How Hair Growth Affects HTMA Interpretation

The Hair Growth Cycle: A Quick Primer

Each hair follicle cycles independently through three main phases: anagen (active growth), catagen (a short regression phase lasting roughly two weeks), and telogen (a resting phase lasting roughly two to three months) (Natarelli, Gahooonia & Sivamani, 2023; Paus & Cotsarelis, 1999). At any given time, only a fraction of scalp follicles are in telogen — most are actively growing — which is why scalp hair grows in a relatively continuous, asynchronous fashion across the head rather than shedding and regrowing all at once.

This matters for HTMA because the technique relies on an assumption inherited from hair biology: while a strand is in the anagen phase, the hair shaft is thought to incorporate minerals present in the body at the time each segment was formed, then carry that record outward from the scalp as the strand elongates (Paus & Cotsarelis, 1999). The mechanism by which elements become incorporated into the growing shaft — and the relative contribution of internal versus external sources — is itself an active area of scientific discussion, addressed in more detail in why hair reflects long-term mineral exposure. What concerns this article is a narrower question: even accepting that hair encodes a time-ordered record, how precisely can we say which period a given sample covers?

Why Growth Rate Defines the Exposure Window

A standard HTMA sample is typically cut close to the scalp, using a defined length (commonly around 3–4 cm) intended to represent approximately the most recent three months of growth. That estimate rests on an assumed average scalp hair growth rate of roughly 1 to 1.25 cm per month (approximately 0.5 inch), a figure widely cited in dermatology and trichology literature (Loussouarn, El Rawadi & Genain, 2005).

If a person’s hair genuinely grows at that average rate, a 3.75 cm sample corresponds reasonably well to a three-month window. But if their actual growth rate is faster or slower — which is common — the same sample length corresponds to a shorter or longer period than assumed. A slow grower’s “three-month” sample might actually represent four or five months of accumulated mineral deposition; a fast grower’s might represent closer to two. The laboratory has no direct way to know which case applies unless growth rate is separately measured — which, in routine commercial HTMA, it generally is not.

This is not a flaw unique to any one laboratory’s method; it is an inherent property of inferring a time axis from a physical length in a biological tissue that does not grow at a fixed, universal rate.

How Much Does Growth Rate Actually Vary Between People?

Published measurements of scalp hair growth rate show a wide range, not a single number. Commonly cited averages cluster around 1 to 1.25 cm per month, but individual measurements in the literature span roughly 0.6 cm to over 3 cm per month depending on the study and population sampled. A large in vivo study of young adults across 24 ethnic groups on five continents found statistically distinct average growth rates by ethnic origin — approximately 280 µm/day for hair classified as African-origin, 367 µm/day for Caucasian-origin, and 411 µm/day for Asian-origin hair (Loussouarn, El Rawadi & Genain, 2005). That is roughly a 45% difference between the slowest and fastest group averages — before accounting for individual variation within each group.

Growth rate also varies with age (generally slowing over the lifespan), and evidence suggests it varies by sex as well, though effect sizes differ across studies (Natarelli, Gahooonia & Sivamani, 2023). Beyond these population-level factors, growth rate can be influenced within a single individual by nutritional status, hormonal changes, certain medications, systemic illness, and localized scalp conditions — meaning even the same person’s growth rate is not necessarily stable from one testing window to the next.

The practical consequence: any fixed rule of thumb translating “centimeters of hair” into “months of history” is a population-average approximation, not a personalized measurement. The companion article on biological variability in HTMA results discusses this alongside other sources of inter-individual variability.

Root, Middle, Tip: Position Along the Strand as a Proxy for Time

Because hair elongates from the follicle, position along the strand functions as a rough proxy for time — the segment closest to the scalp represents the most recently formed hair, and segments further out represent progressively older growth. In principle, this allows a form of retrospective timeline reconstruction, which is the basis for segmental hair analysis used in some toxicology and forensic contexts.

In practice, for routine HTMA sampling, laboratories typically request a bulk sample from a defined proximal length rather than finely segmenting the strand, and report a single aggregated result. This means the reported values represent an average across the sampled growth period, not a resolved timeline within it. A short-lived spike in an element’s incorporation — from a brief dietary change, a transient environmental exposure, or a temporary physiological shift — can be diluted within that average and become difficult to distinguish from a more sustained pattern.

Segmental Analysis: A Partial Solution, With Its Own Limits

Segmental analysis — cutting a hair sample into shorter consecutive sections and analyzing each separately — is one way researchers and forensic laboratories address the averaging problem, and it is well established for reconstructing approximate exposure timelines in toxicology. Applying the same logic to trace element analysis is methodologically appealing, but it does not eliminate the underlying growth-rate uncertainty — it only shifts the problem to a finer resolution. Each segment’s assumed time window still depends on the same population-average growth-rate assumption unless individual growth rate has been separately confirmed.

