Nails grow at rates that differ noticeably between fingers, between hands and between people. The variation traces back to a small tissue structure hidden beneath the cuticle.
Everything happens in the matrix
The matrix sits under the skin at the base of the nail and is the only part that produces new plate. The visible nail is finished material being pushed outward.
Cells in the matrix divide, fill with keratin and die, then compact into the hard layered sheet that emerges past the cuticle as the plate.
Because production happens only there, nothing applied to the visible nail can change growth rate. Products that claim to accelerate growth are acting on strength or on the surrounding skin.
Circulation supplies the matrix
Cell division requires a steady supply of oxygen and nutrients, so blood flow to the fingertip directly limits how quickly the matrix can work.
This explains several familiar patterns. Nails grow faster in warm conditions than in cold, and faster on a dominant hand that is used and moved more.
It also explains why fingernails outpace toenails by a wide margin, since the feet sit at the end of a longer circulatory route and stay cooler.
Age changes the rate in both directions
Growth is fastest in childhood and adolescence, slows gradually through adulthood, and continues slowing into later life as circulation and cell turnover decline.
The plate also changes in character rather than only in speed. Older nails frequently become thicker, more ridged and more brittle at the same time.
Because the whole plate is replaced over a period of months, any change in the matrix takes that long to become fully visible at the free edge.
Minor trauma speeds things up
Repeated mild stimulation of the fingertip appears to increase local circulation, which is why frequently used fingers often carry slightly longer nails.
Middle fingers commonly grow fastest and little fingers slowest, following roughly how exposed and how used each digit is during ordinary activity.
Significant trauma has the opposite effect. Damage to the matrix itself can slow growth, distort the shape of the plate or produce ridges that persist for months.
What health and nutrition actually influence
Serious illness can interrupt matrix activity, leaving a horizontal line or groove that records the interruption as it grows outward over subsequent months.
General nutritional deficiency can slow growth or alter texture, but supplementing beyond normal requirements does not push a healthy matrix to work faster.
Sudden changes in nail growth, colour or shape are worth raising with a clinician, since the plate records systemic events unusually clearly and slowly.