Why Am I So Exhausted? Digestive Health and Nutrient Needs in Perimenopause (Without HRT)

You are not imagining it. The fatigue of perimenopause is not ordinary tiredness. It is a bone-deep, persistent depletion that sleep doesn't fix that standard blood tests frequently fail to explain. It sits somewhere between your body and your brain, and it tends to arrive before most women even realise they have entered a hormonal transition.

Fatigue is consistently reported as one of the most common and most dismissed symptoms of this life stage, though estimates of exactly how many women experience it vary widely between studies, from under half to the large majority, depending on the population and how fatigue is measured. What's consistent across the research is that it's common, and that its causes are rarely explored with the depth they deserve. Most clinical conversations stop at hormones.

What this guide argues, using a growing body of clinical research, is that perimenopause is, at its core, a state of dramatically increased physiological demand. Your body is working harder. Your cells need more. And in most women, the nutrients required to meet that demand are either being depleted faster, absorbed less efficiently, or both.

Understanding why, and which scenario applies to you, is where recovering your energy actually begins.

Perimenopause is the transitional phase preceding menopause, typically beginning in the early-to-mid 40s and lasting anywhere from four to ten years. It is characterised by erratic fluctuations in oestrogen and progesterone as the ovaries become less responsive to hormonal signalling, a process that creates wild swings rather than a gradual, orderly decline.

Researchers increasingly describe it as a metabolic transition window, a distinct physiological state with measurable consequences for cellular energy production, inflammatory signalling, body composition, insulin sensitivity, and cardiovascular function. A 2023 review in Nutrients (Erdélyi et al.) examined how the nutritional requirements of this transition differ from earlier adult life, and how dietary patterns that worked fine in your 30s can become inadequate in your 40s.

At the cellular level, the mechanism is specific. Oestrogen plays a critical role in regulating mitochondrial function — the process by which cells convert nutrients into usable energy (ATP). A 2025 review in the International Journal of Molecular Medicine (Yu et al.) describes how postmenopausal women show an estimated 30% reduction in ATP production efficiency, linked to decreased activity of mitochondrial Complex IV — one of the key enzyme complexes in the chain that produces cellular energy. Neuroimaging studies cited in the same review point to a concurrent decline in how efficiently the brain uses glucose for fuel, beginning in perimenopause.

What this means in plain terms: as oestrogen fluctuates and falls, your cells become measurably less efficient at producing energy. The fatigue you feel is not psychological — it is bioenergetic. And critically, the nutrients that support mitochondrial function — B vitamins, magnesium, iron, CoQ10, zinc, and vitamin D — become not just important but essential.

What Is Actually Happening in Your Body During Perimenopause

Before looking at individual nutrients, there's a dimension that's almost never discussed: the digestive system changes during perimenopause in ways that directly reduce nutrient absorption, meaning the nutritional gap is widening from both ends simultaneously.

Stomach acid often declines with age: A 2025 scoping review in Biomedicines (Vara-Luiz et al.) examined the evidence on this directly and found it more mixed than commonly assumed: rather than ageing itself directly reducing acid output, much of the decline in stomach acid seen in older adults appears to be driven by conditions that become more common with age (atrophic gastritis, H. pylori infection, and long-term use of acid-suppressing medication) and the overall certainty of the evidence was rated low-to-moderate. The practical takeaway is the same either way: a meaningful proportion of midlife women have reduced gastric acid, for one reason or another, and hydrochloric acid is essential for releasing vitamin B12 from food proteins, converting dietary iron into its absorbable form (ferrous iron, Fe²⁺), and supporting uptake of zinc, magnesium, and calcium. The consequence is that a woman eating a nutrient-dense diet may still be running deficient because of absorption, not intake.

The gut microbiome shifts during this window: Oestrogen receptors are present throughout the gastrointestinal tract, and oestrogen actively supports the diversity and balance of gut bacteria. As oestrogen fluctuates and falls, microbial diversity tends to fall with it. Research summarised by the Canadian Digestive Health Foundation links lower microbial diversity to impaired nutrient absorption and slower gut transit. Gut bacteria also help synthesise some B vitamins and produce short-chain fatty acids that regulate inflammation.

Cortisol disrupts the gut lining: Elevated cortisol — a common feature of perimenopause due to changes in the stress-response (HPA) axis — increases intestinal permeability and further disrupts microbiome balance, creating a feedback loop between hormonal stress and impaired absorption.

The practical implication: you may be eating well and still not absorbing well. Testing nutrient status directly is therefore more informative than assessing diet alone.

How Your Gut Makes Things Worse

The Nutrients Most Affected — and Why They Drive Fatigue

For women managing perimenopause without hormone replacement therapy, nutrient depletion is driven by a combination of hormonal change, blood loss, rising metabolic demand, and the digestive factors above. Each nutrient below has a distinct mechanism — and each is a measurable, testable contributor to fatigue.

