IGF-1 LR3: How a Cow's First Milk Led to a Tweaked Growth Signal

In the 1980s, scientists in Adelaide, Australia, were studying a cow's first milk. This thick early milk is called colostrum. In it, they found IGF-1, a growth signal, and a shorter form of it.

The short form was missing three beads at its front. Yet in rat muscle cells, it worked about 10 times as well. That clue led the team to build IGF-1 LR3, a clever lab-made growth signal.

What it is

A peptide is a short chain of amino acids. Think of amino acids as beads on a string. New to this? Start with what a peptide is.

IGF-1 is a chain of 70 beads. Its full name is insulin-like growth factor 1. Growth hormone tells the liver and other parts of the body to make it. IGF-1 is a key messenger for growing taller.

IGF-1 LR3 is IGF-1 with two changes. First, the bead at spot 3 is swapped for a new one, called arginine. Scientists write arginine as "R," so that is the "R3" part.

Second, a tail of 13 extra beads sits at the front. That is the "Long" part. Most of the tail was borrowed from the start of pig growth hormone.

The Adelaide team described it in 1992.

What it does in the body

IGF-1 works through a receptor on cells. A receptor is a lock, and IGF-1 is the key. When the key turns, the cell gets a message to build protein and grow.

But in the blood, more than 80% of IGF-1 is held by helper proteins called binding proteins.

Think of a boat tied to a dock. It stays close, but it can't sail off to work.

IGF-1 LR3 barely ties up. Its two changes make it stick far less to binding proteins. So more of it floats free to reach the locks on cells.

Here is a twist. LR3 does not grip the lock harder. In lab tests, it gripped the lock about 3 times less tightly than IGF-1. Its power comes from being free.

What scientists have found

In cells: In rat muscle cells, LR3 beat natural IGF-1. It did more to help the cells build protein and copy their DNA. These cells make their own binding proteins.

In chicken cells that put out no binding proteins scientists could find, LR3 was weaker than IGF-1. That showed its edge comes from dodging binding proteins.

Labs that grow cells to make proteins have used LR3, too. In a 2000 study, hamster cells making two proteins stayed alive better with LR3 than with insulin.

In animals: Some rats got a steroid medicine that made them lose weight. IGF-1 partly reversed the loss. LR3 was about 2.5 times as strong.

Their guts grew heavier, too, by up to 45%. The tweaked forms had the biggest effect.

Pigs held a surprise. Tweaked forms like LR3 leave the blood faster than IGF-1. Yet in pigs, they lowered blood sugar 2 to 3 times more strongly, and for longer.

In people: No studies of IGF-1 LR3 in people have been published yet. A 2026 review found none.

Its parent, natural IGF-1, has been tested in people. In five studies, 71 children got lab-made IGF-1. They grew very slowly because their bodies made too little IGF-1 of their own.

Before treatment, they grew about 2.8 cm (1 inch) a year. In the first year, that rose to about 8.0 cm (3 inches).