The relation of the composition of the urine to the quality and quantity of the food is highly important.

Independently of renal disease and of excessive perspiration, the occurrence of a scanty urine means that too little fluid is being ingested, and that the nitrogenous waste is accumulating faster than it is being removed.

The urine secretion is normally increased in quantity after meals, and is diminished during fasting and in sleep. The increased rate of secretion lasts for two or three hours, and then lessens. The increase usually begins within an hour after breakfast, but not for two or three hours after dinner. This may be owing to the slower absorption of fluid which occurs with the heavier meal - i. e., while solid food is retained in the alimentary canal, it holds back the fluid like a sponge. The maximum rate of secretion is also reached somewhat later after dinner, sometimes not for four hours.

After mixed meals the quantity of urea is more than doubled, and the phosphates and urates are increased in nearly the same ratio. Proteid foods always increase the output of urea, urates, or uric acid, while vegetable foods increase the phosphates and sulphates.

Klemperer has stated that in diseases characterised by nitrogenous tissue waste the latter is reduced by giving carbohydrates. In other words, the carbohydrates save the consumption of tissue and consequent production of urates or urea.

Meat and albuminous foods in general tend to make the urine more acid, whereas milk and vegetables make it more alkaline.