Metabolic Energy

Human action is an expression of biological energy derived from food. Living cells are glucose-burning machines. Animals take advantage of the ability of plants to manufacture sugar and other nutrients. The energy which supports us is locked into the molecular bonds of a few basic fuel molecules: glucose, fructose, fatty acids, and amino acids. The energy is released as the energy-supplying molecules are dismantled by oxidation. Food-derived energy allows us to move, to do work by muscle contraction, and to keep warm. Body heat is generated by the metabolic activity of every cell.

Stephen Gislason MD

Carbohydrates and fats are the principle sources of energy, although amino acids may be utilized as energy. Combustion of amino acids requires the excretion of nitrogen, which is first converted to ammonia. Glutamine is the shuttle which carries ammonia from rapidly-metabolizing tissues to the liver. The liver converts ammonia to urea, which is delivered to the kidneys for excretion in the urine.

The energy requirement of any individual is determined by physical activity. Your energy balances shift with variations in food intake and activity level. A healthy, active adult will usually spend 1000-3000 Kcal per day of food energy (or approximately 33Kcal/Kg). Daily physical exercise is beneficial and tends to promote normal body weight, with energy intake matching output. With food restriction, increased metabolic efficiency allows the body to do better with less. This increased efficiency, induced by caloric restriction, tends to frustrate people seeking to lose weight.

The basic equation of energy requirement is:

Energy Required =

Basal Metabolic Rate + Work Energy

+ Metabolic Overhead

Body weight remains constant when this equation is balanced. Metabolic overhead is the amount of energy consumed by digestion and absorption of food, plus the work done to synthesize the thousands of compounds we need to stay alive. A basal energy requirement (Basal Metabolic Rate) for a healthy adult with no other energy expenditure would be in the range of 1000-1500 Kcal/day. Excess food energy may be stored as fat at the rate of 7900 Kcal/Kg of fat.

Food energy is also expressed as heat. As caloric intake increases, heat production also increases. Heat production accounts for 8-18% of food intake. There is a relationship between body heat and appetite. An increase in body temperature depresses feeding behaviors, and vice versa. Two types of fat stores are recognized: brown and white fat. White fat is distributed in our subcutaneous tissues and creates the "look" of our bodies. Brown fat produces heat. If the heat production by brown fat is reduced, the body cools abnormally; appetite and fat storage are stimulated.

Cellular Energy Engines

Living cells are basically glucose-burning machines. Glucose, of course, is sugar. The energy which supports us is locked into the molecular bonds of a few basic fuel molecules: glucose, fructose, fatty acids, and amino acids. The energy is released as the energy-supplying molecules are dismantled by oxidation.

Our metabolic machinery must be prepared to cope with a variable and inconsistent food supply. An interchange arrangement prevails in living systems which permits switching among fuel choices. Liver cells can convert sugars into amino acids and fatty acids, or visa versa. The body is prepared to take almost any nutrient combination and turn it into energy. Some food combinations are probably better than others.

The liver is responsible for chemical processing and storage of nutrients derived from food. Absorbed molecules are delivered to liver cells which extract nutrients and decide what interconversions to implement; for example, 60-70% of surplus amino acids entering the liver from a high protein meal may be converted to sugar. Proteins in excess may contribute to sugar or fat synthesis, but not to tissue construction as some body builders hope. The liver manufactures glycogen, a glucose polymer, to serve as a fuel reserve. Fatty acids are also processed and interconverted. If protein intake is inadequate, new amino acids may be synthesized from fragments of sugar or fatty acids.

Mitochondria

The three macronutrient groups can be reduced to the small molecule, Acetyl Co A, which is then burned in the mitochondria, the energy engines in our cells. Sugars are funneled through pyruvate into the mitochondrial engine, known as Kreb's Citric Acid Cycle, KCAC, an elegant rotary engine.

The Kreb's cycle processes fuel molecules through a series of intermediates, extracting energy and exhausting carbon dioxide and water. The energy obtained is transferred to a storage molecule, Adenosine Tri Phosphate or ATP. Phosphate plays an essential role in body energy dynamics. Phosphates are attached to Adenosine to store energy. Adenosine can accept up to 3 phosphate bonds. Three phosphates make a fully charged adenosine or ATP. It is estimated that 47% of the energy available in a glucose molecule is captured in ATP for later use in the cell.

