Signs of hair loss can be subtle at first, but if you're noticing more strands than normal in the shower drain or a thinner ponytail, that could be what's going on. Worrying about hair loss is understandable. Hair is closely linked with identity, confidence, and femininity, and thinning can knock your sense of self more than you'd expect.
The first port of call for most women is external: trying a new shampoo, rosemary oil, or a miracle cure recommended on Instagram. But hair follicles are among the most metabolically active tissues in the body, so they're often among the first things to be affected when there's an internal shift. Hair loss is rarely down to one thing, which is why understanding the cause matters more than reaching for the next product.
Why hair loss happens
There are three phases to the hair growth cycle:
-
Anagen – The active growth phase where follicles produce new hair.
-
Catagen – A transition stage where growth slows.
-
Telogen – The resting phase when older hairs are released.
When your body is under stress, for example due to rapid weight loss, illness, or hormonal changes, more hairs can be pushed into the telogen phase, so you shed more than usual. This is known as telogen effluvium.
The lag: Because of the length of the hair cycle, shedding won't show up until roughly 2–3 months after whatever caused it. So a crash diet in January might not show up as hair loss until March or April - which means women often end up looking for a cause in the present that actually happened weeks earlier.
Telogen effluvium vs. female pattern hair loss: Telogen effluvium causes hair to shed evenly across the scalp and is usually temporary. Female pattern (androgenetic) hair loss is different — triggered by genetics and hormones, it causes gradual follicle miniaturisation, typically showing up as a widening parting and reduced density over time.
What the science and research says
1. Diet and nutrients
Study: "Assessing the relationship between dietary factors and hair health: A systematic review" (Gomes, Silva & Teixeira, 2025)
A 2025 systematic review pooling 17 studies and over 61,000 participants examined how specific nutrients and dietary patterns relate to hair health.1 It found that higher vitamin D and iron levels were associated with lower rates of alopecia, while higher intake of sugary drinks and alcohol was linked to increased hair loss risk.1 The authors were clear that most of the underlying studies were observational, meaning they show association rather than direct cause and effect, but the overall pattern supports the idea that follicles need consistent nutritional support (particularly protein, iron and vitamin D) to function normally.
-
Protein – Hair is formed of keratin, a structural protein that needs a constant supply of amino acids to be repaired and replaced. If the body doesn't have enough amino acids, hair often suffers, as the body prioritises other areas essential for survival.
-
Iron/Ferritin – Follicles need both energy and oxygen. Ferritin (the protein that stores iron) can show up "in range" on a blood test but still be too low to support a healthy hair cycle.
-
Vitamin D – Supports follicle cycling and immune regulation. In the UK, sunlight isn't strong enough from around October to March to generate vitamin D, so levels often dip over winter.
2. Dieting, calorie restriction and hair shedding
Study: general narrative-review evidence on dieting and hair loss (exact source unverified - see reference note below); GLP-1 findings from Buontempo & Santos, 2025
Reviews looking specifically at dieting behaviour describe how reduced calorie intake and inadequate protein can disrupt the hair growth cycle.2 When the body receives fewer calories or insufficient protein, it reduces resources directed toward processes considered less essential for survival, hair growth among them. This is consistent with what's known about telogen effluvium, and it explains why some women notice shedding after successful weight loss rather than during it, thanks to the 2–3 month lag.
This is especially relevant for anyone using GLP-1 medications for weight loss. Pharmacovigilance research has flagged an association between GLP-1 receptor agonists (like semaglutide and tirzepatide) and increased rates of reported hair loss.3 The current thinking is that this is most likely driven by rapid weight loss and reduced nutritional intake rather than a direct drug effect, though researchers note more evidence is needed to separate the two.3 If you're on a GLP-1, protein and micronutrient-rich meals matter even more, since overall intake is often significantly reduced. The same logic applies to intermittent fasting, appetite suppression, and weight cycling.
3. Collagen peptides
Study: "Revealing novel insights on how oral supplementation with collagen peptides may prevent hair loss: Lessons from the human hair follicle organ culture" (Pappelbaum et al., 2024)
A 2024 laboratory study used a human hair follicle organ culture model, intact human follicles studied outside the body, to test the effect of marine and bovine collagen peptides.4 It found that collagen peptides could prolong the hair follicle's growth phase and support a healthier population of follicle stem cells.4 This offers a plausible biological mechanism for why collagen might support hair health, but it's important to note this was a lab-based mechanistic study, not a large clinical trial measuring actual hair regrowth in women. Larger human trials are the next step.
4. What blood tests can (and can't) tell you
Study: "A comprehensive investigation of biochemical status in patients with telogen effluvium" (Durusu Turkoglu et al., 2024)
A 2024 case-control study compared 90 women with chronic telogen effluvium to 90 women without hair loss, measuring ferritin, vitamin D, vitamin B12, thyroid function, zinc, copper, selenium and biotin.5 The main finding was that zinc levels were significantly lower in the women with chronic shedding but ferritin and vitamin D were not significantly different between the two groups.5 This is a useful reality check: hair loss can't always be pinned on one deficiency, and a woman can have "normal" bloods and still be shedding due to stress, hormones, thyroid issues, or genetics.
5. Hormones and the female body
Study: "Menopause and hair loss in women: Exploring the hormonal transition" (Maturitas, 2025)
Hair follicles are sensitive to hormonal shifts throughout life. Postpartum shedding is well known, oestrogen during pregnancy keeps hair in the growth phase, then drops sharply after birth, releasing a lot of hair at once. Shedding typically peaks around 3–4 months postpartum and settles by 6–12 months.
A 2025 review focused specifically on menopause and hair found that as oestrogen declines during the menopausal transition, hair follicles, which are oestrogen-sensitive tissue, respond with decreased density, finer strands, and changes in texture.6 The review also highlighted that hair loss is under-researched relative to other menopause symptoms, despite having a real and often unrecognised emotional impact.6
An underactive or overactive thyroid can also disrupt the hair cycle, as can androgen sensitivity, a feature of PMOS (until recently known as PCOS), which can cause thinning at the crown alongside excess hair growth elsewhere on the body.
Summary: what this means for you
Across all of this research, the same message comes through: hair loss is almost always multifactorial. Nutrition provides the foundation, hormones influence the follicle throughout life, and stress, rapid weight loss, dieting, illness, or medication changes can all tip follicles into a shedding phase.
Practical takeaways:
-
Prioritise protein daily – it's the raw material for keratin.
-
Avoid extreme or rapid dieting – sudden nutritional stress can trigger delayed shedding 2–3 months later.
-
Don't assume "normal bloods" means everything's been checked – ask specifically about ferritin, thyroid function, vitamin D and zinc.
-
Be patient with recovery – if the underlying cause is addressed, shedding usually slows by around 3 months, with visible density changes by 6–12 months.
-
Support your body through hormonal transitions – postpartum, perimenopause, menopause, and PMOS all place extra demands on follicles.
When to see a doctor
A small increase in shedding usually isn't cause for alarm. But if shedding worsens alongside other symptoms such as constantly feeling cold, changes in menstruation, unexplained weight loss, scalp pain, or breathlessness. It's worth seeing a doctor to rule out thyroid dysfunction, anaemia, or autoimmune conditions. Don't self-diagnose with supplements; ask for the specific markers that matter for hair.
Hair is often one of the first ways your body signals that something has changed. There's rarely a single fix, but finding the underlying cause and giving your body time to respond, is the most reliable path back to healthier hair.