It is also worth noting that segmental analysis increases laboratory cost and complexity, and is not standard practice for most commercial HTMA testing aimed at general wellness or nutritional-trend monitoring, as opposed to targeted toxicological investigation (see how laboratories prepare hair samples for HTMA).

What Growth-Rate Uncertainty Means for a Single HTMA Result

None of this means HTMA results are uninformative — it means the time attribution attached to a result (“this reflects roughly the last three months”) should be treated as an approximation with a meaningful margin of error, not a precise statement. Practically, this has a few consequences:

• Trend comparisons are more robust than single-point timelines. Comparing results from the same person across multiple testing rounds, using a consistent sampling protocol, reduces the impact of an individual’s unknown but presumably stable growth rate, because the same systematic offset applies across rounds.
• Cross-person comparisons of “what happened when” are weaker. Because growth rate varies meaningfully between individuals, statements assuming a uniform timeline across different people’s samples carry more uncertainty.
• Reported time windows should be read as approximate, not exact. A “three-month” window is a convention based on an average growth rate, not a measured fact about the specific sample analyzed.

This is consistent with the broader point made in why interpretation quality matters more than the raw numbers: the analytical measurement itself can be accurate while the inferences drawn from it — including assumptions about timing — still require careful, appropriately qualified interpretation.

Common Interpretation Pitfalls

A few recurring interpretive errors are worth naming explicitly, consistent with the field’s broader reliability concerns. A widely cited 2001 JAMA evaluation of commercial hair mineral analysis laboratories — which split a single hair sample and submitted it to multiple commercial labs — found substantial inconsistency in reported results and concluded that such testing should not be used, on its own, to assess individual nutritional status or suspected toxic exposure (Seidel et al., 2001). More recent statistical work has proposed methods to better characterize and reduce measurement variability in hair mineral analysis (Nakamura et al., 2018).

Set against that backdrop, the growth-rate-specific pitfalls to avoid include: treating the assumed time window as a precise, individually verified fact; attributing a specific date or short-duration event to a location within a non-segmented, aggregated sample; assuming two people’s samples of equal length represent equal periods of history; and using a single HTMA snapshot — without a consistent testing history — to draw firm conclusions about the timing of a change in mineral status. None of these pitfalls mean the underlying measurement is meaningless; they mean the narrative built around when something happened needs to be held with appropriate caution, distinct from the measurement itself.

Practical Takeaways

• Treat the standard “three-month window” language as a useful convention rather than a personally verified measurement, since it is built on population-average growth rates that may not match any specific individual.
• Favor repeated testing with a consistent protocol over single-point interpretation when tracking mineral trends over time, since consistent methodology reduces (though does not eliminate) the impact of unknown individual growth rate.
• Treat questions about the precise timing of a specific past exposure or dietary change with particular caution unless segmental analysis was specifically requested and performed — a standard bulk sample is not designed to resolve that level of detail.

Where to Get Tested

Hair tissue mineral analysis of the kind discussed here is offered by laboratories such as LifelineDiag, a European laboratory performing HTMA testing. As with any analytical method, HTMA results are most meaningfully interpreted in context — alongside clinical history, lifestyle factors, and, where relevant, complementary laboratory testing — rather than read in isolation.

This article is part of HTMA.EXPERT’s ongoing series on the scientific fundamentals of hair tissue mineral analysis. HTMA.EXPERT is an educational resource operated and published by Lifeline Diag Sp. z o.o.; it does not perform laboratory testing itself.

Frequently Asked Questions

Bibliography

1. Paus R, Cotsarelis G. The Biology of Hair Follicles. New England Journal of Medicine. 1999;341(7):491–497.
2. Natarelli N, Gahooonia N, Sivamani RK. Integrative and Mechanistic Approach to the Hair Growth Cycle and Hair Loss. Journal of Clinical Medicine. 2023;12(3):893.
3. Loussouarn G, El Rawadi C, Genain G. Diversity of Hair Growth Profiles. International Journal of Dermatology. 2005;44(Suppl 1):6–9.
4. Seidel S, Kreutzer R, Smith D, McNeel S, Gilliss D. Assessment of Commercial Laboratories Performing Hair Mineral Analysis. JAMA. 2001;285(1):67–72.
5. Nakamura T, Yamada T, Kataoka K, Sera K, Saunders T, et al. Statistical Resolutions for Large Variabilities in Hair Mineral Analysis. PLoS ONE. 2018;13(12):e0208816.

Note on sourcing: all references above were verified as real, published works at the time of writing. Precise average growth-rate figures (e.g., “1–1.25 cm/month”) are commonly cited across dermatology sources and are broadly consistent with the peer-reviewed ethnic-comparison data in Loussouarn et al. (2005), but a single, universally agreed precision figure across all populations and ages was not identified in the sources reviewed — this should be read as a general range, not an exact constant.