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People who diligently use sunscreen every day can slow or even prevent for a time the development of wrinkles and sagging skin, a new study found. Although dermatologists have long told people to use sunscreen to prevent aging, this is the first research to show an actual effect on the appearance of skin, researchers said.

The study involved 900 white people ages 25 to 55 in Australia, where intense sun exposure is a fact of life. Most had fair skin, and nearly all burned in the sun. Most were using sunscreen at least some of the time, and two-thirds wore hats in the sun.

But researchers wanted to find out what would happen to skin if people tried to use a broad-spectrum sunscreen all the time over four and a half years. Half of the study participants were told to continue their usual practices, and the other half to slather on sunscreen daily.

The result, the researchers reported on Monday in The Annals of Internal Medicine, is that those assigned to use sunscreen every day had noticeably more resilient and smoother skin than those assigned to continue their usual practices.

The study also included nearly 900 people who were randomly assigned to take beta carotene, a nutritional supplement, or a placebo to see if the supplement prevented skin aging. It did not.

The sunscreen element of the study impressed other researchers. Dr. David R. Bickers, a dermatology professor at Columbia University who was not involved in the research, said it “makes it clear that extensive, consistent use of sunscreen can alter a pattern of what would be an inevitable progression of photo-aging.”

Until now, he said, most studies of sun-damaged skin were conducted with mice, not people, and it was not clear whether the results would be the same.

Dr. Barbara A. Gilchrest, a dermatology professor at the Boston University School of Medicine and the editor of The Journal of Investigative Dermatology, said she, too, found the study convincing.

Dr. Gilchrest, who was not associated with the study, noted that its subjects were not inveterate tanners but rather people who tried to protect their skin.

“They were not taking the worst sun offenders and taking them out of the sun,” Dr. Gilchrest said. “Everyone had pretty darn good sun-protection habits to begin with.”

No one had done such a study before because the very idea is daunting, dermatologists said. Hundreds of healthy people had to agree to follow their assigned regimens for years.

The sunscreen used by those assigned to daily applications had a sun protection factor, or SPF, of 15, which filters 92 percent of the sun’s rays. Someone who would normally burn in 10 minutes would burn in 150 minutes with an SPF 15. Those assigned to use sunscreen even had their sunscreen bottles weighed to make sure they were using it.

“Getting compliance over a sustained period of time is no mean feat,” Dr. Bickers said. “To me, it is remarkable that they were able to get the degree of compliance that they did.”

The sunscreen study was paid for by the National Health and Medical Research Council of Australia. No sunscreen makers contributed.

Researchers studied only white people in Nambour, northeast of Sydney. Participants agreed to let researchers make silicone casts of their skin at the start and the end of the study to assess how their skin had aged.

The study’s principal investigator, Adele C. Green, a senior scientist at the Queensland Institute of Medical Research and her colleagues there and at the University of Queensland had previously shown that this method provides the same sort of information as a skin biopsy. With a biopsy, dermatologists look at elastin, the elastic tissue, which degrades with aging, contributing to wrinkles and sagging skin.

It is easy to see the effects of aging in a biopsy, said Dr. David J. Leffell, a professor of dermatology and surgery at Yale. “Instead of nice pink fibers, you see a purple amorphous material. It is almost like looking at a photograph through a lens covered with Vaseline.”

The silicone molds allowed experts to look at corresponding changes on the skin’s surface. In making a mold, a subject first stretched the skin on the top of a hand by grasping a cardboard tube and making a fist. Then a researcher covered the top of the hand with silicone and peeled it off, forming the mold.

Assessors, who did not know whether the subjects were using sunscreen, examined the lines in the silicone molds and graded them from 0 to 6.

A score of 0 means no photo-aging at all. “It’s like a baby’s skin, resilient,” Dr. Green said. “There is a fine network of lines under a microscope.”

A person with a score of 6 has severely aged skin, with no elasticity and deep lines. Every point on the scale represents coarser skin and increased wrinkling on the hand, the face or wherever the skin is being assessed. On the face, each point is also associated with a greater number of visible small blood vessels.

At the start of the study, the median score in both groups of subjects was 4, which means they had moderate photo-aging. At the end, those assigned to daily sunscreen use still had a median score of 4, but those in the control group had a median of 5.

The study does not answer the question of whether people older than 55 would also have more youthful skin if they used sunscreen, Dr. Green cautioned. After 55, she said, aging’s effects on skin start to predominate. And the effects of ultraviolet light on skin are cumulative.

It is not known how much sunscreen can help if its use is started later in life. But Dr. Green would advise it anyway, she said, because it can protect against skin cancer.

The study also does not answer the question of whether darker-skinned people could protect their skin from wrinkling and sagging by using sunscreen.

People with fair skin have much more of a problem with photo-aging and skin cancer than those with darker skin, Dr. Gilchrest said. But if asked to speculate about whether darker-skinned people would retain more youthful skin if they used sunscreen, she said she thought they would.

It is not clear that anyone will ever get an answer for darker-skinned people, though, said Dr. Robert A. Swerlick, a dermatology professor at Emory University. With less photo-aging in darker skin, it would take longer to see an effect, if there was one, he noted.

“If the study could be extended long enough and people could be enticed to use screening agents diligently, an effect likely could be shown in darker-skinned people,” Dr. Swerlick said. But, he added, “whether it is worth the effort to religiously apply sunscreen over many years when your personal probability of benefit is uncertain and perhaps low is another story.”