Iron and Ferritin

Low ferritin (even without anaemia!) has been linked to apathy, irritability, low mood, fatigue, and cognitive difficulties in women of reproductive and perimenopausal age. A well-known 2012 randomised controlled trial by Vaucher and colleagues, published in CMAJ, found significant improvement in fatigue among non-anaemic women with low ferritin after iron supplementation. Ferritin also plays a role in the synthesis of dopamine and serotonin and is a cofactor for cellular respiration in skeletal muscle, cardiac tissue, and the brain.

Most standard GP panels don't include serum ferritin — they test haemoglobin, report "normal," and stop there. A ferritin below roughly 30 µg/L is associated with symptomatic fatigue even in non-anaemic women; some practitioners use a higher target (around 50–70 µg/L) for optimal function, though individual context matters and this isn't a universally agreed threshold.

Absorption is further compromised by the stomach acid changes above — non-haem iron (from plant sources) is particularly acid-dependent for conversion to its absorbable fo

Vitamin B12 and the B Complex

B12 is the nutrient most exposed to the digestive changes of midlife. Its absorption requires a two-step process: stomach acid and pepsin must first release B12 from dietary protein, and then a protein called intrinsic factor (produced by the stomach lining) carries it to absorption in the terminal ileum. As stomach acid declines, both steps are compromised. A widely-cited 2013 review in the Annals of Clinical Biochemistry (Hughes et al.) discusses how atrophic gastritis — which becomes considerably more common with age — drives B12 deficiency through reduced stomach acid, and notes that a functional deficiency can exist even when serum B12 looks "normal" on a standard reference range.

The consequences for fatigue are direct: B12 is required for myelin production (the protective coating of nerve fibres), red blood cell formation, and cognitive function. Low B12 produces fatigue, brain fog, low mood, and elevated homocysteine — symptoms that overlap almost entirely with perimenopause itself, which is exactly why it's so often missed.

The broader B vitamin family — B1, B2, B3, B5, B6, folate — is equally important for energy production at the mitochondrial level, acting as coenzymes in the biochemical pathways that convert food into ATP. A 2019 randomised trial in Nutrients (Liao et al.) found that B-complex supplementation reduced fatigue and improved energy in healthy adults. During perimenopause, demand for B vitamins rises right when absorption becomes less reliable.

Vitamin B6 is required to synthesise serotonin and GABA — the neurotransmitters that regulate mood, stress response, and sleep onset. Its depletion is linked to the anxiety, irritability, and sleep disruption common in perimenopause.

Folate supports the methylation cycle, which governs gene expression, detoxification, and nervous system function. Elevated homocysteine — a consequence of folate deficiency — is independently associated with fatigue, cognitive decline, and cardiovascular risk.

Magnesium

Magnesium is involved in hundreds of enzymatic reactions in the body, including every step of ATP synthesis. It's essential for sleep quality, muscle relaxation, blood sugar regulation, and modulation of the HPA axis — the system governing cortisol and stress response.

During perimenopause, magnesium comes under pressure from multiple directions at once. Chronic stress raises cortisol, which increases urinary magnesium loss. Disrupted sleep depletes it further. And as gut health shifts, absorption falls.

A 2018 randomised trial (Pouteau et al.) found that magnesium supplementation reduced perceived stress, with the effect amplified by adding B6 — a relevant finding, since both nutrients tend to be depleted together in perimenopause. Magnesium deficiency produces a clinical picture of fatigue, muscle cramps, poor sleep, anxiety, and irritability — symptoms that map directly onto the perimenopausal experience.

Vitamin D

Vitamin D is more accurately described as a prohormone than a vitamin. Its receptors are found throughout the brain, heart, immune cells, muscles, and reproductive tissues, and its role in energy metabolism, mood, immune regulation, and hormonal signalling goes well beyond its commonly discussed role in bone health.

During perimenopause, the body's ability to activate vitamin D is specifically affected by declining oestrogen. Oestrogen supports the activity of the enzyme that converts inactive vitamin D into its active hormonal form in the kidneys, and helps suppress the enzyme that breaks vitamin D down. As oestrogen falls, both of these protective effects weaken which leads to think women may become functionally vitamin D deficient even with reasonable sun exposure or dietary intake.

A 2023 review in Frontiers in Physiology (Mei et al.) discusses how vitamin D deficiency is associated with increased fatigue, low mood, muscle weakness, impaired immune function, and worsened vasomotor symptoms (hot flushes, night sweats) in perimenopausal and menopausal women. In the UK, where effective sunlight is limited for roughly six months of the year, a large proportion of women will have insufficient vitamin D by late winter — compounding an already existing risk.

