Factors affecting protein bioavailability

For a nutrient to be considered completely bioavailable, it needs to be completely digested, absorbed and utilised by the body.

In the case of protein, its bioavailability depends on the quality of the protein, the digestive health of the individual and any other factor that might impact on its absorption.

The Protein-Digestibility Corrected Amino Acid Score (PDCAAS) is the most widely used test to measure the quality of a protein in food. The test is not without its flaws, but it is the gold standard at this time (or when I wrote this post!).

The highest possible PDCAAS is 1.0. A score of 1.0 means that after digestion of the food, it provides 100% or more of the recommended amount of essential amino acids per gram of protein.

Based on the PDCAAS, you can see from the image that most of the high quality proteins come from animal sources.

So, you might think by looking at the table that soy is a good substitute for meat.

Not so fast!

Remember the PDCAAS refers to protein quality only, not bioavailability.

Something the PDCAAS doesn’t take into account is the impact of antinutrients on protein absorption.

Even organic (non-GE) soy contains enzyme inhibitors that interfere with protein assimilation and therefore lower its bioavailability.

As an aside, there are other issues with soy too:

• High levels of phytoestrogens, which are endocrine disruptors
• Extremely high levels of phytic acid (binds to and inhibits absorption of minerals)
• Extremely high levels of goitrogens (inhibit iodine absorption)

To reduce the effect of the antinutients, soy is best eaten fermented (e.g. miso, natto, and tempeh); however, this does not remove the phytoestrogens.

So, can you see how soy is not a good substitute for meat?