Sources and Structure
Isoleucine (2-amino-3-methylpentanoic acid) is one of the three branched chain amino acids, an essential amino acid that possesses a chiral side-chain (only other amino acid to do so is Threonine); although it may exist in four isomers, it tends to naturally occur (in foods) as the double S isomer ((2S,3S)-2-amino-3-methylpentanoic acid). Isoleucine, as evidenced by its name, is the isomer of leucine.
Interactions with Glucose Metabolism
Of the three branched chain amino acids, leucine and valine appear to be somewhat suppressive of cellular glucose uptake during an oral glucose tolerance test while promoting muscle protein synthesis while isoleucine promotes cellular glucose uptake and consumption.
The actions of isoleucine on glucose uptake are dependent on PI3K and PKC activation but independent of mTOR and is not dependent on AMPK activation (this study noting no activation of the α1 subunit and a suppression of the α2 subunit); this differs a bit from leucine signalling, which although it is also dependent on PI3K/PKC and not mTOR leucine itself activates mTOR and (consequently) reduced AMPK signalling which suppresses its own efficacy on increasing glucose uptake. Isoleucine is a fairly weak mTOR activator in vitro with an EC50 of around 8mM (weaker than leucine, stronger than valine) and when looking at Akt/mTOR in vivo leucine depleted amino acid mixtures are no longer effective.
Possibly due to the slight suppressive effect on AMPKα2, a reduction in AMP has been noted in liver cells incubated with isoleucine without affecting ATP or ADP (uncertain if practically relevant for exercise but unlikely).
Additionally, one study that failed to find any influence on Akt and mTOR noted that AS160 (Akt substrate of 160kDa) was activated; AS160 is normally phosphorylated and inactivated by Akt, and this process promotes GLUT4 mobilization secondary to releasing a Rab signalling protein. However, it is possible that this is just due to enhancing insulin signalling (which has been reported elsewhere resulting in increased phosphorylation of AS160) and in this scenario mTOR may be activated as the insulin receptor activates mTOR.
Isoleucine appears to promote glucose uptake into a cell, and this is independent (not associated with) the two classical pathways of signalling via the insulin receptor or via activating AMPK. It is possible that isoleucine is acting upon the same mechanism as leucine to promote glucose uptake, but due to being unable to activate mTOR it does not suppress itself
At 1mM isoleucine can increase glucose consumption 16.8% in muscle cells (leucine and valine inactive) with peak efficacy at 2mM (35%), with the former concentration being somewhat achievable in serum (888+/-265nmol/mL or 0.89mM) following 0.3g/kg ingestion in rats; lower doses (0.1g/kg) were ineffective in reducing glucose, but the 0.3g/kg was associated with a reduction in plasma glucose. A later study using 0.3-1.35g/kg found peak efficacy at 0.45g/kg which elevated serum concentrations to 3mM, and the reduction in serum glucose reached 20% while muscular glucose uptake increased 71% (73% noted elsewhere with 1.35g/kg) and whole body glucose oxidation increased 5.1-6.0% after 30-90 minutes (more efficacy at 30 minutes).
Interestingly, one study using 1.35g/g (same efficacy as 0.45g/kg) noted that the serum concentration was 4352+/-160μmol/L which was fairly similar (a bit higher) than the serum concentration noted with 0.45g/kg; this suggests that the rate limit of isoleucine occurs at either absorption or distribution to the blood.
The increase of glucose uptake has been confirmed in rats with oral isoleucine and appears to reach max efficacy at 450mg/kg in rats (human equivalent of 72mg/kg or, for a 150lb person, 10.8g)
Isoleucine does not appear to positively influence glycogen synthesis in isolated muscle cells and once has been noted to suppress levels of phosphorylated glycogen synthase (slightly confounded with low levels of other amino acids).
Isoleucine does not appear to inherently stimulate insulin release form the pancrease like leucine does (gluconeogenesis of isoleucine into glucose might indirectly, but oral supplementation of 0.45g/kg isoleucine does not appear to significantly increase insulin secretion).
One study using both an amino acid mixture (98% isoleucine by weight) and insulin noted that while the amino acid mixture (2.0334mM) was comparable to submaximal insulin secretion, it was less effective at promoting glucose uptake than maximal insulin secretion; however, the high isoleucine mixture promoted insulin-induced glucose uptake at both submaximal (26%) and maximal (14%) concentrations.
