The Fires of Life--
The Body's Respiratory Metabolism

Weeks 3 and 4

Reading:UCSD Nutrition:Chapters 8-10
New Nutrition:Unit 1

1. Premise: Life requires energy.

A. In order to maintain and sustain the complexities and activities of life, energy must be acquired by the metabolic burning of the dietary "fuels" in the presence of oxygen, a process called respiration. Some of this energy is released as heat. The rest is conserved as adenosine triphosphate (ATP) the universal energy-rich compound in all living systems. ATP is used, transduced, by our cells to provide the five forms of bio-energy:

  • thermal (heat)
  • electrical (nerve impulses)
  • mechanical (muscle contraction)
  • chemical (synthesis, osmotic balance)
  • light (bioluminescence)

B. The primary fuels of our body are supplied in our diet as complex carbohydrates (starches), fats and proteins. These are broken down by digestion to their simple chemical components and then absorbed into the blood circulation.

complex carbohydratesfatsproteins
sugarsfatty acids + glycerolamino acids
C. Within each cell these simpler fuels are chemically transformed by enzymes (metabolism) prior to their burning in the presence of oxygen (O2) to give carbon dioxide (CO2) and water (H2O) via a common pathway, the Krebs Cycle.

In this simplified diagram (consult good biology text for details) the following symbols are used:

O2 - oxygenCO2 - carbon dioxide"C2" - active acetate
C4 - four carbon
organic acids
C5 - five carbon organic
C6 - six carbon organic
ATP - adenosine triphosphateADP - adenosine diphosphatePi - inorganic phosphate

D. The most significant aspect of this respiratory cycle is the conservation of chemical energy as ATP. Over 95% of our body's usable energy is provided through this respiratory cycle.

E. Special and important considerations:

  • Sugars can be converted to fatty acids and fats and some amino acids
  • Less than 1% of our body's energy is stored as carbohydrate (glycogen)
  • Fatty acids cannot be converted to sugars
  • Sugars and some amino acids are interconvertable.
  • Most amino acids are burned directly or indirectly in the Krebs Cycle
  • Ammonia, a waste product from burning amino acids, must be excreted as urea

2. Energy can be both measured and managed

A. The calorie is a unit measure of energy (The more exact and accepted chemical unit is the joule ). All calories are equal. Thus a calorie of energy from the burning of sugar is the same as a calorie from fat or protein. Values commonly seen on labels for foods are given in kilo calories, l000 calories (Kcal), but are usually described as calories (cals).

WARNING!!! A gram of fat contains 9 cals; a gram of sugar or protein, 4 cals; and a ml of alcohol (approximately one gram), 7 cals. The big nutritional problem with fat is not its caloric density (cals/gr), but rather its sensual properties of taste, smell and mouth feel which make fatty foods more enjoyable and less resistible.

B. Two very important and universal laws of nature which describe how energy is managed.

  • The First Law of Thermodynamics: Energy can neither be created nor destroyed. It can only be transduced from one form (thermal, electrical, mechanical, chemical, light) to another.

Thus if we consume and assimilate energy in the form of food and do not burn (oxidize) that food to make and use ATP or generate heat, then that food must be stored in the body as reserve chemical energy i.e. fat or protein.

A simple way to remember the First Law is "If you eat it and don't burn it, you sit on it"

C. The energy balance of the body can be pictured as:
FoodSynthesis and Storage

Basal Metabolic Rate
Muscular exercise

D. Your basal metabolic rate (BMR) is governed by many factors including genetics, eating and digesting food, hormones. However physical activity is one of two principal factors which we can control and modulate to regulate a our body weight. The second is to reduce our caloric intake of food. Remember--all calories count!

  • The Second Law of Thermodynamics: Energy, like water, flows from a higher level (potential) to a lower one. That is to say, highly organized systems, such as the human body, are continually running down and becoming disordered or chaotic (entropy). Energy must constantly be expended to maintain the integrity and complexity of life.

The Second Law can be paraphrased "Ashes to ashes. Dust to dust".

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