IGF-1 for Men Over 40: What the Number Means, Why Both Low and High Shorten Life, and How Peptides, Protein and Fasting Move It

IGF-1 is the blood test that reports how much growth hormone a man is really producing, and it sits in a U-shaped curve with mortality where both the bottom and the top of the lab range carry more risk than the middle. What a normal IGF-1 is at 40, 50 and 60, how peptides, protein, fasting and sleep move it, and the range worth aiming for.

IGF-1 in men over 40 is the most informative growth hormone marker that almost nobody orders. Growth hormone (GH) is released by the pituitary in bursts, mostly during deep sleep, and each burst is gone from the blood within about twenty minutes. A single morning GH measurement is therefore close to useless. Insulin-like growth factor 1, or IGF-1, is the protein the liver produces in response to those bursts. IGF-1 has a half-life of about 12 to 15 hours because it circulates bound to carrier proteins, so a single draw at any time of day reflects the integrated GH output of the previous day or two. That stability is why endocrinologists use IGF-1 to diagnose both growth hormone deficiency and growth hormone excess, and why every peptide marketed to raise growth hormone is, in practice, judged by what it does to IGF-1.

The reason IGF-1 matters to a man over 40 is that GH output falls by roughly 14 percent per decade after age 30, and IGF-1 tracks that decline. A man of 25 typically sits around 200 ng/mL. By 45 the median is closer to 140, and by 65 it is nearer 110. That decline is partly why body fat drifts up, why recovery slows, and why the growth hormone secretagogue peptides have become the second most searched category on this site after testosterone. The complication is that more IGF-1 is not simply better. The largest population studies now show a U-shaped relationship between IGF-1 and death from all causes, with the lowest risk in the middle of the reference range, and higher IGF-1 is linked to modestly higher rates of prostate and colorectal cancer. This guide explains what the number means at each decade, where the safe zone sits, what pushes IGF-1 up and down, and how to use the result to decide whether a growth hormone peptide is a good idea or a bad one.

A normal IGF-1 for a man aged 40 to 49 is roughly 60 to 210 ng/mL on a modern liquid chromatography mass spectrometry assay, with a median of about 140 ng/mL, and the range shifts down by around 10 ng/mL for every additional five years of age. Men aged 50 to 59 typically see a range of about 55 to 190 ng/mL with a median near 125, and men aged 60 to 69 a range of about 48 to 180 ng/mL with a median near 110. The reference intervals differ between laboratories because IGF-1 is measured by several different methods, and results from an immunoassay at one lab cannot be compared directly with a mass spectrometry result from another. A man tracking IGF-1 over time should use the same laboratory each time and pay attention to the age-adjusted range printed on the report rather than a generic adult range.

The most useful way to read an IGF-1 result is as a standard deviation score, sometimes printed as a Z-score, which tells a man how far his value sits from the median for his age. A Z-score between minus 2 and plus 2 is inside the reference range by definition. The practical point is that a result of 65 ng/mL and a result of 195 ng/mL are both technically normal for a 45 year old, yet they describe very different men. The first is at the bottom of the range and, if it comes with fatigue, rising visceral fat and poor recovery, points toward reduced growth hormone output that may deserve a stimulation test. The second is near the top and, if the man is not on any secretagogue, is a reason to confirm the result and to be more cautious about adding anything that raises it further.

The bar chart is the reason age-adjusted ranges matter. A 55 year old man with an IGF-1 of 165 ng/mL is at the 90th percentile for his age. If his laboratory prints a generic adult range of 50 to 250 ng/mL, that same result looks unremarkable. The decline with age is not itself a disease, and there is no evidence that restoring a 55 year old's IGF-1 to the level of a 25 year old improves how long he lives. What the number does well is flag the two tails: the bottom 5 percent, where growth hormone deficiency becomes a real possibility, and the top 5 percent, where acromegaly, exogenous growth hormone, or an aggressive secretagogue protocol should be considered.

The IGF-1 test measures the total concentration of insulin-like growth factor 1 in blood, roughly 98 percent of which is bound to carrier proteins, chiefly IGF binding protein 3 and the acid-labile subunit. Growth hormone released from the pituitary travels to the liver and instructs hepatocytes to produce IGF-1, which then circulates for hours and acts on muscle, bone, cartilage, and nearly every other tissue. Because growth hormone is pulsatile and cleared in minutes, a random GH level in a healthy man can read anywhere from undetectable to 20 ng/mL depending on whether the draw caught a pulse. IGF-1 smooths those pulses into a single number that reflects the previous 24 to 48 hours of GH exposure, which is why it is the standard screening test and why stimulation tests such as the glucagon test or the macimorelin test are reserved for confirming deficiency after a low IGF-1.

