WASHINGTON (Reuters) – Estrogen creams do not help reduce wrinkles, especially those caused by the sun, researchers reported on Monday.

The creams do help skin that is protected from the sun to produce more collagen — the substance that makes skin appear smooth. But this skin is usually far less wrinkled than skin that has seen the light of day, the team at the University of Michigan reported.

"Most of the time you want to get rid of wrinkles on your face, your hands, your neck," said Laure Rittie, who helped lead the study, paid for in part by Pfizer Inc.

She said she was surprised by the finding.

"Generally estrogen is thought to be beneficial for skin. A lot of companies offer products that tell you estrogen can help fight skin aging. This is only partially true," Rittie said in a telephone interview.

"It can only be beneficial for skin on areas that are not exposed to sunlight."

Her team worked with 70 men and women with an average age of 75, although some were in their early 50s.

They made various creams containing estradiol, the active form of estrogen, and had their volunteers apply them to forearms and hips three times every other day for two weeks.

On the hips, which are generally protected from sun, the estrogen-treated skin started producing more collagen, Rittie's team reported.

"Surprisingly, no significant changes in production were observed in women or men after two-week estradiol treatment of photo-aged forearm or face skin," they wrote in the Archives of Dermatology.

Aging alone can cause fine wrinkles, loose skin and sagging. Adding sun damage makes skin appear dry, with uneven pigmentation.

As skin is damaged by sun, the cells stop producing collagen. Dead collagen helps cause a wrinkled appearance.

Creams such as tretinoin and procedures such as carbon dioxide laser resurfacing can stimulate collagen production.

Rittie had hoped to simulate the effects with estrogen cream.

"We are not going to consider estrogen any more," Rittie said.