When kidney function is impaired, adopting a kidney diet minimizes adverse effects, slows disease progression and reduces the burden of medication.
The kidneys are vital organs very involved in maintaining the body’s homeostasis, for example by tightly regulating the levels of fluids, electrolytes, toxins, vitamins and minerals.
The quality of nutrition and hydration can greatly influence the functioning of the kidneys. This article will allow you to better understand what a renal diet consists of and the benefits associated with this type of diet.
Table of contents
- Definition of kidney nutrition and kidney diet
- Kidney function and impact of diet
- Updated kidney nutrition
- Unique dietary needs
- Kidney diet in a nutshell
Estimated reading time : 12 minutes
Definition of kidney nutrition and kidney diet
We name Kidney nutrition an eating pattern associated with kidney health benefits (e.g., slowed progression) or decreased adverse effects of the disease (e.g., phosphorus accumulation). Renal nutrition is a synonym.
The term nutrition refers to the process of eating food and absorbing the nutrients in those foods. We can also talk about kidney diet, a diet referring to a type and range of food that someone eats regularly.
Who should adopt a kidney diet?
Kidney diet is in fact a healthy eating model, consistent with the recommendations of Canada’s Food Guide1, for anyone with kidney disease (kidney failure, kidney stones, proteinuria, polycystic kidney disease) or considered at risk of developing kidney disease.
For example, people with diabetes or high blood pressure are considered to be at high risk of developing kidney disease, with these diseases representing the 2 leading causes of kidney failure. In fact, 1 in 10 people in Canada have kidney failure and most of them are unaware of it2.
Why adopt a kidney diet?
Adopting a kidney diet therefore reduces the risk of developing kidney failure when you are considered at risk, but also slows the progression when you have it. In addition, renal nutrition also minimizes the side effects of the disease and possible complications.
Kidney function and impact of diet
Each kidney is composed of about one million microscopic nephrons. Each nephron has a glomerulus and a network of tubules. The main role of the glomerulus is to filter nearly 180 litres of blood each day, without allowing too much blood components (e.g., protein) to pass through.
This amount of filtered blood is equivalent to a filtration of approximately 120 ml/min in a young and healthy individual. This is considered normal kidney function. Despite this very large amount of filtered blood, the kidneys produce on average only 1.5 to 2 liters of urine per day.
Body pH balance
The kidneys help maintain the acid-base balance of the blood by eliminating excess acidic waste produced by the diet and reabsorbing alkaline bicarbonates. This is essential for maintaining the normal pH of the blood.
When kidney function decreases, acidity builds up in the blood (acidosis). Adjusting the diet therefore reduces the acid load and the side effects of acidosis.
The kidneys eliminate and reabsorb water in order to maintain a normal concentration of all fluids in the body, regardless of the conditions of daily life (physical effort, diarrhea, vomiting, sweating, illness, etc.).
However, when kidney function is weak, the ability to eliminate may be reduced, which can contribute to water retention in the body (swelling or edema). Adjustments in hydration and additions of diuretics are sometimes necessary in some cases.
The kidneys tightly control the levels of electrolytes in the body (sodium, potassium, chlorine, bicarbonates, calcium, phosphorus and magnesium). They filter electrolytes from the blood and control their levels by eliminating them or keeping them in the body as needed. Kidney diet is important to help the kidneys maintain electrolyte balance.
Sodium is the main electrolyte in the body. It is present in many foods and beverages, so sodium intake is considered excessive in the general population.
Processed foods and meals eaten outside are the main sources of sodium in the nutrition. Excess sodium has very adverse effects on kidney health, as it contributes to hypertension, proteinuria, edema and progression of kidney failure.
Potassium is also an electrolyte tightly regulated by the kidneys. The body needs about twice as much dietary potassium as sodium to function well. Potassium has the opposite effect of sodium on blood pressure.
A plant-rich nutrition can meet potassium needs. Many processed foods also contain potassium additives, but these do not have health benefits unlike potassium naturally present in plants. Potassium blood levels that are too low or too high both have negative health effects.
Phosphorus is a mineral mainly present in the bones, like calcium. It is found in large quantities in food, mainly in animal proteins, but also in several food additives used by the food industry.
Phosphorus intakes are generally much too high in the general population. When kidney function declines, phosphorus can build up in the body and lead to complications, especially calcification of blood vessels that increases the risk of cardiovascular disease.
