Splanchnic metabolism of amino acids in healthy subjects: effect of 60 hours of fasting

Metabolic Adaptations to Weight Loss: A Brief Review

ABSTRACT

Martínez-Gómez, MG and Roberts, BM. Metabolic adaptations to weight loss: A brief review. J Strength Cond Res XX(X): 000-000, 2021-As the scientific literature has continuously shown, body mass loss attempts do not always follow a linear fashion nor always go as expected even when the intervention is calculated with precise tools. One of the main reasons why this tends to happen relies on our body's biological drive to regain the body mass we lose to survive. This phenomenon has been referred to as "metabolic adaptation" many times in the literature and plays a very relevant role in the management of obesity and human weight loss. This review will provide insights into some of the theoretical models for the etiology of metabolic adaptation as well as a quick look into the physiological and endocrine mechanisms that underlie it. Nutritional strategies and dietetic tools are thus necessary to confront these so-called adaptations to body mass loss. Among some of these strategies, we can highlight increasing protein needs, opting for high-fiber foods or programming-controlled diet refeeds, and diet breaks over a large body mass loss phase. Outside the nutritional aspects, it might be wise to increase the physical activity and thus the energy flux of an individual when possible to maintain diet-induced body mass loss in the long term. This review will examine these protocols and their viability in the context of adherence and sustainability for the individual toward successful body mass loss.

Metabolic adaptations during negative energy balance and their potential impact on appetite and food intake

This review examines the metabolic adaptations that occur in response to negative energy balance and their potential putative or functional impact on appetite and food intake. Sustained negative energy balance will result in weight loss, with body composition changes similar for different dietary interventions if total energy and protein intake are equated. During periods of underfeeding, compensatory metabolic and behavioural responses occur that attenuate the prescribed energy deficit. While losses of metabolically active tissue during energy deficit result in reduced energy expenditure, an additional down-regulation in expenditure has been noted that cannot be explained by changes in body tissue (e.g. adaptive thermogenesis). Sustained negative energy balance is also associated with an increase in orexigenic drive and changes in appetite-related peptides during weight loss that may act as cues for increased hunger and food intake. It has also been suggested that losses of fat-free mass (FFM) could also act as an orexigenic signal during weight loss, but more data are needed to support these findings and the signalling pathways linking FFM and energy intake remain unclear. Taken together, these metabolic and behavioural responses to weight loss point to a highly complex and dynamic energy balance system in which perturbations to individual components can cause co-ordinated and inter-related compensatory responses elsewhere. The strength of these compensatory responses is individually subtle, and early identification of this variability may help identify individuals that respond well or poorly to an intervention.

Training state and skeletal muscle autophagy in response to 36 h of fasting

The present study aimed at investigating fasting-induced responses in regulators and markers of autophagy in vastus lateralis muscle from untrained and trained human subjects. Untrained and trained subjects (based on maximum oxygen uptake, muscle citrate synthase activity, and oxidative phosphorylation protein level) fasted for 36 h with vastus lateralis muscle biopsies obtained at 2, 12, 24, and 36 h after a standardized meal. Fasting reduced ( P < 0.05) skeletal muscle microtubule-associated protein-1A/1B light chain 3 (LC3)I, LC3II, and adaptor protein sequestosome 1/p62 protein content in untrained subjects only. Moreover, skeletal muscle RAC-alpha serine/threonine-protein kinase (AKT)Thr308, AMP-activated protein kinase (AMPK)Thr172, and Unc-51-like autophagy-activating kinase-1 (ULK1)Ser555 phosphorylation state, as well as Bcl-2-interacting coiled-coil protein-1 (Beclin1) and ULK1Ser757 phosphorylation, was lower ( P < 0.05) in trained than untrained subjects during fasting. In addition, the plasma concentrations of several amino acids were higher ( P < 0.05) in trained than untrained subjects, and the plasma concentration profile of several amino acids was different in untrained and trained subjects during fasting. Taken together, these findings suggest that 36-h fasting has effects on some mediators of autophagy in untrained human skeletal muscle and that training state influences the effect of fasting on autophagy signaling and on mediators of autophagy in skeletal muscle.

NEW & NOTEWORTHY This study showed that skeletal muscle autophagy was only modestly affected in humans by 36 h of fasting. Hence, 36-h fasting has effects on some mediators of autophagy in untrained human skeletal muscle, and training state influences the effect of fasting on autophagy signaling and on mediators of autophagy in skeletal muscle.