Perimenopause frequently brings heavier, more prolonged, and less predictable periods — and with them, sustained blood loss that steadily depletes iron stores over months and years.

The critical point is that the marker that matters is ferritin, not haemoglobin. Ferritin is the body's iron storage protein, and it falls long before haemoglobin does. By the time anaemia shows up on a standard blood test, a woman has typically been iron-deficient for months or longer.

Zinc

Zinc is rarely the first nutrient women or clinicians consider in perimenopause, yet its depletion has specific hormonal consequences. At the molecular level, zinc stabilises oestrogen receptors and is required for their ability to bind DNA. When zinc is deficient, the cellular response to oestrogen weakens — even when some oestrogen is still present. The hormonal signal becomes less effective at the receptor level.

A 2022 study in the Journal of Trace Elements in Medicine and Biology found that zinc supplementation in postmenopausal women significantly improved folate levels and reduced homocysteine — demonstrating that zinc's role extends into metabolic and neurological function, not just immunity and skin. Zinc is also required for thyroid hormone metabolism, and its deficiency can contribute to hypothyroid-like symptoms — fatigue, weight gain, cold intolerance, brain fog — that perimenopausal women frequently report.

Zinc absorption is acid-dependent and therefore directly affected by the stomach acid changes discussed above.

Coenzyme Q10 (CoQ10)

CoQ10 is produced naturally in every cell and plays a fundamental role in the mitochondrial electron transport chain — the final step of ATP production. It's also a powerful antioxidant that protects cell membranes from oxidative damage.

CoQ10 levels decline naturally with age and are further affected by declining oestrogen. A 2022 systematic review and meta-analysis of 13 randomised controlled trials in Frontiers in Pharmacology (Tsai et al.) found that CoQ10 supplementation significantly reduced fatigue across multiple patient populations.

During perimenopause, when mitochondrial efficiency is already falling due to oestrogen withdrawal, CoQ10 depletion compounds the bioenergetic deficit directly. It's one of the nutrients least likely to be tested or considered in standard clinical settings, yet it's directly relevant to the cellular mechanism behind perimenopausal fatigue.

The Thyroid: The Mimicker That Cannot Be Overlooked

One further variable must always be considered alongside nutrients: thyroid function. Perimenopausal women face a notably elevated risk of thyroid dysfunction, particularly Hashimoto's thyroiditis (an autoimmune condition far more common in women, which tends to emerge or worsen during hormonal transitions) and hypothyroidism.

The symptoms of an underactive thyroid — exhaustion, weight gain, cold intolerance, hair thinning, brain fog, constipation, low mood, dry skin — overlap almost entirely with perimenopausal fatigue. Without appropriate testing, thyroid dysfunction can be mistaken for "just perimenopause" and left untreated.

A comprehensive thyroid assessment includes TSH, Free T3, Free T4, and TPO antibodies. TSH alone — the standard test — can look normal while the thyroid is under-functioning at a cellular level, and it will not detect Hashimoto's.

What to Test

- HbA1c and fasting glucose (insulin resistance is common and energy-depleting in this transition)
- Full thyroid panel: TSH, Free T3, Free T4, TPO antibodies
- Oestradiol, progesterone, FSH
- Free and total testosterone + SHBG
- DHEA-S (a marker of adrenal reserve)
- Diurnal cortisol (a four-point saliva test gives a better picture of HPA axis patterning than a single blood draw)

Practical Foundations

Test before supplementing. Iron supplementation without confirmed deficiency can cause harm. Excess zinc interferes with copper absorption. Know your specific picture before reaching for a protocol — and do this with a GP or qualified practitioner, not on your own.

Prioritise bioavailable supplement forms, especially if you suspect stomach acid issues:
- Methylcobalamin (not cyanocobalamin) for B12 — bypasses the acid-dependent conversion step
- Methylfolate (not folic acid) — the activated form that doesn't require conversion
- Magnesium glycinate or magnesium malate — better absorbed and tolerated than magnesium oxide
- Ferrous bisglycinate for iron — gentler on the gut and better absorbed than ferrous sulphate

Support your gut as part of your nutrient strategy. Fermented foods (yoghurt, kefir, sauerkraut, kimchi) support microbiome diversity. Eating slowly and chewing thoroughly maximises digestive efficiency. Limiting alcohol matters clinically — it directly damages the gut lining, depletes B vitamins, and worsens absorption.

