Organic Selenium and the “Rebound Effect”: Risks of Supplementing Selenium

The Key Points in Brief:

• When feeding organic selenium in the form of selenomethionine, the so-called rebound effect can occur: Long after stopping, the selenium level in the blood plasma suddenly rises.
• Selenomethionine is metabolised more slowly by the body and is mainly incorporated into the body’s proteins, which store selenium long-term. In contrast, sodium selenite is absorbed more quickly and excreted again.
• When the proteins containing selenomethionine are broken down, the body gradually releases the selenium back into the blood. This can cause the selenium level in the plasma to rise even after the feeding has stopped.
• Too much selenium can interfere with sulphur binding, leading to brittle hooves, poor coat condition, and chronic coughing. It can also increase the risk of insulin resistance, and consequently, laminitis or overweight.
• To avoid selenium overload in the body, sodium selenite is a better feed supplement than selenomethionine.

What is the rebound effect with selenium?

Most people are familiar with the yo-yo effect of diets: you try to lose weight, and as soon as you return to your normal eating habits, the weight comes back faster than you can notice. It’s somewhat similar with selenium supplementation in the form of selenomethionine.
You may have already stopped feeding it, and suddenly, selenium levels in the blood plasma rise again. This is known as the rebound effect. In this article, we’ll take a closer look at why this happens.

Difference between selenomethionine and sodium selenite

Selenomethionine is often fed to horses that show low selenium levels in their plasma. It is marketed as an organic form of selenium supplementation. Organic selenium is said to be better absorbed by the body compared to inorganic forms. This is only partially true, as sodium selenite, an inorganic form, is absorbed much faster by the body but is also excreted more quickly. In contrast, the organic selenomethionine is indeed absorbed more slowly.

What does “organic” and “inorganic” mean for selenium?

The terms “organic” and “inorganic” have nothing to do with the origin or the bioavailability for the cells, nor do they indicate whether something is better or worse for metabolism or healthier for the horse.
“Organic” is simply a chemical term for this type of selenium compound: the selenium is bound to an organic molecule, namely the amino acid methionine. In contrast, the inorganic sodium selenite is a form of selenium bound to inorganic sodium, which is also found in table salt (sodium chloride).

Selenium compounds in nature

Selenium is found in plants in various organic and inorganic forms, with the main organic form in grass being selenomethionine. In leaves, it is more commonly present as inorganic selenate. However, there are many other forms as well, as shown in the diagram illustrating selenium uptake in plants.

Selenium metabolism in plants, (A) and (B) uptake of selenite (SeO₃²-) and selenate (SeO₄²-) from the soil, (C) takes place only in plants that accumulate selenium. .
Source: Bodnar M, Konieczka P, Namiesnik J. The properties, functions, and use of selenium compounds in living organisms. J Environ Sci Health C Environ Carcinog Ecotoxicol Rev. 2012;30(3):225-52 .

Uptake and storage of selenium in the body

Selenium is not exclusively absorbed by the horse as selenomethionine, as it also occurs in various forms in nature. Once selenium enters the body, it is stored in different pools within the tissue and is released again after varying periods of time. And this is where the rebound effect comes in: unlike sodium selenite, selenomethionine is also non-specifically incorporated into proteins, essentially for “rainy days.”
A study in humans showed that after two years of taking selenomethionine, not as much selenium was excreted as had been ingested. This means the selenium content in the body had increased. The selenium absorbed was “hidden” in the body’s proteins.

Release of selenomethionine from stored proteins

As sodium selenite and selenomethionine target different pools within the tissue, after stopping selenomethionine, there is a redistribution of these pools, which eventually leads to an increase in plasma levels of excess selenomethionine, as it is also released from proteins that are naturally broken down. However, in plasma, more selenomethionine is reabsorbed compared to sodium selenite, and the entire cycle starts over again. This is beneficial when there is little selenium or selenomethionine in the diet – as is the case with the nutritional conditions of wild horses. They are ideally adapted to habitats with selenium-deficient soils.

Risks of overdosing on selenomethionine

However, if it is fed in excess (for example as a selenium supplement due to the low value in the blood count) or over longer periods of time (for example as a mineral feed), these regulatory mechanisms cannot simply be switched off by the body and the body is flooded with selenium.

Schematic representation of the distribution of selenium in the human body.
Source: Patterson BH, Combs GF Jr, Taylor PR, Patterson KY, Moler JE, Wastney ME. Selenium Kinetics in Humans Change Following 2 Years of Supplementation With Selenomethionine. Front Endocrinol (Lausanne). 2021 Mar 29;12:621687

Comparison of elimination time and long-term effect

In humans, selenomethionine is released from the largest selenium pool (“Tissue 3”) after an average of 430 days (about 1.5 years!), whereas sodium selenite is excreted after an average of 213 days. This is exactly what is reflected in the blood test results. At a point when selenomethionine has long been discontinued, the plasma levels of selenium suddenly rise. Additionally, more selenomethionine is measured in the plasma than sodium selenite, which is related to the different ways they are distributed to the pools. As a result, the blood plasma value becomes completely unreliable for assessing the body’s selenium supply.

Transfer to horses and recommendation for feeding

The values and data here are, of course, based on measurements in humans. Measurements in other mammals show that the values are similar to those observed in humans, so these values can also be applied to horses, although the exact values may differ slightly.

Conclusion: Sodium selenite as the preferred source of selenium for horses

As selenomethionine is non-specifically incorporated into proteins and accumulated in the body, the intake of sodium selenite is better for the body to compensate for deficiencies and to avoid an overload of selenium in the body through the natural protein turnover process.
The consequences are the same as with excessive selenium feeding: sulphur is displaced from its binding sites, leading to brittle hooves and hair, resulting in poor hoof horn and coat shedding issues. Mucus from the respiratory tract is no longer efficiently transported, and the horse begins to cough more frequently.

Selenium also has an impact on the rise in blood sugar levels and is suspected of potentially causing insulin resistance. Overall, this results in a rebound effect that is best avoided. As Paracelsus famously said, “the dose makes the poison.” In the case of selenium, there is a fine line between being beneficial for the body and causing negative effects.

Therefore, one should always ensure to include only feed with inorganic sodium selenite in your horse’s diet and avoid organic selenium variants altogether.

Sources

Bodnar M, Konieczka P, Namiesnik J. The properties, functions, and use of selenium compounds in living organisms. J Environ Sci Health C Environ Carcinog Ecotoxicol Rev. 2012;30(3):225-52.

Patterson BH, Combs GF Jr, Taylor PR, Patterson KY, Moler JE, Wastney ME. Selenium Kinetics in Humans Change Following 2 Years of Supplementation With Selenomethionine. Front Endocrinol (Lausanne). 2021 Mar 29;12:621687

Wastney ME, Combs GF Jr, Canfield WK, Taylor PR, Patterson KY, Hill AD, Moler JE, Patterson BH. A human model of selenium that integrates metabolism from selenite and selenomethionine. J Nutr. 2011 Apr 1;141(4):708-17

Gorini F, Sabatino L, Pingitore A, Vassalle C. Selenium: An Element of Life Essential for Thyroid Function. Molecules. 2021 Nov 23;26(23):7084