There are two kinds of fats, animal, and vegetable. Most animal fat is solid at room temperature whilst vegetable and some fish fats are liquids (oils). Fats are comprised of a Glycerol backbone with fatty acid side chains, and it is the length of these chains which determines whether the fat is a liquid oil at room temperature, or a solid fat. Animal fat (tallow) is used extensively in feed formulations for livestock but is unpalatable for horses except in very small amounts, so the fat used in commercial horse feeds is almost exclusively of vegetable origin.

The principal uses of fat in the body are as an energy source and for the formation of phospholipid cell membranes which encapsulate all the cells in the body. The type of fat, and the fatty acid contents can influence how it is used and how it performs right down to the cellular level. Fats are also required for the absorbtion of the fat soluble vitamins in the diet, vitamins A. D. E and K.

The use of fat as an alternative energy source to carbohydrates has gained acceptance in horses over the past twenty years and is now commonplace. Feeding fat enables some reduction in the carbohydrate content of the diet with potential advantages in terms of gut health, temperament and lowered risk of colic and laminitis.

A small amount of fat, 2%-3% is a natural part of the horse diet so they have evolved some capacity to deal with dietary fat effectively, just not at the levels being used recently. Ideally fat should be absorbed in the small intestine but if the amount fed is large, some will pass through into the large intestine and be subject to microbial digestion by the hind gut microbiota. Research has shown that fat (oil) has a negative impact on this microbiota, reducing its normal diversity and disturbing the normal healthy balance of bacterial populations. Oily faeces or manure like a cow may result, and nutrient absorbtion can be reduced.

The key to feeding fat is to habituate the gut flora gradually over a period of time. A reasonable feeding regime for oil is to begin with 50ml per day and add another 50ml every 3-4 days until the desired volume is reached, always split between meals. A reasonable upper limit is to feed no more than 1ml of oil per kg bodyweight per day as a maximum. In addition to conditioning the gut flora it also takes time for fat to be fully utilised efficiently by muscles as an energy source. Getting the full value from fat in terms of performance requires modification to energy pathways used by muscle tissue and those changes may take up to 4 months to fully develop.

The other reason to feed oil is as a source of Omega 3 fatty acids which have a mild anti-inflammatory influence. For this purpose, an oil rich in the desirable omega 3 fatty acids, and low in the pro-inflammatory Omega 6 fatty acids is preferable with flax seed oil having the best ratio of the vegetable oils, matched only by fish oil.

Most oils will oxidize on exposure to air, a process called ‘’going rancid’’ which imparts a sour taste and unpleasant smell. Horses are highly susceptible to rancidity and will refuse feeds contaminated with rancid fat. All oils should be stored in closed airtight containers away from light to reduce oxidation. Rancidity is prevented or delayed by the inclusion of anti-oxidants added to oils or to feeds containing oil. The commonest antioxidant used for this purpose is Vitamin E.

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