Why Fat-Enhanced, Lower Starch Diets Help Maximize Athletic Performance


  1. Definition of Fitness
  2. Types of Exercise
  3. Changes in Nutrient Requirements
  4. Physiological Changes Due to Training (Conditioning)
  5. Impact of Calories Source on Performance
  6. Nutritional Management of “Tying-Up” Syndromes
    1. P.S.S.M .– polysaccharide storage myopathy
    2. R.E.R. – recurrent exertional rhabdomyolysis
  7. Electolytes
  8. Summary

There are a number of ways to define physical fitness. Below are (3) possibilities:

  1. Increased capacity to perform work while increasing the efficiency that work is performed.
  2. Smaller disturbance in homeostasis (i.e. heart rate, blood parameters) during work.
  3. Faster return to normal resting state after performing work.

A horse becomes more fit due to physical and biochemical changes elicited by exercise over time.  This paper will review some of these changes and how horse diets can maximize the benefits of the biochemical responses to short and long-term exercise.


There are two main types of exercise, with a great range in between. The first is maximal exercise, which is; work of high-intensity for a short period of time, such as sprint racing. The main substrate used to supply energy in maximal exercise is non-structural carbohydrates (NSC = starch + sugar).  The second type of exercise is submaximal exercise, which is; work of low-intensity for a long period of time, such as endurance racing.  The main energy substrate for submaximal exercise is fat. Obviously, there a many types of exercise that fit between maximal and submaximal, like; dressage, jumping, longer races, etc.


In a fit horse, the preferred energy substrate is fat.  The preferred energy substrate would depend on how close to maximal or submaximal of the work performed.  The energy substrate preference changes as the horse becomes more fit, allowing a greater portion of the exercise bout to utilize fat, sparing CHO and lessening the production of lactic acid, the end-product of CHO metabolism in the muscle. The end-products of fat metabolism are CO2 and water – Much less disturbing to acid/base balance.

NRC 2007 - Types of Exercise​

Exercise Category Mean Heart Rate (bpm) Description Types of Events
Light 80 1-3 hours/week; 40% walk, 50% trot, 10% canter Recreational riding, beginning of training, show horses (occasional)
Moderate 90 3-5 hours/week; 30% walk, 55% trot, 10% canter, 5% low jumping, cutting, other skill work School horses, recreational riding, beginning of training/breaking, show horses (frequent), polo, ranch work
Heavy 110 4-5 hours/week; 20% walk, 50% trot, 15% canter, 15% gallop, jumping, other skill work Ranch work, polo, show horses (frequent, strenuous events), low-medium level eventing, race training (middle stages)
Very Heavy 110-150 Various; from 1 hour/week of speed work to 6-12 hours/week slow work Racing, endurance, elite 3-day eventing

The main nutrient that increases with increasing exercise is calories.With the exception of electrolytes, primarily sodium and chloride, possibly potassium, the requirements for all other nutrients are satisfied when the horse is fed to maintain desired body condition.

The chart above shows the percent increase over maintenance for critical nutrients for the working horse. Maintenance level is the 100% line (red) and the Digestible energy (D.E.) increase with very heavy work is represented by the yellow dashed line.


Over many weeks of training, more permanent changes in physical and biochemical characteristics of the horse develop.


  • Increased packed cell volume (PCV) and increased blood volume.
  • Possible increase in peripheral O2 utilization.
  • Increased work capacity at lower blood lactic acid concentration.
  • Increased tolerance of lactic acid.
  • Increased free-fatty acid utilization (sparing glucose).


  • Increased resting glycogen concentration.
  • Increased concentration of many enzymes used in energy metabolism.
  • Increased mitochondrial density.
  • Increased proportion of Type IIA fibers in locomotor muscles (may depend on age)
  • Increased capillary density.
  • Increased oxidative capacity (may depend on age).
  • Increased mobilization and utilization of free-fatty acids (especially in submaximal exercise).
  • Increased submaximal work capacity.

