In recent years scientists have discovered that hormones play a role in shaping our microbiome, so how does this affect gut health over the female lifecycle? We asked Dietitian Laura Tilt to explain.
Your gut microbiome: from birth to puberty
Your gut microbiome is as unique as your fingerprint, and it evolves from the day you are born. The first microbes to take up residence in your gut are transferred during birth (whether by caesarean section or vaginal delivery) and from there the microscopic community in your gut takes shape.
The makeup of your microbiome is influenced by many factors in early life such as whether you are breast or bottle fed, the foods you are weaned on, your home environment, the presence of a pet and so on. Interestingly, the gut microbiome looks similar between girls and boys until puberty, when sex specific differences start to emerge. These differences (which persist into adulthood) have been termed the ‘microgenderome’, and because of the role the microbiome plays in our immune function, researchers think it could explain the sex gap in different diseases. So, what’s driving it?
Hormones and the gut
If you hadn’t already guessed, evidence suggests it's sex hormones that are responsible. Hormones are chemical messengers that travel through the bloodstream and trigger or prevent specific functions. In women, the main sex hormones are oestrogen and progesterone, primarily manufactured by the ovaries. Most of us recognise these sex hormones regulate fertility, but they also influence many other aspects of our wellbeing - including it seems, gut function and the microbiome.
For example, receptors for sex hormones have been located on gut cells, suggesting that the gut is designed to sense and respond to them. Studies have shown that female sex hormones can impact gut motility (how the gut moves) gut sensitivity and inflammation. There’s also a two-way relationship between oestrogens and gut microbes. We know that oestrogens can influence the composition of the gut microbiome, but gut microbes are also capable of recycling them back into circulation.
Given that these relationships exist, you might wonder what happens to the gut during periods of hormonal change, like during pregnancy and menopause. Unfortunately, there’s way less research into this area than you might expect - but here’s what we do know so far.
Menstrual Cycle, Gut Health, and the Microbiome
According to hormone mapping experts Forth, women can expect to experience up to 480 monthly fluctuations in their female hormones over a menstrual cycle.
First a quick recap on menstrual cycle biology. The menstrual cycle is split into two halves and divided by ovulation. The first half is the follicular phase, ovulation occurs in the middle, and is followed by the luteal phase. Menstruation (your period) counts as day one of the cycle, and forms part of the follicular phase.
During the follicular phase, oestrogen is dominant, peaking at ovulation (when an egg is released) before dropping sharply. During the luteal phase, progesterone takes centre stage, rising sharply alongside a secondary rise in oestrogen in preparation for a possible pregnancy. If conception does not occur, both hormones drop, and menstruation begins.
So how do these hormonal fluctuations influence the gut? If you’re in the habit of tracking your gut health you might have noticed that your period coincides with symptoms like loose stools, tummy pain and bloating. In one study of healthy women, 73% reported at least one gut symptom just before or during their period, most often abdominal pain and diarrhoea. Women with irritable bowel syndrome (IBS) also tend to experience a worsening of symptoms (like abdominal pain, nausea, and diarrhoea) and increased gut sensitivity just before and during menstruation.
The trigger for these symptoms is believed to be the drop in ovarian hormones which takes place just before your period begins. At the same time, there’s a spike in prostaglandin hormones which cause the uterus to contract and shed its lining, and which cause the gut to contract too.
What about changes to the microbiome? To date, very few studies have investigated this relationship. A recent study of 160 healthy young Danish women found that neither menstrual cycle phase nor hormonal contraceptive use was associated with gut microbiome composition, but more research is needed before we can be sure.
Pregnancy, Gut Function and Gut Microbes
Pregnancy is another period of huge hormonal change, when oestrogen and progesterone rise sharply to support the growing baby and prepare the body for labour and breastfeeding. The impact of soaring hormones tends to be really noticeable in mood and emotions (women often feel more vulnerable, anxious and tearful), but they lead to changes in gut function too.
Increased levels of progesterone relax smooth muscles - including those in the gut. This means food and waste travels more slowly through the digestive system which can contribute to constipation. Other common gut symptoms during pregnancy include feeling (or being) sick, heartburn and haemorrhoids (learn more about these symptoms and how to manage them here). These symptoms are thought to be related to both increased hormones plus a shifting of internal organs to make way for the baby.
Pregnancy alters the mammas microbiome too. In the first trimester, the microbiome of pregnant women looks like that of healthy non-pregnant women but as pregnancy advances, the ratio of two dominant families of bacteria (firmicutes/Bacteroidetes) changes, mimicking what is observed in obesity. This is thought to be a necessary adaptation, ensuring the mother lays down sufficient fat stores for the baby’s growth and milk production. Then, during late pregnancy, progesterone encourages the growth of a group of microbes known as Bifidobacterium. These bacteria are vital members of the new baby’s microbiome because they break down special sugars in breast milk and help train the baby’s immune system. In an incredibly clever exchange, these microbes are passed from mother to baby during vaginal birth and via breastfeeding.
Menopause, Gut Function and Gut Microbes
Outside of pregnancy, women will experience cyclical fluctuations in their hormones alongside the menstrual cycle until perimenopause, the transitional phase that signals the final reproductive years. Perimenopause starts around age 45 and lasts until menopause - which is reached 12 months after the last menstrual period.
Signs of perimenopause include menstrual irregularity and symptoms like hot flashes, vaginal dryness, mood, and sleep changes, which are driven by declining oestrogen levels. Many women also report gut symptoms and changes in their bowel habits, although sadly there are very few studies detailing gut health during this transitional phase. One 1998 study found that perimenopausal and postmenopausal women had a high prevalence (38% vs 14% for premenopausal women) of self-reported changes in gut function, but there’s been no further research in more recent years.
Little is known regarding the influence of menopause – a pivotal event of reproductive aging in women – on the gut microbiome.
Sadly, the influence of hormonal changes during menopause on the microbiome is also underexplored. Some studies have found decreased diversity in post vs. pre-menopausal women and in women with lower levels of oestrogen. Diversity in the microbiome matters as it signals a robust and functional community - so learning how we can increase diversity post menopause seems important (learn more about menopause and the microbiome with Dr Shazadi Harper’s tips).
So where do we go from here?
We know that biological sex impacts gut function and the microbiome, and there’s now convincing evidence that sex hormones drive at least some of the differences between men and women. Frustratingly however, there are big gaps in our knowledge as to how these relationships impact women’s gut health during major hormonal transitions.
This is frustrating, and we know there are sex inequalities in health research. Until we do have more of the answers, being aware of what might be expected during transitional and cyclical hormonal fluctuations can be helpful in understanding our own gut health. With that in mind, here are a few ways you can support your gut health through the lifecycle.
TRACK If you’re experiencing symptoms and suspect there’s a hormonal link, try tracking alongside your menstrual cycle. When do symptoms flare? Is there a pattern? If you’re peri-menopausal, be aware of the potential for changes in your gut health.
SPEAK WITH YOUR GP As a first step, changes in your poo or gut symptoms which don’t go away after a week or two should always be discussed with your G.P., as they can be a sign of an underlying condition
TRY DIET AND LIFESTYLE CHANGES TO EASE SYMPTOMS Dietary and lifestyle changes can often help with symptoms like diarrhoea, constipation, and bloating. The British Dietetic Association has some really handy factsheets that can help here, or you could ask for a referral to a dietitian for personalised advice.
REMEMBER THE BASICS Eating a fibre rich diet, with a diverse range of different plant foods, taking regular movement, and finding ways to rest and manage stress are all ways to care for your gut microbiome whatever your age.
FOLLOW Symprove on social media, for more tips and advice on taking care of your gut.
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