Food sources remain important alongside supplementation:
- Iron: red meat, liver, lentils, dark leafy greens (paired with vitamin C to enhance non-haem absorption)
- B vitamins: eggs, fish, dairy, meat, leafy greens; a methylated B-complex if absorption is in question
- Magnesium: nuts, seeds, dark chocolate, legumes
- Vitamin D: oily fish, eggs — plus a supplement during UK autumn and winter months
- Zinc: shellfish (especially oysters), red meat, pumpkin seeds
- CoQ10: organ meats, oily fish, and supplemental ubiquinol (the active form), particularly after age 40

Stabilise blood sugar. Declining oestrogen impairs insulin sensitivity, making blood sugar crashes more pronounced and more fatiguing than they were earlier in life. Protein at every meal, healthy fats, and reduced reliance on refined carbohydrates and caffeine make a measurable difference to sustained energy through the day.

Key takeaways

Perimenopause creates a state of elevated physiological demand — a metabolic transition in which the body's need for key nutrients increases at the same time as its capacity to absorb and use them decreases. Fatigue is the most visible consequence of this gap.

For women not on HRT, the primary nutrient drivers are iron and ferritin (depleted by heavier periods and impaired absorption), B vitamins (rising demand, reduced acid-dependent absorption), magnesium (lost through stress and poor sleep), vitamin D (impaired activation as oestrogen falls), zinc (deficiency risk linked to both oestrogen decline and acid-dependent absorption), and CoQ10 (declining with age and oestrogen withdrawal).

The path to recovery begins with an honest, comprehensive picture of what's actually happening in your body — not a standard blood panel filtered through a "normal" range, but a targeted look at the specific nutrients and hormonal markers relevant to this stage of life.

Test, don't guess. Treat the root, not the symptom.


Continued in Part 2: if you're on hormone replacement therapy and fatigue, brain fog, or low mood haven't resolved — or improved and then plateaued — the next layer to understand is how oestrogen-containing HRT interacts with these same nutrients, and with your gut.

References

1. Erdélyi A, Pálfi E, Tűű L, Nas K, Szűcs Z, Török M, Jakab A, Várbíró S. The Importance of Nutrition in Menopause and Perimenopause — A Review. Nutrients. 2023;16(1):27. doi:10.3390/nu16010027
2. Yu Y, Yapeng H, Liu Z, Fang L, Li J, Luan Y, Li W, Cong H, Wu X. Mitochondrial Dysfunction in Perimenopausal Mood Disorders: From Hormonal Shifts to Neuroenergetic Failure (Review). Int J Mol Med. 2025;56:215. doi:10.3892/ijmm.2025.5656
3. Vara-Luiz F, Mendes I, Palma C, Mascarenhas P, Nunes G, Patita M, Fonseca J. Age-Related Decline of Gastric Secretion: Facts and Controversies. Biomedicines. 2025;13(7):1546. doi:10.3390/biomedicines13071546
4. Vaucher P, Druais PL, Waldvogel S, Favrat B. Effect of Iron Supplementation on Fatigue in Non-Anaemic Menstruating Women with Low Ferritin: A Randomised Controlled Trial. CMAJ. 2012.
5. Hughes CF, Ward M, Hoey L, McNulty H. Vitamin B12 and Ageing: Current Issues and Interaction with Folate. Ann Clin Biochem. 2013. doi:10.1177/0004563212473279
6. Liao Y, et al. Functional Evaluation of Anti-Fatigue Following Vitamin B Complex Supplementation. Nutrients. 2019.
7. Pouteau E, et al. Superiority of Magnesium and Vitamin B6 over Magnesium Alone on Severe Stress in Healthy Adults. PLOS ONE. 2018. PMC6298677
8. Mei Z, Hu H, Zou Y, Li D. The Role of Vitamin D in Menopausal Women's Health. Front Physiol. 2023;14:1211896. doi:10.3389/fphys.2023.1211896
9. Effect of Zinc Supplementation on Homocysteine, Vitamin B12 and Folate Concentrations in a Postmenopausal Population. J Trace Elem Med Biol. 2022. doi:10.1016/S0946-672X(22)00022-0
10. Tsai IC, Hsu CW, Chang CH, Tseng PT, Chang KV. Effectiveness of Coenzyme Q10 Supplementation for Reducing Fatigue: A Systematic Review and Meta-Analysis of Randomised Controlled Trials. Front Pharmacol. 2022;13:883251. doi:10.3389/fphar.2022.883251
11. Canadian Digestive Health Foundation. Gut Microbiota and Perimenopause (referencing Bharwani et al., 2020; Kim & Kim, 2021).

This article is for informational purposes only and does not constitute medical advice. Any decisions regarding supplementation, testing, or hormone therapy should be made in consultation with a qualified clinician.

Given how many factors are at play, the goal of testing isn't a single normal-or-abnormal result. It's a map of your specific pattern of drivers, so any intervention can be targeted rather than generic.

- Serum ferritin (not just haemoglobin or full blood count)
- Vitamin D (25-OH)
- B12 and folate (request both; consider active B12 if serum B12 is borderline)
- Red cell magnesium (more accurate than serum magnesium)
- Zinc