there is evidence to suggest that certain weight - loss practices ( for example , skipping breakfast , restricting intake , self - initiated dieting ) and weight loss are associated with impairments in various cognitive processes , including memory , attention and processing speed .
one study reported slower reaction times to salient food words on a food stroop task in weight - loss maintainers compared with non - dieting normal weight and obese individuals .
reasons for the decrements in cognitive performance during dieting , weight loss or weight maintenance in these studies are not fully understood but may be due to alterations in blood glucose levels ( as a result of skipping meals , restrictions in food intake ) , preoccupying thoughts of food and weight and emotional reactivity to salient cues . despite these findings , minimal to no cognitive impairment and improvements in executive / attention functioning and memory have been observed in other studies investigating the effects of weight loss on cognitive function .
research over the last decade suggests that low - carbohydrate ( l - cho ) diets are a viable option in the treatment of obesity .
although the effects of these diets on weight and cardiovascular disease risk factors are well described , less is known about secondary outcomes , such as cognitive functioning , that are also important to those deciding which type of weight - loss diet to employ .
given that l - cho ketogenic diets are known to affect brain function as evidenced by their ability to suppress seizure activity , it is reasonable to speculate whether l - cho and high - carbohydrate ( h - cho ) diets differ in their effects on cognitive function under other circumstances like dieting and weight loss .
one study in obese women ( n=21 ) who consumed either a very - low - calorie ketogenic or non - ketogenic liquid formula diet for 28 days showed that attention and information processing did not differ as a function of diet ; however , psychomotor and problem - solving performance were adversely affected in those consuming a ketogenic diet , primarily during the first week of dieting .
a more recent study in overweight and obese women ( n=19 ) showed that those consuming an energy - restricted , l - cho diet for 3 weeks performed worse on a memory - based task but better on an attention - vigilance task than those consuming an energy - restricted h - cho diet . to our knowledge , only one study has compared the effects of consuming an energy - restricted l - cho or h - cho diet on cognitive function in overweight and obese adults ( n=106 ) for a period over 4 weeks . in this study , participants were instructed to consume less than 20  g of carbohydrate per day for the first 8 weeks .
after 8 weeks , they could increase their intake to less than 40  g of carbohydrate per day for the remainder of the study .
although both diet groups showed improvements in speed of processing after 8 weeks , the l - cho group displayed a smaller improvement than the h - cho group .
there were no differences between groups in either working memory or speed of processing after 1 year .
the purpose of this study was to compare the effects of two weight - reducing dietary approaches ( l - cho vs h - cho diets ) on attention , information processing , reaction time , short - term memory and executive function in overweight or obese men and women over a 6-month period .
all study participants were enrolled in three - center , two - year randomized controlled trial comparing the effects of l - cho and h - cho diets for the treatment of obesity .
a detailed description of the recruitment and screening procedures and assessments used in the parent study are available elsewhere .
individuals reporting psychiatric conditions , pre - existing health problems ( for example , chronic diseases such as diabetes , hypertension , cardiovascular disease , stroke ) , use of prescription medications , regular alcohol , tobacco or other drug use were excluded from the study .
the current study was a sub - study of the larger study and only involved the philadelphia site .
all participants at this site ( n=106 ) were invited to participate in assessments of cognitive functioning before and after 1 , 4 , 12 and 26 weeks of treatment and 47 participants agreed to participate .
participants were randomized to treatment conditions and consumed a self - selected diet while participating in a comprehensive behavioral weight control program .
group sessions varied between the two treatment conditions only in the type of diet plan that was prescribed .
participants in the l - cho condition were instructed to follow a diet that was restricted in carbohydrate and unlimited in fat and protein .
participants were provided with information on l - cho diets as well as numerous suggestions for meal plans .
instruction was consistent with the four recommended phases described in dr atkins ' new diet revolution .
participants in the h - cho condition were encouraged to consume a diet consistent with the dietary guidelines for americans ( that is , 30% of calories from fat , 15% from protein and 55% from carbohydrate ) .
participants were instructed to make changes in dietary intake that included reducing fat and increasing the consumption of fruits , vegetables , breads and cereals . suggested caloric intakes for men and women were set at 15001800  kcal  d and 12001500  kcal  d , respectively .
