What is role of protein and amino acid metabolism?
Date: Thursday, November 14th, 2002
Title: “Protein, amino acid metabolism and central signals who drives the system?”
Speaker: Daniel Tomé, PhD, Institut National Agronomique Paris-Grignon, France
Dietary proteins provide amino acids that have major metabolic functions in the body. They are the substrates for protein synthesis, and their capacity to provide essential amino acids is of crucial importance for preserving body functions. They are involved at different level in energy metabolism as substrates (directly or through gluconeogenesis) and as precursors of intermediary components involved in the Krebs cycle. They are also the precursors for numerous nitrogenous components including nucleic acids, nitrogen monoxide, glutathione, polyamines, creatine, taurine, histamine, catecholamines (dopamine, epinephrine, norepinephrine) or serotonin. The capacities to adapt to a large range of protein intake over the specific requirement of nitrogen and essential amino acids for protein synthesis is probably high due to large capacities of metabolic adaptation and to a complex central control of energy nutrient handling.
The input signals associated with the ingestion of amino acid and other energy nutrients originate from visceral and metabolic mechanisms and involve both indirect (mainly vagus-mediated) and direct information (plasma level of nutrients and hormones) recorded by the central nervous system. Protein and energy homeostasis could involve: (i) an indirect visceral sensitivity for both nutrient tasting and signaling mechanisms that mainly records information regarding availability of energy substrates (e.g., ATP- turnover, T°, thermogenesis); (ii) a central nutrient and metabolic chemosensor system for essential amino acid and glucose plasma concentration (and hormones such as insulin and leptin); (iii) specific mechanisms associated with the central availability of specific amino acid precursors (e.g., histidine and tryptophan) of neurotransmitters such as histamine and serotonin (as well as the catecholamines).
Brain levels of some amino acids are influenced by their blood concentration and by the blood levels of other amino acids as well as through blood brain barrier competition. Since diet readily alters blood concentrations of amino acids, it thereby also influences their brain level. The existence of an Amino Acid Chemosensor system, localized in the Anterior Piriform Cortex (APC) of the brain, has been convincingly demonstrated for essential amino acids from experiments using essential amino acid devoid diets which produce an anorectic response. In addition, the rates of neuronal synthesis by brain of the neurotransmitters histamine, serotonin and the catecholamines (dopamine, epinephrine, norepinephrine) depend on the brain levels of the respective limiting precursor molecules (histidine, tryptophan and tyrosine, respectively).
The acute response to a meal involves a cascade of transient metabolic processes in which the rate of dietary amino acid absorption and their resulting post-prandial pattern in blood and in tissues are potent modulators of protein synthesis, breakdown, and oxidation. Dietary amino acids transiently appear in the blood and either participate in post-prandial repletion in a different part of the body, or are routed to catabolic pathways.
As with glucose, plasma amino acid levels are under homeostatic regulation, but fluctuate by approximately 20%. The protein losses of interprandial (post-absorptive) periods is counteracted through meal induced enhancement of tissue protein deposition, but protein mass is precisely regulated and cannot be easily modified. When protein synthesis is satisfied, additional amino acids are increasingly oxidized and used as energy substrates either directly or through gluconeogenesis, thus at the expense of carbohydrates. Under those conditions, increasing protein intake over the requirement for protein synthesis does not further stimulate the protein deposition pathway but increases amino acid oxidation and its contribution to energy metabolism. The regulators of these peripheral processes are nutrients (mainly amino acids and glucose) and hormones (insulin and counteracting hormones). The consequences of these processes, and more particularly the feedback shift between amino acid and carbohydrate utilization, in energy homeostasis, nutritional regulations and body composition remains incompletely understood.
