Nutrition for Fitness Leaders

Energy, Calories and Caloric Balance

Tracey Kobayashi

50 Phelan Ave, NGYM
San Francisco, CA 94112
(415)452-7311
tkobayas@ccsf.edu

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[ Intro ] [ Nutrients ] [ Energy ]
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Calories

One calorie = amount of energy required to raise one gram of water by one degree celsius. Nutritionally, energy units are stated in terms of Calories (capital C).

1 Calorie = 1 kilocalorie = 1,000 calories

Macronutrients all provide energy:

• Carbohydrate -- 4 Cal/g
• Protein -- 4 Cal/g
• Fat -- 9 Cal/g

Energy Requirements

The body needs energy to perform metabolic reactions. In general, glucose or fatty acid is hydrolyzed to form adenosine triphosphate (ATP), the direct energy substrate used by the cells.

Excess energy is stored in the adipose tissues in triglyceride form -- excess glucose or protein is converted into lipid. When necessary, protein can be converted into glucose.

In general, body weight is related to caloric balance, or the difference between calories consumed and calories expended. A weight change of 1 lb is associated with 3500 calories. Approximate basal caloric requirements for an average moderately active person is 1200 Cal for women, 1800 Cal for men.

Energy for Physical Activity

The macronutrients aren't actually used directly by the cells for work. They are, in turn, converted into an energy-rich compound called adenosine triphosphate, or ATP. ATP is an adenosine molecule (composed of adenine and ribose) linked to three phospate molecules. When ATP combines with water in a process called hydrolysis, catalyzed by the enzyme adenosine triphosphatase, the outermost high-energy phospate bond is broken, forming adenosine diphosphate (ADP), and creating energy. Approximately 7.3 kcal of energy is liberated per mole of ATP. ATP splitting takes place whether oxygen is available or not, so is considered non-robic.

Because ATP can't be supplied through the blood, it needs to be recycled in the cell. To quickly resynthesize ATP, another high-energy phosphate compound, creatine phosphate (CP), also exists in the cells in concentrations three to five times greater than that of ATP. Energy from CP breakdown is used to rebond ADP and the free phospate (P). Only a small amount of ATP (enough for a few seconds of work) is stored in the cell, so its relative concentration is influenced greatly by any increase in a cell's energy metabolism. This change stimulates the breakdown of stored nutrients to provide energy for ATP resynthesis.

Carbohydrates. Carbohydrates are the only nutrient that can generate ATP anaerobically. In vigorous activities requiring energy above levels that can be supplied by aerobic reactions, stored glycogen and blood glucose supply the main portion of energy for ATP resynthesis. During light and moderate activity, carbohydrate supplies about half of the body's energy. Continual carbohydrate breakdown is required for fats to be processed and used for energy, too. There are 2 stages for glucose degradation:

• Glucose breakdown to pyruvic acid. At the expense of 2 ATP, glucose is first primed by adding a phosphate. This phosporylated molecule is then split in several reactions to 2 pyruvic acid molecules, yielding 4 ATP molecules, resulting in a net gain of 2 ATP molecules.
• Pyruvic acid breakdown to carbon dioxide and water. Pyruvic acid is converted into acetyle-CoA, a form of acetic acid, which enters the 2nd stage of breakdown, called the Krebs, or citric acid, cycle. Krebs breaks down the acetyl-CoA into carbon dioxide and hydrogen atoms in the mitochondria, which also yields 2 ATP.

Along the way, hydrogen atoms are formed in each reaction. These hydrogen atoms are further oxidized in an electron transfer process, collectively termed oxidative phosphorylation. Once glucose molecule yields 32 ATP through oxidative phosphorylation. If oxygen isn't available (as in high-intensity work), lactic acid is formed, which diffuses into the blood until oxygen is available again.

Fatty Acids and Glycerol. Glycerol can enter the anaerobic glycolytic reactions and degrade to pyruvic acid. Complete glycerol breakdown yields 22 ATP molecules. Glycerol may also provide the carbon skeleton for glucose synthesis, important when carbohydrate is restricted in the diet or during long-term exercise which strains glycogen reserves.

Fatty acid breakdown involves oxidation into acetyl-CoA, which then enters the Krebs cycle if sufficient oxaloacetic acid (generated from carbohydrate metabolism) is available. Fatty acid breakdown is directly associated with oxygen uptake. Each fatty acid molecule yields 147 ATP. Each triglyceride molecule contains 3 fatty acids, so one neutral fat molecule can yield 441 ATP from the FFAs, plus the 22 from glycerol catabolism, totaling 463 ATP. Unlike glycerol, fatty acids can't be used to synthesize glucose.

Protein. Amino acids are first deaminated (nitrogen is removed) and converted into a form that can readily enter the energy pathways. These residues enter the Krebs cycle at various intermediate stages.

Substrate Usage

Balance of macronutrient usage as energy substrates depends upon:

• Level of Activity.
• Psychological and Health Status.
• Nutritional State.
• Endocrine Status (influenced by the above factors).
  • Moderate to intense activity -> growth hormone secretion.
    GH stimulates FFA release and protein synthesis -> FA's available for energy production, proteins not.
  • Emotional or Physical Distress -> cortisol release.
    -> ffa release -> fa usage increase
  • Blood glucose levels control insulin and glucagon secretion.
    High glucose -> insulin release -> inhibit fa release, enhance glucose usage
    Low glucose -> glucagon release -> enhance glycogenolysis and gluconeogenesis, inhibit glucose use

[ Intro ] [ Nutrients ] [ Energy ]
[ Apps ] [ Commercial Diets ]