Supplements for Metabolic Horses

If you have spent any time in barns, chances are that you have seen some over conditioned horses. Equine obesity is a prevalent welfare concern and predisposes horses to serious health issues such as laminitis. This week’s blog post is going to dive into the research on some of the supplements that are marketed for metabolic horses.

First and foremost, it MUST be noted that you cannot supplement your way to weight loss. Adding one, or more dietary supplements is not going to elicit weight loss. If your horse is overweight, proper forage management and exercise is the solution. Not supplements.

With that clarified, we can now look at what science does exist on some of the popular supplements, and if they can be a useful ‘cherry on top’ of our balanced weight loss plan. Please note that this is not a full literature review of each feed ingredient.


Chromium is a micromineral that horses do not yet have an established requirement for. The NRC, 2007 (Nutrient Requirements of Horses), does not have a recommended feeding amount as there was not enough research to establish a precise recommendation when the text was published. In fact, it was only recognized to have a nutritional role by Health Canada in 2010. Chromium is often supplemented to metabolic horses as biologically it does play a role in the clearance of glucose from the blood.

A study published in 2020 evaluated the impact of supplementing 2, 4, or 8 mg of chromium propionate daily to healthy quarter horses. The authors found that the blood insulin and blood glucose levels were lower in those supplemented with 2 mg or 4 mg chromium propionate. However, these findings did not show that feeding chromium equalled increased insulin sensitivity as at 2-hr post feeding, the horses fed 0 mg or 8 mg chromium had higher glucose levels than those fed 2 or 4 mg. Additionally, at 4 h post feeding, the insulin concentrations were greater in horses given 0 or 8 mg chromium than the ones given 2 or 4 mg.

Another study on a supplement containing both chromium and magnesium evaluated the potential impact on horses that had previously been laminitic. The supplement contained 5 mg chromium as yeast and 8.8 g magnesium. Hyperinsulinemia was detected in 12 of the 14 horses prior to being fed the supplement. The authors concluded that this supplement containing chromium did not alter the insulin sensitivity in metabolic horses when fed for 16 weeks.

Overall, chromium propionate is currently the only form approved as an equine feed ingredient, and more research is required to fully elucidate the effects of chromium supplementation on insulin/glucose dynamics in metabolic horses. It does appear to play a role in the glucose dynamics, but we do not yet know enough to be able to provide dosing recommendations for optimal health benefits.

Milk Thistle

Silybum marianum (milk thistle extract) is a popular herbal supplement that is commonly used in horses to support liver function or to improve metabolic conditions. The bioactive compound in this plant is silymarin, a flavonolignan. This compound has anti-inflammatory, antioxidant and even immunomodulating properties.

Most of the research on silymarin has been shown to have positive results on liver function in rodents and humans. Some of the other documented benefits include binding iron to reduce serum ferritin levels,  reduced inflammation, and reduced free radical damage.

A study that used 9 healthy horses evaluated the bioavailability of silibinin (milk thistle derivative) and safety when given through a variety of methods. The authors of this study concluded that the compound was safe to feed but had poor bioavailability in horses.

There are multiple in vitro studies investigating a bio-active flavonoid derived from milk thistle that indicates that it may have an anti-inflammatory impact in horses. Inflammation is involved in laminitis; therefore, this is one of the reasons that milk thistle is marketed for use in these horses.

In an in vivo study with 5 healthy horses, it was concluded that when the horses were fed doses of silibinin phospholipid the plasma antioxidant capacity increased. There was also an impact of dose as the antioxidant capacity increased with the amount fed.

A 2014 study investigated the influence of endotoxins on lamellar tissue integrity, and if milk thistle was capable of inhibiting lipopolysaccharide (LPS) induced effects. LPS is a bacterial toxin. In this study, LPS was used to induce laminar separation in the hoof tissue. The treatment of the hoof tissue with milk thistle or silymarin did reduce the concentration of LPS. It was concluded that milk thistle treatment, and silymarin could improve the tissue integrity as the compounds reduced the endotoxin activity and inhibited the LPS-induced effects on the lamellar tissue.

