Do processed plant sources of phosphorus still have low bioavailability?

Is the bioavailability of micronutrients in plants foods equal? Is the phosphorus bioavailability lower in all plant foods? Or does the level of processing matter? Is the phosphorus bioavailability in unprocessed wheat bran is the same as bran flakes?

Bioaccessible vs Bioavailable – what’s the difference?

A quick stop at the definition shop. While it is not uncommon for me to use these two terms interchangeably, there is a difference between bioaccessible and bioavailable. This article provides a nice definition of these two terms.

Bioaccessibility describes POTENTIAL absorption. Specifically, bioaccessibility measures the amount of the nutrient that the food matrix releases. The body may not necessarily absorb all bioaccessible nutrients. The bioaccessibility amount for any given food will be higher than the bioavailable amount. Researchers can use test tube digestion models to assess bioaccessibility.

Bioavailability describes ACTUAL absorption. Specifically, bioavailability quantifies the amount of the nutrient that the body absorbs and makes available for use. In nutrition, I think we are most interested in the bioavailable fraction of the food since this fraction is the part that will impact our jobs. For example, only the phosphorus absorbed by the body can impact serum levels. Researchers can use human balance studies to assess bioavailability.

Why do minimally processed plants have lower bioaccessibility and bioavailability?

There are several components of a plant that can decrease the bioavailability and bioaccessibility of minerals, including:

1. Fibre can form fibre complexes in the digestive tract. A nutrient trapped in the fibre complex will be harder to absorb. While, fermentation of fiber complexes can occur in the colon, this is likely too late to absorb most micronutrients.
2. Phytic acid is the major storage form of phosphorus in grains, legumes, nuts and seeds. Phytic acid forms phytates when it binds to minerals. Minerals strongly attract phytic acid.
3. Cell Walls – If the digestion process leaves a cell wall intact, it is unlikely that the nutrients inside the cell will be bioaccessible.

How does food processing change bioavailability and bioaccessibility of plant foods?

Minerals (like phosphorus) are more stable than vitamins. Furthermore, light, heat, humidity, oxidation, reducing agents, or acid/base balance do not destroy minerals. However, processing can sometimes reduce the mineral content of food by removing it through leaching or physical separation. For instance, removing the bran from the endosperm to make white flour.

Processing that reduces anti-nutrients or impacts the physical structure of the plant has the potential to increase a mineral’s bioaccessibility and bioavailability. Milling, soaking, dehulling, and fermentation can disrupt the plant cell walls and reduce the fiber and phytate content. This can increase the mineral bioaccessibility in the food.

So is the phosphorus bioavailable in wheat bran?

So while the wheat bran while have undergone some level of processing (physical separation of the wheat kernel), this is unlikely to have removed many of the barriers to absorption in the wheat bran itself.

Therefore, I would hypothesize that the bioavailability of minerals (including phosphorus) would be low. Unfortunately, I couldn’t find a study that has tested this in humans. But, I was able to dig up a 1973 article that investigated this in ponies. And I know, ponies have different digestive tracts than we do. But I would make a guess that ponies would be better at absorbing that we are being that they are herbivores and we are omnivores.

The authors reported that the ponies only absorbed about 30% of the phosphorus from wheat bran. Furthermore, they reported that steeping the wheat bran in 50 degree Celsius water for 1 hour did not change the absorption.

Ok, so based on this old study (not done in humans!) and given our understanding of ways that processing will improve bioavailability, I think we are relatively safe to predict low bioavailability of phosphorus from wheat bran.

How are cereal flakes made?

I started off by watching this youtube video on how cornflakes are made. My summary of steps are:

1. Cleaning
2. Soaking
3. Grinding and removal of the husk
4. Create dough by adding sugar and other ingredients
5. Extrusion
6. Drying and spraying with a sugar solution
7. Cooling

I would consider flakes cereal as more processed than shredded wheat cereal, so I also watched the making of shredded wheat video.

1. Cooking
2. Drying
3. Shredding the kernals into strands
4. Baking/toasting

There were definitely more steps in the flakes process, including a soaking, grinding and extrusion phase. All of these phases would have the potential to remove the anti-nutrients (e.g. fibre and phytates) that make phosphorus from plant sources more difficult to absorb. These steps would also change the physical characteristics of the plant cell structure, which could also increase the bioavailability.

Ok, so back to the bran flakes, what do we know about their bioavailability?

In this 2018 article, the mineral availability of a variety of wheat products was assessed. Here is a copy of their graphical abstract.

This article investigated the content of phytates (antinutrients) on the availability of iron, calcium and zinc in a variety of wheat products. They also used a test-tube digestion model to assess the bioaccessibility of these minerals. They reported that flaked cereal made from wheat contained almost 54% fewer phytates than those found in the whole grain. Additionally, the bioaccessibility of calcium, iron, and zinc was higher for flaked cereals by about 50% compared to the whole grain.

While this study, doesn’t confirm the phosphorus bioavailability specifically, given that the other minerals are seeing increases in bioaccessibility and phosphorus behaves similarly in response to anti-nutrients, I think it is reasonable to conclude that phosphorus in flaked cereals will also be higher.

Take Aways

This was a long one today! Thank you for reading all the way to the end. Or maybe you skipped the middle and went right to the end. That’s cool too.

Here are my take aways. Plant cell structure and naturally occurring anti-nutrients reduce the bioaccessibility and resultant bioavailability of phosphorus in whole plant foods. Processing plant foods will reduce anti-nutrient content and disrupts cell structures which increases the bioaccessibility of other minerals (calcium, iron and zinc) in the plant foods. While we don’t know for sure that this is occurring for phosphorus too, it seems unlikely that it isn’t.

So are all plant foods created equal when it comes to phosphorus bioavailability? Probably not! Processed plant foods are more likely to have higher bioavailability than minimally processed plant foods.