Sixteen subjects with dementia of the Alzheimer type and 16 comparison subjects completed the study. The subjects with dementia consisted of 10 women and six men with a mean age of 75.0 years (SD=5.7) and a mean score of 60.3 (SD=9.8) on the Cambridge Mental Disorders of the Elderly Examination. The comparison subjects consisted of eight women and eight men with a mean age of 73.1 years (SD=4.9) and a mean score of 99.4 (SD=3.7) on the Cambridge Mental Disorders of the Elderly Examination. An additional female comparison subject withdrew after the first visit because of side effects (nausea). There were no significant differences between the groups in age, sex, or years of education.
A considerable depletion of free plasma tryptophan was recorded at 4 and 7 hours after subjects ingested the tryptophan-deficient drink. Free plasma tryptophan decreased by an average of 73% and 62%, at 4 and 7 hours, respectively, in the comparison group and by an average of 69% and 62%, respectively, in the group with dementia of the Alzheimer type. Total tryptophan levels followed a similar pattern. Repeated measures ANOVA showed a significant effect of tryptophan depletion on mean score on the Modified Mini-Mental State (
Table 1) and a significant effect of group on mean score on the Modified Mini-Mental State but no significant interaction of tryptophan depletion and group. In the group with dementia of the Alzheimer type, mean score on the Modified Mini-Mental State was significantly lower after tryptophan depletion (mean=55.7, SD=14.8) than after ingestion of placebo (mean=60.3, SD=13.8) (t=2.38, df=15, p=0.03; 95% CI=0.48–8.64). In the comparison group, there was no significant difference between mean score on the Modified Mini-Mental State after tryptophan depletion (mean=96.0, SD=4.3) and after ingestion of placebo (mean=96.6, SD=3.1) (t=0.74, df=15, p=0.47; 95% CI=–2.19 to 1.07). In both the placebo and depletion conditions, mean scores on the Modified Mini-Mental State were, as expected, significantly lower in the group with Alzheimer’s disease (data not given). There was a significant interaction between order of the study conditions and study group (
Table 1), which probably reflected a learning effect in the group with Alzheimer’s disease. Scores on the mood rating scales showed no significant effects of tryptophan depletion, time, or order of the study conditions and no significant interactions between tryptophan depletion and time (
Table 1). There was a significant effect of group on score on the Geriatric Depression Scale (
Table 1). Post hoc paired t tests showed significantly lower scores on the Geriatric Depression Scale in the comparison group (t=3.27, df=30, p=0.003).