It’s common knowledge that growth hormone and testosterone are key for building lean mass and burning body fat. However, not many bodybuilders or athletes are aware of insulin’s powerful regulatory effects and its synergistic role in producing IGF-1 (Insulin-like Growth Factor-1), the muscle builder.[1]

IGF-1 is vital for the correct glucose and protein metabolism that results in muscle growth. Aside from muscle anabolism, IGF-1 can also reverse a number of ageing defects such as insulin resistance and reduced muscular force. You can buy the best supplements, follow the best training program and listen to the best coaches, but unless you control insulin secretion you will fail repeatedly in your quest for a lean, mean body.

How much IGF-1 your body secretes depends on how well you manage insulin.

The insulin link

When humans release growth hormone from the pituitary gland (located at the base of the brain), some goes to muscle and bone to directly initiate tissue growth.[2] Most GH travels to the liver, where it is destroyed within 60-90 minutes. Before this happens the liver uses this material to manufacture somatomedins, also called growth factors. The best studied growth factors are IGF-I and IGF-II. One of growth hormone’s main roles is to get to the liver and trigger IGF release.[3]

Insulin interacts with growth hormone at the liver to produce IGF-I. In circulation, both IGF-I and IGF- II have very short lives. IGF-II is mainly responsible for nerve growth. IGF-I exerts powerful growth effects on muscle. However, IGF-1 needs a steady supply of insulin to work. Unless you’ve got both insulin and IGF-1 circulating in the right amounts, muscle growth is short-circuited. IGF-1 exerts its potent effects much better when insulin levels are steady.[5] How do we know this?

Research on diabetics provided the earliest examples. Insulin-dependant diabetics have low IGF levels and a hell of a job increasing their muscle mass.[4,5] Diabetes is a disease of inconsistent, ineffective insulin secretion by the pancreas. A diabetic’s insulin supply is like a yo-yo, constantly going up and down depending on how much they inject and what they eat. It’s almost impossible to keep constant. These roller coaster insulin surges kill IGF levels.[4,5]

Mechano IGF-1

While previous research shows conflicting results with regard to IGF-1 secretion and exercise, the latest, well designed research demonstrates that a big squirt of IGF-1 is secreted within the first 12 minutes of intense training.[10]

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More recent work also reveals that when muscles undergo high overload contraction (as in heavy weight training) the result is a muscle specific bioactive form of IGF-1 called mechano-IGF.[10,11] In fact, this IGF circulates in a “system”, consisting mostly of a family of six binding proteins, free IGF- 1 and an acid-labile unit.[11] These six binding proteins in blood and muscle regulate the biological activity (usability) of IGF-1.[10]

New research on IGF-1 shows that intense weight training creates a signal within muscle to rearrange the ratio of these binding proteins and increase the activity and availability of IGF-1. This appears to be essential to the growth and repair process.

The total amount of IGF-1 secreted is not as important as the rearrangement of the IGF-1 binding protein ratios.[11] Heavy resistance training triggers this rearrangement fantastically well.[9] Also, this modulation of the IGF-1 binding proteins to create active IGF-1 is not noticed until six to 12 hours after training.[11] Interestingly, this is just about the time that peak muscle protein synthesis rates kick in. Are you starting to see the link?

To maximize IGF secretion there is a biochemical sequence of events that must be fulfilled, and triggering GH release is the first crucial step.[2] While some athletes inject GH others choose the safer route and enhance its secretion naturally via intense training, increasing sleep frequency and using research-proven nutrients such as GABA and Glutamine.

Using nutrients to spike GH levels is safe. Even the highest endogenous GH spurt consists of bound (inactive) and unbound (active) forms that keep it within tightly regulated physiological levels.[11] However, troubles occur for those that inject large gobs of artificial GH and IGF-1.

Part-2 Next month


1. Arch Biochem Biophys. 2011 Jul;511(1-2):8-13.
2. J Strength Cond Res. 2011 Mar;25(3):767-77.
3. Tillmann V, Patel L, Gill MS, et al. Clin Endocrinol.53(3):329-336,2000.
4. Philips LS. Metabolism, 34:765-770,1985.
5. Carroll PV, Christ ER, Umpleby AM, et al. Diabetes, 49(5):789- 96,2000.
6. Zachwieja JJ, Yarasheski KE. Phys Ther. 79(1):76-82,1999.
7. Parkhouse WS, Coupland DC, Li C, Vanderhoek KJ. Mech Aging Dev. 77;113(2):75-83,2000.
8. Rasmussen BB, Tipton KD, Miller SL, et al. J.Appl.Physiol. 88:386- 392,2000.
9. Br J Sports Med. 2008 Dec;42(12):989-93
10.Endocrinology. 2010 Mar;151(3):865-75
11.Eur J Appl Physiol. 2010 Nov;110(5):961-9

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The Latest Research on IGF, Growth Hormone and Insulin Part-1

by Paul Cribb Ph.D. CSCS. time to read: 4 min