Insulin-Like Growth Factor.

Circulating IGF-1 is produced mainly by the liver under growth hormone drive, and that output falls when protein or energy intake is inadequate. Raising protein intake in an under-fed person raises circulating IGF-1. Supplementation trials in older adults and athletes have measured IGF-1 as a biomarker of that response. Note this is a marker of anabolic signalling, not a measured outcome in itself.

IGF-1 activates mTORC1 through the PI3K and Akt arm, while leucine activates the same complex through Rag GTPase and sestrin sensing. Both inputs are required for a full anabolic signal, which is why amino acid availability and hormonal signalling are described as permissive for each other. This is cell-signalling biochemistry. It does not mean that swallowing IGF-1 adds anything, because the peptide does not survive digestion.

Zinc depletion lowers circulating IGF-1 and blunts growth, and repletion restores it in people who were deficient. The relationship runs through growth hormone signalling and hepatic synthesis. This applies to correcting a deficiency. Adding zinc on top of adequate status has not been shown to raise IGF-1 further.

Arginine given intravenously reliably provokes growth hormone release, which is why it is used as a diagnostic stimulus, and growth hormone in turn drives hepatic IGF-1 production. Oral arginine produces a far smaller and less consistent response because of first-pass metabolism by intestinal arginase. The route matters enormously here. A clinical test result does not transfer to a capsule.

Vitamin D status and circulating IGF-1 track together in observational data, and IGF-1 also increases renal conversion of 25-hydroxyvitamin D to its active form. That makes the relationship bidirectional. Association is not causation in either direction, and supplementation work has been inconsistent. Read it as a linked axis rather than a lever.

Dairy protein raises circulating IGF-1 more than would be predicted from its amino acid content alone, an effect seen across age groups. Comparisons of yoghurt against isolated whey have used IGF-1 among their markers. Whether a higher IGF-1 level is desirable depends entirely on the person and the goal, since the same signal that supports tissue building is not universally wanted across a lifespan.

HMB is a leucine metabolite associated with reduced muscle protein breakdown, while IGF-1 signalling drives synthesis through Akt and mTOR and suppresses the same atrophy-associated ubiquitin ligases. The two act on opposite sides of the same balance in principle. Evidence for the combination in people is thin and mostly in older or clinically compromised groups.

Citrulline raises plasma arginine more effectively than oral arginine does, because it bypasses intestinal arginase, which is the mechanistic argument for using it as a growth hormone stimulus. In the trial setting it was combined with interval training and muscle adaptation markers were followed. The signal is preliminary and confounded by the exercise stimulus, which is by far the stronger input.

Creatine loading increases training volume and cell hydration, both of which sit upstream of the anabolic signalling that IGF-1 participates in. Some resistance training work has reported higher muscle IGF-1 expression with creatine than with training alone. Muscle IGF-1 expression is a marker, and the outcome people care about is measured in strength and lean mass, where creatine has its own direct evidence.