Never eat carbohydrate foods and acid foods at the same meal.
Do not eat bread. potatoes, bananas, dates, or other carbohydrates with lemons, limes, oranges, grapefruits, pineapples, tomatoes or other sour fruits.
The enzyme, ptyalin, acts only in an alkaline medium; it is destroyed by a mild acid. Fruit acids not only prevent carbohydrate digestion, but they also favour their fermentation. Oxalic acid diluted to one part in 10,000 completely arrests the action of ptyalin. There is enough acetic acid in one or two teaspoonfuls of vinegar to entirely suspend salivary digestion.
Tomatoes should never be combined with any starch food. They may be eaten with leafy vegetables and fat foods. The combination of citric, malic and oxalic acids found in tomatoes, (which are released and intensified by cooking), is very antagonistic to the alkaline digestion of starches in the mouth and stomach. They should not be used on salads at a starch meal.
In cases of hyperacidity of the stomach there is great difficulty in digesting starches. Much discomfort is caused by eating them. They ferment and poison the body. Acid-starch combinations are very rare in nature - the sour apple coming nearest to being such a combination.
The highest efficiency in digestion demands that we eat in such a way as to offer the least hindrance to the work of digestion and not that we seek flimsy pretexts for continuing our customary haphazard eating. We should make the best use of our knowledge of the chemistry and physiology of digestion and of the limitations of the digestive enzymes and not try to ignore this knowledge altogether. This is particularly important in diseased states and in cases of crippled digestion.
Don not eat acid fruits with proteins.
This is to say, oranges, tomatoes, lemons, pineapples, etc., should not be eaten with meat, eggs, cheese or nuts.
The active work of splitting up (digesting) complex protein substances is accomplished by the pepsin enzyme. Pepsin acts only in an acid medium; its action is stopped by alkali. When proteins are eaten, the gastric juice becomes acid, furnishing a favorable medium for the pepsin action. Because pepsin is active only in an acid medium, it has been assumed that taking acids with a meal will assist in the digestion of proteins. On the contrary, these acids demoralize gastric digestion either by destroying the pepsin or by inhibiting its secretion. Gastric juice is not poured out when acid is present in the mouth and stomach. The normal stomach secretes all the acid required by pepsin-in digesting a reasonable quantity of proteins. An abnormal stomach may secrete too much acid (hyperacidity). In either case, taking acids with proteins does not aid digestion.
Prof. Pavlov positively demonstrated the demoralizing influence of acids, both fruit acids and the acid results of fermentation, upon digestion. Acid fruits by inhibiting the flow of gastric juice - an unhampered flow of which is imperatively demanded by protein foods - seriously handicaps protein digestion and results in putrefaction. Nuts and fresh cheese are about the only protein foods that do not quickly decompose under such conditions and these have their digestion delayed. Acids do not inhibit the flow of gastric juice anymore or any longer than does the oil of nuts or the cream of cottage cheese.
Instead of orange juice, grapefruit juice, pineapple juice, etc., assisting in the digestion of proteins when taken along with these as is taught in certain quarters, these acids actually retard protein digestion.
So-called health specialists and dietitians, who ignore this fact, and continue to recommend sour salad dressings and acid fruit drinks at meals, are unworthy of the trust placed in them by those who seek their advice. Lemon juice, vinegar, pickles, etc., when mixed with the food, serve as a check to hydrochloric secretion. Just as acids interfere with the secretion of hydrochloric acid, so sodium or alkali intereferes with pepsin secretion and lowers gastric acidity.
I have not been able to find any evidence that acids other than hydrochloric acid activate pepsin. At any rate, there is no need for additional acids as the stomach is capable of supplying all the acids required to provide a favorable medium in which the pepsin can act and supply this at the right time. Additional acids rather than helping in the digestion of protein hinder or suspend the secretion of digestive juice. Gastric juice is not poured out in response to the presence of acids in the mouth and stomach.
Milk and orange juice, while by no means an indigestible combination, is far from a good combination. Orange juice and eggs form an even worse combination. Pineapple juice and flesh is equally as bad. Pineapple juice does not digest flesh. It is well to bear in mind that flesh is not digested by acid but by pepsin. The hydrochloric acid of the stomach supplies the proper environment for the action of pepsin.