Effect of High Starch/Sugar Content in Small Intestine:

Absorption of simple sugars from dietary NSC across the small intestinal wall causes an increase in plasma glucose, which causes an increase in plasma insulin.   ­ Also, there is a small population of microorganisms in horse’s stomach – rapid breakdown of NSC will decrease pH and contribute to ulcers

The elevated insulin level results in:

  • facilitation of glucose and amino acid absorption into cells
  •  increase in glycolysis - end product is lactic acid
  •  increased conversion of glucose to fat           
  •  decrease in lipolysis - but, in fit horse fat is preferred energy source
  •  alters serotonin levels in brain potentially causing hyperactivity and waste of energy that could be used for work

Effect of High Starch/Sugar Content in the Large Intestine:

At high feeding rates, by volume and/or concentration of NSC, excess starch/sugar can bypass small intestine’s capacity to digest and absorb the NSC and it passes into the cecum/colon, resulting in:

  •  rapid fermentation by microbes (structural CHO slowly fermented), producing large amounts of gas
  • favorable environment for the production of Lactobacillus, which produces lactate that is poorly absorbed across the cecum and colon walls causing a reduction in pH.
  • Effects of lower hindgut pH :
    • Grazing horses – normal pH = 6.4 – 6.7
    •  Cecal pH of 6.0 ~ sub-clinical acidosis
    •  pH < 6.0 associated with:
      •  osmotic diarrhea
      •  overgrowth of undesirable bacteria
      •  lysis of beneficial bacteria
      •  Risk of endotoxemia and laminitis

All resulting in some level of discomfort to the performing horse.

Summary of Physiological Changes due to Exercise

Natural response to training shifts from carbohydrate to fat as preferred energy source. Thus:

  • A higher portion of workout/race run uses fat. 
  • Increased glycogen stores in muscle. 
  • Increased time to production of lactic acid.
  • Increased tolerance of lactic acid.
  1. Nutritional Suggestions For Equine Polysaccharide Storage Myopathy (PSSM):

PSSM (polysaccharide storage myopathy; also called EPSM –equine polysaccharide storage myopathy) is a metabolic disorder of skeletal muscle characterized by increased glycogen content, increased glycogen-related enzymes and abnormal amylase (the enzyme which digests starch)-resistant polysaccharide particles.
Glycogen is the storage form of carbohydrates in the muscle and is a major energy source for muscle contractions in the normal horse.  A PSSM horse cannot properly metabolize glycogen, which results in the buildup of glycogen in the muscle. As a result, the horse experiences muscle weakness and cramping.  Outward clinical signs include muscle stiffness and pain, shifting lameness and a “camped-out” stance. PSSM has been documented in several horse breeds, and can be confirmed by biochemical analysis of a muscle biopsy. 
Two forms of PSSM are now recognized according to Dr. Stephanie Valberg of the College of Veterinary Medicine, University of Minnesota. Dr. Valberg has done controlled studies on horses with PSSM.

  • TYPE 1 – due to a mutation in the gene controlling glycogen breakdown. Type 1 PSSM has been identified in at least 20 different breeds and can be diagnosed with a genetic test.
  • TYPE 2 – shown by excessive glycogen storage in the muscle but do not have the genetic mutation. TYPE 2 PSSM accounts for an estimated 25% of PSSM cases in Quarter Horse-related breeds and 80% of PSSM cases in many Warmblood breeds.

Non-Medical Treatment

  • General - Rest and turnout. Regular daily exercise.
  • Nutritional Management.

The suggested nutritional approach is to:

  1. minimize the intake of SUGAR and STARCH (collectively termed as “NSC” – non-structural carbohydrates) to less than 20% of the CALORIES in the TOTAL diet, hay and concentrate combined. NOTE – this suggestion refers to 20% of the CALORIES in the horse’s diet, not 20% of the feed. The calculation is a bit more complicated than merely using a 20% NSC program.
  2. Any additional calories required by the horse to maintain adequate body condition (Body Condition Score 5; simply where the horse’s ribs can be easily felt but not seen) should be supplied by FAT or highly digestible fiber.
    1. There is an opinion out there that EPSM horses need 1 pound of fat per day. There is no controlled studies to support this concept and can result in extra weight gain on horses that are already overweight.
    2. Dr. Valberg has suggested a management approach to afford the working muscle fatty acids as a calories source without contributing to obesity by adding extra fat to the diet. The suggestion is until horses are of acceptable weight, fat metabolism can be enhanced by riding horses after a 5-8 hour fast as a means to elevate plasma free fatty acids. Waiting 5-8 hours after eating allows plasma insulin levels to return to normal. Allowing fat to be utilized during exercise – remember one of the actions of insulin is to inhibit lipolysis and free fatty acids in the bloodstream.
    3. For horses that are underweight, adding additional calories in the form of fat (and digestible fiber) is appropriate, but it may not be necessary to hit a target of 1 pound of fat….just supply the extra calories at a level to attain or maintain a body condition score of 5, as described above.
  3. provide at least 1,000 IU of vitamin E per 
  4. provide 1-2 mg. of selenium per day. 


