Soybean hulls, Peak Performance and milk in the udder – myth or real effect?

Soy, phytoestrogens and mares with milk in the udder – myth or real effect?

In short – the essence for those who just want the most important things:

• Phytoestrogens are found naturally in all legumes – including alfalfa and sainfoin.

• Soybean hulls, which are used in Peak Performance, contain very small amounts of phytoestrogens.

• The laboratory experiments that created the myth of “hormone-disrupting soy” use unrealistically high doses and bypass the body’s regulation.

• Fiber can temporarily change the body's estrogen metabolism, which in some mares can cause a little fluid or milk in the udder for a short time.

• The reaction is typically transient and harmless as the body adapts.

What are phytoestrogens – and where are they found?

Phytoestrogens are natural plant substances found in all legumes – not only in soy, but also in, for example, alfalfa and sainfoin.

They get their name because they resemble the body's own estrogen when you look at their molecular structure. But that doesn't mean they function like estrogen in the body.

The best-known phytoestrogens in soy are isoflavones (including genistein and daidzein). These substances are found primarily in the germ and cotyledons of the soybean – not in the shell.

👉 Soybean hulls, which are the part we use in Peak Performance, therefore only contain very small amounts of phytoestrogens [1–3].

Where does the myth about “hormone-disrupting soy” come from?

The myth stems mainly from laboratory experiments on cell cultures, where extremely high concentrations of isoflavones have been used – far above what a horse can get through regular feed.

In the laboratory, you can see an effect because you:

• Bypasses digestion and the body's microflora

• Prevents liver breakdown and regulation

• Uses concentrations that do not at all reflect reality in living organisms

In the horse's body, phytoestrogens undergo extensive digestion and transformation. Their binding to estrogen receptors is also very weak – approximately 1/400 of the body's own estrogen, and primarily to ERβ (e.g. in bone tissue) rather than ERα, which is the most hormone-sensitive [4–6].

👉 This means that phytoestrogens in practice have a modulating effect, not a hormone-stimulating effect.

Why can the udder still react?

When a mare is given new feed with a different composition of fibers, it can temporarily change the body's estrogen metabolism.

This is mainly because fermentable fibers affect the liver and intestine's metabolism and recirculation of estrogens (the so-called enterohepatic circulation) [7–9].

• Fiber can change how quickly estrogens are excreted or recirculated.

• This may result in a short period where the mare shows some “hormonal expression” – for example, slight swelling or a little milk in the udder.

• In some mares, it can be exacerbated by the heat cycle or natural seasonal hormone fluctuations.

👉 It is therefore not dangerous and not a sign of hormone-disrupting effects, but rather a picture of the body's own adaptation.

Typically transient and completely harmless

In the vast majority of cases, the swelling disappears after a few weeks, once the body has adapted to the new diet.

If the mare is otherwise thriving, eating well and showing no signs of illness or discomfort, there is no need to worry.

You may want to take a picture now and again in a few weeks to monitor progress.

📚 Sources

1. Setchell KD, Cassidy A. Dietary isoflavones: biological effects and relevance to human health. J Nutr. 1999.

2. Murphy PA et al. Isoflavones in soy-based foods: concentration and processing effects. J Agric Food Chem. 1999.

3. Tsukamoto C et al. Factors affecting isoflavone content in soybean seeds. J Agric Food Chem. 1995.

4. Kuiper GGJM et al. Interaction of estrogenic chemicals and phytoestrogens with estrogen receptor β. Endocrinology. 1998.

5. Patisaul HB, Jefferson W. The pros and cons of phytoestrogens. Front Neuroendocrinol. 2010.

6. Messina M. Soy and health update: evaluation of the clinical and epidemiologic literature. Nutrients. 2016.

7. Adlercreutz H et al. Intestinal metabolism of estrogens. J Steroid Biochem. 1984.

8. Goldin BR et al. The effect of dietary fiber on estrogen metabolism in women. Am J Clin Nutr. 1982.

9. Lamp JW et al. Dietary fiber and fecal estrogen excretion in premenopausal women. Cancer Causes Control. 1993.

In short

• Soybean hulls in Peak Performance contain only minimal amounts of phytoestrogens.

• Phytoestrogens do not have an endocrine disrupting effect in the doses that horses receive through feed.

• A little milk in the udder may occur briefly due to altered estrogen metabolism when changing feed – not due to “soy hormones”.

• The reaction is typically transient and harmless.