Chromium and Carbohydrate Utilization during Exercise

Glycogen and the Athletic Horse:

Carbohydrates and fats supply the bulk of the energy to

horses during exercise, and the use of each often depends on the

intensity of the exercise. During lower-intensity exercise,

approximately 42 percent of energy is supplied through the

oxidation of fat, versus 58 percent from carbohydrates.

During high-intensity exercise, the ratio can shift to 30

percent from fat and 70 percent from carbohydrates.

Regardless of intensity, top performance requires both an

adequate supply of and efficient use of carbohydrates.

Cells take up carbohydrates from blood plasma in the form of

glucose, which is stored as glycogen in the liver and skeletal

muscle. For optimal performance, horses must effectively

utilize plasma glucose and maintain cellular glycogen stores.

Carbohydrate availability and its impact on physical

performance in horses are connected in two ways:

1. Increased time to fatigue when supplemental glucose is

administered intravenously (IV) during moderate exercise

2. Negatively impacts performance when muscle glycogen

stores are depleted before exercise.

This research demonstrates the importance of

glucose/glycogen metabolism to physical performance.

Glucose Utilization:

Providing glucose through IV is not a practical solution. Ensuring

the diet provides adequate levels of glucose or glucose

precursors, as well as the efficient use of the available glucose,

is ideal.

Unlike humans, plasma glucose levels in horses can increase

during exercise because of the mismatch between glucose’s

rate of appearance (Ra) due to the breakdown of glycogen in the

liver and the rate of disappearance (Rd) or uptake by skeletal

muscle cells. At moderate levels of exercise, horses can

experience a four-fold increase in both Ra and Rd.3 However, at

higher intensities, while the four-fold increase in Rd is

maintained, Ra can increase to seven times higher than normal –

resulting in a significant increase in plasma glucose levels.4

If skeletal muscle cells could further increase their uptake of

glucose, it might be possible to improve performance or delay

fatigue.

Glycogen Levels:

Low levels of skeletal muscle glycogen (approximately 50

percent of normal) have been shown to negatively impact

performance.2 One 800-meter sprint can deplete glycogen stores

by up to 65 percent.1 Multiple events during the day or

consecutive days of competition or training could quickly deplete

glycogen levels to the point at which performance is impacted, or

horses become easily fatigued. The rate of glycogen synthesis is

considerably lower in horses than in humans. Once significantly

depleted, it can take 48-72 hours to replenish glycogen stores.

There are two thoughts as to why horses have such a slow rate

of glycogen replacement:

1. The gastrointestinal function in horses is not well suited to

digest starch and other soluble carbohydrates, resulting in

limited glucose availability

2. The mechanisms involved in glycogen synthesis are not as

productive as in other species

Impact on GLUT4 Receptors:

GLUT4 is the primary glucose transporter, responsible for

facilitating movement of glucose into cells.

When GLUT4 activity is disrupted, glucose transport and insulin sensitivity are

significantly reduced.

While research in glycogen synthesis is

limited in horses, supplemental chromium in rats and beef cattle

has been shown to increase movement of GLUT4 receptors to the

surface of skeletal muscle, resulting in improved glucose uptake

metabolism.

REFERENCES

1. Jose-Cunilleras, E., & Hinchcliff, K. (2004). Carbohydrate metabolism in exercising horses. Equine and Comparative Exercise Physiology, 1(1), 23–32. doi: 10.1079/ecp20031

2. Lacombe, V. A. (n.d.). Muscle Glycogen Metabolism in Horses: Interactions Between Substrate Availability, Exercise Performance and Carbohydrate Administration.

Retrieved October 24, 2019, from https://etd.ohiolink.edu/!etd.send_file?accession=osu1041621577&disposition=inline.

3. Geor RJ, Hinchcliff KW and Sams RA (2000). Glucose infusion attenuates endogenous glucose production and enhances glucose use of horses during exercise. Journal of Applied Physiology 88: 1765–1776.

4. Geor RJ, Hinchcliff KW, McCutcheon LJ and Sams RA (2000). Epinephrine inhibits exogenous glucose utilization in exercising horses. Journal of Applied Physiology 88:1777–1790.

5. Shaohui Huang and Michael P. Czech. The GLUT4 Glucose Transporter. Cell Metabolism 5, April 2007.

6. Jameson, J. L., & J., D. G. L. (2016). Endocrinology: adult and pediatric. Vol. 1. Philadelphia: Elsevier Saunders.

7. Doerner, P. G., Liao, Y.-H., Ding, Z., Wang, W., Ivy, J. L., & Bernard, J. R. (2014). Chromium chloride increases insulin-stimulated glucose uptake in the perfused rat hindlimb. Acta Physiologica, 212(3), 205–213. doi: 10.1111/apha.12375

8. Effects of KemTRACE Chromium on blood parameters, GLUT4 and muscle fiber characteristics of finishing cattle. TL-16-00031.

9. Qiao, W., Peng, Z., Wang, Z., Wei, J., & Zhou, A. (2009). Chromium Improves Glucose Uptake and Metabolism Through Upregulating the mRNA Levels of IR, GLUT4, GS, and UCP3 in Skeletal Muscle Cells. Biological Trace Element Research, 131(2), 133–142.

doi: 10.1007/s12011-009-8357-2