Insulinlike growth factor-1 is so named because of its resemblance to insulin. It consists of 70 bonded amino acids, which makes it a protein-peptide hormone. That means that like growth hormone, IGF-1 must be injected. Otherwise it degrades in the gut, rendering it useless.
IGF-1 is considered the key to growth hormone’s anabolic effects, and GH release promotes its synthesis in the liver. The liver also synthesizes six binding proteins that work with IGF-1, with one, IGFBP-3, being the primary IGF-1-bonding protein in the blood. A substance called the acid-labile subunit prevents the premature degradation of IGF-1. The complex of IGF-1, binding protein and the acid-labile subunit extend the time that IGF-1 lasts in the blood to 15 hours or more—compared to the 10 minutes that unbound IGF-1 lasts.
Because IGF-1 is so similar to insulin, it can interact with insulin cell receptors and produce some of the same effects as insulin. In fact, the primary side effect of both excess insulin and IGF-1 is hypoglycemia, or low blood glucose, although insulin is 10 times more potent than IGF-1 in that effect. When you train for an extended time—more than one hour—the liver upgrades its release of IGF-binding protein 3 to prevent the onset of hypoglycemia that would otherwise ensue because of the increased release of IGF-1. IGF-1 also amplifies the action of insulin, even at low doses. Insulin helps maintain blood IGF-1 by boosting the synthesis of IGFBP-3.
The primary role of IGF-1, though, isn’t to transport glucose into cells, as in the case of insulin. Instead, it fosters cellular division and growth. It’s also involved in cell repair, particularly in brain, heart and muscle. Its function in cell division has led many scientists to suggest that IGF-1 has a role in several types of cancer. That makes sense, since cancer is a process of uncontrolled cellular division; however, the evidence for that is not yet definitive by any means. True enough, IGF-1 inhibits apoptosis, or cellular suicide. Out of that you get the theory that tumors would upgrade synthesis of local IGF-1 to keep themselves alive and thereby encourage the spread of cancer throughout the body. Yet some researchers suggest that it’s a classic chicken-and-egg scenario, in that IGF-1 doesn’t cause cancer but is instead produced by tumors.
Meanwhile, studies show that people low on IGF-1 have a greater chance of dying from a heart attack. That’s because IGF-1 prevents the death of heart cells and offers protection when the cells are highly stressed, as occurs during a heart attack.
While the liver synthesizes IGF-1 and packages it with the binding proteins for transport into the blood, two variants of IGF-1 that are produced in muscle, one of which is called mechano growth factor, play a major role in muscle gains. They spur the activity of other proteins that are involved in muscle protein synthesis and encourage the activity of muscle stem cells, called satellite cells, which repair damaged muscle—and training does damage muscle. In fact, intense weight training is a primary stimulus of the release of IGF-1 in muscle. (Another protein, myostatin, prevents muscle growth by interfering with satellite-cell proliferation.)
A recent study used specially bred mice that produced only tiny amounts of IGF-1 in their livers, 75 to 85 percent lower than normal mice.1 Despite that, they show normal growth patterns and development. Their bodies compensate by secreting a lot of GH.
The IGF-1-deficient mice have low bodyfat and tend to stay lean as they age. The reason they make so much GH is that IGF-1 is the primary feedback inhibitor of GH release from the pituitary gland. Less IGF-1 in the blood equals more GH release from the brain. Interestingly, human studies show that testosterone also blunts the IGF-1 signal to the brain, thus helping maximize the effects of GH. That’s likely one reason why GH is considered synergistic with testosterone and anabolic steroids, which are synthetic forms of testosterone.
But back to the mice. The lack of systemic release of IGF-1 doesn’t affect its local production in muscle. Researchers showed that by having the mice engage in resistance exercise. No, the mice didn’t do any barbell curls or squats. They climbed a ladder with tiny weights attached to their bodies—a 16-week rodent weight-training routine. Another group of mice not deficient in IGF-1 did the same exercise. The groups produced equivalent muscle gains. The researchers concluded that systemic IGF-1 produced in the liver isn’t required for muscle hypertrophy. The IGF-1 forms produced in muscle following exercise are the important ones.
