Alkaline foods should really be referred to as "alkaline forming foods". Food is considered alkaline forming if food forms alkaline in the body's blood and urine. This does not mean that the food itself is alkaline: it may well be acidic. Orange juice for instance is highly acidic due to its high citrus acid content, but after being metabolized it will cause urine to become alkaline.
But what is really the difference between acid an alkaline forming
First, let's look at those phrases from a chemical perspective:
An acid is a substance that releases hydrogen into a solution and an alkali or base is one that removes hydrogen from a solution. The amount of free hydrogen is measured on a scale ranging from 1 to 14, called pH [pH definition], that denotes the exact level of acidity or alkalinity. A pH value below 7 is considered acid and above 7 alkaline.
pH and Cancer
Virtually all degenerative diseases, including cancer, heart disease, osteoporosis, arthritis, kidney and gall stones, and tooth decay are associated with excess acidity in the body. Alkaline forming foods or good alkaline water can cause a natural cleansing of the human body, spilling acid waste into the urine. A way to monitor the progress in restoring the overall acid/alkaline balance in our body is by using pH test strips.
Do not forget to read this article if you want to restore your acid/alkaline balance: Live Long With Good Health.