Overall, there is some positive research on milk thistle as an anti-inflammatory supplement for horses. However, without having data on bioavailability, it may not be worth investing in for your metabolic horse.


Resveratrol is a polyphenol that has antioxidant properties. A study with mature horses aged 15-22 years old illustrated that supplementation with resveratrol did exert a positive effect by increasing the animal’s antioxidant capacity.

There is one study that evaluated the effects of a supplement containing resveratrol and leucine on insulin concentrations in horses diagnosed with metabolic syndrome. The results indicated that the blood insulin was lower in the horses receiving the supplement at both tested doses.

Overall, there is some research that points towards these active ingredients producing a positive health benefit for metabolic horses. However, with the lack of data on dosing, bioavailability, and clear benefits, they may not be worth your investment. With any equine weight loss program, the focus should be on optimal forage management and exercise. Please remember that each case is individual – therefore some of these products may benefit certain horses while may be unnecessary in others. Always discuss with your nutritionist prior to making diet changes.

By: Madeline Boast, MSc. Equine Nutrition


Chameroy, K. A., Frank, N., Elliott, S. B., & Boston, R. C. (2011). Effects of a supplement containing chromium and magnesium on morphometric measurements, resting glucose, insulin concentrations and insulin sensitivity in laminitic obese horses. Equine veterinary journal43(4), 494-499.

Ememe, M. U., Mshelia, W. P., & Ayo, J. O. (2015). Ameliorative effects of resveratrol on oxidative stress biomarkers in horses. Journal of Equine Veterinary Science35(6), 518-523.

Gugliandolo, E., Crupi, R., Biondi, V., Licata, P., Cuzzocrea, S., & Passantino, A. (2020). Protective effect of silibinin on lipopolysaccharide-induced inflammatory responses in equine peripheral blood mononuclear cells, an in vitro study. Animals10(11), 2022.

Hackett, E. S., Mama, K. R., Twedt, D. C., & Gustafson, D. L. (2013). Pharmacokinetics and safety of silibinin in horses. American journal of veterinary research74(10), 1327-1332.

Hackett, E. S., Mama, K. R., Twedt, D. C., & Gustafson, D. L. (2013). Evaluation of antioxidant capacity and inflammatory cytokine gene expression in horses fed silibinin complexed with phospholipid. American journal of veterinary research74(10), 1333-1339.

Manfredi, J. M., Stapley, E. D., Nadeau, J. A., & Nash, D. (2020). Investigation of the effects of a dietary supplement on insulin and adipokine concentrations in equine metabolic syndrome/insulin dysregulation. Journal of equine veterinary science88, 102930.

Reisinger, N., Schaumberger, S., Nagl, V., Hessenberger, S., & Schatzmayr, G. (2014). Milk thistle extract and silymarin inhibit lipopolysaccharide induced lamellar separation of hoof explants in vitro. Toxins6(10), 2962-2974.

Spears, J. W., Lloyd, K. E., Pratt-Phillips, S. E., Siciliano, P., & Krafka, K. (2021). 43 Safety of chromium propionate as a source of supplemental chromium for horses. Journal of Equine Veterinary Science100, 103506.

Spears, J. W., Lloyd, K. E., Siciliano, P., Pratt-Phillips, S., Goertzen, E. W., McLeod, S. J., … & Rounds, W. (2020). Chromium propionate increases insulin sensitivity in horses following oral and intravenous carbohydrate administration. Journal of animal science98(4), skaa095.

Spears, J. W., Siciliano, P., Pratt-Phillips, S., Lloyd, K., Goertzen, E., Krafka, K., … & Rounds, W. (2020). 105 Chromium propionate improves insulin sensitivity in horses following oral concentrate consumption. Journal of Animal Science98(Suppl 4), 88.

Zholobenko, A., Mouithys‐Mickalad, A., Modriansky, M., Serteyn, D., & Franck, T. (2016). Polyphenols from Silybum marianum inhibit in vitro the oxidant response of equine neutrophils and myeloperoxidase activity. Journal of Veterinary Pharmacology and Therapeutics39(6), 592-601.

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