Owners see it as a cost-effective way to deliver quite large amounts of energy to their animals.

However, there is some disagreement in the horse world over whether it's healthy. Some owners have expressed concern over its effect on a horse's liver, and believe it may adversely affect performance.

Two studies indicate their concerns are unwarranted.

Researchers in Kentucky looked at whether feeding oil had any long-term effect on both health and performance.

They took 12 two-year-old thoroughbreds and split them into groups of six. Both groups were in training.

All received identical diets, but one group had a cup of soybean oil added to their morning and evening meals.

All the horses were exercised on treadmills at intervals of two, four and seven months after the trial began, and levels of blood-glucose were measured. Blood glucose levels decline as an exercising horse tires.

Researchers found that the fall in blood glucose was less in the horses getting fat in their diet.

The results indicated that fat in a horse's diet would appear to delay the onset of fatigue, making it particularly useful when horses are required to perform over longer periods of time, such as in endurance rides.

The study, which was reported the Equine Veterinary Journal Supplement in 1995, also found no evidence of any health problems in the horses supplemented with oil.

So how does fat affect the performance of horses in much shorter bursts, such as in racing?

Another study, also reported in the journal in 1995, was carried out in Australia, looking at just that question.

The horses used for the study were again given a cup of oil morning and night with their usual feed, which meant about 12% of their daily intake came from fat. Corn oil was used.

The horses were then exercised at levels similar to racing, with those receiving the oil able to run harder for a longer period than the control group of horses who received no supplementary oil.

The oil-fed horses ran hard, on average, for 97.2 seconds, compared to 91.7 seconds for the control group.

The researchers noted that horses cannot use fat for an energy source during hard exercise. The dietary oil must therefore have improved the horse's ability to use energy in other ways.

The authors noted that oil would be a good way of providing energy to horses that are off their feed. In other words, oil can allow owners to provide large amounts of energy to a horse, even though it may not be eating much.