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Dispensable amino acids in highest concentration in plasma and muscle tissues- include glutamine, glycine and alanine while the indispensable amino acids include lysine, threonine, BCAA (valine, leucine and isoleucine.


During endurance exercise, changes in protein turnover produce net protein breakdown with release of amino acids in to the free pools. The principal amino acids involved are BCAA, alanine and glutamine.

The dominant features of this interorgan exchange are BCAA movement from the liver to skeletal muscles, with the return of alanine to the liver and glutamine to the gut.

Interorgan exchange,

·   BCAA  move from liver to skeletal muscles

·   With the return of alanine to liver and

·   Glutamine to the gut.

This exchange is related to regulation of skeletal muscle protein synthesis.


The movement of these amino acids among tissues serves to provide

  • Substrate for gluconeogenis (ala)
  • Assist in elimination of nitrogenous waste
  • Maintain glutamine levels
  • Provide substrates for purine nucleotide cycle and
  • Maintain amino acid precursors for protein synthesis


During prolonged exercise, amino acid movement between the liver and gut and skeletal muscle increases. There is a fourfold increase of BCAA released from liver and gut (splanchnic area) and increased uptake by the exercisingmuscles. In return, muscles released alanine, which was removed by the liver for gluconeogenesis. This demonstrates a change in amino acid flux during endurance exercise.


Liver is primary site for degradation of most amino acids. Hepatic tissues contain high concentrations of aminotransferases and deaminases which initiate amino acid degradation by removal of the alpha amino group. Liver possesses very low BCAAT (BCAA transferase) activity which results in BCAA released into circulation. Extra hepatic tissues, kidney, skeletal muscles contain BCAAT and are responsible for initiating the degradative pathway. Among these tissues skeletal muscle appears to be predominant tissue for BCAA degradation.


BCAA catabolism is initiated by the reversible transamination of a BCAA to its corresponding alpha keto acid with transfer of the alpha amino group to alpha keto glutarate, forming glutamate. During periods of increased energy needs, such as starvation, trauma, and exercise increased BCAA concentrations stimulate BCKAD- branched chain keto acid dehydrogenase, and BCAA are oxidized for energy within the skeletal muscle. (During exercise BCKAD is significantly increased.)


While glutamate is formed de novo in skeletal muscle, there is no net release. Glutamate serves as an important intermediate in nitrogen metabolism. The amino nitrogen of glutamate can be transferred to pyruvate or oxaloacetate to form alanine and aspartate, respectively. Alanine is released by the muscle for transport to the liver whereas aspartate is an important component of the purine nucleotide cycle within the muscle. The purine nucleotide cycle serves to maintain muscle energy levels (regenerates the ATP pool) and produce free ammonia. Ammonia may also be generated by glutamate deamination catalyzed by glutamate dehydrogenase, GDH.


The ammonia produced via the purine nucleotide cycle or GDH can combine with glutamate in an ATP dependent reaction catalyzed by glutamine synthase to form glutamine. Glutamine is ultimately released from muscle, with the majority being used by the gut as a primary energy source. Together, alanine and glutamine represent 60-80% of the amino acid released from skeletal muscle while they account for only 18% of the amino acid in muscle protein.


Carbohydrate intake before and during exercise appears to influence plasma BCAA changes. Subjects with depleted glycogen stores at the start of exercise derive greater percentage of their fuel requirements for exercise from amino acids compared to glycogen loaded subjects. Plasma BCAA concentrations also decrease when CHO is consumed during exercise.


Exercise duration can also affect plasma BCAA responses. Short duration endurance exercise (less than 45 minutes) does not appear to alter BCAA levels regardless of the intensity, whereas prolonged exercise (>3 hours) apparently decreases BCAA concentrations.



Mood, performance and sense of wellbeing are all associated with the levels of substances in the brain called as neurotransmitters. A decrease in plasma BCAA concentrations during exercise may negatively affect exercise performance via changes in serotonin. Serotonin is synthesized in the brain from the amino acid tryptophan. Since tryptophan competes with BCAA and the other large neutral amino acids- phenyl alanine, tyrosine and methionine for brain uptake declining plasma BCAA concentrations during exercise would enhance brain tryptophan uptake. Hence, brain serotonin levels would increase, and in turn, compromise exercise performance via a sedative effect on the central nervous system. This is commonly referred to as “central fatigue hypothesis”. As carbohydrate stores become limited during exercise, plasma BCAA concentrations decline. Therefore CHO intake during exercise lowers BCAA concentrations possibly by stimulating muscle BCAA uptake by increases in insulin concentrations.  Such declines in plasma BCAA are hypothesized to play a role in fatigue by favoring brain uptake of tryptophan, the precursor for serotonin synthesis.



Exercise induced changes in amino acid metabolism suggest an increased need for dietary amino acids (that is increased protein requirements). During exercise these changes include a depression of muscle and whole protein synthesis rate and elevations in protein degradations leading to a net increase in amino acid use.

During recovery after exercise, muscle protein synthesis recovers to pre-exercise or non-exercised levels allowing the muscle to shift to an anabolic state for recovery. If anabolic period leads to increased muscle mass, some increase in dietary protein to support this growth. Current recommendations 0.8g/kg/day. Based on urea production, 3MH excretion indicate both strength and endurance exercise increase amino acid utilization. Also N2 balance studies indicate protein requirements of strength and endurance athletes >current RDA.


Urea production

The major end products of amino acid degradation are CO2 and urea. Urea formed in the liver via urea cycle is the major route of N2 removal from body. After formation urea released into blood and removed by kidney and sweat glands. Urea formation indicates amino acid degradation and is dependent on exercise intensity >intensity> urea production. Studies have shown that extra cellular levels of urea nitrogen increases during prolonged exercise and urinary output of urea increases during rest period immediately after exercise. Studies indicate significant contribution by protein to energy during exercise. During recovery after prolonged exercise the rate of protein synthesis increases.

