The Science Behind Building Muscle

Muscle and muscle proteins

Muscle is one of the most adaptable tissues within the body and it has a tremendous capacity to be able to grow, change and repair unlike some other tissues such as tendon.  As a result of training muscle fibres (which are actually individual muscle cells) may increase or decrease in number, size, speed at which they contract and their ability to generate energy for muscle contraction either with oxygen (aerobically) or anaerobically. Like most tissues within the body, muscle is made up predominantly of a combination of proteins, carbohydrates, fats and minerals.  These make up both the structural framework of the muscle, such as connective tissues and the cell membrane, energy stores and the enzymes and other compounds involved in cell metabolism. However, within the muscle the key to the process of contraction are the two proteins actin and myosin which together make up around 50% of the total protein content of muscle. Within the muscle the “chains” or filaments of actin and myosin are all lined up in the same direction – the direction in which the muscle contracts. When energy in the form of ATP is applied the myosin actin filaments  slide in opposite directions and this action in thousands of individual muscle cells leads to the muscle shortening in length i.e. contracting.


Training induced muscle damage and muscle growth

Any form of physical training, whether for endurance/stamina, speed, strength leads to some damage to muscle and this must be repaired. In addition, there is also formation of new muscle structures as a result of training. And why do we train? Because training results in improved performance and a lower risk of injury and most of the response to training is as a result of muscular adaptation. If you are thinking about cardiovascular/aerobic fitness then remember that the heart is also a muscle!


Muscle adaptation can’t be achieved through nutrition alone

The key to getting muscle to adapt through training and to repair and lay down new muscle is exercise itself. There is no way to get muscle to adapt without physical exercise. However, enhancing the response of muscle to training through nutrition has been a theme for over 50 years. Let’s get perhaps the obvious question out of the way first. If actin and myosin are the key proteins for muscle contraction, why not just feed these in a purified form? Unfortunately, like all proteins, actin and myosin cannot be absorbed from the digestive tract and must first be broken down into their constituent amino acids; often referred to as the building blocks of protein. There are 21 different amino acids and different proteins contain different combinations.


Proteins and amino acids

So yes, we can feed protein or amino acids but the proteins will be digested into their amino acids. It doesn’t matter whether the amino acid comes from oats, fish meal, alfalfa, soya, algae or carrots. The amino acid in each source is the same amino acid. What may vary is how much is present in one food source compared with another. There is nothing different to the lysine that is present in oats to the lysine that is present in for example, blue-green algae such as Spirulina. They are the same. In fact the proportions of the different amino acids in Spirulina is very similar to that of forages or cereals such as oats and the US National Library of Medicine also makes the following comment:  “You may have been told that blue-green algae [Spirulina] are an excellent source of protein. But, in reality, blue-green algae is no better than meat or milk as a protein source and costs about 30 times as much per gram”. 1 The key to getting an effect is not what you feed but how much of the right protein or amino acids (see below).


High protein diets – where it all started!

If we look back at the history of nutrition for muscle development in sport, we started with the concept that athletes should consume a high protein diet, for example, eggs, cheese, fish, meat, nuts, seeds, milk. Over time this developed into the concept of consuming concentrated sources of protein, such as whey protein (from the process of making cheese from milk), casein protein (from milk, soy protein, egg white protein and rice protein. Using concentrated sources of protein meant that athletes did not need to eat such large volumes of food. In addition, the concept of timing of intake developed, with recognition that taking large volumes of concentrated protein soon after exercise increased the muscle training response.


From high protein supplements to BCAA

A further development has been that research has shown that response to protein intake in training is not down to all the amino acids but three in particular; leucine, isoleucine and valine. Together these are known as branched chain amino acids (BCAA) and make up around 1/3rd of the amino acid content of muscle. One difference between BCAA and the other amino acids is that they are metabolised within the muscle itself and have been shown to have an anabolic (tissue building) properties2. Even more recent research however, has provided evidence that it is not all the BCAA but one in particular, leucine and its breakdown product HMB (hydroxyl-methyl butyrate), that are responsible for stimulating muscle adaptive responses to exercise when taken in the 1h window after exercise.


Lysine and Leucine – the key amino acids!

When it comes to the amino acid composition of muscle, Leucine and Lysine are the two amino acids present in the highest amount; accounting for 17% each of the total muscle amino acid content. Lysine is also considered to be the amino acid that if not present in sufficient amounts in the diet will limit protein synthesis (e.g. new muscle growth)1,2,3,4,5,6. Not surprisingly, the amino acids present in the highest concentration in mares milk which supplies the foal with all its requirements at the fastest growth stage of its life (up to ~6 months) are lysine and leucine!


Lysine -  considered to be the amino acid that if not present in sufficient amounts in the diet will limit protein synthesis (e.g. new muscle growth).


