If you want to slow down the aging process, it might not hurt to replace whole milk with skim, new research suggests.
The study of over 5,800 U.S. adults found that those who regularly indulged in higher-fat milk had shorter telomeres in their cells -- a sign of accelerated "biological aging."
The findings do not prove that milk fat, per se, hastens aging, stressed researcher Larry Tucker, a professor of exercise sciences at Brigham Young University in Provo, Utah.
But the study does lend support to what U.S. dietary guidelines suggest for adults: If you're going to drink cow's milk, opt for low-fat or skim, Tucker said.
Telomeres are bits of DNA that sit at the tips of your chromosomes -- like the plastic caps at the ends of a shoelace. In essence, they help keep the chromosomes from fraying and sticking together. As people age, their telomeres gradually get shorter, and research has linked shorter telomere length to a higher risk of developing and dying from infections, cancer and heart disease.
"A variety of lifestyle factors are also related to telomere length," Tucker said.
Obese people, for example, tend to have shorter telomeres than lean people, as do smokers versus nonsmokers. And people who eat lots of fiber, vegetables and fruit typically have longer telomeres than those who shun those foods.
So for the latest study, Tucker decided to look at the relationship between telomere length and milk fat.
As diet goes, milk is "an interesting subject," Tucker noted.
"There are dozens of studies that have found, as milk consumption goes up, so do disease risks," he said. "But there are also dozens of studies that say the opposite."
Tucker used publicly available data on 5,834 U.S. adults who took part in a government health study. All were surveyed on their diet and lifestyle; they also gave DNA samples, which allowed measurements of telomere length.
About 60% of study participants said they drank 2% or full-fat milk, while 27% drank nonfat ("skim") or 1% milk, and 13% never drank milk.
On average, Tucker found, people who drank higher-fat milk had skimpier telomeres, versus fans of low- or nonfat milk.
Each percentage-point increase in milk fat -- 2% instead of 1%, for example -- was linked to 4.5 years of biological aging, the study found.
The occasional indulgence did not seem problematic. Among people who drank milk less than weekly, there was no connection between richer varieties and shorter telomeres.
Instead, the link was confined to people who drank milk at least once a week -- and was strongest among daily consumers, according to the report.
The findings were published online recently in the journal Oxidative Medicine and Cellular Longevity. Tucker reported no industry funding.
If there is a direct tie between higher-fat milk and aging, it's not clear why, he noted.
But, Tucker said, there were hints that saturated fat -- the kind found mainly in animal products -- might play a role.
He found that in people who had relatively little saturated fat in their diets overall, milk fat did not make a difference in telomere length. In contrast, the connection was "strong" in the one-third of people with the highest overall intake of saturated fat.
But since many lifestyle factors affect telomere length, it's tricky to pin the blame solely on milk fat.
Tucker was able to account for some other factors -- including people's diet and exercise habits, smoking and body weight. And higher-fat milk was still linked to stumpy telomeres.
"But it's impossible to control for everything," Tucker stressed. "We can't say that this was caused by milk fat."
That point was echoed by Lauri Wright, chair of nutrition and dietetics at the University of North Florida, in Jacksonville.
Still, the study highlights an "interesting relationship," said Wright, who is also a spokesperson for the Academy of Nutrition and Dietetics.
Regardless of whether milk fat speeds biological aging or not, she said, there are already reasons to opt for low-fat.
One is to limit saturated fat. High intake, Wright noted, is associated with heart disease, certain cancers and problems with blood sugar control.
Beyond that, she said, high-fat milk packs more calories, which can pack on pounds.
"Overall," Wright said, "we would recommend skim or 1% milk as part of a healthy diet and lifestyle."
The U.S. Department of Agriculture has recommendations on dairy foods.