3Methyl Histidine (3MH)   excretion

3 MH is an index of contractile protein degradation which is formed by specific Histidine residues in muscle contractile protein. Studies have shown that endurance exercise decreases 3MH excretion while repeated bouts of weight lifting exercise tend to increase 3MH excretion. Muscle growth is associated with higher rates of protein turnover (degradation and synthesis). If muscle protein degradation is elevated during resistance training program, skeletal muscle protein synthesis rate must increase to support muscle repair, growth and function. Weight lifters increase muscle protein synthesis during 24 hours following exercise.

leucine oxidation

Metabolic tracer studies have shown that endurance exercise causes rise in leucine oxidation. The carbon skeleton of BCAA leucine is used for production of CO2 during exercise at a rate that is related to work output. Use of leucine is less when the amount of glycogen available in the muscle is high. Increased oxidation of fat as a result of increased availability of free fatty acids or decreased use of CHO during prolonged exercise apparently increases oxidation of leucine possibly by direct stimulation of oxidative decarboxylation of keto acids formed,



According to ACSM, ADA and Dieticians of Canada (2000) protein requirements of endurance athletes are 1.2g/kg BW/DAY whereas resistance and strength trainers it is 1.6-1.7 g/kg BW /day.

Following endurance bouts urea excretion is increased Urea excretion and leucine oxidation increase at protein intakes of 2.0 g/kg/d
3 MH excretion is unchanged or increases after 12-36 hours and muscle protein synthesis is depressed 3 MH excretion and muscle protein synthesis rates increase. Protein turnover is elevated in strength athletes.
Amino acids are oxidized for energy Amino acids are not oxidized for energy
1.2-1.5 g/kg BW /d 1.4-1.8 g BW /kg/d


Protein requirements for strength sports

There is now sufficient data that protein needs with resistance exercise is divided into 2 areas-

  1. The need for maintenance (minimum protein required for nitrogen equilibrium)
  2. The need for increase in lean tissue (positive nitrogen balance)

At a cellular level, an increased requirement for protein in strength –trained athletes arises due to

  1. Extra protein required to support muscle protein accretion through elevated protein synthesis.
  2. Increased catabolic loss of amino acids associated with strength training activities.

Nitrogen balance studies has shown modest increase in requirements, while others have shown that the anabolic stimulus for protein synthesis actually increase efficiency of use of protein which decreases dietary requirements. Strength trained athletes habitually consume protein intakes higher than required. A positive nitrogen balance is required for anabolism. Therefore extra protein over and above normal is required for nitrogen balance. Inadequate protein intake can result in negative nitrogen balance, which can increase protein metabolism and lead to muscle wasting, training intolerance, and retarded recovery. Measurements of protein synthesis, leucine oxidation and nitrogen balance studies indicate that strength/ power athletes would benefit from protein intakes of 1.4-1.8g/kg BW/d but 2.0 g/kg/d is unnecessary.


CONSUMING more protein than the body can use is unnecessary and should be avoided.

  1. Compromise on CHO status-unable to train and compete at peak levels.
  2. Protein foods are high in fats therefore excess protein difficult to maintain low fat diet.
  3. Hypercalciureric effect of high protein diets affects calcium balance.


Protein requirements for endurance

Physical activity and exercise training are generally assumed to result in maintenance or enlargement of skeletal muscle. However during endurance exercise protein synthesis is suppressed. After exhaustive exercise urea excretion increases by 31% during first 12 hours; urinary 3MH increases after exercise (until 12-36 hours). Thus exhaustive exercise produces a catabolic condition in skeletal muscle by suppressing protein synthesis and stimulating protein degradation. Therefore prolonged exercise produces catabolic state. While acute effect of exercise on protein turnover is catabolic, the long term effect is not muscle hypertrophy. Routine exercise produces maintenance or hypertrophy of muscle mass. Recovery occurs through stimulation of protein synthesis.



ENDURANCE ATHLETES: Decrease in time required to complete an activity orAmount of time a particular activity can be maintained.

STRENGHT ATHLETES: OBVIOUSLY increases in muscle strength which constitutes enhanced performance. For body builders there is increase in lean body mass.





How Does Type, Timing And Amount Of Protein Affect Muscle Hypertrophy?

RESISTANCE training and diet consistently appear to play a role in post workout muscle (PWO) protein synthesis. Many studies support the fact that supplementation of free form amino acids or intact forms of protein can enhance training adaptations when combined with resistance training. Studies indicate-

2005 study-resistance trainers Protein supplements for 14 weeks Increase in type 1 and 2 muscle fibers
Post exercise trial Mixture of CHO & WHEY protein consumed 1 hour after exercise More immediate and overall greater protein synthesis;

Whereas addition of free amino acids caused increase in protein synthesis and balance

Tipton (2004) 20g whey and 20g casein protein with one hour of resistance training Effective in producing protein balance
  25 g whey and casein powder solution When consumed before

Increase in GH testosterone

FFA and Serum insulin

Consumed AFTER

Increased in post exercise O2 consumption

More anabolic environment

  6 g essential amino acid

6 g essential amino acid plus

6 g non essential amino acid

12 g6 g essential amino acid

17.5 g whey protein

20 g casein protein

20 g whey

40 g mixed amino acids

40g essential amino acid

All had similar increase in protein synthesis and balance


Therefore athletes interested in muscle hypertrophy neither-type and amount of protein matters but if protein is within RDA -1.2-2G/KG/DAY for resistance training.