Nutrient muscle tissues and replenishment of glycogen

Nutrient timing is a popular strategy which supports the
consumption of a combination of nutrients- primarily carbohydrates and
proteins- around the individual’s workout session. It has been claimed that this approach can produce dramatic
improvements in body composition. It has even been postulated that the timing
of nutritional consumption may be more important than the absolute daily intake
of nutrients. While some research has demonstrated that the timed ingestion of nutrients
–majorly carbohydrates and proteins- may significantly affect the adaptive
response to exercise, some show that the timing of nutrients hardly play a role
if the target macronutrients are fulfilled by the end of the day. Various
researches have postulated that an anabolic “window of opportunity” exists
after a training session whereby a limited time exists after training to
optimize training-related muscular adaptations. In this paper, we identify
the dietary guidelines for athletes and its physiological and psychological
implications on beginners, intermediate and advanced lifters and if nutrient
(carbohydrates and proteins) timing, in fact helps an individual maximize his
potential. We will also discuss about a popular dieting strategy, Carb Backloading,
in detail and review if there are any additional benefits to consuming carbs
only post-exercise.

Introduction

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Over the past few years, there have been a number of
extensive research studies revolving around the concept of nutrient timing.
This was based on the consumption of nutrients- majorly carbohydrates and proteins-
in and around a training session. The
strategy is designed to maximize exercise-induced muscular adaptations and
facilitate repair of damaged tissue 1. Theoretically, consuming the proper amount of nutrients
around the training session not only helps in the restoration of damaged muscle
tissues and replenishment of glycogen reserves, it also aids in better body
composition and glycogen super-compensation, wherein after a bout of resistance
training, the muscles are able to hold a greater amount of glycogen than they
normally would be able to, hence super-compensating the muscle glycogen
reserves and making it look fuller and bigger. Also, researchers have made
reference to an anabolic “window of opportunity” whereby a limited time exists
after training to optimize training-related muscular adaptations 5. However,
this depends on a number of factors – the duration between training bouts, how
well trained the athlete is – beginner, intermediate or advanced.

 

Nutrient 1: Proteins

 

Pre-exercise ingestion of proteins

Many studies have been conducted to explore the use of pre-exercise PRO and CHO ingestion in
preventing acute exercise-induced muscle damage. Tipton et al. 15 reported that the
ingestion of a mixture of essential amino acids and CHO before resistance exercise
was more effective for the stimulation of post-exercise muscle protein
synthesis than ingesting the same mixture immediately after exercise. This was
attributed to the combination of increased amino acid levels at a time when
blood flow is increased during exercise, thereby offering a greater stimulation
of muscle protein synthesis by increasing amino acid delivery to the muscle.
However, in a following study, the same study group was unable to demonstrate
the same findings when examining the impact of 20 g of whey protein
ingested before as opposed to 1 h after resistance-type exercise on muscle
protein balance measured over a 4- to 5-h recovery period 16. This study,
along with a few others, have concluded that rate of muscle protein synthesis
is stimulated by ingestion of protein sources and the timing of protein
ingestion is not important. 14, 16, 17, 18.

Intra-exercise ingestion of proteins

The ingestion of
protein before or during exercise could be of even more benefit during the
early stages of recovery from more intense exercise bouts and has been
investigated in many short term studies 19. 
In one study, participants completed 3 h of cycling @ 45 – 75% VO2max, followed by a time to exhaustion
trial at 85% VO2max. During each session, participants consumed either a
placebo, a 7.75% CHO solution, or a 7.75% CHO/1.94% PRO solution. While the CHO
only group increased time to exhaustion (19.7 ± 4.6 min) versus the placebo
(12.7 ± 3.1 min), the addition of PRO resulted in even greater performance
(26.9 ± 4.5 min) 20. Another study by Saunders et al. analyzed the impact of
a CHO + PRO combination for its ability to improve performance and minimize
muscle damage 21. Cyclists exercised to
exhaustion on two different occasions separated by 12 – 15 h. During exercise,
the participants were given a CHO solution or a CHO+PRO solution. It was found
that the participants who ingested the CHO+PRO solution had a much higher
increase in performance and the muscle damage markers were significantly lower,
suggesting the CHO + PRO supplement helped to attenuate the muscle damage
associated with prolonged and exhaustive exercise. Similarly, few other short
term studies showed a positive effects of consuming a CHO+PRO solution during
exercise on attenuating muscle damage by lowering muscle damage markers such as
creatine kinase 22, 23. However, in a long term study conducted for one week
on elite cyclists by Mette et al., the supplementation of a PRO+CHO solution
during exercise did not improve recovery or performance in elite cyclists
despite high demands of daily exhaustive sessions during the one-week training
camp. 24