To minimize the effects of PSSM, the energy for muscle function must come from sources other than traditional high-starch grains and molasses. Calories can effectively be supplied from fat highly digestible fiber sources.
Dietary fat can come from a number of sources, typically of vegetable origin, as animal fats tend not to be palatable to horses. Soybean oil, linseed (flaxseed) oil, palm oil and coconut oil are commonly used, though newer sources of omega-3 fatty acids such as descented and flavored menhaden oil are becoming popular.
Highly digestible fiber can come from very soft, high-quality forage (that is low in sugar and starch – typically under 15%), unmolassed beet pulp, soy hulls and dehydrated alfalfa meal.

Dietary fat can come from a number of sources, typically of vegetable origin, as animal fats tend not to be palatable to horses. Soybean oil, linseed (flaxseed) oil, palm oil 
and coconut oil are commonly used, though newer sources of omega-3 fatty acids such as descented and flavored menhaden oil are becoming popular.

In many cases, pasture should be minimized, if not eliminated, to control NSC intake.

  1. Nutritional suggestions for exertional Rhabdomyolysis:

Sporadic exertional rhabdomyolysis can occur when a horse is:

  • exercised beyond the horse’s current fitness  level .
  • fed full rations on their day off from training
  • exposed to respiratory diseases

Recurrent exertional rhabdomyolysis (RER) is an inherited, intermittent, stress-induced defect in the regulation of muscle contraction.

Occurs in 5-10% of Thoroughbred race horses and less commonly in other disciplines. RER is especially common in nervous fillies. Stress, diet, season, estrous cycle and lameness also play a role in RER occurrence.


  1. Decrease NSC intake – high NSC may increase nervousness in the horse, exacerbating RER
    1. reduce % NSC in concentrate (lower corn, molasses)
    2. reduce AMOUNT of concentrate fed per day and per meal
  2. Provide needed calories with fat and digestible fiber – following recommendations for PSSM above will help with RER horses
  3. Electrolyte supplementation may help

Causes of fatigue are poorly understood and are very complex. A simple explanation is that during maximal effort, fatigue is related to waste product accumulation, especially lactic acid - end-product of CHO metabolism.

During submaximal exercise, fatigue appears to be related to substrate depletion, most likely glucose or glycogen.

The physiological response to training helps to delay the onset of fatigue in many ways:

  1. Glycogen Loading: Increases the muscle stores of glycogen. This can be accomplished by feeding a fat-enhanced diet, as will be discussed later. Early efforts to glycogen load horses as is done on humans were largely unsuccessful.
  2. Fiber-type Recruitment: Spares glucose and glycogen. Research suggests that intermediate muscle fiber types are recruited to become either Type I (slow twitch) fibers or Type IIA (fast-twitch oxidative) fibers, which utilize fat as an energy substrate much better than Type IIB (fast-twitch, glycolytic) fibers. This will reduce the production of lactic acid, as fat becomes the preferred energy substrate. This recruitment may depend upon the age of the horse and the type of conditioning (maximal or submaximal) used in the training program.
  3. Enhances Waste Product Removal: The horse’s body appears to become much more efficient in clearing lactic acid from the muscles and CO2 from the blood over time.
  4. Improves Waste Product Tolerance: The horse can tolerate higher levels of lactic acid and CO2 as he becomes more fit. The cumulative benefit of improved waste product removal and waste product tolerance is important to delaying fatigue.

SUMMARY – How do we manage nutrition for the equine athlete?