Nutritional regulation of the anabolic fate of amino acids within the liver in mammals: concepts arising from in vivo studies

At the crossroad between nutrient supply and requirements, the liver plays a central role in partitioning nitrogenous nutrients among tissues. The present review examines the utilisation of amino acids (AA) within the liver in various physiopathological states in mammals and how the fates of AA are regulated. AA uptake by the liver is generally driven by the net portal appearance of AA. This coordination is lost when demands by peripheral tissues is important (rapid growth or lactation), or when certain metabolic pathways within the liver become a priority (synthesis of acute-phase proteins). Data obtained in various species have shown that oxidation of AA and export protein synthesis usually responds to nutrient supply. Gluconeogenesis from AA is less dependent on hepatic delivery and the nature of nutrients supplied, and hormones like insulin are involved in the regulatory processes. Gluconeogenesis is regulated by nutritional factors very differently between mammals (glucose absorbed from the diet is important in single-stomached animals, while in carnivores, glucose from endogenous origin is key). The underlying mechanisms explaining how the liver adapts its AA utilisation to the body requirements are complex. The highly adaptable hepatic metabolism must be capable to deal with the various nutritional/physiological challenges that mammals have to face to maintain homeostasis. Whereas the liver responds generally to nutritional parameters in various physiological states occurring throughout life, other complex signalling pathways at systemic and tissue level (hormones, cytokines, nutrients, etc.) are involved additionally in specific physiological/nutritional states to prioritise certain metabolic pathways (pathological states or when nutritional requirements are uncovered).

Splanchnic regulation of glucose production

The liver plays a key role for the maintenance of blood glucose homeostasis under widely changing physiological conditions. In the overnight fasted state, breakdown of hepatic glycogen and synthesis of glucose from lactate, amino acids, glycerol, and pyruvate contribute about equally to hepatic glucose production. Postprandial glucose uptake by the liver is determined by the size of the glucose load reaching the liver, the rise in insulin concentration, and the route of glucose delivery. Hepatic glycogen stores are depleted within 36 to 48 hours of fasting, but gluconeogenesis continues to provide glucose for tissues with an obligatory glucose requirement. Glucose output from the liver increases during exercise; during short-term intensive exertion, hepatic glycogenolysis is the primary source of extra glucose for skeletal muscle, and during prolonged exercise, hepatic gluconeogenesis becomes gradually more important in keeping with falling insulin and rising glucagon levels. Type 1 diabetes is accompanied by diminished hepatic glycogen stores, augmented gluconeogenesis, and increased basal hepatic glucose production in proportion to the severity of the diabetic state. The hyperglycemia of type 2 diabetes is in part caused by an overproduction of glucose from the liver that is secondary to accelerated gluconeogenesis.

Effects of GH on protein metabolism during dietary restriction in man

The metabolic response to dietary restriction involves a series of hormonal and metabolic adaptations leading to protein conservation. An increase in the serum level of growth hormone (GH) during fasting has been well substantiated. GH has potent protein anabolic actions, as evidenced by a significant decrease in lean body mass and muscle mass in chronic GH deficiency, and vice versa in patients with acromegaly. The present review outlines current knowledge about the role of GH in the metabolic response to fasting, with particular reference to the effects on protein metabolism. Physiological bursts of GH secretion seem to be of seminal importance for the regulation of protein conservation during fasting. Apart from the possible direct effects of GH on protein dynamics, a number of additional anabolic agents, such as insulin, insulin-like growth factor-I, and free fatty acids (FFAs), are activated. Taken together the effects of GH on protein metabolism seem to include both stimulation of protein synthesis and inhibition of breakdown, depending on the nature of GH administration, which tissues are being studied, and on the physiological conditions of the subjects.

Glutamine extraction by the gut is reduced in depleted [corrected] patients with gastrointestinal cancer

OBJECTIVE AND SUMMARY BACKGROUND DATA

Glutamine is an important fuel for the intestinal mucosa. However, glutamine pools may become depleted in the cancer-bearing host as a result of tumor consumption and diminished production due to nutritional depletion. As human data are lacking, the authors investigated glutamine extraction by different sites of the human intestine, including tumor and the potential relation with the degree of nutritional depletion.

METHODS

Thirty-two patients with gastrointestinal malignancies were studied. Blood from an artery and veins draining jejunum, ileum, colon, or tumor were sampled. Depletion was estimated by the percentage ideal body weight.

RESULTS

Fractional glutamine extraction rate in the jejunum was 24%, three times higher than in ileum and colon. Percentage ideal body weight correlated with arterial glutamine levels (r = 0.5275, p = 0.003). In addition, arterial glutamine concentrations were correlated with extraction in the ileum (r = -0.8411, p < 0.001). Colon-containing tumor did not extract more glutamine than did nontumor-containing colon.

CONCLUSIONS

Glutamine is a quantitatively more important substrate for the proximal intestine than for the distal gut. Nutritional depletion results in decreased arterial glutamine concentration, which in turn results in diminished extraction. Colon cancer does not function as a glutamine trap and does not contribute to glutamine depletion.