Except for very high protein and amino acid diets, which are not physiologically relevant and impair well-being, there are two main situations regarding the level of protein intake: 1) adequate levels of protein intake that satisfy nitrogen and essential amino acids requirements for protein synthesis, and 2) high levels of protein intake that increase the contribution of amino acids to energy metabolism. During protein and/or essential amino acid deficiency in the diet, very strong mechanisms exist that detect such deficiency in order to re-equilibrate the diet. They can involve a very rapid fall of the deficient essential amino acid in plasma, immediately detected by the Amino Acid Chemosensor system in the APC, thus initiating an aversive anorectic response. In addition, low protein diets were demonstrated to produce a subsequent increase in brain histidine and histamine with aversive response to the diet. Another situation is the increase in protein intake over the level expected to meet protein and amino acid requirement. When rats are changed over to a high protein diet, they have an immediate depression in food intake followed by a progressive but not complete return to the level of energy intake of the control diet. The overall behavioral response is probably due to a lower initial palatability of the high protein diet together with an enhanced satiety effect of the high protein diet and a delay required for metabolic adaptation.
Interestingly, different results also showed that rats offered the possibility of self-selecting a protein and macronutrient level usually established a high protein intake in the range 30-50% of total energy intake. The reason(s) for the high protein choice remains unclear but is certainly not motivated by the palatability of pure proteins that is known to be low, thus suggesting that some metabolic response associated with high protein intake may bear positive post-absorptive signals. Indeed, the ingestion of a high protein diet produces in the NTS a dramatic decrease of dopamine activity, probably associated with the food reward system. Thus the high protein diet has a low initial palatability and after adaptation, the conditioned palatability increases. In contrast, animals adapted to a high 50% protein diet with a low level of sucrose for 6 months have a reduced adipose tissue, basal insulin, leptin, cortisol and glycemia, and higher glutathione compared to a normal adequate 10-15% protein diet. These observations suggest that, in addition to serving as a source for the renewal of body proteins, dietary amino acids may also function profitably as an alternative energetic substrate for carbohydrates, thereby reducing the postprandial metabolic dependence on insulin. By providing alternative substrates to carbohydrates through gluconeogenesis, amino acids may improve the regulation of energy metabolism. A feedback shift between carbohydrates and protein could be the main mechanism involving both the amino acid and glucose central chemosensor systems, insulin and serotonin (that both depend on tryptophan availability and specifically reduce carbohydrate intake), together with the ischymetric regulation of macronutrient intake.
These effects contribute to the influence of protein on endocrine regulations and body composition. The feedback shift between carbohydrate and amino acids probably represents a central process that allows regulation of glycemia and amino-acidemia in a large range of protein/energy ratio, i.e. 20-40% energy intake. Indeed, the mechanism for precise control of protein intake would have little metabolic advantage. In contrast, the system both responds precisely when protein and essential amino acid intake is inadequate but also allows a large range of adaptive capacities through amino acid degradation and substrate interconversion.
Q. Is there a signal or detector to which the animal responds when they consume these diets, or is this behavior simply a result of other actions?
A. We do not know of any depletion signals that trigger changes in feeding, except for leptin-mediated signaling arising from adipose tissue. There may be no need for a signal in the case of amino acids.
Q. If you give the animals an amino-acid rich diet, do you find increased renal excretion?
A. No; there is no excretory pool, just increased catabolism.
Q. Is there a point at which the animal becomes incapable of eliminating excess amino acid in the diet?
A. We have no evidence that amino-acid elimination becomes insurmountable, but we do know the animal undergoes significant metabolic changes to compensate for excessive amino acid intake. You would potentially see decreased glutamate synthesis on a very high protein diet and a shift from carbohydrate oxidation to protein. So the body would compensate by increasing the oxidation of amino acids. With a very high protein diet, a problem with ammonia handling could probably arise.
Q. Does vagotomy alter protein satiety?
A. Yes; vagotomized rats exhibit an attenuated response to high-protein diets. They reduce overall energy intake but not nearly as much as an intact animal.
