When discussing selenium, instead of quoting batting averages, folks usually quote selenium levels in the blood. Either way – baseball or selenium – numbers count. Numbers also confuse. Blood levels of selenium can be tested in different ways and each number has a different meaning. So let’s look at these numbers closely.
Selenium blood levels
First, let’s clear up some confusion about blood terminology. We can think of blood as a kind of soup composed of two parts: the liquid and the things floating around in the liquid. The things floating in the liquid are mostly red blood cells. The liquid is either plasma or serum, and the difference between the two is simple.
After blood is collected, it is spun down in a centrifuge. The heavy cells settle at the bottom of the tube; the liquid rises to the top. If the blood is allowed to clot prior to centrifugation, the liquid is called serum. If the blood is prevented from clotting, the liquid is called plasma.
The difference is the plasma contains all the clotting factors and related components, while the serum does not. Finally, the term “whole blood” is, well, everything – all the red blood cells, clotting factors and liquid.
In scientific publications, scientists tend to shy away from using the term “soup.” Instead, they refer to the two parts of blood as two compartments, or two metabolic fractions – or better, two metabolic pools.
Thinking about blood as two metabolic pools is a good mental strategy, because these two pools are chemically quite different and measurements of each will give different results, as we shall see for selenium.
Between 67 and 80 percent of all blood selenium occurs inside the red blood cells, primarily as part of an enzyme called “glutathione peroxidase” (GSH-px). This is the molecule that helped shape our current feed laws. Until 1973, we only had a general idea that selenium was an essential nutrient, but we hadn’t pinpointed a precise metabolic use for it, and federal regulations forbade its inclusion in livestock feeds.
In 1973, researchers first reported that GSH-px was a selenium-containing protein, which proved beyond a shadow of a doubt that selenium was required in a metabolic pathway in livestock. Identifying an enzyme that contained selenium convinced the FDA to change its regulations to allow selenium in feeds.
Because GSH-px contains most of the selenium in the blood, researchers initially tried to use its enzymatic activity as an indicator of selenium status. But after analyzing thousands of samples, they found that values for GSH-px activity were notoriously inconsistent, especially between labs. GSH-px is an enzyme, and enzymes are delicate molecules.
Enzyme activity depends on more than just selenium composition – factors such as temperature, reagent quality, handling procedures, technician skill (and also if the samples were left on the dashboard of a pickup for three days before being tested). There is no standardized assay for GSH-px.
Values from one lab cannot be used confidently by another lab. This is not an ideal assay for diagnosing selenium status. True, GSH-px values can be very useful for experiments within a single lab over a short period of time, but their routine use for selenium diagnosis has fallen by the wayside in favor of other measurements.
Today, we typically rely on selenium values from either whole blood or from serum or plasma. (From a selenium perspective, there is little difference between serum and plasma, so for brevity I’ll just refer to serum.) But these values also have very different interpretations.
Professionals don’t agree on the precise selenium levels, but in general, adequate selenium levels in cattle run between 0.200 and 1.200 parts per million (ppm) for whole blood and 0.080 to 0.300 ppm for serum. That is, if you are in the U.S. The rest of the world uses the metric system, so their selenium values often look different. One ppm equals one milligram per liter, which is the same as one microgram per milliliter.
But because selenium values are so small, with so many zeros, some laboratories prefer to report selenium values as nanograms per milliliter (ng/ml), which is the same as parts per billion (ppb). Which is equivalent to taking ppm and moving the decimal three places to the right. Follow me? Anyway, I like these values more because they are larger numbers. Namely, 200 to 1,200 ng/ml for whole blood and 80 to 300 ng/ml for serum.
Selenium pools
Now for the difference between the two pools of selenium.
Red blood cells, which contain most of the blood’s selenium, have definite life spans. After three to four months, old red blood cells are destroyed and replaced by new cells. Selenium remains in a cell’s GSH-px for the entire life of the cell, and this amount is fixed when the cell is formed.
