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The main fuel sources of
energy during exercise are known to be carbohydrates and
fats. In recent years, the role of protein metabolism has
steadily been gaining more attention in the literature. Today it
is known that protein requirements are slightly increased in highly
active individuals. However, several questions still remain as to
a) how much protein does an active person should consume? b) what
are some possible side effects caused by a high dietary protein
intake? c) Does the body metabolize and/or absorb the extra intake
of protein? d) Is there a limit to how much protein the body can
metabolize?
Proteins are made of building
blocks known as “amino acids”. There
are 20 different amino acids in proteins and a few non-protein amino
acids. The body maintains a “pool” of protein and free
amino acids. Skeletal muscle holds the largest pool of free amino
acids in the body. A continuous exchange of amino acids occurs between
these pools as proteins are constantly being synthesized and simultaneously
being degraded (protein turnover). Normally these processes are
in equilibrium such that the breakdown of body protein is replaced
by protein synthesis that uses amino acids from the free pool. These
amino acids are provided from the digestion of foods that are high
in protein (i.e., meat, fish, milk, cheese). Insufficient
intake of protein may cause an imbalance in the amino acid
free pool. Over time this may lead to loss of muscle mass.
In contrast, excessive intakes of protein are converted
to carbohydrate or fat and later stored. Some of the many
functions of protein include the synthesis of new tissue (i.e.,
muscle and skin), synthesis of enzymes, synthesis of plasma proteins
(i.e., albumin, globulin), synthesis of hormones (i.e., epinephrine
and norepinephine) and serve as a minor source of energy (8).
It has been suggested that protein requirements
are slightly increased for both endurance and strength-trained athletes.
The question still remains to whether a higher intake of protein
(above and beyond the recommended values) can bring about further
benefits to the athlete. Thus, the general consensus is that strength
athletes can increase muscle growth with supplemental protein relative
to similar subjects who trained without supplement. However, there
is a limit to where supplementation will
enhance performance. It has been suggested that protein intake at
around 1.4 grams per kilogram of body weight/day appears to reach
a plateau. Thus, there is no need of extra protein or amino
acid supplements, provided that the individual is consuming a diet
containing adequate energy to maintain body weight (1;
3a; 3b; 3c; 7).
Endurance exercise also appears to alter protein
metabolism. Currently, protein recommendations for endurance athletes
(i.e., 2 to 3 hours per day of training) are 1.2 to 1.4grams/kg
body weight/day. As with the strength athletes, there is
no need for further protein supplementation (above and beyond the
recommended levels), provided the individual is consuming a balance
diet (3b; 3c; 6; 7).
Possible side effects to increased protein intake
may include the following but are not limited to: a) an increased
excretion of calcium in the urine which long term can lead to bone
resorption and potentially osteoporosis; b) excessive intake of
single amino acids can be toxic, as large pools of amino acids are
not tolerated; c) an increased stress on the kidney, and finally
d) increased metabolic imbalances and altered neurotransmitter activity
(1; 2; 3b; 3c).
In summary, one cannot deny the importance of
proper protein intake for the highly active individual. Current
research indicates that as long as energy intake is adequate a daily
protein intake of 1.0 to 1.4 grams/kg of body weight may be adequate
for those training vigorously. Thus, there is no need for
further supplementation above and beyond the recommended values
(i.e., most adults require only 0.8 grams of protein per kg of body
weight per day).
1. Garlick,P., McNurlan,M. & Patlak, C.(1999). Adaptation of
protein metabolism in relation to limits to high dietary protein
intake. European Journal of Clinical Nutrition, April(53-suppl.)S34-43.
2. Jackson, A. ( 1999). Limits of adaptation
to high dietary protein intakes. European Journal of Clinical
Nutrition, April(53-suppl.)S44- 52.
3a. Lemon, P.(1989). Influence of dietary protein
and total energy intake on strength improvement. Gatorade Sports
Science Exchange, 2(14).
3b. Lemon, P.(1996). Is increased dietary protein
necessary or beneficial for individuals with a physically active
lifestyle? Nutrition Reviews, 54(4), S169- S175.
3c. Lemon, P.(2000). Beyond the zone: protein
needs of active individuals. Journal of the American College
of Nutrition, 19(5), 513S – 521S.
4.Reeds,P., Schaafsma, G., Tome,D. & Young
V.( 2000). Criteria and significance of dietary protein sources
in humans. Journal of Nutrition, 130, 1874S – 1876S.
5. Tome,D. & Bos,C.(2000). Criteria and significance
of dietary protein sources in humans. Journal of Nutrition,130,
1868S- 1873S.
6.WagenMaker, A. Muscle Amino Acid Metabolism
at Rest and during exercise :Role in human physiology and metabolism.
287 – 309.
7. ADA Report ( 2000). Position of the American
dietetic association, dietitians of Canada, and the American College
of Sports Medicine: Nutrition and Athletic Performance. Journal
of the American Dietetic Association,100(12), 1543 – 1556.
8. The Cooper Institute for Aerobics Research
( 2000). The Physical Fitness Specialist Course.
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