Post-exercise ingestion of proteins

Most of the
studies that have been done on nutrient timing are based on protein consumption
after a training session and whether or not an “anabolic window” exists which
could help an individual maximize his/her training-related muscular
adaptations.  Some of the earlier studies
which were done supported the idea of an anabolic window post training. For
example, Esmarck et al. 25 provided evidence that consuming protein
immediately post training enhanced muscular adaptations better than when the
protein intake was delayed. Thirteen untrained elderly male volunteers were
matched in pairs based on body composition and daily protein intake and divided
into two groups: P0 and P2. P0received a protein supplement immediately
post-exercise while P2 received the same supplement 2 hours following the
exercise bout. After the end of the study period, they noticed that cross-sectional
area (CSA) of the quadriceps femoris significantly increased in the P0 group
while no significant increase was seen in P2, hence suggesting that delaying
post-workout nutrient intake may impede muscular gains. In contrast to this,
there have been numerous studies which have refuted the existence of the
“window of opportunity” whereby a limited time exists after training to
optimize training-related muscular adaptations. Verdijk et al. 26 didn’t
notice any increase in skeletal muscle tissue in elderly men from consuming a
post-exercise protein supplement. 26 elderly men were randomly assigned to a
progressive, 12 week resistance training program with (protein group) or
without (placebo group) protein provided before and immediately after each
exercise session. After 12 weeks, no significant differences in muscle strength
or hypertrophy were noted between groups indicating that timed nutrient
supplementation does not enhance training related adaptations. Similarly, Hoffman
et al. 27 conducted a study on 33 well- trained young men with protein
supplementation given either in the morning and evening or immediately before
and immediately after resistance exercise. At the end of the study, no
significant between-group or absolute changes in body composition was observed.
A lot of other studies have echoed the same sentiment when it came to timing of
protein supplementation, be it in untrained individuals or trained athletes
28-31.

Summary

1.      Pre-exercise/ intra-exercise intake of protein
doesn’t have any additional benefits as compared to any other time of the day. However,
addition of a PRO+CHO solution during prolonged endurance events might offset
muscle damage.

2.     
The post-exercise
‘anabolic window’ lasts a lot longer than what was originally hypothesized,
indicating that ingesting a protein supplement immediately post exercise
doesn’t have any additional benefits.

 

Nutrient 2: Carbohydrates

 

Pre-exercise ingestion of carbohydrates

Glycogen is considered to be quintessential for resistance
training athletes, with 80% of ATP being derived from glycolysis. MacDougall et al. 7 demonstrated that a
single set of elbow flexion at 80% of 1 repetition maximum performed to
muscular failure caused a 12% reduction in mixed-muscle glycogen concentration,
while three sets at this intensity resulted in a 24% decrease. Similarly,
Robergs et al. 8 reported that six sets of 70% one repetition maximum
(1 RM, I-70) and 35% 1 RM performed to muscular failure resulted in a 26.1%
reduction of glycogen stores in the vastus lateralis while six sets at this
intensity led to a 38% decrease, primarily resulting from glycogen depletion in
type II fibers compared to type I fibers. As glycogen levels diminish, ATP
production is hampered, exercise intensity decreases which results in
suppression of the immune system 1. Research involving the ingestion of single
high CHO feedings has also demonstrated the promotion of higher levels of
muscle glycogen and an improvement of blood glucose maintenance (euglycemia) 1. This ensures the athlete is able to
push harder during the training and the ATP production is enhanced via
glycolysis. Also, glycogen availability also has been shown to mediate muscle
protein breakdown. Lemon and Mullin 9 found that nitrogen losses more than doubled following a
bout of exercise in a glycogen-depleted versus glycogen-loaded state. However,
not all studies showed the same result. Hawley and Burke summarized several
studies that administered some form of CHO within one hour prior to exercise:
one study reported a decrease in performance, three studies reported an
increase in performance and some studies reported no effect 10.  In light of these conditions, maintaining a
higher muscle glycogen level before training appears to be beneficial to
resistance training results.

Intra-exercise ingestion of carbohydrates

Most of the research involving the consumption of
carbohydrates during workout have been performed on endurance athletes who
train for longer periods of time (at least 90 minutes) or resistance training
bouts spanning for very long hours. This is most likely because this is when
glycogen stores will be significantly depleted because of the duration of the
workouts, as opposed to a regular resistance training session in which glycogen
can be depleted by about approximately 36-39% only 1. Carbohydrates during an endurance event maintain
high rates of CHO oxidation which undergo glycolysis to provide ATP, offsets muscle damage and prevent
hypoglycemia 11.

