What does 'Bioavailability' mean?

About ten years ago my wife went to see a nutritionist. She came away enthused and with a recommendation to get some supplements. I registered her enthusiasm and, casting around for another category to bolt on to our health publishing business, contacted the company with a view to reselling their products.

They were lovely people and full of information. To be honest, I wasn’t particularly interested myself at the time, as I still thought that if I sourced and cooked food carefully, I was getting my essential nutrition.

I remember sitting across from the boss lady and saying:
“What I really like about your product is that the nutrients come from food and aren’t made in a laboratory”.

She thought for a moment and then replied:
“It doesn’t quite work like that, Mark”.

But I’m ashamed to say I switched off to her detailed explanation — until recently, when I realised how deficient UK supermarket food is in vitamins and minerals, and decided to wise up.

So, what is ‘bioavailability’?
It’s an umbrella term for how easily your body absorbs nutrients. This varies hugely by nutrient. Iron is one of the least easily absorbed minerals, ranging from just 1% (that’s 99% waste!) for the least bioavailable forms, to around 40% for haem iron – effectively blood-based sources.

Think of taking nutrients as like climbing stairs – once the nutrients have reached the top, they’re usable by the body.

Let’s look at a single essential nutrient – iron (Fe) – and its different forms.
At the bottom of the stairs, you’d expect to find elemental iron, e.g. iron filings (remember physics lessons and magnets?).

But actually, the least absorbable form is iron oxide – also known as rust.
Confusingly, the term organic crops up. In its true meaning, organic simply means the compound contains carbon. Inorganic = no carbon.
Adding carbon makes the element more easily absorbable.

Interestingly, plant-based iron – from vegetables – is not as easily absorbed as the better supplement types.

The premium form of supplemental iron is iron bisglycinate, a chelated form – meaning the mineral is bound to an organic compound (like an amino acid or citrate), improving absorption by climbing higher up the stairs.

For iron, the form is critical to whether your body is able to absorb it or just excrete it.
An inferior form may be cheap but also pointless – so it’s worth investing your money here.
Whereas with other nutrients, e.g. vitamin C, simple, cheap-as-chips ascorbic acid is absorbed at around 70–90% – nothing wrong with that!