Bodybuilders and other athletes have been using IGF-1 injections for years. The drug is often used along with GH, anabolic steroids and insulin. One popular formulation is Long R3IGF-1, which is thought to be more potent than IGF-1 produced in the body. The hormones can interact with cellular receptors only when they’re free, or unbound from their plasma-binding proteins. Because the Long R3 IGF-1 lasts longer in the blood than natural IGF-1, it could present a serious threat to health. The free IGF-1 can interact with tumors, causing cancer progression. It could also convert a benign or inactive tumor into an active one. Another experimental form of IGF-1 said to be used by athletes is des(1-3) IGF-1. It’s a short form of IGF-1 that is not protein-bound and is often directly injected into muscles, and it’s rumored to lead to hyperplasia, or the splitting of muscle fibers to form new fibers. It’s strictly conjecture, as there is thus far zero proof of the effect in healthy athletes.
The prescription form of IGF-1 is mecasermin, trade name Increlex. Manufactured using recombinant DNA technology, it’s used to treat IGF-1 deficiency and growth problems. Increlex is also prescribed for patients who have developed antibody resistance to GH therapy. Unlike Long R3 IGF-1, Increlex is identical to natural IGF-1, retaining the 70 amino acid sequence of IGF-1 that the body produces.
Although it appears that only the version of IGF-1 produced in muscle has any true anabolic effects, many bodybuilders and athletes who’ve used IGF-1 claim to have benefited from the drug. There is no scientific evidence for that, but there is some evidence of benefits for people deficient in IGF-1.
Hormone-deficient patients who get IGF-1 experience increased rates of fat loss and fat oxidation. What causes that isn’t known, but one theory is that the IGF-1 may suppress circulating insulin. In addition, fat cells contain IGF-1 receptors, so the hormone can interact with fat cells.
From the standpoint of protein synthesis, IGF-1 injections provide the anticatabolic effects of insulin combined with the increased protein synthesis induced by GH. Like insulin, IGF-1 encourages amino acid uptake into muscle cells. It stimulates peripheral tissue uptake of glucose, which lowers blood glucose levels. It also suppresses liver glucose production, which is actually good for those who are insulin resistant, since the liver under that circumstance tends to produce too much glucose, which perpetuates the insulin insensitivity and can eventually result in diabetes. Indeed, IGF-1 is being considered as a diabetes-prevention drug.
From an athletic point of view, IGF-1 may share insulin’s role in increasing glycogen synthesis, which powers intense training. Possible side effects of IGF-1 injections include jaw pain, facial and hand swelling and heart-rhythm disturbances. The last-named effect is more likely if doses of more than 100 micrograms are injected. That can cause the heart to stop beating (yikes!) and blood pressure to drop dramatically. The effect is caused by an IGF-1-induced drop in blood phosphate and can be prevented by administering phosphate with the IGF-1. An increase in IGF-1 caused by either GH or IGF-1 injections is thought to play a major role in producing the repulsive bloated abdomen seen on some competitive bodybuilders. Adding insulin to the stack exponentially increases the chance of that particular side effect showing up. Note that all internal organs have an extensive supply of both insulin and IGF-1 cell receptors. Providing an abundance of either or both hormones will lead to organ growth, contributing to the abdominal bloat.
Several factors affect IGF-1 production in the body. Insufficient protein or calories cause it to plummet, and excess calories may cause it to increase. One study of normal-weight women who overate found a 19 percent increase in IGF-1 after two weeks of gorging, with 46 percent of the bodyweight gain attributed to lean mass and 54 percent to bodyfat. Fasting insulin doubled in the women, and testosterone levels rose significantly. Thus the lean mass gain produced by overeating could be the result of an increase in IGF-1, insulin or testosterone—or all three. I would quickly add that overeating is not a good method of adding muscle mass, as most of the weight gain consisted of bodyfat. It does, however, explain why bulking up was a popular technique for gaining mass among bodybuilders of the past and, to a certain extent, those of today. Other nutrients necessary to maintain IGF-1 in the body include the minerals magnesium and zinc and thiamine, a.k.a. vitamin B1. Zinc is particularly important.
