This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
Green vegetables do not contain much nutriment in comparison with the cereals and tubers, and they are mainly useful for furnishing a pleasing variation in diet and for supplying a large proportion of salts and some acids which are believed to be serviceable in the prevention of scurvy. The various uses of the salts have been elsewhere described (see p. 45). These vegetables often contain 90 per cent or more of water, which in itself is useful to the system in many ways. They furnish only a small quantity of nitrogenous material, which varies from 1.5 up to 4 per cent. In addition, they contain cellulose, chlorophyll, sugars, gum, pectin, and sometimes a little fat. Their variety of taste depends upon the presence of flavouring materials, chiefly essential oils. As a rule, they have a better flavour, and are more digestible when young than old, when they become tough and " stringy " from a relatively large percentage of cellulose or woody fibre. Since the green vegetables afford so little nutriment in proportion to their bulk, they are not of much service for persons with feeble digestion, and, unless they are young and tender, they are positively harmful by overtaxing the digestive system and irritating the alimentary canal.
On the other hand, they are very useful in overcoming constipation by their bulky waste matter, which acts as a mechanical stimulus to peristaltic action and promotes movement of the bowels.
The digestibility of these vegetables is rendered much greater by careful cultivation in suitable soils. Owing to the large quantity of water which they hold (which readily evaporates), they soon wilt or become dry in market. For the most part they should be eaten when quite fresh, although celery and winter cabbage form an exception to this, as they may be kept for weeks.
Fresh green vegetables as well as roots or tubers are always made more digestible by cooking, which softens them. Their most digestible form for invalids is that of purees.