Blood pressure is the pressure exerted by blood against the walls of arteries when it is pumped by the heart. It is measured in millimeters of mercury (mm Hg) and is expressed as two numbers, e.g. 120/80 mm Hg. The first number, called systolic pressure, is the pressure in the arteries as the heart contracts and pumps blood. The second number, called diastolic pressure, is the pressure in the arteries as the heart relaxes between beats.
The kidneys play an important role in maintaining blood pressure by regulating the volume of blood in the body and producing hormones that act on blood vessels. For example, renin, produced by the kidney, is part of the renin-angiotensin-aldosterone system that tightly controls blood pressure.
Diet also plays a major role in maintaining blood pressure. In general, high sodium intake increases blood pressure, while high intakes of potassium and magnesium have favorable effects on blood pressure.
The kidneys are also responsible for removing waste and unnecessary substances from the body. Among these is urea, which is an end product of protein degradation. The kidneys also remove excess uric acid, acid residues, excess vitamins and minerals, as well as several drugs and toxins.
Therefore, a decline in kidney function is often associated with an increase in waste in the body. This is why the diet must be adjusted to try to produce less waste.
The kidneys are responsible for the production of erythropoietin. This hormone directs the production of red blood cells in the bone marrow. It is therefore essential to prevent anemia. Renal failure is associated with a decrease in erythropoietin, which explains the anemia often present in the more advanced stages.
Be aware that this anemia cannot be corrected by changes in nutrition. Additionally, iron-rich foods like red meat and offal meats are also high in phosphorus, protein, uric acid, and saturated fat, all of which are associated with negative effects on kidney health.
Treatment of anemia in renal failure is therefore based on iron supplementation (tablets or intravenously) and may sometimes require erythropoietin under medical prescription.
Vitamin D activation
Vitamin D that is consumed in the diet or produced through sun exposure must be activated to be used by the body. The kidneys are involved in converting inactive vitamin D into active vitamin D, called calcitriol. This kidney-activated vitamin D allows the intestine to regulate the absorption of calcium and phosphorus for good bone health.
Vitamin D deficiency is common in North America, regardless of kidney function level. In addition, foods rich in vitamin D, such as milk and oily fish, often need to be limited in the kidney diet. Vitamin D supplements (inactive and/or active form) are usually prescribed by the treating team and dosages are individualized according to the specific needs of each person.
Updated kidney nutrition
Nutrition is a rapidly changing science thanks to extensive scientific research and kidney nutrition is one of the areas in nutrition for which knowledge has evolved the most in recent years.
In general, scientific studies have shown that many restrictions on kidney nutrition in the past are not really helpful or desirable for overall health. The old renal nutrition guidelines dated back to 2000 and there were fewer scientific studies at that time.
Research in the field of kidney nutrition has led to a better understanding of the real impact of foods and nutrients on kidney health. For example, we now know that there are several mechanisms involved in the metabolism of certain electrolytes and that an imbalance is not necessarily caused by diet.
Recently, in 2020, international experts in nephrology published the latest recommendations in renal nutrition based on the most recent literature3. These new recommendations represent a very positive wind of change for patients with renal failure who will be able to improve the quality of their diet.
Indeed, the new recommendations place more emphasis on the quality of the foods chosen rather than on the quantities. Thus, we are moving further and further away from the famous lists of foods to avoid, which were based solely on the amounts of nutrients, without taking into account their real impacts on health.
Previously, patients were recommended a daily phosphorus limit that should not be exceeded. It was therefore common practice to avoid foods with higher phosphorus content, regardless of where they came from. Thus, many patients avoided whole grains, nuts and legumes, in addition to restricting dairy products.
However, studies proved a few years ago that the phosphorus contained in products of plant origins was in a form that was poorly assimilated by the body and therefore had little impact on blood phosphorus levels, unlike phosphorus of animal origins4.
In addition, phosphorus added to foods in the form of additives (e.g., sodium phosphate, phosphoric acid) was found to be 100% absorbed with a major impact on phosphatemia.
Thus, the new guidelines no longer suggest a maximum amount of phosphorus to consume, but rather to adjust intakes to maintain normal levels5. Therefore, it is recommended instead to take into account the source of phosphorus (vegetable, animal, additives) in the adaptation of the kidney diet and to limit excess animal protein as well as to avoid the consumption of processed foods.
