Wednesday, July 31, 2013

Nutrition: The Macronutrients



            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:


(Image from positivemed.com)
 

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:


 
(image from thisfitchick.wordpress.com)


(image from www.greenvitals.net)











            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:

 
(image from www.functionalfitmag.com)

            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