Bioavailability, relative absorption and caclium

It’s been a little while since I began writing about nutrients on this blog as I’ve been busy with the kids, running a clinic and the biggest biggest project I’ve ever worked on in my life but finally I have some time to get back to this topic and so I thought I’d take this opportunity to finish off the story for those of you who have been patiently waiting:-)

So if you’re new here, you can catch up on the story here.

So what exactly is bioavailability? Bioavailability is the amount of a vitamin or mineral in a food that is actually taken up and able to be used by the body (Davis et al., 2010).

So if you want to picture this, I’ve got an analogy for you. Think of a car with child car seats installed. Say the car  technically can seat 5 adults, once you have 2 child-seats installed though, you can’t fit 5 adults anymore, only 3 as an adult can’t fit in child seat. In addition, having the car seat in makes it hard for the adult sitting in the back seat to get access to the middle seat as well. So in this example, the “bioavailability” (the bit you can use or that is accessible) is really only 3 adults.

Bioavailability is affected by a few things, when it comes to minerals in foods or supplements, they don’t usually exist in the free form, but generally in “complexes” where they are combined with other molecules eg. calcium with carbonate or phytate, an organic molecule which helps to bind minerals in plant based foods eg. in seeds, to hold the nutrients like a “bank” for when the seed is ready to grow – then certain things (like water) help to inactivate the phytate so it can release the minerals for germination.

When it comes to nutrition there are some things we can do to increase the bioavailability.

Such as:

  • Fermenting
  • Soaking
  • Cooking
  • Refining

While cooking and refining can help make minerals more bioavailable – the important thing to realise is that during cooking some of the vitamins are affected, and during refining, some of the inhibitors to absorption are lost as are some of the minerals.

To make matters more confusing there is also an issue of “relative bioavailability”. Meaning that because of the above factors (eg the “packaging” of the mineral) the mineral (in this case calcium) in different foods is absorbed differently. For example, you might have heard people say things like “cows milk has a lot of calcium in it but your body can’t do much with it” So while this is true, that the calcium in cows’ milk is less efficiently absorbed than the calcium in kale or broccoli, an important factor to realise is that for some people due to their age, taste buds or lifestyle, they might actually find it easier to eat say 50g of milk rather than 100g of kale.  A factor that is taken into account when dietary recommendations are considered. So be mindful when considering your diet that just because it is possible for someone to eat enough greens to get their calcium intake it doesn’t mean they actually do eat enough greens.

Putting this one more way, “consider your nutritional needs based on the lifestyle and diet you actually eat not the one you idealistically wish you did“.

So if you don’t actually eat broccoli, bok choy or kale – the relative absorption of the calcium in it is a moot point. Or if you eat it but only small amounts – then you still might not be meeting your nutritional needs. So you need to be a lot more mindful about the foods that you are eating given your own particular dietary needs and or choices.

So here is a little summary of the relative absorption of the amount of calcium found in various foods:

calcium bioavailability2






Another important factor when it comes to absorption – is the state of your gut. For example if you have food intolerances, then you’re going to be less efficient at absorbing nutrients as well, so it’s also an important factor to consider when looking at the amount of a food (or nutrient) in your diet and considering the topic of the next post – the calcium controversy about whether to supplement or not. In the mean time, if you have any questions about your own particular dietary needs, then you should discuss this with your doctor or appropriate nutritionally qualified health care professional.


  • Mangels, R., Messina, V. and Messina, M. (2011) The Dietician’s Guide to Vegetarian Diets: Issues and applications (4th Ed). Jones & Bartlett Learning USA.
  • Davis,B., Melina, V. and Berry, R. (2010) Becoming Raw. The essential guide to raw vegan diets. Book Publishing Company, USA
  • Gropper, S., Smith, JL., Groff, JL. (2009) Advanced nutrition and human metabolism (international student edition)(5th Edn). Wordsworth, Cengage Learning USA.