Just like phosphorus, potassium has always been on the list of nutrients to avoid in case of kidney failure. Some patients even believe that potassium is harmful to kidney health and restrict their intake even though they have never had an episode of hyperkalemia (excess potassium).
In fact, the majority of people do not consume enough potassium in their diet even though it is beneficial for health, especially in blood pressure control. In recent years, many advances have been made in understanding the mechanisms involved in potassium homeostasis.
Thus, we now know that the potassium level in blood is influenced more by internal mechanisms than by dietary intake. In addition, as with phosphorus, dietary potassium is absorbed differently depending on the source from which it originates.
For example, potassium in plant proteins and unprocessed fruits and vegetables is only about 60% absorbed, while potassium in animal and processed plant proteins (e.g., juices, sauces) is more absorbed. Potassium additives (e.g., potassium chloride, potassium lactate) are absorbed quickly by the body and may increase the risk of hyperkalemia.
This new knowledge therefore calls into question popular restrictions of plants, especially since we now know that a high consumption of fruits and vegetables and a high intake of dietary fiber reduce certain risk factors for hyperkalemia such as metabolic acidosis and constipation6.
Plant consumption should therefore not be limited in renal insufficiency, although this practice is unfortunately still too widespread7. Once again, it is clear that the focus should instead be on avoiding processed foods containing food additives and moderating the consumption of animal protein.
Unique dietary needs
Regardless of the type of kidney disease and the level of kidney function, diet is always at the heart of treatment. However, kidney nutrition requirements may vary during the course of the disease, depending on the evolution of blood and urine results, medication and the specific needs of each individual.
Thus, there is no single “kidney diet” for kidney disease. For example, a person on dialysis will not have the same needs as a person not on dialysis or on a transplant. Also, some people need to increase their intake of certain nutrients while others need to reduce them (eg, protein, potassium, calcium).
Therefore, it is best to ask your health professionals about your specific needs and avoid dietary restrictions that are unnecessary or even harmful to your health.
Kidney diet in a nutshell
Kidney diet may seem complicated to many people at first glance, but when you think about it, it is similar in many ways to the dietary patterns recommended for the prevention of several diseases.
The kidney health plate is the same as the one suggested by Canada’s Food Guide: half the plate of vegetables accompanied by a moderate serving of protein and whole grains8. The consumption of plants (fruits, vegetables, vegetable proteins and whole grains) should be the basis of the kidney diet.
In fact, the two most important aspects to limit in the renal diet are processed foods and animal proteins. Reducing processed foods helps limit sodium as well as potassium and phosphorus additives. As for it, the decrease in animal proteins makes it possible to reduce phosphorus intake in addition to reducing the waste produced by the body (urea, uric acid, acid residues).
These two dietary changes alone also lead to reduced blood pressure and proteinuria. The updated kidney diet is therefore much less restrictive than before and above all, it is more varied and colorful than ever, to the greatest benefit of patients!
3. Ikizler TA, Burrowes JD, Byham-Gray LD, et al; KDOQI Nutrition in CKD Guideline Work Group. KDOQI clinical practice guideline for nutrition in CKD: 2020 update. Am J Kidney Dis. 2020;76(3)(suppl 1):S1-S107.
4. Picard K, Mager DR, Richard C. The Impact of Protein Type on Phosphorus Intake, Serum Phosphate Concentrations, and Nutrition Status in Adults with Chronic Kidney Disease: A Critical Review. Adv Nutr. 2021 Dec 1;12(6):2099-2111.
5. Ikizler TA, Burrowes JD, Byham-Gray LD, et al; KDOQI Nutrition in CKD Guideline Work Group. KDOQI clinical practice guideline for nutrition in CKD: 2020 update. Am J Kidney Dis. 2020;76(3)(suppl 1):S1-S107.
6. Babich JS, Kalantar-Zadeh K, Joshi S. Taking the Kale out of Hyperkalemia: Plant Foods and Serum Potassium in Patients With Kidney Disease. J Ren Nutr. 2022 Nov;32(6):641-649.
7. Picard K, Griffiths M, Mager DR, Richard C. Handouts for Low-Potassium Diets Disproportionately Restrict Fruits and Vegetables. J Ren Nutr. 2021 Mar;31(2):210-214.