Vol. 5, No. 3, 1999 Page 3

Tryptophan depletion increases aggression
in normal males

The body uses the amino acid tryptophan to manufacture the neurotransmitter serotonin, and deficiencies in brain levels or brain usage of serotonin are strongly linked to aggression and impulsiveness. But can dietary deficiencies in tryptophan itself actually lead to aggression?

To find out, J. M. Bjork and colleagues measured the aggressive behavior of eight men under two conditions: tryptophan depletion, and tryptophan "loading" (in which large amounts of the amino acid were provided). The tryptophan-depletion and tryptophan- loading conditions followed an overnight fast.

In each condition, subjects were told that they were losing money due to the actions of another (in reality, fictitious) study subject. The subjects had the opportunity to retaliate, by reducing the earnings of the imaginary subject. Under the tryptophan-depletion condition, the researchers say, "aggressive responding was significantly elevated" in relation to baseline levels, with aggression becoming more pronounced as trypotophan levels continued to decline. In contrast, no changes in aggression were seen in subjects given ample amounts of tryptophan.

Bjork et al. note that previous studies have shown that sharp reductions of tryptophan concentrations in plasma result in increased aggression, while raising tryptophan levels can blunt aggression. "These effects," they say, "are presumably due to impaired or enhanced serotonin synthesis and neurotransmission in the brain."


"The effects of tryptophan depletion and loading on laboratory aggression in men: time course and a food-restricted control," J. M. Bjork, D. M. Dougherty, F. G. Moeller, D. R. Cherek, and A. C. Swann, Psychopharmacology (Berl), Vol. 142, No. 1, February 1999, pp. 24-30. Address: J. M. Bjork, Department of Psychiatry and Behavioral Sciences, University of Texas, Houston Medical School, Houston, TX 77030.

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