Calcium : phosphorus ratio (Ca:P)

Calcium and phosphorus are always considered together, because it is not so much their absolute amounts as their mutual proportion that determines the health of bones and teeth [1]. The optimal Ca:P ratio is roughly 1.2:1–1.4:1 in the dog and 1.1:1–1.3:1 in the cat [1] [2].

Why the proportion, not the amount alone

Pure meat is rich in phosphorus and poor in calcium. Feeding meat alone without a source of calcium drastically lowers the Ca:P ratio — and this despite a "sufficient" amount of phosphorus. When there is too much phosphorus relative to calcium, it binds with calcium already in the digestive tract, forming unabsorbable compounds. As a result the body draws the missing calcium from its own skeleton, which with a chronic deficiency leads to decalcification and weakening of the bones (secondary hyperparathyroidism) [3].

What it means in practice

• Too low a Ca:P (too little calcium, too much pure meat) is the most common mistake of home-prepared diets [4]. The solution: add a source of calcium — meaty bones or a targeted supplement (eggshell meal, calcium citrate). More in the article Calcium.
• Too high a Ca:P (too much bone or calcium supplement) blocks the absorption of zinc, copper and iron, and in puppies — especially of large breeds — disrupts proper skeletal development [1].
• In growing puppies of large and giant breeds the margin is the narrowest — an excess of calcium is here just as dangerous as a deficiency and requires particular caution [1].

Diets based on meaty bones do not have to balance out perfectly in every meal — the body evens out minor fluctuations over the scale of a few days. What counts is the average proportion in the diet, not a single bowl.

Sources

1. National Research Council (2006). Nutrient Requirements of Dogs and Cats. National Academies Press. https://nap.nationalacademies.org/catalog/10668/nutrient-requirements-of-dogs-and-cats
2. FEDIAF. Nutritional Guidelines for Complete and Complementary Pet Food for Cats and Dogs. https://europeanpetfood.org/self-regulation/nutritional-guidelines/
3. Hand MS, Thatcher CD, Remillard RL, Roudebush P, Novotny BJ (2010). Small Animal Clinical Nutrition, 5th Edition. https://www.markmorrisinstitute.org/sacn5.html
4. Pedrinelli V, Zafalon RVA, et al. (2019). Concentrations of macronutrients, minerals and heavy metals in home-prepared diets for adult dogs and cats. Scientific Reports 9:13058. https://doi.org/10.1038/s41598-019-49087-z