This is article is not intended to be a complete
discussion on nutrition. Rather it is to set-up the reader for an article on
bioenergetics. The main goal of this article is to discuss the role of each of
the nutrients in body. As such, the article gives a simple discussion on each
nutrient and its role in exercise. It is recommended that one should consult
with a licensed nutritionist on proper nutrition practices.
There are three macronutrients the
body needs. Each with a specific role in order for the body function and in
certain amounts only. Any excess of these macronutrients are stored as fat. Nutrition
and physical activity are part of the bioenergetics equation where energy
intake is equal to energy expenditure. A qualified nutritionist should be able
to compute the energy intake portion of the equation based on information given
by a client. Physical activity will most often be handled by both the client
and a trainer.
Nutrition is part of being healthy
and fit. A qualified and licensed nutritionist can prescribe how much to eat
for each macronutrient. Listed below are
the macronutrients and their role in exercise.
Carbohydrates:
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Excellent sources of carbohydrates are
whole grains, pasta, fruits and vegetables (2). Carbohydrates are also an
excellent source of dietary fiber. Which aid in giving bulk to the stools in
the intestinal tract (2). They have been getting a bad rap recently due to the
myth of the low carbohydrates diets. Moreover, they have been unfairly
implicated in obesity (1). However, carbohydrates play an important role in the
body during physical activity and exercise. The body stores carbohydrates in
two forms: glucose and glycogen. The role carbohydrates play in the body is
enumerated below (3, 5):
1. Energy Source: Carbohydrates serves primarily as an
energy source. Particularly in high intensity exercise. Therefore, sufficient
daily intake of carbohydrates helps active individuals maintain the body’s
limited supply of glucose and glycogen stores. Cells in the human body can only
store so much glycogen. Any excess intake of dietary carbohydrates is often
converted to fat for storage. This interconversion helps explain the fat
increase even when lipid intake is low.
2. Protein Sparer: Adequate intake of dietary
carbohydrates helps preserve protein tissue. Proteins primary role in the body
is tissue growth, maintenance, and repair. It will serve as an energy source to
a lesser degree. Glycogen depletion, which occurs during starvation, reduced carbohydrate
intake, and strenuous exercise affects the fuel mixture for exercise. Glycogen
depletion triggers fat catabolism and synthesis of glucose from amino acids.
This gives the body options in augmenting carbohydrate availability. This helps
maintain the plasma glucose levels in the blood even when the glycogen stores
are insufficient. The trade-off is that the body’s protein stores are strained,
most specially the muscle protein. This reduces lean muscle tissue and strains
the kidneys, forcing them to excrete nitrogenous by-products of protein
breakdown.
3. Metabolic Primer: One of the more important roles of
carbohydrates is that it serves as metabolic primer. By-products of
carbohydrate catabolism serve as primers for fat oxidation. When insufficient
breakdown of carbohydrates occurs, this causes fat mobilization to exceed fat
oxidation. The lack of glycogen catabolism by-products causes an incomplete
oxidation of fat. This produces ketone bodies. Excessive accumulation of ketone
bodies increases the body’s fluid acidity. This situation is called acidosis,
or ketosis, in the case of ketone bodies.
4. Fuel for the Central
Nervous System:
Carbohydrate is the only fuel the central nervous system (CNS) will use as
fuel. The brain metabolizes glucose exclusively as fuel. The CNS requires a
continuous stream of carbohydrates for proper functioning. Under extreme
conditions, such as poorly regulated diabetes, low carbohydrate diets, and
starvation the brain will metabolize fat (as ketones) for fuel. Low carbohydrate
diets also induce changes in the skeletal muscle that increase fat use in low
to moderate exercise.
Blood sugar is regulated within narrow
limits for two reasons:
1. Glucose serves as a primary fuel
for nerve tissue metabolism.
2. Glucose represents the sole
energy source for red blood cells.
At rest and during exercise, the body
maintains normal blood glucose levels at 100mg per dL (or 100mL). However,
during prolonged exercise this level eventually falls below normal. Liver
glycogen becomes depleted and the working muscles continue to metabolize
available blood glucose. Reduced levels of blood glucose is termed
hypoglycemia, and is often <45 mg per dL of blood. Symptoms of hypoglycemia
include hunger, weakness, mental confusion, and dizziness. This would
ultimately impair exercise performance and contributes to fatigue of the
central nervous system associated with prolonged exercise.
