Energy content of diets of variable amino acid composition

Ingested non-essential amino acids recruit brain orexin cells to suppress eating in mice

Ingested nutrients are proposed to control mammalian behavior by modulating the activity of hypothalamic orexin/hypocretin neurons (HONs). Previous in vitro studies showed that nutrients ubiquitous in mammalian diets, such as non-essential amino acids (AAs) and glucose, modulate HONs in distinct ways. Glucose inhibits HONs, whereas non-essential (but not essential) AAs activate HONs. The latter effect is of particular interest because its purpose is unknown. Here, we show that ingestion of a dietary-relevant mix of non-essential AAs activates HONs and shifts behavior from eating to exploration. These effects persisted despite ablation of a key neural gut → brain communication pathway, the cholecystokinin-sensitive vagal afferents. The behavioral shift induced by the ingested non-essential AAs was recapitulated by targeted HON optostimulation and abolished in mice lacking HONs. Furthermore, lick microstructure analysis indicated that intragastric non-essential AAs and HON optostimulation each reduce the size, but not the frequency, of consumption bouts, thus implicating food palatability modulation as a mechanism for the eating suppression. Collectively, these results suggest that a key purpose of HON activation by ingested, non-essential AAs is to suppress eating and re-initiate food seeking. We propose and discuss possible evolutionary advantages of this, such as optimizing the limited stomach capacity for ingestion of essential nutrients.

Isolated Leucine and Branched-Chain Amino Acid Supplementation for Enhancing Muscular Strength and Hypertrophy: A Narrative Review

Branched-chain amino acids (BCAA) are one of the most popular sports supplements, marketed under the premise that they enhance muscular adaptations. Despite their prevalent consumption among athletes and the general public, the efficacy of BCAA has been an ongoing source of controversy in the sports nutrition field. Early support for BCAA supplementation was derived from extrapolation of mechanistic data on their role in muscle protein metabolism. Of the three BCAA, leucine has received the most attention because of its ability to stimulate the initial acute anabolic response. However, a substantial body of both acute and longitudinal research has now accumulated on the topic, affording the ability to scrutinize the effects of BCAA and leucine from a practical standpoint. This article aims to critically review the current literature and draw evidence-based conclusions about the putative benefits of BCAA or leucine supplementation on muscle strength and hypertrophy as well as illuminate gaps in the literature that warrant future study.

Single amino acids in sucrose rewards modulate feeding and associative learning in the honeybee

Obtaining the correct balance of nutrients requires that the brain integrates information about the body's nutritional state with sensory information from food to guide feeding behaviour. Learning is a mechanism that allows animals to identify cues associated with nutrients so that they can be located quickly when required. Feedback about nutritional state is essential for nutrient balancing and could influence learning. How specific this feedback is to individual nutrients has not often been examined. Here, we tested how the honeybee's nutritional state influenced the likelihood it would feed on and learn sucrose solutions containing single amino acids. Nutritional state was manipulated by pre-feeding bees with either 1M sucrose or 1M sucrose containing 100mM of isoleucine, proline, phenylalanine, or methionine 24h prior to olfactory conditioning of the proboscis extension response. We found that bees pre-fed sucrose solution consumed less of solutions containing amino acids and were also less likely to learn to associate amino acid solutions with odours. Unexpectedly, bees pre-fed solutions containing an amino acid were also less likely to learn to associate odours with sucrose the next day. Furthermore, they consumed more of and were more likely to learn when rewarded with an amino acid solution if they were pre-fed isoleucine and proline. Our data indicate that single amino acids at relatively high concentrations inhibit feeding on sucrose solutions containing them, and they can act as appetitive reinforcers during learning. Our data also suggest that select amino acids interact with mechanisms that signal nutritional sufficiency to reduce hunger. Based on these experiments, we predict that nutrient balancing for essential amino acids during learning requires integration of information about several amino acids experienced simultaneously.

