Calcium is the most abundant mineral in the body. 99% of this calcium is stored in bones, where it also has an important structural function. Calcium is also needed for muscle function, clotting, nerve conduction among other functions.
The National Institutes of Health's Recommended Dietary Allowances (RDAs) for Calcium :
*Adequate Intake (AI)
Because some research links a high calcium intake to an increased risk of cardiovascular disease, I would advise against very high intakes.
For healthy adults I think that 1000mg of calcium* per day is a good target.
*Mostly from highly absorbable sources of calcium; see below for examples.
However, postmenopausal women and older men should speak to their general practitioner to discuss the possible need for greater amounts due to an increased risk of osteoporosis.
There is evidence indicating that vegans tend to not consume enough calcium1.
Calcium from many plants is often not well-absorbed compared to animal-based sources; this is because it can be bound to other compounds such as oxalates and phytic acid. Spinach for instance is rich in calcium, but only 5% of this is bioavailable2. But foods fortified with calcium such as calcium set tofu, fortified juices, soy milk/yoghurts are rich in bioavailable calcium. The bioavailability of calcium in certain low oxalate greens such as kale, bok choy and mustard greens is comparable to that found in milk3.
Good animal-free sources of calcium:
(Source: https://fdc.nal.usda.gov/ + food labels)
It's important to note that not all low-oxalate greens are particularly good sources of calcium. Also, high-oxalate greens such as (spinach, chard, beet greens) are poor sources of calcium because oxalate can bind to calcium rendering it much less bioavailable. High-oxalate foods are not unhealthy, but individuals with a history of calcium oxalate kidney stones may need to limit their consumption.
If you decide to take a calcium supplement, be aware this may interact with some medications and supplemental calcium has been consistently associated with an increased risk of cardiovascular disease. As such, it is highly advisable to discuss this with your physician first.
References
1. Davey, G.K., Spencer, E.A., Appleby, P.N., Allen, N.E., Knox, K.H. and Key, T.J., 2003. EPIC–Oxford: lifestyle characteristics and nutrient intakes in a cohort of 33 883 meat-eaters and 31 546 non meat-eaters in the UK. Public health nutrition, 6(3), pp.259-268.
2. Heaney, R.P., Weaver, C.M. and Recker, R.R., 1988. Calcium absorbability from spinach. The American journal of clinical nutrition, 47(4), pp.707-709.
3. Heaney, R.P. and Weaver, C.M., 1990. Calcium absorption from kale. The American journal of clinical nutrition, 51(4), pp.656-657.