Exercise boosts IGF-1. Some studies suggest that the antiaging effects of DHEA use come from an increase in IGF-1 in the body. IGF-1 maintains both muscle and connective tissue, as well as brain and heart cells, so it’s not a stretch to think that having more IGF-1 will make you feel and possibly look younger. Recent human studies confirm the antiaging effects of IGF-1 and GH. Yet animals deficient in IGF-1 live longer and show no cancer whatsoever. Clearly, that’s an example of how animal physiology may differ from that of humans. On the other hand, countless people who’ve used GH therapy say that they feel younger, but that’s rarely evident in their appearance. Excess bodyfat is associated with lower IGF-1 and GH. One recent study examined lifestyle factors that affect IGF-1 in college-age women and found a positive correlation with soy protein and the mineral selenium.2 Drinking alcohol blunted the effects of IGF-1 in the women.
The greatest future use of IGF-1 will without doubt involve gene therapy, which directly places genes that produce IGF-1 in muscle, usually by attaching them to an inactive virus or vector that penetrates the muscle cells. Studies with young mice show that the procedure results in a 15 percent increase in muscle mass, along with a 14 percent increase in strength. Gene therapy in old mice led to a 27 percent increase in strength, along with a total regeneration of aged muscle. In another mouse study, the IGF-1 gene was placed in the animals’ glutes and calves, which resulted in a 17 to 115 percent increase in muscle-cross-sectional area. One hopes the growth occurred mainly in the calves rather than the glutes!
Studies with the muscle-specific form of IGF-1 have yielded similar or better results. Some scientists speculate that once the procedure is perfected for humans, it will spell the end of age-related muscle weakness and frailty. They foresee an 80-year-old man who can produce the same muscle gains as a 19-year-old. Older people don’t gain as much muscle as younger people because satellite cell activity either doesn’t occur or is ineffective. That defect is completely repaired with IGF-1 gene therapy.
Some predict that gene therapy will replace drugs as the main form of doping in the future. No one has any idea of how to detect gene therapy doping yet. The only possible way would be a muscle biopsy, but even that would prove problematic because complete uptake of the IGF-1 gene may not occur, and the biopsy may reveal just normal muscle tissue.
Rumors abound that some athletes have already subjected themselves to IGF-1 gene therapy. That isn’t hard to believe when you consider that one of the therapy’s developers, H. Lee Sweeney, Ph.D., of the University of Pennsylvania School of Medicine, says he’s besieged by athletes and coaches from around the world who offer to be his guinea pigs. In truth, however, the technique is not ready for prime time, for some earlier gene-therapy experiments resulted in patient deaths. Future subjects could experience fatal immune reactions to the vectors used to place the gene in the body. Another danger is an inability to control the expression of the gene, which could translate into a rapidly spreading cancer. Or the expression of the gene could extend from skeletal muscle into heart muscle, resulting in excessive heart muscle growth that portends premature heart failure.
Last and perhaps not least, while IGF-1 injections work great on paper, real-world results are mixed. Most athletes suggest that using IGF-1 alone does little or nothing to boost muscle gains, which makes sense in light of the mouse study that linked only local muscle IGF-1 to mass gains. Many steroid manuals suggest that IGF-1 injections are best used with other anabolic agents, such as GH, testosterone and insulin. In that case, how do you ascertain just how well IGF-1 is working? The gains attributed to IGF-1 may in fact result from the other drugs in the combo. Nor can you discount the placebo effect. If you think something will work and truly believe that it will, it often does. Perhaps those who tout the “massive muscle gains” they’ve allegedly made from IGF-1 injections made those gains because they trained harder and believed from their head down to their diamond-shaped calves that the drug would work.
And for them, it did. Or did it?
1 Matheny, W., et al. (2009). Serum IGF-1-deficiency does not prevent compensatory skeletal muscle hypertrophy in resistance exercise. Exp Biol Med. 234:164-170.
2 Karl, J.P., et al. (2009). Diet, body composition, and physical fitness influences on IGF-1 bioactivity in women. Growth Hor IGF-1 Res. In press.