Q. Since vagotomy alters the response to amino-acid diets, do you think there are detectors in the gut for amino acids?
A. There is no direct evidence to support the notion of a very specific detection of amino acids in the gut, however but Helen Raybould is exploring this possibility. It appears more likely that the effects in the gut are secondary to other metabolic processes that occur there.
Q. Are amino acid or protein diets ever palatable?
A. We haven’t observed any instance in which amino acid or protein compounds are palatable.
Q. Your data suggest that the specific effect of amino-acid depletion only occurs when some threshold has been reached; is this possible?
A: Our data do suggest that there’s a requisite level of depletion before the animal comes to prefer the replete diet.
Q. In your studies, the high protein diets were compared to high carbohydrate diets, but if you had also compared high-protein diets to those high in fat, rather than carbohydrate, would your findings be the same?
A. The main problem is the shift between carbohydrate and protein. Indeed, the high carbohydrate diet increases insulin and thus inhibits lipolysis and gluconeogenesis. With a high-protein, low-carbohydrate (high-fat) diet, lipolysis is inhibited to a lesser extent during the post-prandial phase.
Q. Have symptoms of energy deficiency been evident in the animals receiving the threonine-deplete diet?
A. Yes; they will forage for food in their cages; however, they continue to avoid the threonine-deplete diet, sometimes to the point of starvation.
Q. Has anyone examined how cachexic patients (e.g., those with anorexia nervosa or cancer) respond to a threonine-replete diet? Might they represent a human condition analogous to the threonine-deplete animals?
A. To my knowledge this type of study has not been done, but such patients are unlikely to respond to the threonine-replete diet, as their cachexia is due to a deficiency in total energy, and not the result of an imbalanced amino-acid diet.
Q. Can a low-protein diet impact feeding as rapidly as the threonine-depleted diet? In other words, would you see increased activation of C-fos and Ca2+ flux within 30-60 minutes?
A. Very low-protein diets also reduce feeding. The mechanism remains incompletely understood but probably differs from that responsible for the threonine-depleted diet.
Q. Are there macronutrient-specific differences in feeding patterns among rats adapted to a high-protein diet?
A. Yes- we observed that these rats start their meals with carbohydrate (and fat), but then shift to protein. This pattern seems associated with increased insulin production from carbohydrates.
Q. Does energy intake eventually return to baseline levels, once rats have adapted to the high-protein diet?
A. No; we do see food intake increase compared to the initial suppression induced by the high-protein diet, but they do not resume intake at their previous level.
Q. Do you have any idea where dopaminergic neurons in the NTS originate?
A. We do not know the mechanism by which dopamine activity is modified in the NTS among rats receiving a high protein diet.
Q. In your studies, approximately 10% of the rats completely avoided protein; do you think genetic differences account for this subpopulation?
A. We are addressing this possibility in our current study.
You demonstrated that the APC mediates the response to amino acid depleted diets, but have you also observed shifts in the APC when animals are fed a protein-replete diet?
A. We haven’t measured the effect yet. The results from Dorothy Gietzen clearly demonstrated that the APC was the main chemosensor for essential amino acid deficiency. The role of the APC as the main amino acid chemosensor system remains to be completely established.
Q. What other central or peripheral sites exhibited neuronal differences in response to the diet?
A. When animals consume a high-protein diet for the first time (i.e., before adaptation), post-prandial plasma amino acid levels increase; subsequently the APC and different areas of the hypothalamus are activated compared to that observed following a normal diet. In contrast, once adapted (for at least a week) to a high protein diet, differences between APC activation among rats consuming normal or high protein diets are no longer evident. In contrast rats adapted to high protein diets present a higher activation of the NTS.
Q. Did glucagon levels differ in response to high- and low- protein meals in humans?
A. In humans we have observed increased basal glucagon levels after 7 days on a high protein diet. In contrast, postprandial responses of insulin and glucagon to the same meal was not different between subjects adapted to a normal or a high protein diet for 7 days.