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Creatine
Phosphate
By Dick Rafoth MD for cyclingforums
(adenosine
triphosphate) is the chemical compound which
provides the energy to power muscle cell (actin-myosin
fibers) contraction. A single ATP molecule contains
an amino acid base (adenosine), a sugar (ribose) and
three phosphate groups. The chemical energy of the
ATP molecule is stored in the phosphate groups, and
when these high energy phosphate bonds are cleaved
or broken during the processes of cellular
metabolism, the energy is then available for muscle
contraction and other vital cellular functions.
However
the cellular storage capacity for ATP is limited,
and at maximum work levels ATP stored in the muscle
is depleted within several seconds. To sustain
physical activity, the cell must continually
resynthesize ATP (which is the ONLY molecule able to
provide energy to the muscle fibers to power muscle
contraction). ATP is resynthesized via one of three
metabolic pathways depending on the degree and
duration of the physical activity.
The
preferred path involves the breakdown of creatine
phosphate (CP) - another high energy, phosphate
bearing molecule - to supply the chemical energy to
resynthesize ATP. Like ATP, CP is also stored in
finite amounts in the cell and provides at most an
additional 5 to 10 seconds of energy which limits
its usefulness to sprint type activities. Once CP
has been exhausted, the cell uses two slightly
slower metabolic pathways to resynthesize ATP - one
is oxygen dependent (aerobic) and the other is not
(anaerobic).
Muscle
cell ATP is resynthesized from CP at 4 to 8 times
the maximal rate from aerobic pathways. This
explains why performance in sprint activities of 5
to 20 second duration is positively correlated with
muscle CP levels. For sprints of longer duration,
the slower, but higher capacity aerobic and
anaerobic pathways then take over to resupply ATP.
Examples of sprint activities that benefit from high
cell AP levels are weightlifting, field events,
short track events (100 meter dash), baseball, and
volleyball . For sustained muscle activity of more
than 10 to 20 seconds duration, and to replenish
both CP and ATP, the cell relies on aerobic
metabolism.
Creatine
is an amino acid occuring naturally in fish and
meat, and is found in skeletal muscle, heart, brain,
retina, testes and the uterus. Normally less than 1
gram per day is supplied by the diet and another
gram is synthesized (mainly by the kidneys).
Synthesis is the only source for vegetarians and
suggests a role for creatine supplementation in this
group of athletes. Creatine is eliminated through
the kidneys, and there is concern that high doses
might potentially injure the kidneys. However at the
doses mentioned below, no adverse affects have been
proven.
Demant
TW and Rhodes EC (Sports Med 1999 Jul;28(1):49-60)
published a nice review on creatine. To quote
"While creatine has been known to man since
1835, when a French scientist reported finding this
constitutent of meat, its presence in athletics as a
performance enhancer is relatively new. Creatinine
is synthesised from the amino acids glycine,
arginine and methionine in the kidneys, liver and
pancreas, and is predominantly found in skeletal
muscle, where it exists in 2 forms. Approximately
40% is in the free creatine form (Crfree), while the
remaining 60% is in the phosphorylated form,
creatine phosphate (CP). The daily turnover rate of
approximately 2 g per day is equally met via
exogenous intake and endogenous synthesis. Although
creatine concentration (Cr) is greater in fast
twitch muscle fibres, slow twitch fibres have a
greater resynthesis apability due to their increased
aerobic capacity. There appears to be no significant
difference between males and females in Cr, and
training does not appear to effect Cr. Creatine
supplementation of 20 g per day for at least 3 days
has resulted in significant increases in total Cr
for some individuals but not others, suggesting that
there are 'responders' and 'nonresponders'. These
increases in total concentration among responders is
greatest in individuals who have the lowest initial
total Cr, such as vegetarians. Increased
concentrations of both Crfree and CP are believed to
aid performance by providing more short term energy,
as well as increase the rate of resynthesis during
rest intervals. Creatine supplementation does not
appear to aid endurance and incremental type
exercises, and may even be detrimental. Studies
investigating the effects of creatine
supplementation on short term, high intensity
exercises have reported equivocal results, with
approximately equal numbers reporting significant
and nonsignificant results. The only side effect
associated with creatine supplementation appears to
be a small increase in body mass, which is due to
either water retention or increased protein
synthesis."
Supplements
of creatine will increase muscle cell CP levels. To
quote the abstract of one study "The effect of
dietary creatine and supplementation on skeletal
muscle creatine accumulation and subsequent
degradation and on urinary creatinine excretion was
investigated in 31 male subjects who ingested
creatine in different quantities over varying time
periods. Muscle total creatine concentration
increased by approximately 20% after 6 days of
creatine supplementation at a rate of 20 g/day. This
elevated concentration was maintained when
supplementation was continued at a rate of 2 g/day
for a further 30 days. In the absence of 2 g/day
supplementation, total creatine concentration
gradually declined, such that 30 days after the
cessation of supplementation the concentration was
no different from the presupplementation value.
During this period, urinary creatinine excretion was
correspondingly increased. A similar, but more
gradual, 20% increase in muscle total creatine
concentration was observed over a period of 28 days
hen supplementation was undertaken at a rate of 3
g/day. In conclusion, a rapid way to "creatine
load" human skeletal muscle is to ingest 20 g
of creatine for 6 days. This elevated tissue
concentration can then be maintained by ingestion of
2 g/day thereafter. The ingestion of 3 g creatine/day
is in the long term likely to be as effective at
raising tissue levels as this higher dose." If
you decide to give it a try, at a dosage of 15 to 20
grams per day for a loading period of 7 days and
then a maintenance of 3 to 5 grams per day, the cost
of a creatine supplementation program is about 50-70
cents a day.
