Menopausal symptoms and soya

The menopause is often accompanied by a number of complaints: hot flushes, night sweats, a feeling of tiredness, irritability, headaches, etc. These symptoms occur because during the menopause the oestrogen level in the body decreases.
The frequency of these types of disorders is much lower in the Asiatic countries compared to the Western countries. A possible explanation could be due to  the high consumption of soya amongst Asians.
Soya contains isoflavones, phyto-estrogens, that have a similar structure to endogenous estrogen. Their activity is nevertheless much lower. In menopausal women, isoflavones have the potential to bind to estrogen-receptors and  exert a weak estrogenic activity.

Research

The variability in results from early studies is thought to be due largely to widely differing study designs and sample sizes. In order to overcome this, recent meta-analyses have been published that select out the very best trials, using specific pre-defined criteria, and pool the studies into one analysis in order to increase statistical power. One recent meta-analysis of 12 randomized, controlled trials reported that isoflavone supplementation was associated with a reduction in hot flashes of 34% (95% confidence intervals - 47% to – 21%, P<0.0001) , with an increase in efficacy as the baseline number of hot flashes and isoflavone dose increased (1).

A second critical review of 11 studies using well-characterized isoflavone supplements confirmed the importance of dose in determining whether or not isoflavones reduce hot flashes (2). In this review, all five studies providing more than 15 mg genistein showed significant decreases in hot flash frequency, while only one of the six studies providing less than 10 mg genistein showed a significant decrease. The conclusion of these researchers was that consumption of genistein, rather than total isoflavones, is responsible for the reduction in symptoms associated with soy intake, and that 15 mg genistein is the minimal quantity for benefit.

Furthermore, different randomized placebo-controlled trials came to similar conclusions. When 51 Swedish postmenopausal women consumed either 60 mg isoflavones or a placebo for three months, hot flash symptoms were significantly reduced by 50% in the isoflavone group, while no change occurred in the placebo group (3).

Hot flashes were reduced by 57% and severity was reduced by 38% in Italian women who consumed 54 mg genistein daily for 12 months compared to women on placebo (4).

A third randomized controlled crossover trial was performed over an eight week period in the United States in 60 postmenopausal women. During the treatment period, the women consumed one-half cup of soy nuts (containing 25 g soy protein and 101 mg isoflavones) spread throughout the day (5). Although there was no change in hot flash frequency during the control period, the women experienced a 40% greater decline in frequency of hot flashes and significant improvements in a number of other menopausal symptoms while consuming the soy nuts (5).

Although soy isoflavones have been shown to be effective in reducing menopausal symptoms, it is generally accepted that this effect is far smaller than that of exogenous estrogens. Countering this assumption are results of a randomized, controlled, double-blind pilot study published in 2006 in which the effects of soy consumption were compared directly with the effects of exogenous estrogen administration. Seventy-nine Brazilian women consumed either 100 mg isoflavones split into two doses, or 0.625 mg conjugated equine estrogens daily for six months (6). In this direct comparison study, the Kupperman Menopausal Index of menopausal symptoms declined similarly in both the isoflavone and estrogen groups, by about 80% by the end of the trial.

All these data suggest that consumption of as little as 30 mg/day of soy isoflavones (or at least 15 mg genistein), in soy protein or tablet form, may reduce hot flashes by as much as 50% or more. This total reduction includes that provided by “the placebo effect”, the greatest benefit of soy isoflavones may be realized when the isoflavone rich food or supplement is taken in divided doses throughout the day by subjects who experience at least four hot flashes per day.

References :

1. Howes LG, Howes JB, Knight DC. Isoflavone therapy for menopausal flushes: a systematic review and meta-analysis. Maturitas. Oct 20 2006;55(3):203-211. 
2. Williamson-Hughes PS, Flickinger BD, Messina MJ, Empie MW. Isoflavone supplements containing predominantly genistein reduce hot flash symptoms: a critical review of published studies. Menopause. Sep-Oct 2006;13(5):831-839. 
3. Cheng G, Wilczek B, Warner M, Gustafsson JA, Landgren BM. Isoflavone treatment for acute menopausal symptoms. Menopause. May-Jun 2007;14(3 Pt 1):468-473. 
4. D'Anna R, Cannata ML, Atteritano M, et al. Effects of the phytoestrogen genistein on hot flushes, endometrium, and vaginal epithelium in postmenopausal women: a 1-year randomized, double-blind, placebo-controlled study. Menopause. Jul-Aug 2007;14(4):648-655. 
5. Welty FK, Lee KS, Lew NS, Nasca M, Zhou JR. The association between soy nut consumption and decreased menopausal symptoms. J Womens Health (Larchmt). Apr 2007;16(3):361-369. 
6. Kaari C, Haidar MA, Junior JM, et al. Randomized clinical trial comparing conjugated equine estrogens and isoflavones in postmenopausal women: a pilot study. Maturitas. Jan 10 2006;53(1):49-58