Estrogen Balance

How Gut Bacteria Recycle Oestrogen

Several human studies point the same way here, and a small number have gone further and changed what people ate. Oestrogen is not simply made and then thrown away. The liver packages it for removal and sends it into the gut in bile, the fluid the liver makes to help digest fat. Certain gut bacteria can unwrap that package and set the hormone free again. Freed oestrogen is reabsorbed into the bloodstream instead of leaving the body. The collection of gut bacteria able to do this even has a name, the estrobolome. So how much oestrogen is circulating depends partly on which bacteria are living in your gut. Most people assume the ovaries alone set that level. After the menopause, and in men, the gut's share of the job grows larger.

The enzyme that sends oestrogen back round

The unwrapping is done by a single enzyme, beta-glucuronidase, made by a range of common gut bacteria. The liver attaches a chemical tag to oestrogen that marks it for disposal. Beta-glucuronidase cuts that tag off. The hormone becomes active again and is free to re-enter circulation.

Researchers measured this in 51 adults, a group made up of men, premenopausal women and postmenopausal women. They tested urine and stool for oestrogen and its breakdown products, then analysed which bacteria were present. In the men and postmenopausal women, total oestrogen in urine tracked closely with how rich and varied the gut bacteria were. Every measure of variety they used showed a correlation of at least 0.50.

Certain groups of bacteria stood out. Members of the Ruminococcaceae family, which make their living fermenting fibre, showed correlations between 0.57 and 0.70 with those oestrogen levels.

Source: Fecal microbial determinants of fecal and systemic estrogens and estrogen metabolites: a cross-sectional study

A second study, and an honest limit

A larger group was studied next, and the result was similar but softer. Researchers recruited 60 healthy postmenopausal women aged 55 to 70. They compared gut bacterial variety against the ratio of oestrogen breakdown products to the parent hormone in urine. Women with more varied gut bacteria had a higher ratio, meaning more of the hormone had been processed into its breakdown forms.

The caveat deserves stating. That result held for only one of the three ways the researchers measured variety. Two other standard measures showed nothing. It is a real limitation, and it is why this belongs in the category of a consistent signal rather than a settled fact.

Both studies are snapshots taken at a single moment in time.

Source: Associations of the fecal microbiome with urinary estrogens and estrogen metabolites in postmenopausal women

What happened when the diet actually changed

Both of those studies watched without intervening. An older study did change the diet, under tight control. Researchers fed 48 premenopausal women through a metabolic kitchen, where every meal was prepared and measured. Participants began on a diet high in fat and low in fibre, at around 12 g of fibre a day. They then moved to a diet low in fat and high in fibre, at around 40 g a day.

Levels of oestrone, one of the main forms of oestrogen, fell significantly in the blood. So did the stored form of it. When the researchers separated out the effects, fibre on its own accounted for the drop in oestradiol, the most active form of the hormone.

An earlier comparison pointed the same way. Vegetarian women eating around 28 g of fibre a day passed more oestrogen out in their stool than non-vegetarian women eating around 12 g. The more stool they passed, the more oestrogen left the body with it.

These were small studies in premenopausal women, and the diets changed fat as well as fibre. They remain the closest thing here to a direct test.

Source: The effect of dietary fat and fiber on serum estrogen concentrations in premenopausal women under controlled dietary conditions

Source: Estrogen excretion patterns and plasma levels in vegetarian and omnivorous women

What this means for you

So the argument runs like this. The amount of oestrogen circulating in the body depends partly on an enzyme made by gut bacteria, the studies that have measured it find variety in those bacteria tracking with oestrogen levels, and raising fibre under controlled conditions moved the hormone in the blood. The controlled work is decades old and small, so this supports a sensible everyday habit rather than a treatment. If you have unusual bleeding, persistent pelvic pain or breast changes, see a doctor rather than adjusting your diet and waiting. The habit the evidence points to is fibre from a wide range of sources, because the bacterial families involved are fibre fermenters and a narrow diet leaves them with less to eat. Pulses, whole grains, vegetables, fruit, nuts and seeds each feed different members of that community. That is exactly the idea behind GutLab, where everyday gut health starts with regularly feeding the microbiome, the full community of microbes living in the gut, a broad range of plant-rich ingredients.