Muscle Work

Every rotation of the Kreb's cycle produces 12 new high-energy phosphate bonds. ATP releases its energy when it gives up one phosphate and becomes ADP, the two-phosphate version of adenosine. ATP is distributed throughout all cells of the body, acting as a local battery to fuel all molecular transactions. If the battery runs down, cellular processes slow-you run out of gas. ADP must be recharged by the mitochondrial rotary engine.

The branch-chain amino acids branch-chain amino acids-valine, leucine, and isoleucine-are processed through mitochondria also. We can call this the Mitochondrial Amino Acid Processor (MAAP). MAAP dismantles the amino acids in a sequence which requires 3 enzymes and the co-factors, Biotin, VM.B12, and magnesium. Amino acids are reduced to a common fragment, succinyl choline, which enters Kreb's cycle (KCAC) for further processing into carbon dioxide (CO2), water (H20), and high-energy phosphate bonds.

Fatty acids are important fuels, burned by cellular oxidation engines. Two methods are recognized:

A or alpha oxidation (We can call these A engines).

B or beta oxidation (We can call these B engines).

The A engines work by nibbling carbon atoms from the end of the FA chain. Brain cells tend to have A engines. B engines, in the mitochondria work in a step-wise manner. First, the fatty acid is brought into the mitochondria and activated. The amino acid, carnitine, is essential at this step. Without carnitine, B engines will not work and triglycerides accumulate. Carnitine, synthesized from lysine in the liver, is not an essential amino acid. Whenever there is clinical evidence that B engines are not working well, it is reasonable to supply carnitine as a supplement.

(Note: carntine is not creatine)

Once a fatty acid is activated in the mitochondria, the B engine produces acetyl Co A which then passes to the all-purpose energy machine, the Kreb's cycle (KCAC), for the production of ATP. The liver has a special version of B engines, which produces another fuel, ketone bodies of which there are three: acetoacetate, hydroxybutyrate, and acetone. Ketones are alternative fuels preferred by brain cells whenever glucose is in short supply.

Skeletal Muscle Energetics

The total creatine content of muscle cells is controlled by an active creatine uptake in which beta 2-receptor stimulation and the activity of the Na(+)-K(+)-ATPase play a significant role. Recovery after exercise is entirely oxidative; the rate of ATP synthesis is largely controlled by ADP, the concentration of which is determined by the creatine kinase equilibrium that includes the concentration of H+. At the onset of aerobic dynamic exercise, ATP is maintained largely by glycolysis, producing lactic acid, and by phosphocreatine breakdown. After vasodilation, ATP synthesis becomes predominantly oxidative. (Radda GK.Diabetes, 45 Suppl 1:1996 Jan, S88-92)

Transition from anaerobic to aerobic energy

Mechanical power output reached its peak immediately after the beginning of exercise, then rapidly declined, becoming gradual after 60 s. The O2 debt and O2 deficit were highest immediately after the beginning of exercise, then rapidly decreased to nil in 60 s. The O2 intake was small at the beginning, then rapidly increased to attain a steady state in 30 s at 80%-90% of the maximal O2 intake of the subject. Energy supply from the lactic acid system  reached its highest value during the period between 5 and 15 s, then rapidly decreased to nil in 60 s.

The results would suggest that anaerobic supply was the principal contributor during the initial stage of exercise, but that aerobic supply gradually took over. In 60 s anaerobic supply ceased, and aerobic supply became the principal contributor. The cessation of anaerobic energy supply took place much sooner than the 2 min that is conventionally suggested. ( Yamamoto M; Kanehisa H. Eur J Appl Physiol, 71: 4, 1995, 320-5)

Four anaerobic variables

short-term peak power (SPP)

short-term anaerobic capacity (SAC),

intermediate-term peak power (IPP)

total work (TW)

Interval training increases aerobic power and also enhanced performance in repeated high intensity, short duration work.

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