This ends, of course, when they're put on the feed lot, and they develop a nice layer of fat and fatty deposits in their muscles, if a grain-based diet doesn't kill them first.
I stumbled across this link looking for some information about athletes on a fat-based diet:
Feeding Oil or Fat for Horses - A Review
I got what I was looking for, and it turns out it's fascinating. Stance Equine is a company that "...pioneered the feeding of no grain, low sugar and starch feeds to horses to avoid the metabolic chaos caused by high NSC [non-structured carbohydrate] feeds. The Stance Equine feeds are based on the unique attributes of copra, coconut meal and coconut oil."
"The Domesticated LifestyleHuh. That sounds familiar... Horses on the Paleo diet.
"The modern, domesticated horse lives in an environment markedly different to the evolutionary one. Horses are often kept in paddocks and stables, restricting access to quality pasture. This, coupled with the high-energy demands of performance, can make safely meeting horses energy needs, a tall order...."
Well, not quite, as (from the first link): "The option of feeding horses the balanced and natural diet on which they evolved is, in most cases, unfeasible due to space, time and financial constraints. Besides which it is most unlikely that the natural diet of horses would suffice to meet the energy needs demanded of many performance pursuits."
Of course, as it requires a lot of time and chewing. Nevertheless, this puts horses squarely in the same position as a human athlete, who also can't afford to spend all their time hunting ruminants, and so must make do with a second-best diet.
"The Grain SolutionBut unlike human athletes, it turns out that there's been a good bit of work done on the effect of a fat-based diet on horse's athletic performance. Now mind you, they're not people, or even mice, but based on the similarities of the ill effects on the horses from a high carbohydrate diet, it's an interesting possibility that the benefits would be the same.
"Traditionally, horse diets have often included large quantities of grain (ie. starch) in an attempt to sate energy needs. However, horses have a limited capacity to digest starch and high starch feeding practices can result in starch overload into the hindgut (Kohnke et al 1999; Rowe et al 2001). A number of metabolic disorders - such as tying up, laminitis, colic, and excitable behaviour - are associated with high grain diets. Advances in feed processing technologies have reduced the risks associated with high grain feeding, but have not eliminated them...."
It turns out there's also a Maffetone Method for horses:
"Training to Facilitate the Aerobic PathwayThe article goes through a variety of information about feeding fat to horses. "Horses exhibit a preference for vegetable oils over animal fats (Potter n.d.).", thank heavens, but that got me worried about the ill effects of vegetable oils on horses...
"The aerobic metabolism provides energy for slow-twitch muscle fibres and therefore stamina. It also provides much of the energy for explosive exercise, which cannot be met by energy from the anaerobic process alone. Training the horse to facilitate aerobic metabolism, is advantageous to performance in all disciplines as a result of the elevated ATP production.
"With training, horses have a higher number of mitochondria and enzymes in their muscles, which produce more energy. Training also increases the blood's ability to carry oxygen to the muscles, through increased haemoglobin concentration in blood. Training allows horses to carry out aerobic energy production more efficiently...."
"The Link Between Dietary Fats and Cancer.I won't get into beef tallow at the moment, but I agree that feeding lots of linoleic acid to anything is a bad idea.
"In humans nd [sic] rodents, dietary fats have been implicated in carcinogenesis, partly via the mechanism of oxidative DNA damage (Loft et al 1998). Loft et al (1998) found high total energy intake (rather than simply high fat intake), to be the major cause of oxidative DNA damage in rats, irrespective of degree of saturation of dietary fat consumed. D'Aquino et al (1991) reported that unlike coconut oil, fish oils are highly susceptible to oxidative deterioration and challenge the antioxidant defence system in rats, thereby increasing susceptibility of tissues to free radical oxidative damage.
"An experiment involving tumour-bearing mice indicated that the level of dietary linoleic acid consumed was proportional to the weight of their tumours and to the number of macroscopic metastases. Mice that consumed proportionally more saturated fatty acids (in the form of coconut oil) had lighter tumours and fewer macroscopic metastases (Rose et al 1993).
"Reddy (1992) noted that diets containing coconut oil, olive oil or fish oil had no colon-tumour enhancing effects regardless of whether they were fed at rates of 23% or less. However rats fed diets containing 23% corn oil, safflower oil, beef tallow or lard had increased incidence of colon tumours.
"The links between dietary fats -their type and level of consumption- and impact on CHD and carcinogenesis are poorly understood. However, it appears that coconut oil may play a non-promotional role with regard to carcinogenesis...."
"Benefits of Feeding FatNeedless to say, read the whole thing. I regard the effects of a fat-based diet on human athletes as somewhat contradictory. Glycogen storage is reduced, according to some studies, but according to others, training in the fasted state increases glycogen stores. I was looking for information to shed some more light when I came across this article. I find it very interesting that a fat-based diet is a full-on benefit to glycogen management in horses. After all, in some respects, humans are better runners than horses...
"Fat supplemented diets for horses have proven to be beneficial beyond that mentioned above. Adding fats to the diets of growing and breeding horses has increased milk energy yield in lactating mares and increased growth rate in weanlings (Scott et al 1989; Davison et al 1991). However, the most exciting effects of feeding fat to horses have been observed in the equine athlete.
"The effect of fat supplementation on muscle glycogen storage and utilisation has been widely tested. Other areas - such as effect of extra dietary fat on thermoregulation, energy requirement, management of Equine Rhabdomyolysis, and other exercise parameters - have also been explored...."
Of course these folk are selling fat-based feeds for horses, so they have an axe to grind, but it seems consistent with what I've been reading about other species.
I'm beginning to come around to Gary Taubes' view... Easily-digested carbohydrates may be best avoided, regardless of what species you are.
"Should We Feed Fat to Horses?Sounds to me like these folks are on the right path.
"Current research indicates that fat supplementation is a viable energy source for performance horses which is of greater benefit than simply caloricifically. However, almost all studies investigating the effects of fat supplementation on horses have been relatively short term, involved a small number of horses and produce results which are not always highly repeatable. In order that more confidence be placed in the long term feeding of fats to horses, studies need to be of longer duration, involve more horses and more consistent results are necessary. The best type of fat for horses should also be ascertained...."