For every woman who complains about how messy her new live-in boyfriend is, add a new concern: his effect on her health. Dietitians have found that women tend to gain weight once they move in with male partners. ''Living with a male seemed to put pressure on females to consume more of the 'unhealthy' choices,'' Amelia Lake, a research fellow at the Newcastle University Human Nutrition Research Center in Britain, wrote this year in the journal Complete Nutrition, ''while females had a positive influence on the diets of the males.''

Lake and her colleagues collected evidence from dozens of studies in Britain, the United States and Australia (including work by the Cornell researchers Caron Bove and Jeffery Sobal, who traced patterns of dietary convergence). They found that both partners tend to change their dietary habits to accommodate each other at the beginning of a relationship (what the researchers call the honeymoon period), with men eating better than usual and women eating worse. Over time, both partners drift back toward their starting points -- but not completely. Because women tend to take over control of the couple's food shopping and cooking, they often retain a positive impact on their partner's food consumption. Meanwhile, women tend to stay in the honeymoon period longer, and inasmuch as they are open to influence by their partners' lifestyles in the long run, they will tend to eat fattier, less healthful foods. CLAY RISEN

Chart: The chart shows patterns of Dietary Convergence among couples.(Chart by Catalogtree)