Isoleucine does not appear to promote glycogen resynthesis or insulin secretion (anabolic mechanisms of glucose metabolism) but may augment insulin-induced glucose deposition
In rats fed an amino acid supplement very high in isoleucine (5.28mg cysteine, 3.36mg methionine, 6.68mg valine, 944.8mg isoleucine, and 6.68mg leucine) was able to reduce plasma glucose following an oral glucose tolerance test. Doubling the leucine content increased insulin secretion (while the low leucine supplement had no significant influence) suggesting it mediated the trend to increase insulin.
Several studies measuring glucose uptake in muscle tissue note an increase in glucose uptake with either an amino acid mixture with a high (78%) isoleucine content or with isolated isoleucine which appears to peak in efficacy at 0.45g/kg bodyweight in rats (10.8g for a 150lb human).
In rat studies, isoleucine either alone or as the major amino acid in a mixture promotes skeletal muscle uptake of glucose while suppressing the area under curve (AUC) of oral glucose tolerance tests, which is probably secondary to increasing uptake of glucose from the blood into cells
One study using a high dose of isoleucine (12.094g) paired with low doses of leucine (0.084g), valine (0.086g), methionine (0.043g) and cysteine (0.088g) in otherwise healthy active adults noted that, when taken alongside 100g glucose, the amino acids were able to reduce plasma glucose at all measured time points before 180 minutes and the overall AUC without affecting insulin secretion (glucagon increased at 60 minutes but was otherwise not significantly influenced).
The increase in glucose uptake seen with isoleucine appears to occur in animals with maximal efficacy at around 0.45g/kg, and the approximately human equivalent of this dose has been tested in humans (albeit alongside very low doses of other amino acids) and has been noted to reduce glucose spikes following a meal
Skeletal Muscle and Metabolism
One study investigating the mechanisms underlying the glucose uptake effect of isoleucine in muscle cells (that did not appear to occur to a significant level in adipose tissue) noted that there was a suppression of the mRNA for two enzymes that are involved in gluconeogenesis (PEPCK and G6Pase mRNA) which was thought to underlie less oxidation of alanine observed ex vivo in liver cells as assessed by valine secretion (a biomarker of proteolysis). It has been noted that the suppression on hepatic gluconeogenesis by insulin at levels following ingestion of 100g carbohydrates in humans (around 1200pmol) is likely sufficient to suppress gluconeogenesis by itself and may render the effects of isoleucine irrelevant if not in a fasted state.
Isoleucine potentially has anticatabolic actions at the level of amino acids by reducing the rates of gluconeogenesis. Signalling properties of isoleucine on the nucleus and thus muscle preservation is not yet known
Inflammation and Immunology
β-defensin is an antimicrobial peptide produced by human epithelial tissue (intestines, skin, lungs) (while α-defensins are produced by neutrophils) and it is thought that inducing defensins and other antimicrobial peptides is a protective therapeutic strategy for bacterial infections; the production of β-defensin appears to be increased by L-Isoleucine while the straight chain analogue (norvaline) are inactive and this increase is dependent on an isoleucine induced increase in activity of NF-kB.
The addition of 2g L-isoleucine to an oral rehydration solution (ORS) given to children with acute diarrhea and consequent dehydration appears to either reduce symptoms of diarhhea (stool output) or at least trend towards increased β-defensin in stool without affecting duration has been noted.
It is possible that isoleucine supplementation has antibacterial properties in the intestines and preliminary evidence looks a bit promising, but it doesn't appear to be remarkably potent (rather than a bit protective) and the evidence currently available needs to be replicated
The glucose uptake into muscle cells appears to be increased with incubation of the mTOR inhibitor rapamycin which may be due to the role of mTOR as a negative regulator of glucose uptake in cells. mTOR inhibitors (resveratrol) may be potentially additive with isoleucine supplementation (possibly at the cost of muscle protein synthesis), and resveratrol has also been noted to be synergistic with leucine via other mechanisms related to mitochondrial biogenesis.
Possible interactions with isoleucine and mTOR inhibitors (of which resveratrol is the common supplemental one referred to), but whether this is good or bad depends on the context of the situation as it may increase glucose uptake at the cost of muscle protein synthesis; this would be good for diabetics but adverse for persons wishing to build muscle