The fundamental responses are first to efficiently use consumed nutrients for energy production, mainly carbohydrates and fat, then to generate sufficient energy for current needs.

Also an energy reserve or some fat mass, must be maintained in case of increased needs or decreased intake.

Secondly, there must be sufficient protein synthesis, using consumed protein, and an anabolic stimulation from hormones and growth factors, to repair and preserve the lean mass.

What Regulates Metabolic Activity?

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Hormonal Regulation of Metabolic Activity

The next step in complexity is to understand how energy and protein synthesis can be so closely regulated in uninjured man. This regulation is performed by hormones, produced by the endocrine system, which become the chemical messengers to the metabolic pathways to energy and protein balance.

Insulin is the key hormone responsible for converting carbohydrates or glucose to energy and converting excess glucose to fat and maintaining a normal blood glucose.

Catecholamines are always present but increased release occurs with any stress so as to provide and immediate increase in energy. Metabolic rate increases, relative to the stress, as does blood glucose due to the anti insulin properties.

Cortisol is a hormone released to counterbalance inflammation, such as a burn and is released in response to stress. It has potent catabolic activity leading to protein breakdown and also causes excess glucose production as an attempt to increase available energy.

Growth Hormone is the body’s most potent stimulus to anabolism or protein synthesis, responsible for growth and also preservation of lean mass during starvation. Blood glucose is also increased via a separate pathway.

Glucagon is a hormone also released in increased quantities after stress. With stress this hormone is most responsible for the excessive increase in glucose production (gluconeogenesis) using amino acids, (a catabolic effect).

All hormones in the non stress state must be balanced to achieve the appropriate energy and protein balance. The stress hormones catecholamines, cortisol and glucagon are released with any stress response, especially burns.

NORMAL METABOLISM - NON STRESS SYSTEM NORMAL NUTRIENT INTAKE

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NORMAL METABOLISM

All the components of the normal metabolic response in the non stress system is schematically described: 

1) An efficient pathway to energy production by the macronutrients, carbohydrate, fat, and to a much lesser extent protein.

2) Adequate oxygen available for the required energy production.

3) And protein consumed is used primarily for protein synthesis to repair and maintain lean mass and all its components. Normal metabolic rate is  25/30/kcal/kg/day with at least 55% coming from carbohydrates intake and 20-30% coming from fat intake. Less than 10% of Kcals comes from protein with the typical intake of protein being 0.8 g/kg/day. Energy production is adequate to meet demands including the energy for protein synthesis. The fat store is available if needed. Lean mass is maintained and there is no net protein loss.

Of importance is the sensitivity of the lean mass compartment to anabolic or catabolic stimuli as will be seen in the stress state. A balance of the metabolic hormones is responsible for the well tuned system. An intact skin is essential for avoiding excess energy losses via heat and water loss.