Sustained and profound hypoglycemia
can cause unconsciousness and irreversible brain damage, according to Mcardle
et al.
How much glycogen does the body
actually store? The body store approximately 400g of muscle glycogen; 90-110g
of liver glycogen, and about 2-3g of blood glucose. For an 80kg man, it would
be around 500g. Each gram of glucose, or glycogen, contains about 4 kCal of
energy. The average person contains about 2000kcal, enough to power a high
intensity run for 20 miles (3).
Carbohydrate rich diets can double
the body’s carbohydrates stores compared a regular well balanced diet. The
upper limit to which the body can store glycogen is about 15g per kg of
bodyweight (3).
Carbohydrate Dynamics in
Exercise:
Glycogen stored in the active muscle
supply almost the entire energy requirement during the transition from rest to
moderate exercise. Glycogen stores last for about 20 minutes wherein they
supply about 40% to 50% of the required energy. The remainder is supplied by
fat and protein. (3)
Recommended Dietary
Intake of Carbohydrates:
Complex carbohydrates, such as rice,
bread, pasta, fruits, and vegetables should be the principal source of calories
from carbohydrates. Simple carbohydrates found in candy and soda lead to quick
increases in blood sugar, thus their high rating in in the gylcemic index
(G.I.). They also tend to increase fat deposition. (1)
The recommended amount of
carbohydrate intake is 55% to 70% of total caloric intake for physically active
people. This is estimated to be around 400g to 600g. For sedentary, the
recommendation is 40% to 50% of total caloric intake, about 300g. (3, 4, 5) A
nutritionist should be able to give a more precise recommendation.
Fats and Lipids:
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Fats are the most common type of
lipid stored in the body. The fat content of the body for young adults is about
15% for males, and 25% for females (3).Sources of fats are fish, seeds, nuts,
vegetable oils, meats, butter, lards, fried foods, and baked items. There are
several types of fat (2). This is a short list. It is recommended that the
reader look for a more complete resource for nutrition for a complete
discussion on Lipids and fats:
1. Unsaturated fats: This includes monounsaturated fats
and polyunsaturated fats. Usually found in plants (nuts, seeds, grains, and
vegetable oils) and are liquid at room temperature. Polyunsaturated fats are
found usually in fish, such as mackerel, herring, tuna, and salmon.
2. Saturated fats: Solid at room temperature. These
generally come from animal sources, meat and dairy products.
3. Transfatty acids: Found primarily in baked and fried
foods. Derived from the partial hydrogenation of unsaturated corn, soybean, or
sunflower oil. Other sources are vegetable shortening, margarines, crackers,
candies, cookies, snack foods, and other processed foods (3).
Lipids play an important role in the
body. Listed below is how the body uses lipids (3, 5).
1. Energy source and
reserve: It may
surprise some people that at rest, lipids actually supplies 80% to 90% of the
energy requirement for well-nourished individuals. In addition, one gram of fat
is equivalent to 9 kCal of energy. More than twice that of carbohydrate and
protein.
2. Protection of the
vital organs: About
45% of body fat provides protection for the vital organs against physical
trauma.
3. Thermal regulation: Subcutaneous fat (fat under the
skin) provides insulation. This permits individuals to tolerate the cold.
4. Vitamin carrier: Fat is used a transport medium for
fat-soluble vitamins. Specifically vitamins A, D, E, and K. As per Mcardle et
al, approximately 20g of dietary fiber provides a sufficient source and
transport medium for these vitamins. Severely reducing lipid intake, such as in
dieting, can reduce the body’s store of these vitamins, leading to vitamins
deficiency.
5. Hunger Depressor: Fat depresses hunger and gives a
feeling of fullness, satiety. It takes about 3.5 hours for the stomach to empty
lipids.
6. Component of cell
membranes: A derived
lipid, cholesterol forms part of the cell plasma membranes, and a precursor to
synthesizing vitamin D.
7. Component in certain
hormones: Again,
cholesterol helps build adrenal gland hormones and the sex hormones, estrogen,
androgen, and progesterone.
Fat Dynamics in Exercise
(3):
As per Mcardle, intra- and
extracellular fat supply anywhere between 30% and 80% of energy requirement for
physical activity. Depending on the nutritional status, fitness level, exercise
intensity and duration.
