You would not believe how much scientists know about toenail and fingernail growth.
Photo Credit: Radarsmum67 via flickr | CC BY 2.0
The short answer is that, like many of the body's physiological mechanisms, the reason for differential growth rates isn't 100% clear. That said, we do know a surprising lot about nail growth (more than I would have guessed, anyway). Based on decades of observations, researchers have developed a couple of pretty compelling hypotheses, not just about the difference between toenail growth and fingernail growth, but about differential growth rates among individual digits.
For instance, we know that the rate at which nails grow varies pretty widely from person to person. Numbers tend to differ slightly between studies, but various investigations conducted over the course of the 20th century put the average growth of fingernails in the range of 0.5 to 1.2 mm per week, and toenails about 3- to 4-times slower than that. We also know that nail growth is determined by the turnover rate of what are called "matrix cells," and while we won't cover their physiology here (for a clear rundown of that, see this primer from the American Academy of Dermatology), studies have shown that this rate is associated with everything from sex, to exercise level, to diet; the mechanistic details underlying the turnover rate of matrix cells are less than clear, but the variables they've been found to correspond with are extensive, to say the least.
Just for fun, here are some notable correlations: Studies dating at least as far back as the 1930s have observed that the rate of nail growth tends to be greater in the summer than the winter. Observational studies conducted on humans in temperate as well as arctic conditions, as well as on rats in cold experimental climes, have corroborated these findings. Rate of growth has also been shown to vary not just between individual fingers (most studies observe that the middle digit of the dominant hand tends to sprout nails fastest, while pinkies or thumbs usually take last place), but between hands, as well; while the difference is always small, rate of nail-growth on the dominant hand has been found to outpace that of the non-dominant one by about a tenth of a millimeter per month. And, thanks in large part to decades of meticulous self-study by one William Bean, we know that the rate of nail growth tends to decrease with age. In January 1980, following thirty-five years of self-examination, Bean published the following summary of his observations in the American Medical Association's Archives of Internal Medicine (which now goes by JAMA Internal Medicine):
A 35-year observation of the growth of my nails indicates the slowing of growth with increasing age. The average daily growth of the left thumbnail, for instance, has varied from 0.123mm a day during the first part of the study when I was 32 years of age to 0.095mm a day at the age of 67.
It's also worth noting – if only to further emphasize the thoroughness of his measurements – that Bean never observed any seasonal trend in his nail-growth. He suspected that "spending so much time indoors with central heating and air conditioning," insulated him from the climatic extremes that would result in seasonal variations in the rate of fingernail growth.
All this is to say that there has been a lot of careful study devoted to nails and their growth. From these investigations, two related hypotheses have emerged.
Use And Blood Flow
The first is the so-called "terminal trauma" hypothesis. The definition of the word "trauma," in this instance, isn't as dramatic as you might think; basically, the hypothesis states that the rate of a nail's growth is linked to how often its corresponding finger is used. This would explain why so many studies have found the rate of fingernail growth to be faster on the dominant hand than the non-dominant one, and why longer digits – which are presumably more liable to be used, tapped, or stubbed – tend to push out nails faster than their stubbier counterparts. "According to [proponents of the trauma theory]," wrote Wired's Nick Stockton when he wrote on this subject last year, "frequent fingertip use indicates to the nail matrix that the nail is probably being worn down faster, so it calibrates by increasing the rate of growth." To my knowledge, no studies have attempted to quantify or compare the traumas endured by fingers with those experienced by toes. However, at least one has sought to compare the differential effect of trauma on nail growth in individual people; writing for Clinical Correlations, the NYU Langone Online Journal of Medicine, Alice Drain cites this study, which found that, among jazz bassists, the left fifth finger (which is used to play their instrument) tends to grow a nail that is significantly flatter than the right fifth finger.
In any case, an increased rate of growth almost certainly requires an increase in nutrient supply, which in turn demands an increase in blood flow. This line of reasoning brings us to the second, related explanation for fast fingernail growth: That fingernails, by being closer to the heart, experience better circulation than your toenails. The observation that nail growth tends to increase in warmer conditions (which is also associated with improved circulation) and decrease the older we get ( blood flow in the limbs decreases with age) lends empirical ballast to this explanation.
Interestingly, one of the more recent investigations to examine nail growth – the straightforwardly titled "Growth rate of human fingernails and toenails in healthy American young adults," published in 2009 – concluded that "nail growth rates have increased compared with previous estimates conducted decades ago." The researchers – led by Ka He, then assistant professor of nutrition and epidemiology at the University of North Carolina at Chapel Hill School of Public Health – observed average fingernail and toenail growth rates of 3.47- and 1.62-mm per month, respectively. They also observed no significant difference between right and left growth rates of toenails or fingernails.
It's not hard to fit these observations into the terminal trauma hypothesis. Here, I'll go first: We could speculate, for example, that the rise of computers has led us to use the fingers of both hands on a more regular basis. Please feel free to weigh in with your own ad hoc reckons, below.
H/t The Guardian, Wired, Clinical Correlations
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