1 assessment was based on a previous study showing impairments in cognitive function 1 week after initiation of a l - cho diet .
a minimum of one 30  min training session was required before the baseline assessment to familiarize participants with the computer tasks .
together with study staff each participant reviewed written instructions describing the procedures for completing the four computer - based performance tasks .
participants were given the opportunity to ask questions and to briefly practice each task in order to demonstrate that they understood how to complete the task .
if a participant was unable to demonstrate comprehension of a particular task during the initial training session , ( s)he would be given one additional opportunity to practice that particular task and demonstrate understanding ( that is , there was a maximum of two training sessions ) .
on testing days , participants were instructed to consume lunch at noon from a list of suggested foods with known macronutrient compositions ( consistent with the dietary prescriptions of the group to which they were randomized ) , eat a standard l - cho snack ( 3% cho , 180 calories ) 2  h later , and to abstain from alcohol .
testing was conducted approximately 2  h following consumption of the snack to ensure that all participants were in the same fed state . upon arrival
, participants were asked to provide a urine sample ( to assess ketones ) and to complete a check - in questionnaire ( to assess adherence to the pre - session procedures ) as well as a vas questionnaire ( to assess hunger , craving and anxiety ) .
the stim complete system ( compumedics neuroscan ) was used , and the assessment battery included : the color and food stroop task ( attention and information processing ) ; the continuous performance task ( cpt , attention and reaction time ) ; word recall ( verbal short - term memory ) and the wisconsin card - sorting task ( problem solving , set - shifting and cognitive flexibility ) .
test order was randomly administered , and the performance battery took approximately 30  min to complete .
comparisons between treatment group means over time were conducted with proc glimmix in sas v. 9.3 ( sas institute inc . , cary , nc , usa ) .
this procedure was designed for generalized linear mixed models , also called random coefficients or multilevel / hierarchical models . to build the models , baseline ( visit=0 )
next , time was coded as 1 , 4 , 12 and 24 ( visit months ) and entered as a continuous variable .
overall trends for time were evaluated using linear and up to third order polynomial terms , and differences in trajectories between treatment groups were tested with interaction terms with dummy ( 0/1 ) coding for treatment groups . to accommodate the dependence caused by repeated measures for each subject over visits , initial models were parameterized with random intercepts and slopes along with the covariance between the variance components .
lack of significant p - values for regression coefficients as well as confidence intervals including zero for variance / covariance components led to final or reduced models with only random intercepts and linear or curvilinear ( quadratic ) fixed time slopes .
differences between groups were based on the p - values from the  fixed ' effects using =0.05 as level of statistical significance .
significant time ( p<0.05 ) by condition interactions were followed up with comparison between lsmeans at each time point employing bonferroni adjustment for multiple comparisons ( p<0.0125 ) .
comparisons between treatment group means over time were conducted with proc glimmix in sas v. 9.3 ( sas institute inc . , cary , nc , usa ) .
this procedure was designed for generalized linear mixed models , also called random coefficients or multilevel / hierarchical models . to build the models , baseline ( visit=0 )
next , time was coded as 1 , 4 , 12 and 24 ( visit months ) and entered as a continuous variable .
overall trends for time were evaluated using linear and up to third order polynomial terms , and differences in trajectories between treatment groups were tested with interaction terms with dummy ( 0/1 ) coding for treatment groups . to accommodate the dependence caused by repeated measures for each subject over visits , initial models were parameterized with random intercepts and slopes along with the covariance between the variance components .
lack of significant p - values for regression coefficients as well as confidence intervals including zero for variance / covariance components led to final or reduced models with only random intercepts and linear or curvilinear ( quadratic ) fixed time slopes .
differences between groups were based on the p - values from the  fixed ' effects using =0.05 as level of statistical significance .
significant time ( p<0.05 ) by condition interactions were followed up with comparison between lsmeans at each time point employing bonferroni adjustment for multiple comparisons ( p<0.0125 ) .
the sample consisted of 47 ( 25 males , 22 females ) participants who were 70% caucasian , 28% african american , 2% others , with a means.d .
age of 47.48.7 years and body mass index of 35.33.4  kg  m. there were no significant differences between participants in the l - cho ( n=22 ) or the h - cho ( n=25 ) groups on any baseline variable .
participants in the h - cho group lost 0.61.0% , 3.41.6% , 7.44.3% and 9.75.6% of their baseline body weight at 1 , 4 , 12 and 24 weeks , respectively .
participants in the l - cho group lost 0.51.6% , 4.02.1% , 8.53.8% and 11.45.6% of their baseline body weight at 1 , 4 , 12 and 24 weeks , respectively .
there were no significant differences in weight loss between groups at any of the assessment points .
however , there was a significant main effect for time ( p<0.0001 ) and a significant interaction between time and condition ( p=0.0002 ) for urinary ketones . compared with the h - cho group ,
urinary ketones increased to a greater extent in the l - cho group , particularly during the first week on the diet , and gradually declined over time , suggesting good adherence to l - cho prescriptions , which increased carbohydrate intake over time ( figure 1 ) .
