Urine, like other body fluids, can be either acidic or alkaline. Acidic substances have a pH of less than 7 and alkaline substances have a higher pH. Urine normally is slightly acid, with a pH around 6, although it can range from 4.5 to 8. Urine pH changes, depending on your diet, certain disease processes and the medications you take. Excreting acid or alkaline urine helps maintain the body's acid-base balance, the balance between acidity and alkalinity.
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What you eat can alter the pH of your urine, although it doesn't change the pH of your blood. Vegetarians generally have more alkaline urine than meat eaters, because meat and dairy produce acidic urine and most vegetables and fruits a more alkaline urine. Although you may think of fruits as acidic, once eaten most fruits and vegetables produce alkaline urine. Cranberries, one of the exceptions to the rule, produce a more acidic urine, but that's not the reason they help prevent urinary tract infections, which occur more frequently when you have alkaline urine. Instead, they prevent bacteria from attaching to the bladder walls and multiplying there.
If you have a disorder that causes acidosis in the blood, your body tries to excrete the excess in the urine, causing acidic urine. Diseases that cause acidosis include respiratory problems that interfere with air exchange, diarrhea, dehydration, severe diabetes and starvation. Your urine may have a high pH or alkalinity if you have kidney disease, vomiting, diseases that causes rapid breathing or urinary tract infection.
Drugs that can cause alkaline urine include acetazolamide, a diuretic used to treat glaucoma, some types of seizures and congestive heart failure. Other drugs that increase urine alkalinity include sodium bicarbonate and potassium citrate. Drugs that make the urine more acidic include thiazide diuretics, methenamine mandelate, an antibiotic used to prevent or control but not treat urinary tract infections, and ammonium chloride, used to treat alkalinity in the blood.
The first urine specimen in the morning is usually more acidic than urine produced later in the day, because you breathe less and more shallowly while you sleep, producing a slight respiratory acidosis in the blood. If you don't test the urine specimen immediately after urinating and leave it in an open container, bacteria may accumulate and multiply. Bacteria in the urine turn it more alkaline, so you may get inaccurate results.