Are Tofu & Plant Milks A Poor Source of Bioavailable Calcium?

A recently published study has concluded that the calcium in plant-based foods may not be bioavailable1. Now the idea that calcium in whole plant foods tend to be low in bioavailable calcium is nothing new. Many (but not all) unrefined plant foods are either low in calcium or high in anti-nutrients that block calcium absorption. However, the study claims to have found evidence indicating that even calcium-fortified plant milks and calcium-set tofu (usually taken to be among the best animal-free calcium sources) may not provide much in the way of bioavailable calcium.  
 
However, this is an in vitro study. In other words, we are talking about experiments that have taken place outside of a living organism. Roughly speaking, they simulated the digestion of various plant-based foods (plus some skimmed milk) and observed the amount of calcium that would be theoretically “accessible” for absorption.

 

The bar chart above shows that compared to skimmed milk; most plant-foods were found to yield relatively small amounts of “bioaccesible” calcium per portion. Admittedly, the soy milk tested was not calcium-fortified and consequently it would not be expected to provide much calcium. However, the rice, oat and almond milks also did not perform well, despite calcium fortification. In addition, contrary to what I’d predict, the calcium-set tofu was also found to contain little bioaccessible calcium.  
 
These results do not tell us how much of the calcium would be absorbed in vivo (in the animal). However, for a nutrient to be bioavailable, it must be bioaccessible. As such, if these estimates are accurate, it would suggest that we absorb much less calcium from these products than is commonly believed. 

But this is a single study. Let’s put this research into context by looking at the highest quality studies – namely, in vivo clinical trials in humans. All the trials below estimated absorption by “marking” calcium in food products and tracking its presence in blood, faeces or urine.  


Tofu 
 
Weaver et al. conducted two small randomised crossover trials which compared the calcium absorption of 2% milk to 1) tofu set with calcium chloride and 2) tofu set with calcium sulphate2. Calcium absorption was estimated by “fecal recovery” of marked calcium in study 1). In study 2), they tracked marked calcium in serum.  
 
Calcium absorption from tofu in both studies was comparable to that of the 2% milk.  
 

Soy milk 
 
Zhao et al.3 used a three-way crossover study design to compare the absorption of:

1) soy milk fortified with calcium carbonate,
2) soy milk fortified with tricalcium phosphate
3) 2% dairy milk.  
 
Fractional calcium absorption was not significantly different between soy milk fortified with calcium carbonate and 2% dairy milk. But the soy milk fortified with tricalcium phosphate resulted in somewhat lower absorption rates.  


 
Heaney et al. found that mean absorption of intrinsically labelled* tricalcium phosphate in 2% fat soy milk had a fractional absorption rate of 0.237%, which was significantly lower compared to 2% fat dairy milk4.   
 
*Intrinsically labelled means that the fortificant was marked at the manufacturing stage, as opposed to adding marked calcium to the finished product.  
 
In Tang et al. So Good™ brand calcium-fortified soy milk was extrinsically labelled with calcium chloride and showed a comparable absorption rate to that of dairy milk5.  
 
Note: I am unsure what form of calcium So Good™ uses as this information has not been revealed by the company.  

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Unfortunately, I have not been able to find any other relevant studies. But based on the above, I believe that tofu and soy milk remain decent sources of calcium. While the soy milks fortified with tricalcium phosphate did not perform equally well as dairy milk, they ostensibly do provide a significant amount of bioavailable calcium: much more than one would predict on the basis of the estimates of bioaccessibility mentioned earlier (incidentally, all of the fortified plant milks in the in vitro study were fortified with tricalcium phosphate).  

I have not been able to locate any studies looking at the bioavailability of other plant milks, but for the time being we can assume that calcium fortified almond milk, oat milk, rice milk etc. are also good sources of calcium. I will keep a look out for new studies and update this article as and when.  
 
References 
 
1. Muleya, M., Bailey, E.F. and Bailey, E.H., 2024. A comparison of the bioaccessible calcium supplies of various plant-based products relative to bovine milk. Food Research International175, p.113795. 

2. Weaver, C.M., Heaney, R.P., Connor, L., Martin, B.R., Smith, D.L. and Nielsen, S., 2002. Bioavailability of calcium from tofu as compared with milk in premenopausal women. Journal of food science67(8), pp.3144-3147. 
 
3. Zhao, Y., Martin, B.R. and Weaver, C.M., 2005. Calcium bioavailability of calcium carbonate fortified soymilk is equivalent to cow’s milk in young women. The Journal of nutrition135(10), pp.2379-2382. 
 
4. Heaney, R.P., Dowell, M.S., Rafferty, K. and Bierman, J., 2000. Bioavailability of the calcium in fortified soy imitation milk, with some observations on method. The American Journal of Clinical Nutrition71(5), pp.1166-1169. 
 
5. Tang, A.L., Walker, K.Z., Wilcox, G., Strauss, B.J., Ashton, J.F. and Stojanovska, L., 2010. Calcium Absorption in Australian Osteopenic Postmenopausal Women: an Acute Comparative Study of Fortified Soymilk to Cows’ Milk. Asia Pacific journal of clinical nutrition19(2), pp.243-249. 

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