One of the most harmful nutrients to the kidney is phosphate. Yet, the body needs phosphate for many essential biological functions. In this blog, I will discuss and clarify the role of dietary phosphate in kidney health. I will use the term phosphorus instead of phosphate for ease.
Daily requirements of phosphorus
There is no doubt that the average person requires phosphorus in the diet to live a healthy life. On average, the normal person requires 900-1250 milligram of phosphorus a day. Higher intake is especially required for growing children and pregnant women. Unfortunately, the standard American diet often contains higher amounts of phosphorus.
Role of phosphorus
Phosphorus plays an essential role in bone and teeth formation. It is also required for metabolism and energy production in the body. It is crucial for normal heart and nerve functions. Phosphorus serves as a building block for many structural and functional proteins. Because of its important role, the gut, hormones, kidneys, bones, and even vitamin D are involved in regulating phosphorus levels in the body.
The different types of dietary phosphorus
To simplify, we divide dietary phosphorus into two major categories. The first is organic phosphorus which comes “naturally” from animal and vegetarian proteins. The second is inorganic phosphorus, which is present in food additives.
How the kidneys handle phosphorus
The kidneys are critical for keeping a balanced, steady state of phosphorus in the body. Most of the phosphorus ingested in food gets absorbed in the gut. Some of it leaves the body in the stool. Circulating phosphorus gets filtered by the glomeruli. In this process, tiny capillaries in the kidney take phosphorus out of the blood. However, the kidney tubules help reclaim that phosphorus. They add phosphorus back to the blood, a process called reabsorption. The glomeruli and kidney tubules work together to filter waste out of the blood, but also recycle nutrients that are needed, like phosphorus. The end result is that the amount of phosphorus excreted in the urine equals that absorbed in the gut.
Hormonal control of phosphorus balance
The reabsorption of phosphorus in the kidneys depends on several hormones. The parathyroid hormone (PTH) decreases phosphorus reabsorption. When phosphorus isn’t recycled, more phosphorus leaves the body through the urine. On the other hand, 1,23 (OH) vitamin D increases phosphorus reabsorption, meaning that the body holds on to more phosphorus. Vitamin D also increases phosphorus absorption by the gut.
Bones also influence phosphorus levels. They produce fibroblast growth factor 23 (FGF-23), which decreases phosphorus reabsorption. Simply put, FGF-23 tells the kidneys to excrete phosphorus in the urine, instead of holding on to it. The following picture summarizes phosphorus balance.
Phosphorus in kidney disease
In kidney disease, the ability of the kidney to excrete phosphorus progressively declines. Data shows that the decline begins early in kidney disease. In response, there is an increase in the production of PTH and FGF-23. Both hormones attempt to rid the body of phosphorus by decreasing the reabsorption of phosphorus by the kidneys. In the beginning, this effort maintains normal phosphorus in the blood. But as kidney disease advances, the ability of the kidneys to excrete phosphorus decreases further. Thus, in advanced kidney disease both PTH and FGF-23 are high, but phosphorus accumulates in the blood.
Elevated phosphorus binds to calcium and deposits in the blood vessels and under the skin. This leads to inflammation. In addition, several studies have shown that elevated levels of PTH and FGF-23 are associated with poor outcomes in kidney patients. Elevated levels of FGF-23 lead to poor cardiovascular outcomes and progression of CKD. Elevated PTH causes poor bone health in kidney patients.
Dietary phosphorus in kidney disease
Recent data showed that patients on plant-based diet are less likely to develop kidney disease. In fact, a plant-based diet is now recommended for kidney disease patients. Some worry that plant-based diets lead to higher intake of phosphorus. Many practitioners believe that a diet high in legumes, nuts and seeds will increase intake of phosphorus.
Yet, when we look at serving size, animal protein and dairy have the highest content of phosphorus. Besides, phosphorus in vegetarian sources is bound to phytic acid. Humans don’t have the enzymes needed to digest phytic acid. So, we actually do not absorb a lot of the phosphorus that is present in plant-based foods.
Actually, we only absorb 40% of vegetarian phosphorus as compared to 60% of animal-based sources of phosphorus.
But the food industry is actually adding phosphorus to many foods. This is part of food processing. Some use it as a preservative. Some use it to prevent discoloration. Others use it to improve taste. You often see names that are not easily recognizable. These names include calcium phosphate, disodium phosphate, phosphoric acid, or sodium triphosphate. These types of phosphorus are the inorganic phosphorus. We absorb almost 100% of phosphorus as food additives.
This is why it is so important to look at the ingredients. Any ingredient that has the four letters, “Phos” in it means that it contains phosphorus. You can find great resources about dietary phosphorus here.
The bottom line
Higher intake of dietary phosphorus leads to worse kidney outcomes. Processed food has the highest absorbable phosphorus, followed by animal proteins. Vegetarian sources have the lowest absorbable phosphorus. This supports the use of a whole foods, plant-based diet in kidney disease.
