A comparison of the bio-accessible calcium supplies of various plant-based products relative to bovine milk.

For a food to be considered a good source of calcium (Ca), it must have a high Ca concentration and the Ca must be highly bioavailable. Dairy products have traditionally been considered excellent sources of Ca due to both a high Ca density and bioavailability. For example, a glass of 240 mL milk is estimated to contain 300 mg Ca, providing about 96 mg absorbable Ca and a bioavailability of 30 %, which is considered to be high. In the case of plant-based products, most cannot match both the high Ca density and bioavailability of milk, although some dark green vegetables match or may even surpass the Ca density and bioavailability of milk. However, these are normally consumed in smaller quantities such that the amount of absorbable Ca per serving achieved per day remains lower than that of a milk serving. It is therefore important to identify plant-based sources of calcium that can make a meaningful contribution to calcium intakes for populations following diets with a minimum supply of dairy products. Accordingly, the objective of the study cited was to assess the bio-accessibility (in vitro bioavailability) of several plant-based food products.

The gross and bio-accessible calcium supplies of 25 plant-based products from 5 food groups considered to be good and important sources of calcium were evaluated. The food products were categorised into the following groups: 1. Cereals (maize, finger millet, white and brown bread, rice, oats), 2. Legumes (kidney beans, lentils, chickpeas, black chickpeas, garden peas, calcium-set tofu), 3. Dark green vegetables (broccoli, cabbage, kale and spinach), 4. Plant-based beverages (soy, rice, oat and almond milk), and 5. Dried fruits & other (raisins, prunes, figs, apricots, tahini). Bio-accessible Ca was examined using the INFOGEST static digestion model in which isotopically labelled ⁴³Ca was used as a tracer of reagent Ca to improve accuracy.

The gross Ca content varied widely amongst all the food products, ranging between 7.48 and 959 mg/100 g fresh weight, with approximately 50 % of the products being equivalent to or surpassing the Ca content of skimmed milk. Bio-accessibility of Ca was equally variable, ranging from about 0.1 to 50 %. The lowest bio-accessibility (<10 %) was found in spinach, plant-based beverages, tofu, dried figs and tahini and was attributed to: (a) the high content of oxalate and phytate in some of the products, and (b) the low solubility of tri-calcium phosphate which was used for fortification in the plant-based beverages. The remaining products generally had a high bio-accessibility that was similar to, or higher than that of skimmed milk (~30 %). When both bio-accessibility and recommended serving portions were considered, only three products were identified as good sources of Ca, requiring 0.2 – 1.4 servings to equal the bio-accessible supply from skimmed milk. The top three sources of plant-based Ca identified were kale, finger millet and fortified white bread in that order, with kale providing five times more bio-accessible Ca than one serving of skimmed milk. Moderate sources of Ca where 1.5 – 3 servings were equivalent to one serving of skimmed milk included wholemeal bread, some bean varieties (black chickpeas, chickpeas, kidney beans, peas), broccoli, cabbage and almond drink. The rest of the products were either of low calcium content, poor bio-accessibility, and/or not consumed in sufficient quantities to make a significant contribution to daily requirements. White bread was a good source of calcium as it was fortified with calcium carbonate and this suggests that mandatory wide-scale fortification of staple cereals with this form of Ca should be considered a viable approach to augment dietary Ca intakes in vulnerable populations. Low bio-accessibility of fortified Ca in plant-based beverages, often marketed as good sources of Ca, suggests the need for regulation and for further in vivo studies to validate bioavailability of Ca in these products.  

In summary: The gross compositions of about 50 % of the analysed 25 plant-based products were equivalent or even higher than that of milk. However, when the bio-accessibility and recommended serving portions were considered, only 3/25 products could provide bio-accessible Ca equivalent to milk, with kale supplying 5 × more bi0-accessible Ca than milk. The bio-accessible Ca supplies from plant-based beverages were surprisingly low, despite their high Ca content which raises doubt about the Ca health claims often associated with most brands. Bio-accessibility from these products was < 5 %, compared to 30 % for milk and was attributed to the low solubility of the tricalcium phosphate used for fortification and the potential presence of phytate. This shows that for fortification programmes to effectively augment dietary Ca supplies, the right food vehicle and chemical form of the Ca used must be carefully considered. White bread fortified with calcium carbonate was amongst the best source of Ca identified, indicating the potential of mandatory wide scale Ca fortification in meeting recommended Ca intakes.