Nutrients During Training
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1. What is the efficiency of digestion during training?
It depends on the intensity of the training. During tougher training, more blood flow
goes to the muscle, meaning less goes to the intestines for digestion. I estimate that
at competitive triathlon intensities it is less than one-fourth the normal digestion rate. I
base this on the fact that during training an athlete: (1) can only digest 250 kcals of
carbohydrate in one hour and (2) can not easily digest any fat or fiber.
2. Does digestion change with age?
There is no evidence for this that I have been able to find thus far. I suspect that
anyone with digestion difficulty greater than the average athlete is not drinking
enough water (you need 1 L or Qt for each 1000 kcals that you eat).
3. How does the consumption of carbohydrates and protein during training
affect energy levels / fatigue?
Protein intake may spare muscle but will not delay fatigue in highly trained athletes
(see Article #1). Protein intake has not been shown to change energy levels during
training but can lower your perceived exertion and improve mental focus slightly (see
Article #1). Carbohydrate intake (up to 250 kcals / hour) increases power output (by
2%) and endurance (by 20%). Taking in more than 250 carbohydrate kcals/hour
results in body fat storage during exercise and will not aid in post-exercise recovery
any more than the presence of your current body fat. Putting ~100 kcals of protein
into your training fluids per hour would supply your body with the same amount of
protein that it is burning as fuel (~10% of the total). However, no research has shown
that taking in this much protein provides a benefit beyond that seen with only ingesting
half that much (see Article #1).
4. How do I avoid intestinal cramps / gastric distress during training?
Avoid excess protein (> 20% of kcals) and fat intake and avoid fiber altogether while
you are exercising. Most importantly, make sure that you are hydrated. You need 1 L
or Qt to digest each 1000 kcals that you eat or drink. If you take in 250 kcals of
carbohydrate and 50 kcals of protein per hour during training you require ~1/3 Qt of
water to digest and absorb it. Until you drink that much you really aren’t replacing any
perspiration losses. Ideally, you would consume a dilute solution of carbohydrate with
some protein rather than solid food (energy bars or concentrated packets of calories)
to ensure that food-related dehydration does not occur. Beware of eating a large
amount of fruit or fruit products in the hour or two before training; the fructose content
can cause osmotic diarrhea that may set in once you start exercising.
5. Is a low-glycemic index sugar like fructose or galactose better than
maltodextrin?
Fructose and galactose are absorbed by the liver and converted into glucose. This
glucose is then released into the bloodstream to be carried to the muscles where it is
absorbed. The muscles can only absorb and use glucose...this is the case for all
organs, and the liver's job is to convert all sugars that are not glucose into glucose.
The reason that fructose and galactose have a low glycemic index is because they
have to go through this conversion process, which takes time, and energy. Insulin
does not respond to fructose and galactose, so eating these sugars does not raise
insulin. Insulin is the hormone that shuttles sugar into all organs. Muscle is the only
tissue that can absorb significant amounts of glucose without the presence of insulin
(this occurs during muscle contraction, i.e. exercise). Therefore, fructose and
galactose force your liver to use up energy stores to do the conversion to glucose.
This is energy you will wish you had at the end of a race. On the flip side, however,
the body is more likely to bonk if sugar reserves in the liver get too low so it is good to
include SOME fructose and/or galactose. Maltodextrin is pure glucose and goes
straight to the muscles once it is broken down in the intestine. If you drink it in large
amounts every half hour this is bad because insulin will be released by the pancreas
and will shuttle the sugars from the maltodextrin to all organs, not just the muscles
(including fat cells). For a person taking in calories infrequently it is better to go with
the fructose and galactose and lose the liver energy doing the conversion. For a
person taking in calories at a low dose but frequently, maltodextrin is by far more
efficient. The question is this: do you want your liver to ration out your sugar to you or
do you want to do it yourself? There is an energy cost either way: in the first case you
lose energy because the liver does work, in the second you lose energy because you
have to plan your intake ahead of time. In the case of the latter, the energy loss is not
during a race. An additional consideration is that 250 kcals per hour of fructose makes
all humans who have ever been tested sick. In fact, at San Jose State Univ. the
graduate nutrition course does a lab experiment each year proving this to their
students. The reason is that the number of fructose transporters in the intestine is
limited so that not all of the fructose can be absorbed. The excess fructose remaining
in the intestine, as well as all the water required to solvate it, results in bloating and
osmotic diarrhea. A general conclusion from all this would be that if you are ingesting
calories during training or competition, don’t try to "catch up" with a large amount of
calories all at once if you fall behind for an hour.