The major energy source of light to
moderate exercise comes from fatty acids released from triacyglycerol storage
sites, which is delivered to the muscle as free fatty acids (FFA), and
intramuscular triacylglycerol.
The
start of exercise sees as drop in plasma FFA as the muscle increases it’s
uptake of FFA. An increase in FFA release follows owning to the hormonal
stimulation from the sympathetic nervous system and a drop in insulin levels.
During moderate intensity exercise, carbohydrates and fat supply approximately
equal amounts of energy. If exercise continues at this level for greater than
an hour, fat catabolism gradually supplies a greater percentage of the energy
requirement. This coincides with a drop in glycogen levels.
If there are enough glycogen reserves,
carbohydrate becomes the preferred energy substrate due to its rapid catabolic
rate. Towards the end of prolonged exercise, fat may supply up to 50% to 70% of
the total energy requirement.
Recommended Dietary
Intake of Lipids:
The recommended intake of lipids is
20% to 35% of total caloric intake depending on the type of lipid consumed (3,
4, 5). Saturated and transfatty acids should not be greater than one-third of
the total fat intake, about 10% (4).
The remaining fat intake should be
evenly divided between polyunsaturated fats, 10% and monounsaturated fats, 10%
(4).
Protein:
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The basic structure of proteins is
amino acids. There are two kinds of amino acids, essential and non-essential.
Essential amino acids are those that cannot be synthesized by the body and thus
need to be consumed daily. Non-essential amino acids are those that are
synthesized by the body. There are 9
essential amino acids and 11 non-essential amino acids. (2)
There are also two kinds of protein,
complete and incomplete. Complete proteins are those that contain all of the
essential amino acids. While the incomplete proteins are missing one or more of
the essential amino acids. (2)
High quality protein foods come from
animal sources. Examples are meat fish, eggs, and milk. Vegetables also contain
some proteins, however lack one or more of the essential amino acids. Again,
there are other sources where the reader may get information on different nutrional
values for different foods. (3)
Blood plasma, visceral tissue,
muscle tissue represent the major sources of protein in body. It makes up 12%
to 15% of body mass. (3) The role of protein in the body is listed below (3, 5):
1. Major structural
component of the cell:
This varies considerably. A brain cell may have about 10% protein, while a red
blood cell may have about 20%.
2. Used for growth,
repair, and maintenance of body tissues.
3. Hemoglobin, enzymes
and hormones are produced from protein also.
4. Proteins also help
maintain the acid-base balance in the body.
5. Energy can be derived
by the catabolism of protein.
Proteins Dynamics during
Exercise (3):
With depleted glycogen stores, the
liver may maintain glycogen output via the production of glucose from amino
acid skeletons. This process is called gluconeogenesis. Increased metabolism of
protein during endurance exercise and intense training mirror that of extreme
starvation. One of the processes of converting glucose from amino acids is the
alanine-glucose cycle. This can generate about 10% to15% of the total energy
requirement.
Recommended Dietary
Intake of Proteins:
Proteins should account for 5% to
15% of total caloric intake (1, 4, 5).
A recommended amount is 0.83g per kilogram
of body mass. For infants and growing children, the recommended intake is
suggested to be 2.0g to 4.0g per kilogram of body mass. Pregnant women are
recommended to increase their protein intake by 20g, nursing mothers by about
10g. Recommendations for athletes who train intensely are 1.2g to 1.8g per
kilogram of body mass.
Bibliography:
1.
Corbin,
Charles B., Gregory J. Welk, William R. Corbin and Karen A. Welk “Concepts of
Fitness and Wellness: A Comprehensive Lifestyle Approach 9th
edition” 2011
2.
Powers,
Scott K. and Stephen L. Dodd “Total Fitness and Wellness 5th
Edition” 2009
3.
Mcardle
William D., Frank I. Katch and Victor L. Katch “Exercise Physiology:
Energy, Nutrition, and Human Performance
7th edition” 2010
4.
Heyward,
Vivian H. “Advanced Fitness Assessment and Exercise Prescription 4th
edition” 2002
5.
Wilmore,
Jack H. and David L. Costill “Physiology of Sport and Exercise 2nd
Edition” 1999
