Carbohydrates and The Distance Runner: A Scientific Perspective
by Jason R. Karp, M.S.
Jason Karp provides a summary of what the
latest scientific research tells us about glycogen storage and synthesis, and
the most effective amounts, types, and frequency of carbohydrate ingestion for
endurance running training.
A classic view of muscle fatigue is the limitation in energy
supply. During prolonged endurance exercise, muscle glycogen (the stored form of
carbohydrates) represents the greatest limitation, with fatigue coinciding with
glycogen depletion .
It has been known since the late 1960s that the ability to
perform endurance exercise is strongly influenced by the amount of pre-exercise
glycogen stored in skeletal muscles, with muscle glycogen depletion becoming the
decisive factor limiting prolonged exercise . With the well-documented decrease
in muscle glycogen content that accompanies endurance exercise, an empty-refill
cycle becomes evident. When muscle glycogen is depleted by prolonged exercise,
muscles respond to the empty tank by synthesizing and storing more than what was
previously present. Empty a full glass, and you get a refilled larger glass in
its place. Much like college fraternity parties.
For most individuals, the synthesis of muscle glycogen and
subsequent endurance exercise performance can be supported by eating a normal
diet. There just simply isn't enough of a chronic drain on muscle glycogen
stores to concern oneself with a scientific strategy for optimal recovery.
Distance runners, however, are a unique breed. Doing two
workouts a day and running 15 miles on a Sunday morning places a chronic strain
on their storage of fuel. Recognizing the importance of carbohydrates as a fuel
source, distance runners may be the only people who regularly gather for pasta
Over 30 years ago, Costill and colleagues showed that running
10 miles a day at 80% VO2
max for just three consecutive
days was enough to markedly decrease muscle glycogen, despite eating a mixed
diet (e.g., 40-60% carbohydrates, 30-40% fat, 10-15% protein). While early
investigations reported that it takes nearly 48 hours to replenish glycogen
stores to pre-exercise values, more recent evidence shows that glycogen can be
restored within 24 hours when using an optimal dietary strategy.
It is not uncommon for elite distance runners to train twice
a day, as recent research from our laboratory on U.S. Olympic Marathon Trials
qualifiers supports. Surely the performance of two workouts a day would
necessitate an even greater need to recover quickly. Not only do the adaptations
to training occur during the recovery period from the training rather than
during the training itself, the rapidity with which athletes recover from a long
or intense workout will dictate how often they can perform other long or intense
workouts, which may ultimately influence their ability to reach their athletic
potential. Therefore, strategies for optimal recovery are needed for competitive
The human body responds rather elegantly to
situations that threaten or deplete its supply of fuel. A metabolic priority of
recovering muscle is to replenish muscle glycogen stores. And the more the tank
is emptied, the faster the rate of refilling.
For example, Zachwieja and colleagues found that glycogen was
synthesized significantly faster during six hours post-exercise in a leg that
was exercised to elicit a large degree of glycogen depletion compared to the
opposite leg that was exercised to elicit only a small degree of glycogen
depletion. The rate at which muscle glycogen is replenished depends primarily on
the hormone insulin and the availability and uptake of glucose from the
circulation. Using two different muscles extracted from mice, Bonen and
colleagues observed that glycogen synthesis significantly increased when both
muscles were incubated with insulin and increased linearly with increasing
concentrations of glucose.
Examining muscle glycogen synthesis in exercising rats,
Johnson and Bagby observed that glucose infusion for three hours post-exercise
resulted in a greater glycogen synthesis in soleus, gastrocnemius, and
quadriceps muscles compared to lactate or saline infusion. The infused glucose
following exercise was directly used by the oxidative muscle fibers (slow-twitch
and fast-twitch A) for glycogen synthesis, while the non-oxidative fibers
(fast-twitch B) also used lactate through an indirect pathway.
Ingestion of carbohydrate elevates blood glucose
concentration, thus providing a substrate for the synthesis of new glycogen, and
increases insulin concentration, stimulating cellular uptake of glucose.
