Insulinlike growth factor-1 (IGF-1) is a hot item in bodybuilding circles. According to gym gossip, the drug version
of this hormone is partially responsible for physiques that top the 250-pound mark while simultaneously boasting
bodyfat levels under 5%.
While it's debatable that these astounding mass gains accrued from IGF-1 use, extensive research on the substance does suggest definite anabolic properties. IGF-1 itself is a single- chain polypeptide composed of a string of 67 amino acids. Structurally, it resembles proinsulin, the form of insulin stored in the pancreas. It appears to share or even amplify many of insulin's known anabolic and anticatabolic effects.
While IGF-1 is produced in most organs of the body, it's found primarily in the liver. The synthesis of IGF-1 is spurred on by growth hormone; in fact, many of the anabolic properties ascribed to GH are caused by IGF-1. When it circulates in the blood, IGF-1 binds to certain proteins. Its production is also influenced by insulin.
Several studies show that IGF-1 synthesis is acutely affected by nutrition, especially the number of calories consumed and the amount of protein ingested. Consuming too few calories for an extended period halts IGF-1 production. Several diseases characterized by decreased absorption of food, including anorexia, also negatively affect IGF-1 status. AIDS patients also usually show decreased levels of IGF-1.
Fasting lowers IGF-1 levels dramatically. Such levels begin to decline within 24 hours of the start of fasting and often drop 15% within 10 days. As IGF-1 values drop, the body loses nitrogen, which leads to a catabolic loss of muscle tissue. Both adequate protein and caloric intake are vital in restoring IGF-1 levels to normal after a period of decreased food consumption. After fasting, the body requires 12 calories per kilogram (2.2 pounds) of body-weight over a 24-hour period to restore IGF-1 levels to normal.
On the other hand, overeating can slightly increase IGF-1 synthesis. One study showed that overfeeding women at normal bodyweights for two weeks increased IGF-1 levels by 19%. But lest you take that as an invitation to gluttony, be aware that IGF-1 decreases with obesity, and is inversely correlated with abdominal fat.
The effect of consuming too few calories on IGF-1 production is extremely potent. Studies involving rats show that even giving the animals growth hormone fails to increase IGF-1 levels that have been depressed by caloric deprivation. Scientists explain this by noting that excessive food restriction leads to GH resistance in the body.
IGF-1 levels in the liver are extremely sensitive to the availability of amino acids. For example, another study showed that when rats' incubated liver cells were placed in a medium containing only 20% of the normal liver amino-acid content, IGF-1 decreased to 56% of control values within one day.
Insufficient protein also leads to a more rapid breakdown of IGF-1 in the body. One study showed a 29% decrease in serum IGF-1 levels from a protein-restricted diet.
Other studies reveal that when protein and calories are both restricted, even a combination of IGF-1 and growth hormone fails to produce an anabolic effect in rats. In effect, the animals become insensitive to the hormones under near-starvation conditions.
The implication for bodybuilders is that under a drastically decreased caloric intake, it's essential to maintain an adequate diet of high-quality protein to ensure optimal IGF-1 and growth hormone release. This is particularly true for those who don't resort to using drugs such as anabolic steroids or growth hormone.