should be monitored carefully and diet modified accordingly.
Vegetables and fruits are some of the best sources of potassium. Grains, dairy products, lentils, beans, meat and fish also have potassium (EatRight Ontario, 2014a).
Because foods rich in potassium also tend to be rich in Vitamin C, individuals on a low potassium diet may benefit from Vitamin C supplementation. It is important to note Vitamin C deficiency causes scurvy, and Vitamin C enhances iron absorption. Vitamin C supplements should not be taken beyond the Tolerable Upper Limit (Health Canada, 2010) as this may contribute to the development of oxalate kidney stones and exacerbate renal failure.
Phosphorus (P) is one of the major minerals with approximately 80% of the body’s P found in bones and teeth as calcium phosphate (CaPO). However, it is the role P plays in the maintenance of intracellular fluid volume that makes this mineral a major concern in renal disease.
High levels of serum P will bind to calcium. Low Ca levels in the blood causes PTH to be released which stimulates bones to release Ca into the blood to maintain P balance. This can cause both tissue calcification and weak bones.
Individuals with more advanced CKD (Stage 3 or 4): A dietary phosphate restriction is recommended when plasma PTH levels are increased or when serum phosphorus levels are elevated (Dietitians of Canada, 2009a). A healthy adult is recommended to consume 700mg/day (Health Canada, 2010).
Phosphorus is found in high amounts in protein rich foods such as milk and milk products, meat and meat alternatives, such as beans, lentils and nuts. Grains, especially whole grains, also provide phosphorus. Phosphorus is found in smaller amounts in vegetables and fruit (Dietitians of Canada, 2014b).
It is important to note phosphate binders can cause constipation. As renal diets tend to be lower in fibre and fluids, stool softeners and laxatives may be helpful.
Whether Vitamin D comes from the sun or food, it enters the blood circulation as a prohormone bound to a protein. It then must be metabolized in the liver and in the kidneys to become the active form of hormone Vitamin D3 (calcitriol). People with CKD will have decreased ability to synthesize adequate amounts of Vitamin D3. As the major biological function of Vitamin D3 is to maintain normal blood levels of calcium and phosphorus, impaired Vitamin D synthesis contributes to impaired intestinal calcium absorption, resistance to the actions of parathyroid hormone on bone and renal osteodystrophy.
Individuals with more advanced CKD (Stage 3 or 4): Serum calcium levels may decrease as kidney function deteriorates; in contrast high serum calcium levels can occur due to treatments with vitamin D metabolites or calcium-based phosphate binders. Thus, regular monitoring of serum calcium and serum levels of 25-hydroxyvitamin D (25(OH)D) is important with nutrition therapy adjusted accordingly.
To treat hypocalcemia consider calcium supplements (i.e. calcium carbonate) and/or vitamin D sterols. Be cautious that calcium intake (from both diet and supplements) does not exceed the Tolerable Upper Limit (Health Canada, 2010).
To treat hypercalcemia, consider adjusting therapies that increase blood calcium such as reducing the dose or eliminating calcium-based phosphate binders and/or reducing or discontinuing vitamin D sterols.
The most common source of vitamin D is from the sun. Vitamin D is not found naturally in many commonly consumed foods, with the only good natural dietary sources being certain kinds of fish and egg yolks (Dietitians of Canada, 2014c).
Energy requirements are similar to that of healthy individuals and should be established to achieve a healthy Body Mass Index (BMI). In the absence of dyslipidemia, fat recommendations are similar to healthy individuals (Dietitians of Canada, 2009a). The Dietary Reference Intakes set the Acceptable Macronutrient Distribution Levels (AMDR) at 20-35% of calories from fat with saturated and trans fat intake as low as possible (Health Canada, 2010).
Recommend foods high in mono- and polyunsaturated fats such as vegetables oils, avocados, nuts and seeds. Trimming visible fats of of meat is recommended, as well as removing chicken/poultry skins.
Dietitians of Canada (2009a) Practice-Based Evidence in Nutrition. Nephrology - Chronic Kidney Disease: Non-Dialyzed (Adult) Practice Guidance Summary.
Dietitians of Canada (2009b) Practice-Based Evidence in Nutrition. Nephrology - Acute Renal Faliure (Adult) Evidence Summary
Dietitians of Canada (2014a). Food Sources of Sodium. Retrieved from: http://www.dietitians.ca/Nutrition-Resources-A-Z/Factsheets/Minerals/Food-Sources-of-Sodium.aspx. Accessed Oct 9, 2014.
Dietitians of Canada (2014b) Food Sources of Phosphorus. Retrieved from: http://www.dietitians.ca/Nutrition-Resources-A-Z/Factsheets/Minerals/Food-Sources-of-Phosphorus.aspx Accessed Oct 9, 2014.
Dietitians of Canada (2014c) Food Sources of Vitamin D. Retrieved from: http://www.dietitians.ca/Nutrition-Resources-A-Z/Factsheets/Vitamins/Food-Sources-of-Vitamin-D.aspx Accessed Oct 9, 2014.
EatRight Ontario (2014a). What you need to know about potassium. Retrieved from: http://www.eatrightontario.ca/en/Articles/Nutrients-%28vitamins-and-minerals%29/What-you-need-to-know-about-potassium.aspx#.VC97cBZB7k0 Accessed Oct 9, 2014.
EatRight Ontario (2014b) Introduction to Protein. Retrieved from: http://www.eatrightontario.ca/en/Articles/Protein/Introduction-to-protein.aspx#.VExlnBZB7k1 Accessed Oct 9, 2014.
Health Canada (2010). Dietary Reference Intake Tables. Retrieved from: http://www.hc-sc.gc.ca/fn-an/nutrition/reference/table/index-eng.php Accessed Oct 10, 2014.