The catecholamines adrenaline, noradrenaline and dopamine are produced from the non-essential amino acid L-tyrosine through a series of enzymatic reactions that are controlled by a rate limiting step catalysed by tyrosine hydroxylase. Catecholamines are important neurotransmitters in the brain that are involved in facilitating arousal, motivation and mood. Outside of the brain the catecholamine adrenaline is involved in the response to external stressors, and is also a dominant hormone in controlling metabolic rate and fat oxidation. Following food consumption, catecholamine levels in the peripheral tissues rise and this increases body temperature in a process called the thermic effect of food (TEF). A high TEF is associated with increased energy wastage, while a low TEF is associated with preservation of energy. It has been shown that obese and overweight individuals have a blunted TEF when compared to those of normal weight. The catecholamines are therefore involved in motivation, arousal and energy utilisation.
The importance of a correct TEF to weight maintenance and body composition is demonstrated by the effect of ephedrine, a compound derived from the ma huang herb. Ephedrine can activate the adrenaline receptors in humans and studies show that it causes beneficial fat loss and motivation effects that facilitate weight loss and exercise. If brain levels and postprandial circulating levels of the catecholamines are low in obese subjects this would be expected to cause a demotivated and unaroused individual who found it difficult to oxidise energy and who gained weight easily. And when we look at those who are obese this is what we find. There is an association between physical activity levels and body weight, and this is often used to justify the stance that the obese are lazy and that they should just exercise more. But the cause and effect cannot be ascribed in this association, and it is perhaps the case that a faulty catecholamine metabolism is actually the cause of both of these variables.
As with ephedrine, administration of L-tyrosine in normal healthy individuals is able to increase catecholamine production, and this in turn increases motivation and arousal and may increase body temperature. The effects of L-tyrosine supplements on catecholamine production has been studied in obese and lean subjects by measuring the amount of catecholamine metabolites detected in the urine1. Obese and lean women were fed a liquid formula diet with or without L-tyrosine at 0.26 grams per kg of body weight and metabolites of catecholamines were monitored. Lean subjects responded to L-tyrosine supplementation with elevations in plasma tyrosine concentrations as well as increases in the excretion of the catecholamine metabolites 3-methyl-4-hydroxyphenyl ethylene glycol, vanilmandelic acid and homovanillic acid. However, the obese women had no increases in 3-methyl-4-hydroxyphenyl ethylene glycol but did show increases in vanilmandelic acid and homovanillic acid.
These results therefore suggest that catecholamine metabolism in obese women may be faulty. The authors noted that in lean subjects, levels of 3-methyl-4-hydroxyphenyl ethylene glycol were related to the degree of fatness, while levels of vanilmandelic acid were related to the degree of lean tissue and to the energy utilisation. However, these associations were not apparent for the obese women. The faulty catecholamine metabolism in obese individuals has been observed in other studies, and in particular a blunted thermogenic response to food ingestion is thought to be present in those who are overweight. However, whether this faulty catecholamine metabolism is the cause of or a result of the obesity is not fully understood. The fact that iron is the cofactor required for the rate limiting enzyme in catecholamine synthesis, and that the obese have been found to be at greater risk of iron deficient anaemia suggests (here) that a faulty iron metabolism might in some way play a contributory role in this dysfunctional catecholamine metabolism.
Dr Robert Barrington’s Comments: Obesity is characterised by a dysfunction in normal metabolic regulation, particularly that involving energy utilisation. A faulty catecholamine synthesis might be part of this dysfunction. Those who successfully lose weight in the long term do not try to limit energy and perform physical activity in the face of such a metabolic dysfunction. Instead those who are successful reverse the metabolic dysfunction through consumption of a high quality diet that provides all the required essential nutrients and which is absent of the metabolic poisons that cause metabolic aberrations. This then allows normal utilisation of energy and weight loss occurs without the need to force the issue.
RdB