there was a significant main effect for time ( p<0.02 ) and a significant quadratic effect for time ( p<0.05 ) for the percentage of correct responses made on the color stroop task .
the percentage of correct color responses significantly increased over time in both the h - cho and l - cho groups , but there was no difference between the groups ( figure 2 ) .
there were no significant effects on accuracy as a function of word type ( that is , neutral or salient foods ) on the food stroop over time or between groups .
there were no significant differences over time or between groups in response time or number of correct responses on the cpt .
the dependent measures for this task were percentage of correct responses and number of incorrect responses .
there were no significant differences in the percentage of correct responses or number of incorrect responses over time or between groups .
there were no significant changes in the performance ( that is , mean response time or number of errors ) over time or between groups .
participants in the h - cho group lost 0.61.0% , 3.41.6% , 7.44.3% and 9.75.6% of their baseline body weight at 1 , 4 , 12 and 24 weeks , respectively .
participants in the l - cho group lost 0.51.6% , 4.02.1% , 8.53.8% and 11.45.6% of their baseline body weight at 1 , 4 , 12 and 24 weeks , respectively .
there were no significant differences in weight loss between groups at any of the assessment points .
however , there was a significant main effect for time ( p<0.0001 ) and a significant interaction between time and condition ( p=0.0002 ) for urinary ketones . compared with the h - cho group
, urinary ketones increased to a greater extent in the l - cho group , particularly during the first week on the diet , and gradually declined over time , suggesting good adherence to l - cho prescriptions , which increased carbohydrate intake over time ( figure 1 ) .
there was a significant main effect for time ( p<0.02 ) and a significant quadratic effect for time ( p<0.05 ) for the percentage of correct responses made on the color stroop task .
the percentage of correct color responses significantly increased over time in both the h - cho and l - cho groups , but there was no difference between the groups ( figure 2 ) .
there were no significant effects on accuracy as a function of word type ( that is , neutral or salient foods ) on the food stroop over time or between groups .
there were no significant differences over time or between groups in response time or number of correct responses on the cpt .
the dependent measures for this task were percentage of correct responses and number of incorrect responses .
there were no significant differences in the percentage of correct responses or number of incorrect responses over time or between groups .
there were no significant changes in the performance ( that is , mean response time or number of errors ) over time or between groups .
there was a significant main effect for time ( p<0.02 ) and a significant quadratic effect for time ( p<0.05 ) for the percentage of correct responses made on the color stroop task .
the percentage of correct color responses significantly increased over time in both the h - cho and l - cho groups , but there was no difference between the groups ( figure 2 ) .
there were no significant effects on accuracy as a function of word type ( that is , neutral or salient foods ) on the food stroop over time or between groups .
the dependent measures were response time and number of correct responses . there were no significant differences over time or between groups in response time or number of correct responses on the cpt .
the dependent measures for this task were percentage of correct responses and number of incorrect responses .
there were no significant differences in the percentage of correct responses or number of incorrect responses over time or between groups .
the dependent measures were mean response time and number of errors . there were no significant changes in the performance ( that is , mean response time or number of errors ) over time or between groups .
this study investigated the cognitive effects of l - cho and h - cho weight - reducing diets over a 6-month period .
the main findings from this study are : ( 1 ) weight loss has neither a positive or negative effect on cognitive function and ( 2 ) l - cho and h - cho weight - loss diets have similar effects on cognitive performance .
although others have reported some improvement in executive / attention functioning and memory with weight loss , little improvement was observed in the current study .
more specifically , with the exception of improved accuracy over time in both groups on the color stroop task , assessing attention and information processing , there were no significant differences in performance over time or between groups on tasks assessing attention and reaction time ( cpt ) , short - term memory ( word recall ) or problem solving ( wisconsin card - sorting task ) . in the most restrictive period of the current study ( weeks 112 ) , participants in the l - cho group consumed 20  g of carbohydrate .
carbohydrate intake gradually increased by 5  g per day per week , depending on the rate of weight loss , for the remainder of the study . to our knowledge
, only one other study has investigated the effects of consuming 2040  g of carbohydrate as part of an energy - restricted diet on cognitive function in humans . in this study , participants were randomized to either a h - cho diet ( that is , 46% carbohydrate ) or a l - cho diet consisting of 920  g of carbohydrate per day for the first 8 weeks of the study and no more than 40  g of carbohydrate per day for the remaining 44 weeks of the study .