 A study conducted by Kulik et al.
studied the effects of supplemental carbohydrate (CHO) ingestion on the
performance of squats to exhaustion 12. In this study, the subjects were made
to do sets of 5 reps to exhaustion at 85% of their 1RM.  Subjects consumed
0.3g.kgCHO.bodymass or a placebo of equal volume immediately before exercise
and after every other completed set of squats. There was no significant statistical
difference between groups in total sets, volume, work or the rate of perceived
exertion. This study suggests that intra workout carbohydrates might not be as
beneficial for an athlete performing resistance training.

Post-exercise ingestion of carbohydrates

It is common lore that the post- exercise carbohydrates must
have a substantial glycemic and insulinemic response in order to optimize
recovery and there are studies which support this claim. One such study showed that glycogen storage was
2–3 times faster during four hours post-exercise resulting in greater glycogen
storage at four hours rather than later in the day, suggesting that delaying
the ingestion of a carbohydrates post-exercise will result in a reduced rate of
muscle glycogen storage 4. However, there are numerous studies which
suggest that consuming carbohydrates
immediately post-workout may not aid in faster glycogen replenishment. In a
study conducted by Parkin et al., 6 trained cyclists cycled at 70% of VO2 max
for 2 hours followed by four 30 second sprints. Post exercise, all subjects
were given 5 high glycemic index meals over a 24 hour period, the first three
being fed immediately post workout and 3 being fed starting 2 hours after the
training session. Muscle biopsies taken at 8 and 24 hours revealed that there
was no difference in muscle glycogen or glucose-6-phosphate in either trial
13.

These studies
present conflicting opinions but taking all the relevant points into
consideration, it becomes quite clear that accelerating glycogen resynthesis is
important for endurance athletes when the duration between two events is less
or for resistance training athletes who perform 2 training sessions in a day
provided the same muscles are being worked in both the sessions.

Summary

1.      Pre-exercise intake of carbohydrates may
help with increased glycogen levels which translates to higher energy output in
the training session.

2.      Intra-exercise intake of carbohydrates are
more beneficial for endurance/resistance training athletes who train for very
long hours and are at risk of significant glycogen depletion.

3.      Post-exercise intake of carbohydrates help
with glycogen resynthesis but the rate remains the same even at 24 hours,
indicating that completing the CHO requirements before the next training
session is optimal enough for athletes.

 

Carbohydrate Backloading: A review

Carbohydrate Backloading
is a dieting strategy that was popularized by John Keifer. This strategy emphasizes
on keeping carbs at an absolute minimum throughout the day and ingesting
carbohydrates after the training session. 34

Mechanism of carbohydrate Backloading:

 It was hypothesized that carbohydrate backloading
takes advantage of the non-insulin mediated uptake of glucose by the muscle
tissues post-exercise, because of GLUT4 translocation, as opposed to in the
morning, when insulin sensitivity in both muscle and fat tissue is generally
higher 32.

Similar to GLUT (Glucose
Transporter) 1-3, GLUT4 and GLUT12 are a set of glucose transporters which are
present in muscle and fat tissue. While GLUT 1-3 are exposed to the cell surface,
GLUT4 are tucked below the surface within the cellular membranes. Due to this
withdrawn nature of GLUT4, this only reacts to the presence of insulin by
moving from the interior of the cell to its surface.  Thereby, this insulin-mediated transport of
glucose transports high volumes of glucose in the cells containing these GLUTs
(both muscle and fat tissue). 33, 34

However, resistance
training mimics the function of insulin in muscle cells and GLUT4 rises to the
surface shuttling glucose into the muscle tissue. This non-insulin mediated
uptake of glucose by the muscle tissues post-exercise is postulated to have
increases in the skeletal muscle tissue without any significant increases in
adipose tissue. 34

Research on carbohydrate backloading:

The most popular research
to support the theory of CBL is a 6 month study. In this study, Sofer et al. 35
authors compared the effects of carbs eaten mostly at dinner (experimental
group) vs. eaten throughout the day (control group) in a group of 78
Israeli police officers.  It was
found that reductions in weight, body fat and waist circumference were greater
in the evening-carb experimental group vs. the control group. In addition,
glucose control, inflammation, blood lipids and satiety were improved to a
greater degree in the evening-carb group.

However, there
are a few limitations to the design of the study. The subjects were fed a daily
average protein intake of 0.66-0.76 g/kg, which is much less than what
is consumed by resistance trainees aiming for a better body composition, which
questions the applicability of the research to this population. Also, the
experimental group lost an average 11.8 percent of their body weight in 6
months as compared to the control group who lost an average 10.9 percent, which
isn’t statistically significant (