Fat metabolism in exercise – with special reference to training and growth hormone administration

Abstract

Despite abundance of fat, exclusive dependency on fat oxidation can only sustain a metabolic rate corresponding to 50-60% of VO(2max) in humans. This puzzling finding has been subject to intense research for many years. Lately, it has gained renewed interest as a consequence of increased obesity and physical inactivity imposed by Western lifestyle. Why are humans so poor at metabolizing fat? Can fat metabolism be manipulated by exercise, training, diet and hormones? And why is fat stored in specialized adipose tissue and not just as lipid droplets inside muscle cells? In the present review, human fat metabolism is discussed in relation to how human fat metabolism is designed. Limitations in this design are explored and examples of different designs for fat metabolism from animal physiology are included to illustrate these limitations. Various means of manipulating fat metabolism are discussed with special emphasis on exercise, training, growth hormone (GH) physiology and GH administration. It is concluded that fat stores, non-esterified fatty acids (NEFAs) availability and enzymes for fat oxidation can be increased substantially. However, it is almost impossible to increase fat oxidation during endurance exercise at higher intensities. It seems that, for some reason, the human being is far from optimally designed for fat oxidation during exercise. Acute GH administration has several unexpected effects on fat and carbohydrate metabolism during aerobic exercise, and future research in this area is likely to provide valuable information with respect to GH physiology and the regulation of fat and carbohydrate metabolism during aerobic exercise.

Role of proton MR for the study of muscle lipid metabolism

“…Boesch et al (2) revealed a decrease of IMCL in one subject performing a mountain bike ride over 3 h at about 70% of VO 2max , for the first time using 1 H-MRS to demonstrate that IMCL are metabolized during this form of exercise. In the following years, numerous reports on reduced IMCL levels during exercise were published and reviewed (17,20,(34)(35)(36)(37)(38)(39)(40)(41)(42)(43)(44)(45)(46)(47)(48)(49). It became obvious that both exercise duration and intensity are important factors in determining the reduction of IMCL (39).…”

Hormônio do crescimento e exercício físico: considerações atuais

“…Além destes dois peptídeos, outras isoformas e fragmentos da molécula principal de GH estão presentes em quantidades variáveis na circulação em conseqüência de modificações após a etapa de tradução protéica (Baumann, 1991). Acredita-se que estas isoformas exerçam, também, importantes influências no desenvolvimento placentário, gravidez e lactação, porém ainda são pouco descritos na literatura (De Palo et al, 2001;Lange et al, 2004).…”

“…Os mecanismos responsáveis por estas alterações não estão totalmente esclarecidos na literatura. Apesar disso, outros diversos fatores podem influenciar a secreção do GH, incluindo o estado nutricional (Tirapegui et al, 2005), a quantidade de sono (Van Cauter, Copinschi, 2000) e de gordura corporal (Lange et al, 2004), o estresse (Bonifazi et al, 1998) e a prática da atividade física ou grau de treinamento (Gomes et al, 2004).…”

“…Dentre os hormônios mais estudados pela ciência está o hormônio do crescimento (GH) (Lange, 2004;Doessing, Kjaer, 2005). Alguns aspectos fisiológicos, contudo, ainda permanecem controversos e desconhecidos, incluindo fatores que regulam sua síntese, mecanismos de ação e efeitos no metabolismo protéico e lipídico.…”

Influence of Vascular Occlusion in Concentration of Growth Hormone and Lactate in Athletes during Strengthening Quadriceps Exercise

“…This can be explained by the heterogeneity of the groups, which was beyond our control, since the division of the groups in question occurred randomly. In addition, variables such as level of training [29], body composition [30], gender [31] and the age of individuals studied [32] may also modulate the release of this hormone [33,34].…”