although there were no differences in working memory between the groups at 8 weeks , there were greater improvements in the speed of processing in the h - cho diet group compared with the l - cho group .
there were , however , no differences in working memory or speed of processing between groups after 12 months . those data along with findings from the current study
suggest an overall minimal impact of l - cho weight - loss diets prescribing 20  g of carbohydrate per day or more on cognitive function .
it is important to note that these findings are limited to relatively healthy , overweight and obese , middle - aged adults and to the cognitive processes assessed in these studies .
other studies suggest that l - cho diets restricting carbohydrate intake to less than 20  g per day can affect performance in both positive and negative directions depending on the cognitive function assessed .
one non - randomized study , in which participants completely withdrew from carbohydrates for the first week of the study , consumed 58  g of carbohydrate during the second week , and consumed 1016  g of carbohydrate during the final week of the study , showed that individuals consuming l - cho diets displayed faster reaction times on the cpt than those consuming a h - cho diet ( that is , dietary guidelines for americans ) .
in contrast , performance on memory tasks was worse in the l - cho group compared with the h - cho group .
in addition , participants consuming a h - cho diet displayed a practice effect over time and consistently responded faster to non - food words compared with food words on the stroop task , whereas those consuming a l - cho diet did not show a consistent practice effect and displayed little difference in reaction time as a function of word type . in another randomized study that prescribed either
a h - cho diet ( that is , 52% carbohydrate ) or a l - cho diet consisting of 10  g of carbohydrate a day , no differences in performance were observed between groups at any time on the stroop task or another task requiring sustained attention ( digit vigilance task ) but participants consuming the l - cho diet performed worse on a task requiring mental flexibility ( trail making task ) between baseline and week 1 on the diet .
these studies suggest that under conditions of more severe carbohydrate restriction , cognitive performance can either be enhanced or impaired depending on the type or complexity of the task .
this study had several strengths including a randomized design and dietary conditions ( that is , self - selected foods , dietary prescriptions that followed popular weight - reducing diet strategies ) that simulated a real - life setting .
as such , the present findings can be more easily generalized to similar populations in the real world .
in addition , all participants were given a standardized snack at a set time to ensure that they were all in the same metabolic state during testing .
further , weight loss was the same between groups so there was no confound of differential weight loss between the groups . despite these strengths ,
the study has several weaknesses including a small sample size and a limited array of cognitive tasks .
larger sample sizes would allow for a greater diversity in participants ( that is , ethnic background , age and so on ) and a better ability to detect subtle differences in cognitive performance between groups .
it is also possible that the testing parameters in this study were not stringent enough to tease out differences in cognitive performance or elicit differential responding between groups .
previous research has shown poorer performance on complex cognitive tasks but not on more basic cognitive tasks following l - cho intake
. a larger array of cognitive tasks would provide a better understanding of the types of mental functioning that can potentially be affected by diets with different macronutrient compositions .
although it is possible that the initial training session raised baseline performance , which might make it difficult to detect changes in subsequent performance , we do not feel that it made a significant impact because of the brevity of the training session .
in conclusion , these findings suggest that individuals participating in a 6-month behavioral weight - loss program randomized to either a l - cho ( that is , dr atkins ' new diet revolution ) or a h - cho ( that is , dietary guidelines for americans ) diet experienced similar effects on cognitive performance .
performance on the color stroop task improved over time in all participants but there were no differences in performance between groups .
there were no differences in performance over time or between groups on any other task ( that is , the food stroop , cpt , word recall or wisconsin card sorting task ) .
 objective : low - carbohydrate ( l - cho ) diets are often used for weight loss but their effects on cognitive function are not well understood . 
 the present study compared the effects of a l - cho and high - carbohydrate ( h - cho ) weight - loss diet on cognitive function adults.design:participants were randomized to either a l - cho ( n=22 ) or h - cho ( n=25 ) weight - loss diet . 
 cognitive function was evaluated by four computerized cognitive tasks ( stroop task , continuous performance task , word recall and wisconsin card sorting task ) presented in random order before and at 1 , 4 , 12 and 24 weeks after the initiation of the l - cho or h - cho diet.participants:forty-seven adults ( 25 males ) with a means.d . 
 age of 47.48.7 years and body mass index of 35.33.4  kg  m2.results:there were no significant differences in weight loss between groups at any time point . 
 there were significant improvements on color stroop task accuracy over time in both diet groups ( p<0.05 ) , but there were no differences in performance between groups on this or any other cognitive task at any time period.conclusion:these findings suggest that weight loss has neither a positive nor a negative effect on cognitive function and that l - cho and h - cho weight - loss diets have similar effects on cognitive performance . 