Involvement of the neutral amino acid transporter SLC6A15 and leucine in obesity-related phenotypes

Brain pathways, including those in hypothalamus and nucleus of the solitary tract, influence food intake, nutrient preferences, metabolism and development of obesity in ways that often differ between males and females. Branched chain amino acids, including leucine, can suppress food intake, alter metabolism and change vulnerability to obesity. The SLC6A15 (v7-3) gene encodes a sodium-dependent transporter of leucine and other branched chain amino acids that is expressed by neurons in hypothalamus and nucleus of the solitary tract. We now report that SLC6A15 knockout attenuates leucine's abilities to reduce both: a) intake of normal chow and b) weight gain produced by access to a high fat diet in gender-selective fashions. We identify SNPs in the human SLC6A15 that are associated with body mass index and insulin resistance in males. These observations in mice and humans support a novel, gender-selective role for brain amino acid compartmentalization mediated by SLC6A15 in diet and obesity-associated phenotypes.

Genetic variation of dietary restriction and the effects of nutrient-free water and amino acid supplements on lifespan and fecundity of Drosophila

We measure genetic variation in lifespan and fecundity at two food levels in 34 core lines of the Drosophila Genetic Reference Panel collection. Lines were significantly different from each other in lifespan and fecundity at both restricted and full food. There was a strong food-by-line interaction for the slope of age-specific mortality, fecundity and proportion of fertilized eggs, indicating the presence of genetic variation for the strength of the dietary restriction effect, likely to represent standing genetic variation in a natural population from which the lines used have originated. No trade-off between fecundity and lifespan manifested in life-history variation among inbred lines. Our data partially corroborate the recent proposition that availability of nutrient-free water eliminates the apparent dietary restriction at least in some conditions. Although flies on full food with water added had lifespan slightly higher than those without a water source, it was still significantly lower than that in flies on restricted food, with no indication of interaction. We fully corroborate the recently discovered effect of addition of essential amino acids to the medium: addition of 1.5 mM methionine to restricted food significantly increased fecundity without a measurable decrease in lifespan; addition of each of 10 essential amino acids increased fecundity and decreased females lifespan to the levels observed on full food, again with no evidence of line-by-food interactions. We propose a mechanistic hypothesis explaining the observed data, based on the assumption that food consumption by flies is adjusted according to flies' saturation in water and methionine.

Elevated plasma phenylalanine concentrations may adversely affect bone status of phenylketonuric mice

Children with phenylketonuric (PKU) are at risk for fractures. This study used a PKU murine model (PAH(enu-2)) to evaluate effects of moderate dietary protein restriction and elevated plasma phenylalanine concentration impact upon bone status. Fifty-four male weanling PKU and control mice were assigned to either an elemental phenylalanine (Phe)-restricted diet (treated) or Phe-unrestricted diet (untreated) with low or normal protein levels for 56 days. Untreated mice and control mice received equal amounts of dietary Phe; treated mice consumed prescribed dietary Phe to maintain plasma Phe concentrations between 120 and 480micromol/L. Plasma Phe, osteocalcin, and urine deoxypyridinoline (DPD)/creatinine were analysed at baseline and at days 28 and 56. Femur strength, bone mineral density (BMD) and bone mineral content (BMC) were analysed at day 56. Moderate protein restriction did not significantly affect bone status. Mean plasma Phe concentrations were significantly greater in untreated vs treated and control mice (p < 0.0001). Total body weight was significantly less in untreated vs control mice (p < 0.01). Mean femur weight was reduced in untreated mice vs both treated and control mice (p < 0.03). Untreated mice had smaller mean femur length than control mice (p < 0.002). Femur strength was greater in treated mice compared to control mice (p < 0.01) but not compared to untreated mice. No significant difference among groups was found in BMD and BMC. At day 56 there was a statistical trend (p < 0.056) towards higher urine DPD/creatinine excretion in untreated mice than in treated mice. Plasma Phe concentration was positively correlated with urine DPD/creatinine. These data suggested that hyperphenylalaninaemia may adversely affect bone status in PKU mice.