An attempt to measure the availability of calcium in edible leaves commonly consumed by South African negroes

Abstract

Most primitive and emerging populations obtain their calcium, largely or almost wholly, from vegetable sources. Of these, edible leaves (wild and cultivated), in certain contexts, are very important. To throw light on the availability of the calcium content, five common types of cooked edible leaves or "spinaches", all having low concentrations of oxalic acid, were fed in amounts containing about 200 mg calcium, to groups of South African Negro children. Urinary calcium excretions, accruing over 6 hour periods, were determined. The mean amounts of calcium excreted were of the same order and similar to that resulting from consumption of a volume of milk containing 200 mg calcium. Using the methodology described, it was confirmed that the calcium in Swiss Chard "spinach", (Spinacia oleracea) with its high oxalic acid content, is poorly absorbed. Nevertheless, it is inferred that the calcium contained in leaves of "spinaches" commonly consumed is available for utilization, a finding in agreement with observations on South African Negroes with respect to the satisfactory composition, dimensions, and other related indices of bone which reflect calcium status.

Comparison of nutrient intakes of South African elderly rural Black women in 1969 and 1989

“…A number of self-help schemes are actively seeking to improve the situation. A further matter of regret is the decreased consumption, previously high, of wild 'spinaches', many of which contain significant amounts of minerals, especially calcium, iron, and of vitamins, especially vitamin C and &carotene (Quin, 1959;Walker et al, 1975). However, consumptions of these foodstuffs are still high in some areas, e.g.…”

Absorption of calcium from a leafy vegetable rich in oxalates

“…Numerous studies carried out on laboratory animals have shown that Ca from green leaves, such as spinach (Spinacia oleraceu) which is rich in oxalic acid, is not utilized (Fincke & Sherman, 1935;Kohman, 1939;Speirs, 1939). However, studies on human subjects are few and the results contradictory (McLaughlin, 1927;Bonner, Hummel, Bates, Horton, Hunscher & Macy, 1938;Basu & Ghosh, 1943;Johnston, McMillan & Falconer, 1952;Walker, Walker & Wadvalla, 1975). The FAO/WHO Expert Group on Calcium Requirements (WHO, 1962) decided not to give serious consideration to the effect of these interfering substances on the utilization of Ca.…”

“…For these reasons, it was considered worth while obtaining further information about availability of Ca from commonly-consumed green leafy vegetables. The simple experimental approach suggested by Walker et al (1975) to determine availability of Ca was used in this study and the results obtained are reported in this communication.…”

“…For studying the absorption of Ca from a single dietary source it is more appropriate if the investigation is carried out over a short period of time. This type of experimental approach was first used by Walker et al (1975)~ where the authors took the Ca excreted in the urine in a 6 h period after consumption of a diet as an approximate index of the amount of Ca absorbed within that period. This was based on findings of Szymendera, Heaney & Saville (1972)~ who, using a technique developed for measuring absorption of Ca by administering two isotopes of Ca orally and intravenously to human subjects, found that Ca absorption occurs maximally in the duodenum and proximal jejunum and that it was essentially complete within 4 h. Although the rates of absorption of Ca from different foods may be variable, a period of 6 h can be considered sufficient for maximal absorption of Ca to have occurred from the two types of food used in this study.…”