Effect of Stress, Exercise, Feed Form, and Meal Size on Gastrin Levels in Horses

Gastrin is a hormone that stimulates the production of gastric (stomach) acid in horses. Meals of hay and concentrates cause gastrin levels to rise, increasing gastric acid production. Intense exercise has also been reported to cause a rise in gastric acid. Excess gastric acid production is a factor in the development of gastric ulcers in performance horses.

A research project was conducted to answer the following questions:

• Is there a difference in gastrin level after a large hay meal, small grain meal, or meal of hay and grain?

• How does exercise affect gastrin production?

• Is there a link between gastrin level and stress associated with exercise, medical procedures, or fasting?

To explore the answers to these questions, a study was designed using six mature Standardbreds (four geldings and two mares). One group of horses was given unlimited hay. The second group was fed twice a day with about five pounds (2.2 kilograms) of oats and a limited amount of hay; animals on this diet were finished eating in about an hour. Horses in the third group were given about five pounds of oats twice a day, but no hay, and were finished eating in about 15 minutes. All horses had free access to water.

After a fast of at least 14 hours regardless of diet, the horses were exercised on a non-inclined treadmill. Two exercise patterns were followed. In the incremental test, horses warmed up at a walk for two minutes. They then trotted at 5 meters per second for two minutes, with speed increasing at a rate of 1 meter per second every two minutes through a speed of 9 meters per second. In the long-distance running test, horses ran at a steady speed of 4 to 6 meters per second for 30 minutes.

Jugular catheters were placed in all horses and blood samples were taken before and immediately after insertion and then periodically before, during, and after feeding and exercise. Blood samples were analyzed for levels of gastrin and cortisol, a known indicator of stress.

In this study, horses that ate only hay or hay and grain showed a peak in plasma gastrin level about 30 minutes after eating. Horses eating unlimited hay showed the most prolonged gastrin response. When only the small grain meal was given, gastrin response was delayed compared to response time when horses were fed hay, reaching a peak about 2.5 hours after eating.

Differences in gastrin level were seen after horses ate a large hay meal (highest and most prolonged response), small hay meal (rapid response but fairly quick return to base level), or small grain meal (delayed response and fairly quick return to base level). An immediate and large increase in plasma gastrin concentration was found when high-volume meals (large amount of hay) were given, whereas a small meal (concentrate/grain) evoked a later onset of gastrin release, suggesting that gastric distention plays an important role in inducing gastrin release during a meal.

The researchers suggested that the stomach distention that follows a large hay meal may be a mechanical trigger for the continuing release of gastrin, while a smaller meal does not distend the stomach enough to cause such a release. This explanation is backed up by studies of colicky horses in which there was a correlation between degree of stomach distention and level of gastrin in the blood.

Gastrin levels after feeding were higher than during fasting, while cortisol levels tended to be higher while horses were fasting. Treadmill exercise had little effect on gastrin concentrations, while cortisol levels were found to increase with exercise. No changes in plasma gastrin or plasma cortisol were noted in response to venipuncture for installation of the jugular catheters, a procedure that is associated with sharp increases in gastrin production in human subjects.

Because rises in cortisol did not show consistent correlation with gastrin levels in this experiment, it is suggested that gastrin secretion is not a useful measure of stress in fasting or exercised horses.

Although evidence suggests that gastric ulcers can be caused or aggravated by production of gastric acid, the saliva stimulated by hay consumption is effective in buffering the acid. High levels of gastrin are therefore not harmful to the horse as long as food is present in the stomach. Unlimited hay may not be an option for feeding certain performance horses, but managers should consider allowing horses access to grass, hay, or another fiber source as much as possible.

If access to fiber is not possible and horses do not eat for long periods of time, gastric acid may lead to abdominal discomfort from ulcers or other factors. These horses can be kept more comfortable by the use of supplements such as equine antacids, hindgut buffers, and digestive tract conditioners.