Daily sunscreen users were significantly less likely than discretionary sunscreen users to show signs of skin aging after 4.5 years, according to a study of young and middle-aged adults.
However, beta-carotene supplements appeared to have no effect on skin aging.
The findings from the randomized controlled trial were published online June 3 in Annals of Internal Medicine (2013;158:781-90).
"No known randomized studies in humans have evaluated the effect of sunscreen on surface changes associated with skin aging," wrote Maria Celia B. Hughes, MMedSci., of the Queensland Institute of Medical Research, Australia, and her colleagues.
The investigators used data from the Nambour (Australia) Skin Cancer Prevention Trial, in which 1,621 adults were studied from 1992 to 1996 to test the effect of sunscreen use or dietary supplements on skin cancer risk, photoaging, and actinic keratosis development.
To determine whether consistent, daily sunscreen use could prevent progression of skin aging, researchers randomized Nambour study participants under age 55 into four groups: daily use of broad-spectrum sunscreen plus 30 mg of beta-carotene; daily sunscreen use plus a placebo supplement; discretionary sunscreen use plus 30 mg of beta-carotene; and discretionary sunscreen use plus a placebo supplement.
The investigators focused on 903 adults younger than 55 to exclude the potential effects of growing old on participants’ skin aging.
Skin aging was assessed by comparing skin microtopography based on impressions taken of the backs of participants’ hands at baseline in 1992 and 4.5 years later in 1996. Assessors were blinded to the treatment groups.
"Most of the study participants were fair skinned, and more than 90% burned on acute sun exposure," the researchers noted. The groups were similar in terms of phenotype, sun exposure, and pretrial sunscreen use. All groups reported similar amounts of sun exposure during the study period; 78% of daily sunscreen users and 76% of discretionary sunscreen users reported being outdoors for less than 50% of their weekend time. In addition, the use of other sun protection measures, including seeking shade and wearing a hat, was similar among the groups.
By the end of the study, 77% of the daily sunscreen users applied sunscreen at least 3-4 days per week, vs. 33% of the discretionary users.
Overall, 58% of the participants in the current study met criteria for moderate photoaging at baseline in 1992, and 49% met those criteria in 1996. However, at the end of the 4.5-year period, daily sunscreen users were 24% less likely to show signs of skin aging than were discretionary users, a statistically significant difference.
When the odds of having a higher microtopography grade in 1996 than in 1992 were adjusted for sunburns and photoaging of the neck, the researchers noted, "only the daily sunscreen intervention group showed no detectable increase in microtopography grade."
No significant differences in skin aging were seen in participants randomized to beta-carotene vs. placebo.
The study was limited by several factors, including limited outcome data, which reduced the power to detect moderate treatment effects. In addition, the data were insufficient to rule out an effect of beta carotene on skin aging, the researchers noted.
However, "these results have important clinical implications," the researchers said. "A unit increase in microtopography significantly correlates with risk for actinic keratoses and skin cancer." Thus, the cosmetic benefits of reducing skin changes in middle age also may reduce cancer risk.
The National Health and Medical Research Council of Australia funded the study.