Americans have lower average vitamin D levels today than they did 20 years ago — a finding that suggests that overzealous sun protection in the past two decades might be lowering vitamin D levels across the board.

Researchers compared vitamin D levels from just under 20,000 participants in 1988-1994 with a group in 2000-2004. The latter group showed a reduction of 5-20 nanomoles per liter in vitamin D blood concentration. The study was published this week in the American Journal of Clinical Nutrition. “In an adult subgroup, combined changes in (body mass index) milk intake, and sun protection appeared to contribute to a real decline in vitamin D status,” the authors wrote in their paper.

According to the study, men appeared to be particularly affected.

Many in the vitamin D research community are already talking about this paper. “The result is potentially disturbing if it represents the beginning of the downward trend in (vitamin D) concentrations that has been seen over the past 10-15 years,” Dr. Anthony Norman, a vitamin D expert at University of California, wrote in an accompanying editorial in the American Journal of Clinical Nutrition.

Sun exposure to the skin is the body’s most natural and prolific source for vitamin D.