Phosphorus is an essential element in the feline diet, so it must be present in adequate amounts. It is, however, hard on diseased kidneys. For that reason, IF your cats have any degree of renal insufficiency (as demonstrated in bloodwork), it's best to choose foods with lower levels of phosphorus. If your senior cats have healthy kidneys, then phosphorus levels found in most cat foods should present no problem.
The best thing you can do for your seniors is to have regular bloodwork run on them (at least once yearly - twice would be better, if you can afford it). Bloodwork will help catch an early diagnosis of most of the disease processes common to elderly cats, and early diagnosis is the key to successful management or cure of these diseases. Bloodwork will also identify increases in serum phosphorus that will indicate when dietary adjustments need to be made.
Laurie
The best thing you can do for your seniors is to have regular bloodwork run on them (at least once yearly - twice would be better, if you can afford it). Bloodwork will help catch an early diagnosis of most of the disease processes common to elderly cats, and early diagnosis is the key to successful management or cure of these diseases. Bloodwork will also identify increases in serum phosphorus that will indicate when dietary adjustments need to be made.
Laurie