Cognitive functions are those abilities that we use everyday; remembering, reacting, paying attention and recognising people and our environment.

There is considerable interest in the effects of diet, food and food components on mental performance (1, 2).

The knowledge about the possible beneficial effects of soy on cognitive function is still premature and more research is needed.

The effect of soy isolfavones on the cognitive performace of young premenopausal women has been investigated in four studies, two of which report improved verbal memory (3, 4). File et al’s (2001) 10-week study reported that 100 mg soy per day was associated with improved verbal and non-verbal episodic memory and mental flexibility in both genders. Verbal fluency and planning ability were also improved in women, but not in men (4). Hill et al.’s (2005) double-blind, randomised, placebo-controlled trial of 68 mg soy isoflavones (aglycone weight) per day, administered in a soy food matrix across two consecutive menstrual cycles per treatment found no effects on other cognitive functions but symptoms of breast tenderness and headaches were reduced (5, 6).

Perimenopausal and postmenopausal women commonly report decline in cognitive functioning, especially regarding memory (7). A handful of trials have been conducted in postmenopausal women from 45 years up to 75 years of age, ranging in duration from 6 weeks to 1 year, with doses of soy isoflavones ranging from 60 to 100 mg/day. These generally show stronger cognitive effects of soy isoflavones than in premenopausal women. Effects are evident in terms of improvements in memory and aspects of frontal lobe function (9, 10). Some of the beneficial effects are confined to women within 10 years of the menopause suggesting that there may be a critical time point for isoflavone action (8, 11). On the other hand, 2 very recent studies both failed to find positive effects of 70-80 mg/d isoflavones on a range of cognitive functions in younger and older postmenopausal women (12,13).

Possible mechanisms for soy isoflavone effects on cognitive function
Isoflavones, which mimic the effects of estrogens, could influence cognitive function. Furthermore, phytoestrogens are able to enter the brain and act centrally as well as peripherally and evidence from human and animal studies suggests that phytoestrogens could affect cognition via direct as well as by modulating the hormonal milieu (14).

More studies are desperately needed, not only to confirm these reports, but also to understand the influence that long-term consumption of a soy diet may have on cognition.

1. Dye L, Lluch, A and Blundell, JE. Macronutrients and Mental Performance. Nutrition 2000;16:1021-1034.
   
   
2. Dye L, Blundell J. Functional foods: psychological and behavioural functions. British Journal of Nutrition 2002;88:S187-211.
   
   
3. Hill C, Dye L, Bryant M, Powell J, Talbot D and Cassidy A. ‘Phytoestrogens and cognitive performance in premenopausal women.’ Journal of Nutrition 2004;134:1292S.
   
   
4. File SE, Jarrett N, Fluck E, Duffy R, Casey K and Wiseman H. Eating soya improves human memory. Psychopharmacology 2001;157:430-6.
   
   
5. Hill C, Dye L, Bryant M, Powell J, Talbot D, Cassidy A (2005) ‘Phytoestrogens and cognitive performance in postmenopausal women. Paper presented at 3rd International Conference on Soy and Health, Chicago, IL.
   
   
6. Bryant M, Cassidy A, Hill C, Powell J, Talbot D and Dye L. Effect of consumption of soy isoflavones on behavioural, somatic and affective symptoms in women with Premenstrual Syndrome (PMS) British Journal of Nutrition 2005;93(5):731-739.
   
   
7. Brinton RD. Cellular and molecular mechanisms of estrogen regulation of memory function and neuroprotection against Alzheimer's disease: Recent insights and remaining challenges. Learning and Memory 2001;8:121-133.
   
   
8. Kreijkamp-Kaspers S, Kok L, Grobbee DE, De Haan EHF, Aleman A, Lampe JW and van der Schouw YT. "Effect of soy protein containing isoflavones on cognitive function, bone mineral density, and plasma lipids in postmenopausal women." Journal of the American Medical Association 2004;292:65-74.
   
   
9. Casini ML, Marelli G, Papaleo E, Ferrari A, D’Ambrosio F and Unfer V. Psychological asessment of the effects of treatment with phytoestrogens on postmenopausal women: a randomized, double-blind, crossover, placebo-controlled study. Fertility and Sterility 2006;85(4):972-978.
   
   
10. File SE, Duffy R and Wiseman H. Improved memory and frontal lobe function in post-menopausal women after 3 months' treatment with soya supplements. European Neuropsychopharmacology 2002;12:S406.
    
    
11. Kritz-Silverstein D, Von Muhlen D, Barrett-Connor E and Bressel MAB. Isoflavones and cognitive function in older women: the Soy and Postmenopausal Health In Aging (SOPHIA) Study. Menopause 2003;10:196-202.
    
    
12. Fournier LR, Ryan-Borchers TA, Robison LM, Wiediger M, Park J–S, Chew BP, Mcguire MK, Sclar DA, Skaer TL, Beerman KA. The effects of soy milk and isoflavone supplements on cognitive performance in healthy, postmenopausal women. Journal of Nutrition, Health and Aging 2007;11(2):155-164.
    
    
13. Ho SC, Chan ASY, Ho YP, So EKF, Sham A, Zee B, Woo JLF. Effects of soy isoflavone supplementation on cognitive function in Chinese postmenopausal women: a double-blind randomized, controlled trial. Menopause 2007;14(3):489-499.
    
    
14. Hill CE and Dye L. Phytoestrogens and Cognitive Function. Current Opinions in Nutraceutical Research 2003;1(3):203-212.