Does human growth hormone have antiaging powers? Or does it contribute to heightened cancer risk and earlier death? The complex science in the area suggests the answer is yes to both.
Growth hormone, a biochemical that helps stimulate cell growth and division, is given to children and teens with low natural supplies to increase their growth. Increasingly, healthy older individuals also are taking it to improve the appearance of skin, increase muscle tone and for other purported benefits.
Taking growth hormone for antiaging purposes is hugely controversial in the medical community but nonetheless appears to be gaining popularity in parts of the world. The global market for human growth hormone, or HGH, will reach an estimated $4.7 billion by 2018, up from $3.5 billion in 2011, according to Global Industry Analysts Inc., a market research firm.
New studies published this year, however, offer the strongest indication yet that lower levels of a compound related to growth hormone called insulin-like growth factor-1, or IGF-1, are related to longevity and lower risk of cancer as people reach old age.
"These studies suggest that growth hormone for healthy aging might not be a good idea," says Nir Barzilai, an endocrinologist at Albert Einstein College of Medicine in New York City who published one of the papers in the journal Aging Cell in February.
That levels of testosterone, estrogen, growth hormone and other biological chemicals fall with age is well known among medical professionals. But whether replacing or supplementing hormones is good for the health of an aging individual is a complex question.
One cautionary tale comes from estrogen replacement, once thought to benefit women post-menopause. Data from a large trial, the Women's Health Initiative, indicated that giving estrogen to women 50 and older appeared to increase the risk of stroke and perhaps breast cancer.
That's because what's good for young people biologically isn't necessarily good for older adults. The same hormones may have a different effect across the life span and the outcome may be different, says Dr. Barzilai, also director of Einstein's Institute for Aging Research.
HGH prompts the liver and other organs to make IGF-1, which affects many tissues and organs in the body. Studies usually measure IGF-1 rather than growth hormone directly because IGF-1 levels remain more constant.
The Food and Drug Administration has approved many synthetic growth hormone products for treatment of people who need more in their systems. But since 2010 the agency has monitored people receiving treatment due to data suggesting that adults who were treated with HGH during childhood had a 30% increased risk of early death compared with the general population. The FDA continues to believe the benefits outweigh the risks in that population, according to its website.
Also, even supposed antiaging benefits may not truly be healthy. There's evidence that the increased muscle from growth hormone doesn't actually increase strength. Growth of muscle alone due to HGH use won't necessarily improve functioning if the well-worn neural pathways to the brain aren't repaired.
And growth hormone does stimulate cartilage growth, but this can actually cause carpal tunnel syndrome in older people, says William Sonntag, director of the Reynolds Oklahoma Center on Aging.
There aren't any randomized controlled trials—and probably won't be for ethical reasons—that would provide more definite evidence of benefit or harm of use of IGF-1 in healthy older people.
Few in the scientific community dispute that there are some modest benefits to increasing IGF-1 levels, such as tightening of the skin. There also appear to be cognitive benefits.
Dr. Sonntag and his colleagues bred mice with the idea of removing an IGF-1 gene—and therefore lowering the amount of IGF-1—in their brains at various ages. They found that mice growing up with a normal amount of IGF-1 who had their IGF-1 levels reduced in later life showed cognitive impairment as a result.
But the relationship between growth hormone and cognitive function is complex, Dr. Sonntag says. When the group studied mice that lived all their lives with low IGF-1 levels—as some people with genetic mutations are known to do—they showed no cognitive impairment.
In fact, these mice didn't show deficits of IGF-1 in the brain. Somehow, it seems, the brain tissue, which also makes IGF-1, compensated for the lower amount of IGF-1 levels circulating in the blood, researchers at Southern Illinois University of Medicine have shown.
"Although on the surface it looks like reducing IGF-1 is going to be good for us, it's a lot more complicated than that," Dr. Sonntag says.
There also are clear risks involved with higher levels of growth hormone, many scientists say. Among the strongest is cancer. Studies show that lowering IGF-1 by 50% decreases cancer risk significantly and that increased levels of IGF-1 are linked with higher cancer risk.
Longevity is another area where low growth hormone levels appear to be better than high. Across many species, those with low lifetime IGF-1 levels, including mice, fruit flies and humans, live longer than those with higher levels, a number of studies have shown. But the data isn't as clear about typically aging people who likely have normal levels of growth hormone growing up but experience age-related decline.
A new study is among those offering better evidence that lower growth hormone is linked with longevity. Valter Longo, director of the Longevity Institute at the University of Southern California, and his team, published the study in Cell Metabolism in March.
They found that in those ages 50 to 65, people with higher IGF-1 levels showed a fourfold increased risk for cancer and 75% increase in overall mortality compared with those with lower levels.
"Overwhelmingly the human data and the research and the science will say that, for the majority of people, [taking HGH is] just a bad idea," says Dr. Longo.
It doesn't rule out the possibility that there's some benefit for some individuals, but generally exercise, muscle training and eating better should be the focus of efforts to improve health, he says.
Yet this treatment may induce a strong placebo effect.
Dr. Sonntag recalls hearing from a retired salesman in his 80s who said he had been taking growth hormone and was feeling great because of it. The man also said he exercised regularly and watched his diet. He was in town to take his 101-year-old mother to lunch.
Moreover, the man was taking pills to stimulate HGH production—which it does in children but not for older people, Dr. Sonntag says. "He had good genes. He was doing the right thing" with exercise and diet, the doctor says. But "he thought it was the growth hormone he was taking."
Write to Shirley S. Wang at email@example.com
Corrections & Amplifications
The participants of the study by the Longevity Institute at the University of Southern California were ages 50 to 65 years old. An earlier version incorrectly stated that the study's participants were over 65.