IGF-1 does not need a fasting draw and does not vary meaningfully with time of day, which makes it one of the easier hormones to test. It does, however, respond to several things a man might not think of as hormonal. Protein and total calorie intake over the previous week move IGF-1 by 20 to 50 percent. Poorly controlled diabetes, liver disease, hypothyroidism and chronic inflammation all suppress hepatic IGF-1 production even when GH output is normal, a state called GH resistance. Obesity is the trickiest case: obese men produce less GH, sometimes dramatically less, yet their IGF-1 often reads normal because high insulin levels sensitise the liver to whatever GH is present. This is why a normal IGF-1 in a man with a 42 inch waist does not fully rule out reduced GH output, and why waist, insulin and IGF-1 should be read together.

Neither the top nor the bottom of the IGF-1 range is where men live longest. The relationship between IGF-1 and all-cause mortality is U-shaped, and the lowest risk in middle aged and older men sits roughly in the 100 to 160 ng/mL band on most modern assays. A 2011 meta-analysis in the Journal of Clinical Endocrinology and Metabolism pooled 12 cohort studies and found that both the lowest and the highest IGF-1 categories had about 25 to 30 percent higher all-cause mortality than the middle category. Analyses of more than 350,000 UK Biobank participants published between 2020 and 2023 replicated the shape: mortality from cardiovascular disease was highest at low IGF-1, mortality from cancer was highest at high IGF-1, and the two curves crossed near the middle of the range.

The two arms of the curve have different explanations. Low IGF-1 is linked to sarcopenia, low bone density, higher visceral fat, worse lipid profiles and heart failure, and in older men with heart disease a low IGF-1 predicts poorer survival. High IGF-1 is a growth signal, and cell growth is exactly what cancer needs. The clearest human evidence comes from people with Laron syndrome, a genetic condition in which the GH receptor does not work and IGF-1 is close to zero. In a 22 year follow-up of an Ecuadorian cohort of about 100 people with Laron syndrome, there was one non-lethal cancer and no cases of type 2 diabetes, against 17 percent cancer incidence and 5 percent diabetes in their unaffected relatives. The trade is not free, since people with Laron syndrome are short and carry more body fat, but it shows the direction of the effect.

The line chart is an illustration built from the shape and magnitude reported across the Burgers 2011 meta-analysis and the UK Biobank analyses, with the 100 to 130 ng/mL band set to a relative risk of 1.0. The exact values vary with assay, age and follow-up length, and men should not read the chart as a prediction for any individual. The message that survives every study is the same: cardiovascular risk rises as IGF-1 falls, cancer risk rises as IGF-1 climbs, and the overlap of the two puts the safest zone in the lower middle of the reference range. For a man over 40 this argues against chasing the top of the range with a growth hormone secretagogue, and equally against the very low IGF-1 produced by chronic under-eating.

High IGF-1 is associated with a modestly higher risk of several cancers, most consistently prostate, colorectal and breast, but the increase is small in absolute terms and it has not been shown that raising IGF-1 with a peptide for a few months causes cancer. The Endogenous Hormones and Prostate Cancer Collaborative Group pooled 12 prospective studies with 3,700 prostate cancer cases and found a 38 percent higher odds of prostate cancer in the highest fifth of IGF-1 compared with the lowest fifth, with a roughly linear trend across the fifths. For colorectal cancer, the increase per standard deviation of IGF-1 is about 10 to 15 percent. UK Biobank Mendelian randomization analyses, which use genetic variants to test causation, support a causal role for IGF-1 in prostate and colorectal cancer, which makes the association harder to dismiss as a marker of something else.

What this means in practice is proportional caution. A man with a family history of prostate or colorectal cancer, a rising PSA, or a history of colon polyps has a stronger reason than most to keep IGF-1 in the lower half of the range and to avoid secretagogues, especially the long-acting ones such as CJC-1295 with DAC and MK-677 that keep IGF-1 elevated around the clock. A man with no such history, a low baseline IGF-1 and a clear reason to use a growth hormone peptide, such as documented visceral fat with a low IGF-1, is making a different and more defensible trade, provided the dose is kept to what returns IGF-1 to the middle of the age range rather than the top. PSA and a colonoscopy on schedule are reasonable safeguards for any man over 45 who raises IGF-1 deliberately.