Insulin, which is secreted from the pancreas, is the primary signal for glycogen
synthesis. Doyle and colleagues reported that the glucose and insulin responses
to carbohydrate ingestion together accounted for 94% of the variance in glycogen
replenishment, highlighting the importance of this substrate and this hormone.
Muscle glycogen concentration is a powerful mediator of insulin sensitivity,
with sensitivity greatest within the first two hours after glycogen-depleting
exercise when carbohydrate is available.
Relatively little glycogen synthesis occurs when your
athletes do not consume carbohydrates after they run. The synthesis of glycogen
between training sessions occurs most rapidly if carbohydrates are consumed
immediately after exercise. Indeed, delaying carbohydrate ingestion for two
hours after a workout can significantly reduce the rate of glycogen synthesis
within the first few hours.
For example, Ivy and colleagues found that glycogen was
synthesized significantly faster when ingesting carbohydrates immediately after
exercise compared to delaying ingestion for two hours, as both the insulin and
glucose levels were significantly reduced when carbohydrate ingestion was
To maximize the rate of glycogen synthesis, your athletes
should consume 0.7 gram of carbohydrate per pound of body weight (g/b) within 30
minutes after their workouts, and continue to consume 0.7 g/b every two hours
for four to six hours afterward. It would be even better if they can eat or
drink more often, since a more frequent ingestion (e.g., every 15 to 30 minutes)
of smaller amounts of carbohydrates better maintains blood glucose and insulin
For example, a study published in Journal of Applied
in 1993 found that when subjects ingested 0.2 gram of
carbohydrates per pound of body weight every 15 minutes, glycogen was
synthesized at nearly double the rate found in other studies in which
carbohydrates were ingested every 1 to 2 hours. Another study published in
American Journal of Clinical Nutrition
in 2000 found that the rate of
glycogen synthesis significantly increased when subjects ingested 0.3 compared
to 0.2 gram of carbohydrates per pound every 30 minutes. Therefore, the rate of
glycogen synthesis following exercise seems to be maximized when 0.7 g/b is
ingested every two hours or when carbohydrates are ingested every 15 to 30
minutes at a rate of 0.5 to 0.7 g/b/hr.
In addition to the amount
of carbohydrates, the
of carbohydrates ingested may influence glycogen synthesis rate and
subsequent exercise performance, since different types of carbohydrates may
produce different blood glucose and insulin responses. Since glycogen is derived
from glucose, it is not surprising that glucose has been found to be the most
effective type of ingested carbohydrates at enhancing the insulin response and
restoring glycogen immediately after exercise.
For example, Blom and colleagues found a significant
difference in the rate of glycogen synthesis between glucose ingestion (0.3 g/b)
and an equivalent amount of fructose ingestion. Ventura and colleagues found
higher plasma glucose and insulin concentrations when glucose was ingested 30
minutes before exercise compared to when either fructose or a placebo was
ingested. However, subsequent endurance time to exhaustion at 82% VO2
was only significantly longer when comparing ingestion of glucose to placebo.
Comparing glucose to sucrose tells a slightly different
story, as Bowtell and colleagues found that the rate of glycogen synthesis was
significantly greater with an 18.5% glucose solution (0.4 g/b) compared to
either an 18.5% or 12% sucrose solution (0.4 g/b). However, two studies did not
find significant differences in the rate of glycogen synthesis between glucose
and sucrose ingestion. Casey and colleagues, who also used 18.5% glucose and
sucrose solutions (0.45 g/b), admit that a lack of a significant difference was
likely due to the large interindividual variability in their data.
Using a more concentrated solution (30%), which may have
slowed gastric emptying, Blom and colleagues found a small, non-significant
difference in the rate of glycogen synthesis between glucose ingestion (0.3 g/b)
and an equivalent amount of sucrose ingestion. Taken together, these studies
suggest that, while glucose ingestion has a greater effect on insulin level and
glycogen synthesis compared to fructose ingestion, it has a similar effect as
ingestion of sucrose.
