Women who cut calories to lose weight may inadvertently slow their reaction times, an effect that can continue for weeks after the women have stopped dieting.

That’s the finding from a study by scientists with USDA’s Agricultural Research Service and the Institute of Food Research of the British Biotechnology Sciences Research Council. The scientists report their findings today (April 9) in New Orleans at a meeting of the Federation of American Societies for Experimental Biology.

Reaction time lengthened by 11 percent in women volunteers who went on a reducing diet directed by researchers from the ARS Western Human Nutrition Research Center in San Francisco. The volunteers’ reaction times continued to slow for 3 weeks after they had stopped dieting and started eating enough to maintain their new, lower weights.

Researchers want to determine whether this slowdown lowers dieters' alertness--and thus increases their risk of accidents--or whether the even longer-term regimens sometimes needed to achieve healthful weights might increase such risks.

Fourteen women, age 25 to 42, participated in the study. Overweight but otherwise healthy, the volunteers lost an average of 27 pounds. During the 15-week reducing-diet-phase of the 21-week study, the women ate only half of the number of calories needed to maintain their beginning weight. The study was longer than other experiments on dieting and mental performance reported during the past five years.

At five intervals during the experiment, scientists measured reaction times by determining how long it took the volunteers to strike the space bar on a computer keyboard when a white star appeared on their screen. This "simple reaction time" does not require thinking through options and making a choice.

Further study of dieting's effects on reaction time could lead to new understanding of how the body uses calories and nutrients for thought and action. That new information could be used by healthcare professionals to improve diet and exercise regimens for the estimated one-half of all American women who, at any given time, are on a reducing diet.

Scientific contact: Mary J. Kretsch, USDA-ARS Western Human Nutrition Research Center, San Francisco, Calif., phone (415) 556-6225, mkretsch@whnrc.usda.gov.