Mineral relative bioavailability misconceptions explained

While visiting EuroTier 2016 last November, I had several discussions that involved minerals in animal feeds: the likes of copper, zinc, manganese, phosphorus and calcium. Regardless of the purpose of the discussion or the mineral in question, I often encountered a misconception.


While visiting EuroTier 2016 last November, I had several discussions that involved minerals in animal feeds: the likes of copper, zinc, manganese, phosphorus and calcium. Regardless of the purpose of the discussion or the mineral in question, I often encountered a misconception that I managed to resolve for myself only during my graduate years. It helped that I did some relevant studies with minerals under the late Dr. D.H. Baker at the University of Illinois — the same prominent researcher who co-authored the book “Bioavailability of Nutrients for Animals,” a seminal title, and highly recommended.

What is availability?

Availability, in absolute terms, refers to the amount of a nutrient that is available to the animal for metabolism and incorporation into tissues. It is extremely difficult, and often impractical, to measure absolute availability. Thus, we measure relative bioavailability, and here some confusion exists because many take this to be the same as digestibility, which it is not.

The terms bioavailability, relative bioavailability and availability are well-known, but they are often used interchangeably, which is imprecise.

The terms bioavailability, relative bioavailability and availability are well-known, but they are often used interchangeably, which is imprecise. First, bioavailability and availability are identical terms as we are measuring them in vivo (that is, using animals as “test tubes”), but the correct term is bioavailability because the term availability can include in vitro tests, which are just lab test-tube tests without involving animals.

Relative bioavailability (RBV) is the easy way in comparing two sources of minerals, taking one as 100 percent bioavailable (arbitrarily and by general consensus among scientists) and measuring the relative performance of the second source.

How is RBV determined?

RBV is measured using in vivo assays, the most common of which is the so-called “slope ratio assay.” This test compares the bioavailability of a specific nutrient from a standard source to that of an unknown source. For this test to be successful, the RBV should be determined in the linear phase of response (believed to be between 30 and 70 percent of the actual requirement) and also for the two response lines to intersect or have a common beginning.

measuring RBV in trace minerals

Slope ratio assay is the most common test measuring RBV in trace minerals, especially using chicks. In this example case, the standard source of a trace mineral is more bioavailable than the test source. The slope ratio between the two lines determines the RBV of the test material.

To give a very practical example, let’s look at corn for broilers. In this ingredient, phosphorus is 24 percent as bioavailable (INRA, 2004) as phosphorus in monosodium phosphate (the standard taken as 100 percent RBV). In contrast, the RBV of phosphorus in wheat is 54 percent, and this explains partially why the two ingredients require different levels of phosphate supplementation. These values have been associated with relative dietary specifications, and thus most diets for poultry are now formulated on RBV basis for phosphorus — but not for other minerals, and especially trace minerals.

Digestibility is different to availability

We assumed 100 percent RBV for phosphorus in monosodium phosphate, the golden standard in phosphorus assays. Yet only 88 percent of phosphorus in monosodium phosphate is actually digestible (de Blas et al., 2003). Thus, with pigs at least, this more precise methodology has created a new set of digestibility coefficiency factors associated with digestibility dietary specifications. Most nutritionists now formulate pig diets based on digestibility phosphorus values but continue to use RBV to formulate poultry diets, as digestibility values for poultry are not forthcoming. In pigs as well as in poultry, trace minerals are still supplied on a total basis.

The case of trace minerals

We have RBV for all common trace mineral sources, and it is rather easy to determine such figures for new sources. The usefulness of these numbers is limited to comparing two sources, and often a new source versus the industry standard. When it comes to formulate diets, specifications are rather generous for a number of reasons, and even then, we tend to ignore the RBV of the sources used to supply the required levels of each mineral. Thus, in some cases, animals may receive excess trace minerals on a RBV basis, whereas in others they are undernourished. Although the first is not impacting the environment directly, the second does, as we tend to add generous safety margins.

Determination of trace mineral requirements on a RBV will help us reduce the amount of trace minerals excreted into the environment as we can use the most useful sources and at the correct amount. Perhaps we should even start thinking about taking into account the amounts of trace minerals supplied by natural ingredients. Here, an old industry incidence will clarify the huge problem we face today.

A certain form of copper oxide is virtually unavailable; animals simply cannot use it to cover their copper needs. But it is the least expensive source of copper so it was used frequently, at least back in the days when little attention was paid to RBV issues. This copper source was added in trace mineral premixes to provide about 10 ppm copper in the final diet of the animal, as the requirement was perceived to be from 5 to 8 ppm (for most monogastric animals). The fact that animals receiving this supplemental copper, which ended up contaminating the environment, did not develop any deficiencies due to the fact that natural ingredients such as corn, wheat and soybeans provide about 8 ppm copper in the final diet to cover the needs of most animals.

Trace minerals are environmentaly hazardous materials

It is superfluous to mention, I believe, that trace minerals such as copper and zinc are heavy metals. Once deposited on soil they take forever “to move,” and as such, they accumulate. This would not be necessarily a bad thing if plants growing in overburdened soils did not suffer from chlorosis: copper and zinc render soil iron unavailable to the plants, which in turn without iron cannot synthesize chlorophyll and become pale — hence the term chlorosis (pale).

The condition is rather serious because certain areas in Europe are only 80 years away from becoming useless, according to the most pessimistic estimates. To this end, fine-tuning trace mineral nutrition can extend the time we are allowed to raise pigs and poultry in sensitive areas.

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