  • As a horse becomes more fit, the muscles prefer fat as the energy substrate
  • Providing more fat (and digestible fiber) in the working horse’s diet can help maximize his performance and minimize issues such as hyperactivity, ulcers and tying-up.
  • Supply up to 15-20% of needed calories with fat during training and events.
  • Decrease starch in diet (less corn, molasses etc.).  Increase digestible fiber in diet (excellent quality grass hay, dried beet pulp). 
  • Control feed intake – feed more often during the day; make good grass hay available at all times.
  • Supply electrolytes when horse is sweating
  • Use professionally designed diet.  

Burke DJ, Albert WW. Methods for measuring physical condition and energy expenditure in horses. J Anim Sci. 1978 Jun;46(6):1666-72.
Burke DJ, Albert WW.
Comparative Training Effects of the Walker, Treadmill and Riding on Quarter Horse Mares. Proc. Sixth ENPS Symposium, April, 1979.
Burke DJ, Albert WW. And Harrison PC. 
Effect of Training, Racing and Diet on Heart Rate, Respiratory Quotient, Energy  Expenditure Rate and Muscle Lactic Acid Production in Horses. Proc. Seventh ENPS Symposium, April, 1981.
Hambleton PL, Slade LM, Hamar DW, Kienholz EW, Lewis LD.Dietary fat and exercise conditioning effect on metabolic parameters in the horse. J Anim Sci. 1980 Dec;51(6):1330-9.
Hinchcliff, K.W., et al, 2004. Equine Sports Medicine and Surgery: Basic and Clinical Sciences of the Equine Athlete, Saunders, St. Louis, Mo.
Hoffman RM, Boston RC, Stefanovski D, Kronfeld DS, Harris PA. 
Obesity and diet affect glucose dynamics and insulin sensitivity in Thoroughbred geldings.
J Anim Sci. 2003 Sep;81(9):2333-42.
Kronfeld DS. Dietary fat affects heat production and other variables of equine performance, under hot and humid conditions. Equine Vet J Suppl. 1996 Jul;(22):24-34.
McKenzie EC, Valberg SJ, Godden SM, Pagan JD, MacLeay JM, Geor RJ, Carlson GP. 
Effect of dietary starch, fat, and bicarbonate content on exercise responses and serum creatine kinase activity in equine recurrent exertional rhabdomyolysis. J Vet Intern Med. 2003 Sep-Oct;17(5):693-701.
Pagan JD, Geor RJ, Harris PA, Hoekstra K, Gardner S, Hudson C, Prince A. 
Effects of fat adaptation on glucose kinetics and substrate oxidation during low-intensity exercise.
Equine Vet J Suppl. 2002 Sep;(34):33-8.
Potter GD, Hughes SL, Julen TR, et al. A review of research on digestion and utilization of fat by the equine. Pferdeheilkunde 1992; 1:119-123.
Sloet van Oldruitenborgh-Oosterbaan MM, Annee MP, Verdegaal EJ, Lemmens AG, Beynen AC. 
Exercise- and metabolism-associated blood variables in Standardbreds fed either a low- or a high-fat diet.
Equine Vet J Suppl. 2002 Sep;(34):29-32.
Valentine BA, Hintz HF, Freels KM, Reynolds AJ, Thompson KN. Dietary control of exertional rhabdomyolysis in horses. J Am Vet Med Assoc. 1998 May 15;212(10):1588-93.
Zeyner A, Bessert J, Gropp JM. Effect of feeding exercised horses on high-starch or high-fat diets for 390 days. Equine Vet J Suppl. 2002 Sep;(34):50-7.
Prevalences and clinical signs of polysaccharide storage myopathy and shivers in Belgian Draft Horses. Anna M. Firshman, BVSc, PhD; John D. Baird, BVSc, PhD; Stephanie J. Valberg, DVM, PhD JAVMA, Vol 227, No. 12, December 15, 2005
The Management of Tying-Up in Sport Horses: Challenges and Successes Stephanie J. Valberg University of Minnesota, St. Paul, Minnesota; Proceedings of the 2010 Kentucky Equine Research Nutrition Conference

D.J. Burke, Ph.D.