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The Source of Calcium Phosphate Kidney Stones

Calcium phosphate kidney stones form when calcium atoms combine with phosphoric acid (instead of oxalic acid like we discussed in last week’s post HERE). To lay the foundation for the following content, we will begin with a molecular description of the offending compound.

Phosphoric acid (shown below) is made up of a phosphorus atom (illustrated as the “P”) with 4 oxygen atoms bound to it. 

Phosphoric Acid Molecule

One of the oxygen atoms in the above illustration has two lines, which indicates its bond to phosphorus. This bound oxygen atom cannot provide any charge with which to bind with calcium ions (or any other compound) to form a crystal. The other three oxygen atoms, however, have ordinary bonds and carry a negative charge. This makes them susceptible to binding with other positively charged compounds in the body (think of this like a magnet again). 

The ability for other compounds to bind with the three available negatively charged oxygen atoms (such as calcium in this instance) is heavily influenced by urine pH. In exceedingly acidic urine, the three negatively charged oxygen atoms will almost always be bound to positively charged hydrogren atoms (Figure A below). In this case, it is impossible for crystals to form because all charge sites (negatively charged oxygen atoms) are occupied by hydrogen atoms. 

In normal urine (ph of around 6), two of the negatively charged oxygen atoms are bound to positively charged hydrogen atoms (Figure B below). This allows the phosphoric acid molecule to exist with one negatively charged oxygen atom, which is not sufficient to form a crystal (Figure D below).

Where problems start to occur are with individuals who have abnormally alkaline urine (pH above 6.3-6.5). Alkaline urine forces the phosphoric acid molecule to expose two negatively charged oxygen atoms (Figure C below). These two open negatively charged sites allow for positively charged compounds such as calcium to bind in two separate locations and then stack on top of each other over and over again to form crystals (Figure E below). For this reason, calcium phosphate kidney stones tend to occur in individuals who produce a more alkaline urine than those that produce calcium oxalate kidney stones. 

Phosphoric Acid in Urine

Calcium Phosphate Crystal Formation

The initial crystal that is formed is called Brushite. Brushite is an equal mix of calcium and phosphate ions. However, Brushite crystals can convert to another specific type of calcium phosphate stone called hydroxyapatite (HA) when the ratio of calcium to phosphoric acid becomes unbalanced. Unfortunately, we do not yet know what causes this conversion to happen. 

Calcium Phosphate stones have a higher tendency to form in people dealing with systemic diseases such as Primary Hyperparathyroidism (causes an increase in blood calcium levels) and Renal Tubular Acidosis (medical condition that involves an accumulation of acid in the body due to a failure of the kidney to appropriately acidify the urine). These two conditions raise average urine alkalinity (higher pH) and foster crystal formation. Many uncommon genetic diseases do the same.

If not from disease, the rest of the population forms calcium phosphate stones due to a mix of reasons. Some unknown. What is known, however, is that limiting phosphoric acid through diet (just as like with oxalic acid) can have a positive impact on limiting calcium phosphate stone formation. 

The recommended daily amount (RDA) of phosphorus needed for normal bodily function is 700 mg. You can get this easily from natural food sources. Foods that are high in protein (e.g. meat, beans, eggs, chicken, and fish) are usually high in phosphorus as well. This means additional phosphoric acid from processed food and soda is likely more than the body needs.

Because so many of us drink sodas and eat processed foods, some health professionals are concerned about the American diet when it comes to phosphoric acid. For example, one soda may have up to 500 mg of phosphoric acid. People who take in 4,000 mg per day of phosphorus are considered at high risk for negative health effects associated with phosphorus.

Adults with kidney disease are recommended to have no more than 800 to 1,000 mg of phosphorus a day. Kidneys help the body get rid of extra phosphorus, but too much phosphorus can build up in the blood if they’re not functioning well.

Just as with an excess of oxalic acid in the body due to the food choices we make, an excess of phosphoric acid can lead to an increased susceptibility to calcium phosphate kidney stones.

For those who struggle with calcium phosphate kidney stones, it is suggested that the following steps be taken:

  1. Ensure proper hydration- the more water (H2O), the more opportunity for phosphoric acid to bind with water molecules and get passed through the system in the urine. 
  2. Reduce phosphoric acid intake from soda and processed foods to recommended levels- less phosphoric acid, the less opportunity to bind with calcium to form crystals
  3. Introduce more citrates into the diet (ex: lemons/limes); citrate, like calcium, has the ability to bind with phosphoric acid to help prevent crystal formation.

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