It depends on the intensity of the training. During tougher training, more blood flow
goes to the muscle, meaning less goes to the intestines for digestion. I estimate that
at competitive triathlon intensities it is less than one-fourth the normal digestion rate. I
base this on the fact that during training an athlete: (1) can only digest 250 kcals of
carbohydrate in one hour and (2) can not easily digest any fat or fiber.
2. Does digestion change with age?
There is no evidence for this that I have been able to find thus far. I suspect that
anyone with digestion difficulty greater than the average athlete is not drinking
enough water (you need 1 L or Qt for each 1000 kcals that you eat).
3. How does the consumption of carbohydrates and protein during training
affect energy levels / fatigue?
Protein intake may spare muscle but will not delay fatigue in highly trained athletes
(see Article #1). Protein intake has not been shown to change energy levels during
training but can lower your perceived exertion and improve mental focus slightly (see
Article #1). Carbohydrate intake (up to 250 kcals / hour) increases power output (by
2%) and endurance (by 20%). Taking in more than 250 carbohydrate kcals/hour
results in body fat storage during exercise and will not aid in post-exercise recovery
any more than the presence of your current body fat. Putting ~100 kcals of protein
into your training fluids per hour would supply your body with the same amount of
protein that it is burning as fuel (~10% of the total). However, no research has shown
that taking in this much protein provides a benefit beyond that seen with only ingesting
half that much (see Article #1).
4. How do I avoid intestinal cramps / gastric distress during training?
Avoid excess protein (> 20% of kcals) and fat intake and avoid fiber altogether while
you are exercising. Most importantly, make sure that you are hydrated. You need 1 L
or Qt to digest each 1000 kcals that you eat or drink. If you take in 250 kcals of
carbohydrate and 50 kcals of protein per hour during training you require ~1/3 Qt of
water to digest and absorb it. Until you drink that much you really aren’t replacing any
perspiration losses. Ideally, you would consume a dilute solution of carbohydrate with
some protein rather than solid food (energy bars or concentrated packets of calories)
to ensure that food-related dehydration does not occur. Beware of eating a large
amount of fruit or fruit products in the hour or two before training; the fructose content
can cause osmotic diarrhea that may set in once you start exercising.
5. Is a low-glycemic index sugar like fructose or galactose better than
maltodextrin?
Fructose and galactose are absorbed by the liver and converted into glucose. This
glucose is then released into the bloodstream to be carried to the muscles where it is
absorbed. The muscles can only absorb and use glucose...this is the case for all
organs, and the liver's job is to convert all sugars that are not glucose into glucose.
The reason that fructose and galactose have a low glycemic index is because they
have to go through this conversion process, which takes time, and energy. Insulin
does not respond to fructose and galactose, so eating these sugars does not raise
insulin. Insulin is the hormone that shuttles sugar into all organs. Muscle is the only
tissue that can absorb significant amounts of glucose without the presence of insulin
(this occurs during muscle contraction, i.e. exercise). Therefore, fructose and
galactose force your liver to use up energy stores to do the conversion to glucose.
This is energy you will wish you had at the end of a race. On the flip side, however,
the body is more likely to bonk if sugar reserves in the liver get too low so it is good to
include SOME fructose and/or galactose. Maltodextrin is pure glucose and goes
straight to the muscles once it is broken down in the intestine. If you drink it in large
amounts every half hour this is bad because insulin will be released by the pancreas
and will shuttle the sugars from the maltodextrin to all organs, not just the muscles
(including fat cells). For a person taking in calories infrequently it is better to go with
the fructose and galactose and lose the liver energy doing the conversion. For a
person taking in calories at a low dose but frequently, maltodextrin is by far more
efficient. The question is this: do you want your liver to ration out your sugar to you or
do you want to do it yourself? There is an energy cost either way: in the first case you
lose energy because the liver does work, in the second you lose energy because you
have to plan your intake ahead of time. In the case of the latter, the energy loss is not
during a race. An additional consideration is that 250 kcals per hour of fructose makes
all humans who have ever been tested sick. In fact, at San Jose State Univ. the
graduate nutrition course does a lab experiment each year proving this to their
students. The reason is that the number of fructose transporters in the intestine is
limited so that not all of the fructose can be absorbed. The excess fructose remaining
in the intestine, as well as all the water required to solvate it, results in bloating and
osmotic diarrhea. A general conclusion from all this would be that if you are ingesting
calories during training or competition, don’t try to "catch up" with a large amount of
calories all at once if you fall behind for an hour.
© Clyde F. Wilson