The largest day-to-day drivers of IGF-1 in a man over 40 are protein and calorie intake, sleep, body composition, thyroid and liver function, and any growth hormone secretagogue or anabolic hormone he is using. Testosterone therapy raises IGF-1 by about 10 to 20 percent, because testosterone increases the size of GH pulses. Deep sleep is where most GH is released, and a week of five-hour nights lowers IGF-1 measurably. Resistance training produces acute GH pulses but changes resting IGF-1 only modestly over months, usually by 5 to 15 percent. The table below lists the interventions with the approximate size of their effect, which is the information a man needs to decide which lever to pull first.

Growth hormone peptides raise IGF-1 by anywhere from 10 percent to more than 100 percent depending on the compound, the dose and how long it acts, and the size of the rise is the single best predictor of both the benefit and the risk. Short-acting compounds such as sermorelin and ipamorelin produce a GH pulse that lasts one to two hours and then clears, which preserves the body's own feedback loop and typically lifts IGF-1 by 10 to 40 percent. Tesamorelin, a stabilised GHRH analog dosed nightly, raised IGF-1 by roughly 100 ng/mL in its phase 3 trials and is the only secretagogue with randomized evidence of visceral fat reduction. The long-acting agents, CJC-1295 with DAC and oral MK-677, keep GH and IGF-1 elevated around the clock, produce the largest rises, and are the ones most likely to push a man above the upper reference limit and to raise fasting glucose along the way.

The chart makes the practical decision fairly clear. A man whose baseline IGF-1 sits at the 20th percentile for his age, say 95 ng/mL at 50, needs roughly a 30 to 40 percent rise to reach the middle of the range, which is what the short-acting compounds deliver. Putting that same man on MK-677 or CJC-1295 with DAC risks doubling his IGF-1 to around 190 ng/mL, which is above the 95th percentile for a 50 year old and squarely on the rising cancer arm of the mortality curve. The MK-677 trial in older adults is instructive: IGF-1 was restored to young adult levels, fat-free mass rose by about a kilogram, and fasting glucose and HbA1c rose too, without any improvement in strength or function. The metabolic cost of a large, continuous IGF-1 elevation is real, which is why the short-acting compounds remain the sensible default and why the dose should be set by the follow-up IGF-1, not by the vial.

A man over 40 with an IGF-1 in the bottom quarter of his age range can usually raise it by 15 to 30 percent within three months by fixing sleep, eating at least 1.6 grams of protein per kilogram of body weight, correcting hypothyroidism or a deficient testosterone level, reducing liver fat, and training with weights. Sleep comes first because the largest GH pulse of the day occurs in the first ninety minutes of slow-wave sleep, and a man who is short on deep sleep, whether from late nights, alcohol or untreated sleep apnea, is cutting off the signal at the source. Treating sleep apnea with CPAP raises IGF-1 measurably within months. Protein comes second: dairy protein in particular raises IGF-1 more than any other food, and men who eat under 1.2 grams per kilogram often see IGF-1 climb 20 percent when intake is brought to 1.6 or above.

Body composition and metabolic health are the slower levers. Visceral fat and fatty liver both blunt the liver's response to GH, and men who lose 5 to 10 percent of body weight through diet and training often see IGF-1 rise even though calorie restriction itself lowers it, because the improvement in hepatic GH sensitivity outweighs the deficit. Resistance training three to four times a week raises resting IGF-1 by a modest 5 to 15 percent over six months, mostly through better sleep and muscle-derived IGF-1 signalling. Testosterone matters as well: a man with a total testosterone under 300 ng/dL who is brought to the mid-normal range typically sees IGF-1 rise 10 to 20 percent, which is one of the several ways testosterone therapy and growth hormone output are linked. A man who has done all of this and still has an IGF-1 below the 5th percentile for his age, with symptoms, is the man for whom a GH stimulation test and an endocrinology referral are appropriate.

A man over 40 should test IGF-1 once as a baseline alongside his testosterone, thyroid, glucose and liver panels, read the result against the age-adjusted range as a percentile rather than a pass or fail, and then act on it in one of three ways depending on which third of the range it lands in. Bottom third: fix sleep, protein, thyroid, testosterone and liver fat first, and consider a short-acting secretagogue only if the number stays low with symptoms. Middle third: leave it alone and retest every one to two years. Top third without any secretagogue: confirm with a repeat, check for causes such as recent high protein loading, and be wary of adding anything that raises it. The twelve month plan below turns that into a sequence.

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