CARBOHYDRATE VS. CARBOHYDRATE. PROTEIN INGESTION
While the amount and type of ingested carbohydrates for
maximal glycogen synthesis have been identified, the effect of other
macronutrients in combination with carbohydrate is less clear. For example,
research that has examined protein ingestion along with carbohydrates on the
rate of glycogen synthesis or endurance performance (Table 1) has yielded
inconsistent results, as some studies have shown this strategy to hasten the
rate of glycogen synthesis and improve endurance performance, especially when
the amount of carbohydrates ingested is less than that needed for maximal
glycogen synthesis, while other studies have reported no benefit with the
simultaneous ingestion of protein.
At least some of the discrepancy in the literature may be
attributed to the use of post-workout beverages that contained different amounts
of calories or different amounts of carbohydrates. It is possible that
Carrithers and colleagues and Tarnopolsky and colleagues did not observe
differences in muscle glycogen content between the treatments because the
carbohydrate-protein beverages contained less carbohydrates than the
carbohydrate-only beverages (0.32 vs. 0.45 g/lb/hr and 0.34 vs. 0.45 g/lb/hr,
respectively). In the study of Rotman and colleagues, the carbohydrate and
carbohydrate-protein beverages, which were ingested every two hours, already
contained the recommended amount of carbohydrates for maximal glycogen
synthesis, which may have obscured any added benefit of protein.
The specific type of carbohydrates contained in the beverages
has also varied between studies. Tarnopolsky and colleagues and Rotman and
colleagues used an equal mix of glucose and sucrose in both their carbohydrate
and carbohydrate-protein beverages, Van Hall and colleagues used sucrose,
Zawadzki and colleagues used a mix of dextrose and maltodextrin, van Loon and
colleagues used an equal mix of glucose and mal to dextrin, and Ivy and
colleagues used a mix of sucrose and maltodextrin. Carrithers and colleagues did
not even use the same type of carbohydrates between the different treatment
beverages, as their carbohydrate-only beverage contained glucose while their
carbohydrate-protein beverage contained fructose and dextrose.
All three studies that reported a beneficial effect of
combining carbohydrates with protein used maltodextrin, a complex carbohydrate,
as one of the carbohydrate ingredients. Of the studies that reported no
additional benefit with the co-ingestion of protein, three used sucrose and
three used glucose as one of the carbohydrate ingredients. Despite some
similarities in the type(s) of carbohydrates used between studies with similar
findings, the type of carbohydrates used does not help explain the contrasting
results in the literature.
Another potential reason for the conflicting results may be
due to differences in the frequency of supplementation. For example, studies
finding a beneficial effect with the co-ingestion of protein have most often
used feeding intervals of two hours, while studies reporting no benefit with the
co-ingestion of protein have most often used feeding intervals of less than one
hour (Table 1). It appears, therefore, that both a high carbohydrate content of
the beverage (enough to elicit maximal glycogen synthesis rates) and the more
frequent ingestion of carbohydrates negates any benefit of added protein.
The only thing that seems to be clear, obvious as it may be,
is that beverages containing carbohydrates or carbohydrates plus protein are
more effective than plain water or a placebo at restoring glycogen after
exercise and lengthening time to exhaustion during exercise. Despite the many
highly-advertised commercial sports drinks like Gatorade, any beverage that
contains a large amount of carbohydrates will be very helpful for recovery. For
example, research from our laboratory has shown that chocolate milk, which has a
high carbohydrate and protein content, is an effective alternative to commercial
sports drinks for recovery from exhausting exercise.
So, to get your athletes to fill their empty tanks and
recover as quickly as possible after their next long run, tell them to drink 1
to 2 glasses of chocolate milk per hour for a few hours after running. And tell
them to skip the fraternity party.
FROM: TRACK COACH 176