July 15, 2013–Selenium serves an important role in the equine body’s antioxidant defense mechanism. The chemical contains glutathione peroxidase (GSH Px), an enzyme that neutralizes the effects of oxidative stress. Although naturally generated by the horse’s body on a daily basis, oxidative stress has been shown to increase after strenuous exercise. Without an adequate amount of selenium in the horse’s diet, the horse’s antioxidant defense mechanism is compromised, which could result in issues such as musculoskeletal problems and a weakened immune system. In regions around the United States where forage and soil contain low or deficient levels of selenium, equine nutritionists often advise owners to supplement the equine diet with a selenium source.
Currently, the National Research Council (NRC) advises a selenium intake of 1 milligram per day for a mature 500-kilogram horse, or 0.1 parts per million. These recommendations are based on shorter-term studies conducted in the 1970s and 1980s. Due to the duration of those studies, the resulting research did not measure the impact of selenium supplementation through the course of the equine red blood cell lifespan in the horse, which is approximately 150 days.
Researchers at the University of Kentucky and Alltech’s Center for Nutrigenomics and Applied Animal Nutrition recently completed a longer-term study to determine whether horses might benefit from a higher amount of selenium than recommended by the NRC. The research study sought to determine how the activity of GSH-Px, an important component of the antioxidant system that regulates levels of hydrogen peroxide concentrations in the cell, was influenced by dietary intake of selenium. The objectives of the research were to evaluate the effects of selenium depletion and repletion on GSH-Px activity and antioxidant status and oxidative stress in the horse over a long-term period.
The researchers divided a group of 28 mature, idle horses blocked by age and gender into four dietary treatment groups. The horses were randomly allocated within their block to one of four groups: low selenium (LS), adequate selenium (AS), high organic selenium (SP) and high inorganic selenium (SS). The first phase of the study was a depletion phase aimed to lower the horses’ selenium status. It spanned more than 196 days, with the LS, SP and SS groups ?receiving a low level of selenium in the diet (60 percent of the NRC-recommended amount), and? the AS groups serving as control and receiving a slightly higher amount of selenium than the NRC-recommended amount at 0.12 parts per million. The horses received a custom formulated balancer pellet in addition to unlimited forage intake from pastures closely monitored for levels of selenium to ensure study consistency.
In the second phase of the study, the research team aimed to replenish the horses’ selenium stores. Each group of horses received specific variations in amount and type of selenium in their diets during a course of 189 days. The control AS group continued to receive the same adequate selenium diet as before and similarly the LS group received the same low-level selenium diet administered previously. However, the SP and SS groups now received their selenium supplements in addition to the diets they received before – the SP group received an organic selenium supplement and the SS group received an inorganic selenium supplement in the feed. During the repletion period, the total amount of dietary selenium intake (balancer pellet, forage and supplements where applicable) for the LS group was 0.06 parts per million and for the AS group was 0.12 parts per million, and the SP and SS groups were both regulated at 0.3 parts per million.
During the repletion and depletion phases, blood samples were taken routinely to measure the level of selenium in the blood as well as the GSH-PX activity. During the repletion phase, both selenium concentration and GSH-Px activity dropped in the SS, SP and LS groups. In the control AS group, selenium concentrations dropped until day 84 and thereafter stabilized during this phase. The researchers noted a correlation between whole blood selenium concentration and GSH-Px activity, with activity lower in horses on the low selenium diet and higher in horses on the adequate control selenium diet.
Within 28 days of the repletion phase of the study, the two groups of horses with supplemented selenium in their diets (groups SS and SP) regained comparable levels of whole blood selenium with the AS control and surpassed the levels in the low-selenium group. A blood sample collected on day 154 and day 189 showed that the selenium levels in the SS and SP groups had risen above the level of the control AS group.
When examining the important GSH-Px function in all four groups during the repletion phase, researchers found that the dietary treatment and time were both factors in GSH-Px activity.? As the study progressed to the final blood sample at day 189, the GSH-Px activity reached a plateau in the group of horses receiving the supplemented inorganic selenium (SS). However, GSH-Px activity showed continued increase in horses that received the organic selenium at day 189.
In this study, the changes in whole blood selenium and GSH-Px activity during the depletion and repletion phases indicate that the selenium status of the horse can be adjusted through dietary intake if sufficient time is allowed. The researchers also observed that the AS group, the control group receiving a diet with slightly above the NRC-recommend amount of selenium, experienced a decline in GSH-Px activity during the depletion phase regardless of maintaining a whole blood selenium concentration above what was previously reported to sustain GSH-Px activity. However, the GSH-Px activity eventually stabilized in this treatment group.
The study also identified a distinction between the groups receiving the inorganic selenium supplementation and the organic selenium supplementation, with the later group continuing to increase in GSH-Px activity with a trend to surpass its counterpart upon the conclusion of the study period. Researchers note that further studies to fully understand the role of GSH-Px activity in the antioxidant mechanism are necessary. Still, their current data suggests that the NRC-recommended level of selenium does not support the maximum potential of GSH-Px activity that serves an important role in horses subjected to oxidative stress, especially in regions of the world deficient in selenium.
When evaluating the complete set of data collected over the two year study period, a positive correlation was found between selenium status and total antioxidant capacity, while a negative relationship was found between selenium status and an indicator of oxidative stress in these horses.
“This study demonstrates that under certain conditions horses may benefit from a total dietary selenium intake, higher than what is currently recommended. Some commercial feeds are already formulated to allow for a higher dietary selenium intake of 0.3 parts per million,” said Mieke Brummer, Ph.D, an equine nutritionist with Alltech who conducted this research project for her doctorate in partnership with the University of Kentucky. “We found a strong, positive correlation between whole blood selenium and GSH-Px activity.”
Brummer said the findings of this long-term research serve to show the link between selenium intake and the antioxidant enzyme GSH-Px in the horse. She believes this research will provide evidence in favor of a higher total dietary selenium recommendation, which currently stands at 0.1 parts per million for a 500-kilogram horse. The higher selenium-supplemented study groups in the study received 0.3 parts per million of selenium, determined to be capable of promoting GSH-Px activity. However, selenium can be very toxic at relatively low levels of inclusion. Therefore, horse owners should work closely with equine nutritionists to evaluate the total level of selenium their horses are receiving through both the forage and concentrate components of their horses’ diets.
View the complete study, “Measures of antioxidant status of the horse in response to selenium depletion and repletion,” online.