"You are what you eat" is the slogan from an almost 40-year-old Danish school campaign for healthy eating habits. A poster displayed carrots, rye bread, milk, and fish crossing the finish line victorious and happy. Ice cream, French fries, and candy, however, trail behind and dejected. The message was unmistakable.

But what about animals? To what extent does feed composition affect their endurance, health, and their temper?

A lot of starch – wild horse, a lot of fat – calm horse?

Among riders, it is commonly said that if you feed with high amount of grain (starch) you are kicked off. On the other hand, if you feed it more fat, you’ll have a calmer horse. The few studies performed until now on this topic have mainly been with rather extreme levels of starch or fat. Therefore, researchers at the Department of Animal and Veterinary Sciences have conducted an experiment with a group of 4-year-old Danish Warmblood stallions, where the starch and fat contents of the concentrates were more relevant to practical use.

Feed composition did not affect the horses' physiology

In the experiment, 20 stallions were divided into two groups. One group received a feed high in starch and low in fat. The other group received a feed high in fat and low in starch. Both groups were fed these mixtures for 9 weeks together with hay. Afterward, they performed a test on a treadmill. The horses' heart rate was measured before and after training. A blood sample was also taken to assess the animals' physiological condition.

The feed composition did not affect the animals' physiological endurance. Previous international studies have shown that a high starch content in feed could disrupt the function of the hindgut. Such imbalance increases the risk of laminitis. It has also been shown that a high starch content can decrease the body's sensitivity to insulin, leading to various diseases.

So why didn't the researchers find the same results? According to Assistant Professor Saman Lashkari, there are two reasons: First, in this experiment, they chose to use feed mixtures within the normal market range. The differences were therefore less extreme than in previous studies. Second, the intensity of the training was lower. The horses were not exhausted, and there was no shift from aerobic to anaerobic metabolism.

Fear or curiosity?

Horses are naturally fearful animals that react to novelty with avoidance responses. Therefore, testing the horses' behavior involved assessing their fearfulness or curiosity toward unfamiliar objects. Neither heart rate measurements nor behavioral responses suggested that the horses were significantly more fearful with one feed over the other. However, there was a tendency for horses fed a low-starch, high-fat diet to show more exploratory behavior which may be linked to reduced fearfulness.

The connection between the gut and the brain

At first glance, a connection between feed composition and horses' fearfulness might not seem apparent. But going back to the slogan 'You are what you eat,' there is increasing research suggesting a link between diet and mental health in both humans and animals. This connection involves the gut and its bacteria—the so-called gut-brain axis. For example, studies have shown that mice became more fearful when fed starch that altered gut bacteria. It has also been shown that foals were more prone to crib-biting, less curious, and more fearful of strangers when fed a high-starch diet. However, none of these previous results could be confirmed in this experiment, aside from the fact that horses fed a high-fat diet were slightly more curious.

For the horse owners the implications of the study are, that they are free to choose the type of feed they like within the range of concentrates on the marked without being concerned about the horses’ physiology and behavior. It is of course a prerequisite that the horses get all the roughage they need.