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All living cell functions require an ongoing supply of energy derived from carbohydrates, lipids and proteins with their own pathways of breakdown. All of them end up in the oxidation of reduced coenzymes, yielding chemically-bound energy in the form of adenosine triphosphate (ATP). One broad definition of energy would be the capability to do work and, therefore, the more work that has to be done, the more energy is needed, which may under extreme conditions put the cell into a state of energy metabolic stress. This complex of problems has been examined in the present thesis, where individuals representing different degrees of training status, have been subjected to various types of stressful work-loads as regards intensity and duration. Meanwhile, the energy turnover has been monitored on different levels as whole body (organism)-, single organ/tissue-, cellular and molecular levels.

Combined methodologies have been developed and utilized to examine carefully and in some detail energy expenditure and biochemical variables with study subjects under long-term, (outfield) physically and mentally stressful conditions.

When the individuals were in a well-controlled energy balance, a diet rich in saturated fatty acids did not elicit any major metabolic stress signs concerning serum lipoproteins and/or insulin/glucose homeostasis during the test period including high volume and low intensity energy turn over. Only a slight decrease in the Apo-B / Apo-A1 ratio was observed, despite a period of totally sedentary life style among the participants. Mental stress combined with a varying energy balance during off-shore sailing races was shown to cause such an energy metabolic stress situation that development of abdominal obesity and signs of a metabolic syndrome in embryo affected the participants who were young, non-obese men and despite their fairly healthy lifestyle concerning the diet they were on and their physical activity habits. Even well-trained young individuals of both sexes, subjected to exhaustive endurance (high intensity exercise session), developed signs of insulin resistance with a deteriorated intracellular glucose availability leading to a supposed ion pump failure and a disturbed osmoregulation on a cellular level. Hence, they presented themselves as having acquired an energy metabolic stress like condition.

In conclusion, an energy metabolic stress syndrome has been described, basically due to impaired fuelling of ion pumps with a cluster of signs and symptoms on single organ/tissue-, cellular and molecular levels manifested by muscular intracellular swelling, tendency towards erythrocyte shrinkage as a consequence of a relative insulin resistance concomitant with ion distribution disturbances (Gardos effect), oxidative stress and osmoregulatory taurine leakage.