The results of this randomized controlled trial indicate that calorie restriction or dieting, marked by significant weight loss and quantification of average daily energy deficit from change in energy stores, is not associated with a consistent pattern of deficits in verbal memory, visual memory, or attention/concentration performance. Only one of the verbal memory subtests, Trial B on the RAVLT, demonstrated a decrease in performance during the trial, but change in verbal memory was not a function of group assignment and the control group experienced a decrease commensurate with the three dieting groups (see ). Verbal memory performance, measured with the ACT, improved during the trial and change scores did not differ among the groups. Performance on the BVRT indicated that visual perception and memory was not negatively affected by calorie restriction/dieting. Additionally, attention/concentration was not negatively affected by calorie restriction, as demonstrated by improved performance on four of the CPT-II subtests. Change in performance on the CPT-II was not associated with group assignment. Lastly, effect size calculations (generalized eta squared) indicated that the proportion of variance in change scores accounted for by treatment was small across cognitive tests (0.00 to 0.07), and daily energy deficit was not associated with change scores on any test.
The results of this randomized controlled trial do not support previous studies reporting that self-reported dieting was associated with cognitive functioning deficits.5-9
Rather, this study supports research that found no relation or small inconsistent relations between dieting/calorie restriction and cognitive impairment.12-15
Daily energy deficit or calorie restriction in this study was documented through weight loss and change in body energy stores. Interpretation of the results of this study in the context of previous findings suggests that overt calorie restriction is not the reason for diminished cognitive function among self-reported dieters. Rather, self-reported dieters appear to have greater preoccupation with food and their body size/shape, and this preoccupation frequently mediates the association between dieting and cognitive impairment.6-11
Therefore, cognitive deficits among self-reported dieters could be due to the allocation of mental resources to task irrelevant cognitions, which limits mental resources for other cognitive tasks.10
If this is true, it suggests that cognitive deficits of self-reported dieters are similar to information processing biases that have been demonstrated in eating disorder samples and people who are overly concerned about their body weight/shape,32
many of whom score high on measures of dietary restraint. Indeed, self-reported restraint has been found to influence performance on cognitive tests. Green et al.33
found that high restraint dieters had the worst performance on cognitive tests, with a high restraint/nondieting group performing intermediately between nondieters and the high-restraint dieters. These results demonstrate the effect of restraint on cognitive function, even in the absence of self-reported dieting.
This study represents an important step toward examining the full effect of calorie restriction on cognition, which is necessary when studying calorie restriction in humans.3
The results of the present study add to the literature by addressing some of the limitations of previous studies. First, a group of nondieting middle-age men and women were recruited for this randomized controlled trial. Second, participants were randomized to one of three dieting conditions or a control condition, and cognitive function was measured at baseline (before dieting) and follow-up with validated neuropsychological instruments that are appropriate for serial evaluations. Third, daily energy deficit was quantified by change in body energy stores and the association between average daily energy deficit and change in cognitive function was examined.
The results of the present study must be interpreted in the context of its limitations. The most significant limitation includes the small sample size of each group (n = 12) and limited statistical power to detect group differences. This weakness affects the confidence with which the conclusions of the study are supported by the results. Effect sizes were calculated for each cognitive test to obtain a quantitative measure of the proportion of variance in change scores accounted for by treatment. The effect sizes are independent of sample size and consider variability, and they indicated that the proportion of variance in change scores accounted for by treatment was small. To further explore the extent to which the conclusions of the study were limited by small sample size, the number of participants per group needed to detect significant (α = 0.05) differences between groups was calculated based on the observed effect size.
The calculations for the verbal memory tests indicated that 32 participants per group were needed for the RAVLT Trial VI subtest, which had the largest effect size (0.07). For the RAVLT Trial I-V score and the ACT 36 second delay subtests, 51 and 72 participants per group were needed to detect a significant difference between groups. Nevertheless, the ACT 36-sec delay data indicated that every group except the CR group experienced increased performance. All other verbal memory tests required 87 (RAVLT, Delayed Recall) to more than 167 participants per group to detect significant differences. These analyses suggest that group differences could be detected with relatively large group sample sizes for two or three of the verbal memory subtests, but the other five to six subtests require quite large sample sizes that would likely be considered impractical in randomized controlled trials.
Furthermore, the small effect sizes for these tests (effect sizes range from 0.00 to 0.07) suggest that the effect of CR on cognitive function may not be clinically meaningful. Consequently, the conclusion that no consistent pattern of verbal memory performance deficits was apparent during the trial appears valid, although this conclusion is qualified by low statistical power and additional research with larger sample sizes is needed.
The visual memory measures had very small effect sizes (0.01) and required more than 492 participants per group to detect changes. Similarly, the attention/concentration subtests of the CTP-II had small effect sizes (0.00 to 0.04) and the number of participants per group required to detect significant differences ranged from 90 for the CPT-II Perseverations subtest to more than 190 for the other seven subtests. Therefore, it appears that the conclusion that no consistent patterns of visual memory or attention/concentration deficits are present is valid, although this conclusion is qualified by the small sample size of the study. Future research is needed that includes larger samples and additional tests of visual memory.
The small sample size also limited the ability to examine potential differences in change in cognitive function between dieting groups (CR and LCD) and participants who achieved similar energy deficit through exercise. Exercise has been found improve psychological well-being, particularly among men,34
but differential effects of energy deficit from exercise and dieting on cognitive function have not been thoroughly examined.
In conclusion, this study found no consistent evidence of deficits in verbal memory, visual memory, or attention/concentration associated with calorie restriction or dieting. Further research is needed with larger sample sizes to determine if self-reported dieters display information processing biases that occupy mental resources and negatively impact performance on certain cognitive tests.