Is
there any benefit to ingesting more than 20 grams
per day initially? Casey A and Greenhaff PL (Am J
Clin Nutr 2000 Aug;72(2 Suppl):607S-17S) report that
"ingestion of creatine monohydrate at a rate of
20 g/d for 5-6 d was shown to increase the total
creatine concentration of human skeletal muscle by
approximately 25 mmol/kg dry mass, some 30% of this
in phosphorylated form as phosphocreatine. However,
there is no evidence that increasing intake >
20-30 g/d for 5-6 d has any additional effect on
creatine uptake or performance." Another
interesting finding was that those already on a high
creatinine diet will have decreasing benefits from
these supplements. " In individuals in whom the
initial total creatine concentration already
approached 150 mmol/kg dry mass, neither creatine
uptake nor an effect on phosphocreatine resynthesis
or performance was found after
supplementation." And chronic oral supplements
may decrease the level of the cell wall transport
protein responsible for moving creatine from the
blood into the cell, thus minimizing benefits with
time (Guerrero-Ontiveros ML, Wallimann T Mol Cell
Biochem 1998 Jul;184(1-2):427-37)
There
is evidence that carbohydrate supplementation will
improve the benefits of a creatine supplementation
program. Muscle biopsy, urine, and plasma samples
were obtained from 24 males before and after
ingesting 5 g Cr in solution (group A) or 5 g Cr
followed,30 min later, by 93 g simple CHO in
solution (group B) four times each day for 5 days.
Supplementation resulted in an increase in muscle
phosphocreatine (PCr), Cr, and total creatine (TCr;
sum of PCr and Cr) concentration in groups A and B,
but the increase in TCr in group B was 60% greater
than in group A (P < 0.01). There was also a
corresponding decrease in urinary Cr excretion in
group B (P < 0.001). This again supports the
importance of adequate carbohydrates in any dietary
training program.
Higher
muscle CP levels also improve performance in
repetitive sprint events and activities that
involves several heats or sets. Presumeably the
additional muscle CP, recharged from the aerobic and
anaerobic pathways, aid in a rapid resynthesis of
ATP.
The
most recent summary of creatine as a supplement is
found in an American College of Sports Medicine
roundtable on the physiological and health effects
of oral creatine supplementation (Med Sci Sports
Exerc 2000 Mar;32(3):706-17). To excerpt: "Creatine
(Cr) supplementation has become a common practice
among professional, elite, collegiate, amateur, and
recreational athletes with the expectation of
enhancing exercise performance. Research indicates
that Cr supplementation can increase muscle
phosphocreatine (PCr) content, but not in all
individuals. A high dose of 20 g x d(-1) that is
common to many research studies is not necessary, as
3 g x d(-1) will achieve the same increase in PCr
given time. Coincident ingestion of carbohydrate
with Cr may increase muscle uptake; however, the
procedure requires a large amount of carbohydrate.
Exercise performance involving short periods of
extremely powerful activity can be enhanced,
especially during repeated bouts of activity. This
is in keeping with the theoretical importance of an
elevated PCr content in skeletal muscle. Cr
supplementation does not increase maximal isometric
strength, the rate of maximal force production, nor
aerobic exercise performance. Most of the evidence
has been obtained from healthy young adult male
subjects with mixed athletic ability and training
status. Less research information is available
related to the alterations due to age and gender. Cr
supplementation leads to weight gain within the
first few days, likely due to water retention
related to Cr uptake in the muscle. Cr
supplementation is associated with an enhanced
accrual of strength in strength-training programs, a
response not independent from the initial weight
gain, but may be related to a greater volume and
intensity of training that can be achieved. There is
no definitive evidence that Cr supplementation
causes gastrointestinal, renal, and/or muscle
cramping complications. The potential acute effects
of high-dose Cr supplementation on body fluid
balance has not been fully investigated, and
ingestion of Cr before or during exercise is not
recommended. There is evidence that medical use of
Cr supplementation is warranted in certain patients
(e.g.neuromuscular disease); future research may
establish its potential usefulness in other medical
applications. Although Cr supplementation exhibits
small but significant physiological and performance
changes, the increases in performance are realized
during very specific exercise conditions. This
suggests that the apparent high expectations for
performance enhancement, evident by the extensive
use of Cr supplementation, are inordinate."
Bicycling specific studies
There
have been at least 8 good studies on the effects of
creatine supplementation on cycling performance -
and with conflicting results. A quick review of the
literature turned up 9 contolled studies with 4
indicating a positive effect, and 5 demonstrating no
benefit.
Although
there is tantalizing evidence that creatine
supplements may benefit single and repetitive sprint
performance in events of 15 to 30 seconds duration,
this is far from proven. And there is no evidence
to support its use in cycling events lasting more
than a minute.
THE
BOTTOM LINE (FACT VS FICTION)
- sprint activities of 5 to 20 second duration rely on
muscle CP
- although there is tantalizing evidence that creatine
supplements may benefit single and repetitive
sprint performance in events of up to 15 to 30
seconds (and even several minutes in a rare
report) duration, this is far from proven and
there is no evidence to support its use in
cycling events lasting more than a minute
- there is no significant difference between males and
females in cell CP levels
- training does not appear to effect cell CP levels
- those already on a high creatinine diet will have
decreasing benefits from these supplements
- vegetarians can definitely benefit from creatine
supplements
- a high carbohydrate diet will improve the benefits of a
creatine supplementation program
- there is no evidence that increasing intake > 20-30
g/d for 5-6 d has any additional effect on
creatine uptake or performance. Any additional
creatine is just eliminated through the urine.
- creatine supplementation leads to weight gain within the
first few days, likely due to water retention
- it is likely that only one
athlete in many thousand will suffer from kidney
problems when taking creatine supplements ie the
risk is very low,but it is certainly not zero
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