Leucine - BCAA make up around one third of the protein in muscle. Leucine, isoleucine and valine are three of the nine essential amino acids because they must be obtained from the diet as they cannot be synthesised from other components in the diet. In human medicine and sport there is an increasing number of scientific studies which suggest that of the three BCAA, leucine may play the most important role in stimulating protein synthesis15.  In human sport, BCAA have been shown to aid in recovery processes from exercise such as stimulating protein synthesis, aid glycogen replacement, delaying the onset of fatigue and helping maintain mental function in aerobic-based exercise (see review 9). In horses, one study has shown a reduction in blood lactate after exercise. Although two studies which fed either 22.5g, 1h before exercise10 or 33g three times a week11 both failed to show any benefit, neither of these studies followed the current recommendations for human athletes of 90mg BCAA/kg/day of BCAA, equivalent to 45g per day for a 500kg horse, fed within 1h after exercise! 


HMB (B-hydroxy B-methylbutyrate): HMB is a metabolite of the amino acid leucine. That is if leucine is consumed in food, it will be “processed” in the intestinal tract, muscle and finally liver to release HMB. HMB is present in small amounts in some feeds that horses consume naturally, such as alfalfa. HMB is thought to work by speeding up the process by which muscle is able to repair and regenerate itself following exercise. Some of the effects of HMB that have been clearly established in scientific studies in human subjects include increasing muscle strength and power and speeding recovery from exercise by reducing muscle damage. In horses, feeding 10-15 g/day HMB for 6-32 weeks has been reported to improve endurance, reduce muscle damage, aid maintenance of bodyweight, increase red blood cell number and win rate in racing12, 13, 14.


So why feed a supplement?

Why not just feed hay or cereals if these contain the right amino acids. The first issue is that to achieve the required intake for leucine and lysine to have an effect on muscle after exercise would require feeding ~4kg of oats as soon as possible after exercise. Of course this would also have to be without hay and so could represent a big risk for colic and or laminitis. If you were to try and achieve this with forage, then your horse would need to consume between 5 and 10kg of hay within 1h of finishing exercise.



When it comes to helping muscles adapt more effectively to training we have moved from high protein diets to concentrated protein supplements to specific amino acid supplements. These achieve much higher intakes of the specific amino acids that have been shown to influence muscle responses to training. Remember, an amino acid is an amino acid irrespective of whether it was digested from oats, hay, soya, spirulina or fish. It’s how much you feed and when that counts!



  1. Accessed 27/06/2015

  2. Karlsson HK, Nilsson PA, Nilsson J, Chibalin AV, Zierath JR, Blomstrand E. Branched-chain amino acids increase p70S6k phosphorylation in human skeletal muscle after resistance exercise. Am J Physiol Endocrinol Metab. 2004 Jul;287(1):E1-7.

  3. Breuer, L.H., Kasten, L.H. and Word, J.D. (1970) Protein and amino acid utilization in the young horse. Proceedings of the 2nd Equine Nutrition Research Symposium, 16-17.

  4. Hintz, H.F., Schryver, H.F. and Lowe, J.E. (1971) Comparison of a blend of milk products and linseed meal as protein supplements for young growing horses. J. Anim. Sci. 33, 1274-1277.

  5. Potter, G.D. and Huchton, J.D. (1975) Growth of yearling horses fed different sources of protein with supplemental lysine. Proceedings of the 4th Equine Nutrition and Physiology Symposium. 19-20.

  6. Ott, E.A., Asquith, R.L. and Feaster, J.P. et al (1979) Influence of protein level and quality on the growth and development of yearling foals. J Anim Sci, 49, 620-628.

  7. Graham PM, Ott EA, Brendemuhl JH and TenBroeck SH. The effect of supplemental lysine and threonine on growth and development of yearling horses. J Anim Sci. 1994 Feb;72(2):380-6.

  8. Graham-Thiers PM and Kronfeld DS. Amino acid supplementation improves muscle mass in aged and young horses. J Anim Sci. 2005 Dec;83(12):2783-8.

  9. Campbell, B., Kreider, R.B., Ziegenfuss, T., La Bounty, P., Roberts, M.,  Burke, D., Landis, J., Lopez, H. and Antonio, J. (2007) International Society of Sports Nutrition position stand: protein and exercise. J. Int. Soc. Sports Nutr. 4, 1-8.

  10. Stefanon, B., Bettini, P. and Guggia, P. (2000). Administration of branched-chain amino acids to Standardbred horses in training. Journal of Equine Veterinary Science 20: 115–119.

  11. Casini, L., Gatta, L. and Magni, B. (2000) Effect of prolonged branched-chain amino acid supplementation on metabolic response to anaerobic exercise in Standardbreds. Journal of Equine Veterinary Science 20: 120–123.

  12. Nissen, S., Fuller, J. and Rathmacher, J. (1997). ß-hydroxy ß-methylbutyrate (HMB) supplementation in training horses. Metabolic Technologies Bulletin, Ames, Iowa.

  13. Miller, P. and Fuller, J.C. (1998). The effects of supplemental ß-hydroxy-ß-methylbutyrate (HMB) on training and racing Thoroughbreds. Abstract from the 17th Annual Meeting AESM, Leesburg, VA, p.13.         

  14. Ostaszewski, P., Kowalska, A., Szarska, E., Szpotański, P., Cywinska, A. Bałasińska, B. and Sadkowski, T. (2012)  Effects of β-Hydroxy-β-Methylbutyrate and γ-Oryzanol on Blood Biochemical Markers in Exercising Thoroughbred Race Horses. J. Equine Vet. Sci. 32(9), 542-551.