Influence of insulin on peripheral uptake of branched chain amino acids in the 60-hour fasted state

The influence of insulin on branched chain amino acid (BCAA) metabolism was investigated in healthy subjects faster for 60-64 h, using the euglycemic insulin clamp technique and hepatic venous catheterization. As compared to the postabsorptive state, fasting resulted in a 50-80% decrease in glucose disposal during the clamps, indicating insulin resistance. However, the arterial concentrations of BCAA, which were increased by 200-220% after the fast, decreased to a similar extent during hyperinsulinemia, regardless of the fasting situation. The splanchnic exchange of BCAA was unaltered both in response to fasting itself and to fasting and hyperinsulinemia. The results suggest that insulin resistance during fasting does not influence BCAA metabolism. Furthermore, the changes in BCAA concentrations after a prolonged fast are due to altered peripheral metabolism of BCAA.

Ornithine alpha-ketoglutarate and glutamine supplementation during refeeding of food-deprived rats

The aim of this study was to compare the efficiency of ornithine alpha-ketoglutarate (OKG) and glutamine supplementation in an experimental model of denutrition that provides well-characterized disturbances of amino acid patterns. Male Wistar rats (187 +/- 11 g; five in each group) were starved for 3 days and then refed for 7 days with an oral diet (192 kcal kg-1.day-1 and 2.25 g of nitrogen kg-1.day-1), supplemented with 0.19 g of nitrogen kg-1.day-1 in the form of OKG, glutamine, or casein (control group). Food deprivation induced a fall in most tissue amino acids, with the notable exception of muscle leucine and liver glutamate, which increased by 43% (p < .01), and 11% (p < .05), respectively. The main effect of OKG was seen in the viscera, with a normalization of most amino acid pools (including proline and branched-chain amino acids) in the small bowel and liver. The main effect of glutamine was observed in the muscle, with a normalization of the glutamine and leucine pools. We conclude that, in this model and with the doses used, OKG and glutamine act in different target tissues, ie, splanchnic areas and muscle, respectively.

Quantitation of tryptophan and other plasma amino acids by automated pre-column o-phthaldialdehyde derivatization high-performance liquid chromatography: improved sample preparation

Pre-column derivatization with o-phthaldialdehyde is a rapid and sensitive method for the quantitation of amino acids in biological fluids. This method uses acetonitrile as a deproteinizing reagent which gives improved recovery of tryptophan compared with 5-sulfosalicylic acid and permits the measurement of aspartic acid which coelutes with 5-sulfosalicylic acid. The method is automated to increase reproducibility and convenience. Mean coefficients of variation for peak areas relative to internal standard were 3.2 and 5.2% for amino acid standards and plasma samples, respectively. The presence of nitrilotriacetic acid stabilized the o-phthaldialdehyde reagent which is important in an automated system. The method is suitable for the analysis of large numbers of plasma samples where total tryptophan and aspartic acid are of interest.

Decreased glucose oxidation during short-term starvation

Prolonged fasting (for days or weeks) decreases glucose production and oxidation. The effects of short-term starvation (ie, less than 24 hours) on glucose metabolism are not known. To evaluate this issue, glucose oxidation and glucose turnover were measured after 16-hour and subsequently after 22-hour fasting. Glucose oxidation was calculated by indirect calorimetry in 12 healthy men (age 22 to 44 years); glucose turnover was measured by primed, continuous infusion of 3-3H-glucose in eight of these 12 volunteers. After 16-hour fasting net glucose oxidation was 0.59 +/- 0.17 mg x kg-1 x min-1 and glucose tissue uptake 2.34 +/- 0.12 mg x kg-1 x min-1. No correlation was found between net glucose oxidation and glucose tissue uptake. Prolonging fasting with an additional 6 hours resulted in decreases of respiratory quotient (0.77 +/- 0.01 v 0.72 +/- 0.01) (P less than .005), plasma glucose concentration (4.7 +/- 0.1 v 4.6 +/- 0.1 mmol/L) (P less than .05), glucose tissue uptake (2.10 +/- 0.12 mg x kg-1 x min-1) (P less than .05), net glucose oxidation (0.09 +/- 0.04 mg x kg-1 x min-1) (P less than .005), and plasma insulin concentration (8 +/- 1 v6 +/- 1 mU/L) (P less than .005). Net glucose oxidation expressed as a percentage of glucose tissue uptake decreased from 22% +/- 8% to 2% +/- 1% (P less than .05). There was no net glucose oxidation in seven of 12 controls after 22-hour fasting.(ABSTRACT TRUNCATED AT 250 WORDS)