These are terms that have produced considerable confusion for those trying to follow the Food Combining Lifestyle since Dr Hay wrote his books in the 1930s.
The problem has arisen because the terminology has changed!
Acids such as hydrochloric acid, nitric acid or sulphuric acid are strong acids. A molecule of hydrochloric acid consists of one negative ion of chlorine and one positive ion of hydrogen. In water, the ions separate completely and can react quickly and strongly with other ions in solution.
pH is the measure of the hydrogen ion concentration in a solution. A pH of 1 is a strong acid with many hydrogen ions. A pH of 14 has few hydrogen ions and is a strong alkali. A ph of 7 is a neutral solution.
Acids such as citric acid or ethanoic acid (vinegar) are weak acids. Although molecules of these acids still contain positive hydrogen ions, they do not separate completely in water.
Substances that can neutralise acids are called bases. If a base will also dissolve in water it is called an alkali.
Alkalis such as sodium hydroxide and potassium hydroxide are strong alkalis. A molecule of sodium hydroxide consists of one positive sodium ion and one negative hydroxide ion (OH-). In water the ions separate completely.
Alkalis such as sodium hydrogencarbonate (bicarbonate of soda) and ammonium hydroxide are weak alkalis. The hydroxide ions do not separate completely in water.
Calcium carbonate is a base. It is not soluble in water, but will react to neutralise an acid and to release carbon dioxide gas.
Blood, lymph and the fluids that bathe our cells must stay within a very narrow pH. Any tendency to change has to be counteracted straight away. The body produces substances called buffers, which react with any acid or alkali to maintain a constant pH. The most important buffer is sodium hydrogencarbonate.
The mineral ions of sodium, calcium and potassium used to be called the 'alkaline earth elements' because they form the positive ions of strong alkalis. If food containing these elements is burnt in oxygen and the ash dissolved in water, an alkali is obtained.
Dr Hay's references to alkaline foods are simply an injunction to eat foods that have useful levels of calcium and potassium salts: to eat plenty of fresh fruit and vegetables every day!
Protein in the diet which is not needed for growth or repair of tissue has to be destroyed. The amino acids are converted into urea and uric acid which have to pass into the bloodstream to be eliminated by the kidney. A high protein diet results in the production of excess acid which can put the acid / alkali balance under strain, as well as causing problems for the kidney which has to eliminate the acid.