Brief exposee

In human beings, phytoestrogens are readily absorbed, circulate in plasma and are excreted in the urine. Metabolic influence is different from that of grazing animals due to the differences between ruminant versus monogastric digestive systems.

Read on effects in

1. Man
2. Female
3. Infants

In the last few years, there has been a great deal of research into the possible beneficial effects of phytoestrogens in both diabetes and coronary heart disease.

The use of phytoestrogens (as soy protein) in fast food meals and other processed foods as a low-cost substitute for meat products may lead to excessive consumption of isoflavonoids by fast food eaters. A research team at the Queen's University in Belfast, in a review article, speculate that such intake may lead to a slight decrease in male fertility, including a decrease in reproductive capability if isoflavones are taken in excess during childhood.

In theory, exposure to high levels of phytoestrogens in males could alter their hypothalamic-pituitary-gonadal axis. However, studies have shown that such a hormonal effect is minor. 

Isoflavones supplementation has no effect in sperm concentration, count or motility and show no changes in testicular or ejaculate volume.  Phytoestrogens may have health benefits for males.

"Researchers are studying if phytoestrogens can prevent prostate cancer".

There are conflicting studies, and it is unclear if phytoestrogens have any effect on the cause or prevention of cancer in females.  

Epidemiological studies showed a protective effect against breast cancer.  In vitro studies concluded that females with current or past breast cancer should be aware of the risks of potential tumor growth when taking soy products, as they can stimulate the growth of estrogen receptor-positive cells in vitro. 

The potential for tumor growth was found related only with small concentration of genistein and protective effects were found with larger concentrations of the same phytoestrogen.

A 2006 review article stated the opinion that not enough information is available, and that even if isoflavones have mechanisms to inhibit tumor growth, in vitro results justify the need to evaluate, at cellular level, the impact of isoflavones on breast tissue in females at high risk for breast cancer.

A Cochrane Review of the use of phytoestrogens to relieve the vasomotor symptoms of menopause (hot flashes) demonstrated that there was no evidence to suggest any benefit to their use.

HRT may also be effective at reversing the effects of aging on muscle. A future aim is to target therapy to molecular mechanisms that work specifically in selected tissues.

Some studies have found that some concentrations of isoflavones may have effects on intestinal cells. At low doses, genistein acted as a weak estrogen and stimulated cell growth; at high doses, it inhibited proliferation and altered cell cycle dynamics. This biphasic response correlates with how genistein is thought to exert its effects.

Some reviews express the opinion that more research is needed to answer the question of what effect phytoestrogens may have on infants, but their authors did not find any adverse effects. Multiple studies conclude there are no adverse effects in human growth, development, or reproduction as a result of the consumption of soy-based infant formula compared to conventional cow-milk formula.  While it should be noted that all infant formulas are inferior to human milk, soy formula presents no more risk than cow-milk formula.

One of these studies, published at the Journal of Nutrition,  concludes that:
"...there is no clinical concerns with respect to nutritional adequacy, sexual development, neurobehavioral development, immune development, or thyroid disease. SBIFs provide complete nutrition that adequately supports normal infant growth and development. FDA has accepted SBIFs as safe for use as the sole source of nutrition"

Clinical guidelines from the American Academy of Pediatrics state: "although isolated soy protein-based formulas may be used to provide nutrition for normal growth and development, there are few indications for their use in place of cow milk-based formula.
These indications include

• (a) for infants with galactosemia and hereditary lactase deficiency (rare) and
•  (b) in situations in which a vegetarian diet is preferred.