USDA Nutrient database

Use the USDA Nutrient database to get details on all the stuff that you eat. For example, "healthy foods" are dangerous to people with kidney disorders or patients with Renal Tubular Acidosis as the Phosphorus in whole grain cereals leaks into the body and causes considerable damage. Phosphorus is typically low enough not to be listed in the label, but high enough to cause considerable damage to kidney patients. Here is an example for Milk and Soymilk as listed in the USDA nutrient database.

Milk, lowfat, fluid, 1%, fortified with Vitamin A - Contains Phosphorus and Potassium

Minerals
Calcium, Ca	mg	142	0	0
Iron, Fe	mg	0.06	0	0
Magnesium, Mg	mg	16	0	0
Phosphorus, P	mg	111	0	0
Potassium, K	mg	180	0	0
Sodium, Na	mg	58	0	0
Zinc, Zn	mg	0.45	0	0
Copper, Cu	mg	0.010	0	0
Manganese, Mn	mg	0.002	0	0
Selenium, Se	mcg	2.5	0	0

Soymilk - Does not contain Phosphorus and also lower Potassium. The Std Error on Potassium (last column) is quite high, so it may have the same amount of Potassium as regular milk.

Minerals
Calcium, Ca	mg	140	4	24.47
Iron, Fe	mg	0.49	4	0.125
Potassium, K	mg	141	4	29.953
Sodium, Na	mg	50	4	5.739
Zinc, Zn	mg	0.24	2	0
Copper, Cu	mg	0.123	1	0
Selenium, Se	mcg	2.3	1	0

The database has all the food items you find at stores too. So you can check most of the store items. Below is an example.

Puddings, rice, dry mix, prepared with whole milk

Minerals
Calcium, Ca	mg	103	0	0
Iron, Fe	mg	0.37	0	0
Magnesium, Mg	mg	13	0	0
Phosphorus, P	mg	85	0	0
Potassium, K	mg	128	0	0
Sodium, Na	mg	108	0	0
Zinc, Zn	mg	0.38	0	0
Copper, Cu	mg	0.020	0	0
Manganese, Mn	mg	0.059	0	0
Selenium, Se	mcg	1.8	0	0

Phosphorus is an extreme example in that it is found in just about anything. But if you substitute milk with Soymilk here, you provide an enormous improvement in Quality of Life for kidney and RTA patients. Your kryptonite mineral may be something else depending on your ailment; the database gives you a great starting point to figure out what to eat and what to avoid.

You can also get a PDF file of Phosphorus content for all items, but it does not seem to be as accurate as a specific lookup and does diverge quite a bit from the specific lookup. So the PDF file is good if your primary focus is what to take, but not good if your primary focus is avoidance.