Therefore, the selenium value for all the red blood cells in the body is really a rolling average – a reflection of the selenium status averaged over different periods of red blood cell formation, including values for old cells as well as for new cells. This lag period shows up as a delayed response to supplementation or a delayed response to reduced selenium intake.
This lag period therefore applies to the selenium values of whole blood, because whole blood levels are overwhelmingly influenced by the amount of selenium in the red blood cells. For example, supplementing selenium to a deficient animal takes three months to maximize the selenium levels in whole blood.
Thus, the selenium level of whole blood depicts the relatively long-term selenium status of an animal. While this number may be useful historically, it may be quite misleading as an indicator of current events.
What will selenium levels tell me?
In practice, we are usually interested in the current selenium status of our animals because we need to make timely decisions about supplementing selenium or treating deficiencies. White muscle disease, retained placentas, sudden death from heart failure, weak and dead calves, reduced immune response, reduced reproductive performance, etc., are all critical symptoms of selenium deficiency, but some are not very precise.
We need accurate numbers in our herd so we can react quickly and properly. Serum selenium is a better gauge of this than whole blood selenium. Because serum is primarily a liquid transport system, not a final, long-term destination, serum selenium levels rise and fall relatively quickly in response to supplementation or shortage.
For example, supplementing selenium to deficient animals causes serum levels to rise quickly and reach maximum levels within weeks, while whole blood levels may show little change during this period. The slow response time for whole blood could easily mislead producers into thinking their selenium supplementation was ineffective.
One interesting characteristic of serum selenium is these levels respond slightly differently to different forms of supplementation. Inorganic forms of selenium (selenates or selenites) cause serum levels to plateau at lower values than the organic forms (seleno-amino acids).
Both types of supplementation can be effective against selenium deficiencies, but remembering how serum responds may be useful when looking at marketing claims or trying to interpret laboratory reports.
So which selenium value should you use? Which should you believe? Actually, the best place to test for selenium is the liver, not the blood. But in a practical sense, the liver is a rather inconvenient tissue to sample – so we are stuck, so to speak, with blood. No problem, as long as we understand which value is which and we don’t try to “pool” our samples. 
PHOTO: If you have identified a selenium deficiency, what blood sample numbers should tell you whether your mineral block supplementation is effective? Photo by Lynn Jaynes.
Woody Lane, Ph.D., is a livestock nutritionist and forage specialist in Roseburg, Oregon. He operates an independent consulting business and teaches workshops across the U.S. and Canada. His book, From The Feed Trough: Essays and Insights on Livestock Nutrition in a Complex World, is available through Woody Lane.
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Woody Lane, Ph.D.
- Lane Livestock Services
- Roseburg, Oregon
An example: is this enough selenium?
Q: If I feed my cows a trace-mineral mix free-choice, year-round, which contains 15 ppm of selenium, is that adequate enough to avoid selenium deficiency?
A: Well, maybe, if every individual cow eats enough of it every day. Do you feel lucky? At 15 ppm, selenium is rather low. The FDA permits a maximum level of 120 ppm selenium in a free-choice, trace-mineral mix for beef cattle.
Feed companies may include lower selenium levels in their trace-mineral mixes if they also include palatable ingredients that encourage higher intake. It’s a trade-off. The FDA also limits the total selenium intake from free-choice minerals to 3 mg per head per day.
The issue is simply will the cows consume enough of that free-choice mineral mix to meet their requirements? The answer is the classic “it depends.” Seriously.
Other factors will affect this situation: background selenium levels in the forage and grain, palatability of the trace-mineral mix, block mineral versus loose mineral, number of mineral feeders in a pasture, availability of salt from other sources, weather factors like temperature and humidity, etc.
Personally, I prefer the higher selenium levels in a mineral mix, because then there are enough selenium atoms to reduce the risks and cover more bases.
—Woody Lane, Ph.D.
