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Quick Summary Points

  • Isoleucine is a branched chain amino acid (BCAA) and by default also an essential amino acid (EAA)
  • Isoleucine cannot be made in the body and we must obtain it from food.
  • Isoleucine is important for protein synthesis and other anabolic and anti-catabolic processes in the body.
  • Isoleucine is best taken together with the other BCAAs or leucine and valine.

What is L-Isoleucine?

L-Isoleucine is one of the essential amino acids that cannot be made by the body and is known for its ability to help endurance and assist in the repair and rebuilding of muscle. This amino acid is important to body builders as it helps boost energy and helps the body recover from training. There are eight Essential Amino Acids in total, with the others being: L-Leucine, L-Lysine, L-Methionine, L-Phenylalaline, L-Threonine, L-Tryptophan and L-Valine. (L-Histidine and L-Arginine are often included as EAAs).1 L-Isoleucine is also classified as a branched-chain amino acid (BCAA). There are three branched-chain amino acids in the body, with the others being L-Valine, and L-Leucine, and all of them help promote muscle recovery after exercise. Isoleucine is actually broken down for energy within the muscle tissue.

What Does L-Isoleucine Do?

Isoleucine, like Valine is a BCAA, and they both serve as precursors for glutamine and alanine, used during intense weight training. It is used as fuel by muscle cells, sparing other amino acids from being burned up. Like the other BCAAs, Isoleucine plays an important role in protein synthesis, anabolism and anti-catabolism. Without adequate BCAAs, muscle cells won’t heal and grow. BCAAs such as this can be ingested as a BCAA supplement, or via a whey containing supplement.2

Ideal Ratio of L-Isoleucine to Other BCAAs

Isoleucine should always be taken together with the other two branched-chain amino acids, leucine and valine. Some believe that to ensure optimum absorption, you need two parts L-Leucine and two parts L-Valine for every one part L-Isoleucine. Others believe a ratio of 1:1.8:1.2, their ratio in the muscle, is more effective. Some essential amino acids are even available in supplement form in the 4:1:1 and 3:1:1 ratio. The jury is still out on this matter.

Who Needs L-Isoleucine?

As L-Isoleucine is an essential amino acid, it is required by everyone for health reasons.

Importance of Isoleucine

People that exercise a lot or have a low-protein diet should consider supplementation with BCAAs, of which isoleucine is one. BCAAs are crucial for muscle protein synthesis and decreased protein catabolism. When taken in the correct ratio of two parts leucine and two parts valine for every one part isoleucine, muscle anabolism and thus growth is enhanced. Short changing yourself of these important BCAAs will limit your bodybuilding success.

1 Essentials of Sports Nutrition and Supplements, p. 486
Sports Supplement Review, p. 170

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