You can change some aspects of how your hair grows, but not all of them. Hair texture, growth direction, and curl pattern are largely set by the shape of your follicles and your genetics, and no product or habit will permanently reprogram those. But you can influence how fast your hair grows, how thick it appears, and how healthy it looks by addressing nutrition, scalp health, hormones, and chemical treatments. The key is understanding which factors are fixed and which ones you actually have leverage over.
What Controls Your Hair’s Natural Pattern
The curl or straightness of your hair comes down to the cross-sectional shape of each strand as it exits the follicle. Round hair grows straight. The more oval the cross-section, the curlier the hair. This shape is determined by your follicle’s geometry, which is genetically programmed. You inherit follicle shape the same way you inherit eye color.
Growth direction is even more fixed. Hair whorls (the spiral patterns on your scalp, including cowlicks) are determined before birth. Their formation is set during fetal development, and there is no evidence that any treatment can permanently change the direction hair grows out of the follicle after birth. You can temporarily train hair with styling, heat, or products, but the follicle itself keeps producing hair in the same direction.
How Fast Hair Actually Grows
Hair grows about half an inch per month, or roughly six inches per year. Each strand grows approximately 0.35 millimeters per day. You can’t dramatically speed this up, but you can remove obstacles that slow it down.
Your hair cycles through three phases. The growth phase (anagen) lasts two to eight years, and about 90% of your hair is in this phase at any given time. A short transitional phase follows, lasting about two weeks, during which the strand detaches from its blood supply. Then comes a resting phase of two to three months before the hair sheds and a new one begins growing. How long each phase lasts determines your hair’s maximum length and overall thickness. Factors that shorten the growth phase or lengthen the resting phase, like extreme dieting, illness, stress, pregnancy, or certain medications, can make hair thinner and slower-growing.
Nutrients That Affect Hair Growth
Nutritional deficiencies genuinely impair hair growth. Iron is one of the most well-studied: dermatologists typically look for stored iron (ferritin) levels above 50 to 70 micrograms per liter for optimal hair health, and recommend dietary sources or supplements when levels fall short. Low iron, even without full-blown anemia, can push more hairs into the resting and shedding phases.
Vitamin D deficiency also correlates with hair thinning, particularly in women. Studies have found that lower vitamin D levels track with increased severity of hair loss. Correcting a deficiency won’t transform thin hair into thick hair, but it removes a bottleneck. If your hair has changed and you suspect a nutritional gap, a blood test can clarify whether iron, vitamin D, or another nutrient is part of the problem.
Your Scalp Matters More Than You Think
The scalp acts as an incubator for hair before it even breaks through the skin. When that environment is unhealthy, hair suffers before you ever see it. Conditions like dandruff, seborrheic dermatitis, and psoriasis create oxidative stress on the scalp, which damages hair follicle cells and can trigger premature shedding.
The mechanism is specific: oxidized fats on the scalp push hair follicles out of their growth phase early, triggering cell death in the follicle. A yeast called Malassezia, naturally present on everyone’s scalp, becomes a proven source of oxidative stress when it overgrows. The result is weaker anchoring of hair in the follicle, more shedding, and thinner-looking hair overall. Smoking, UV exposure, pollution, and untreated scalp inflammation all compound this effect. Keeping your scalp clean and managing any chronic skin conditions there can meaningfully improve the quality and retention of new hair growth.
How Hormones Reshape Your Hair Over Time
Hormones are one of the few biological forces that genuinely change how your follicles behave over the course of your life. The most common example is pattern hair loss, which affects at least 80% of men and 50% of women by age 70. It’s driven by a hormone called DHT, a potent form of testosterone that binds to receptors in genetically susceptible hair follicles. Over time, DHT causes those follicles to shrink, producing progressively thinner, shorter hairs until the follicle essentially stops producing visible hair.
This process involves two changes to the hair cycle: the growth phase gets shorter, and the resting phase gets longer. Topical treatments like minoxidil work by extending the growth phase, keeping follicles active longer. Minoxidil activates a signaling pathway in the cells at the base of the follicle that maintains their “actively growing” state. It doesn’t reverse genetic programming, but it counteracts the shrinking process while you use it.
Aging also changes hair independent of hormones. In women, hair diameter increases from the twenties through the mid-forties, then starts declining. Hair density peaks in the twenties and drops steadily after that. Until about age 35, thicker individual strands offset the loss of total hairs, so the change isn’t noticeable. After the mid-forties, both diameter and density decline together, and thinning becomes visible.
Chemical Treatments Change the Strand, Not the Follicle
Perms, relaxers, and keratin treatments can dramatically alter how your existing hair looks and feels. They work by breaking and reforming the chemical bonds that give hair its shape. Hair is made of a protein called keratin, held together by strong sulfur bonds between amino acids. A perm solution breaks those bonds using a reducing agent, allowing you to reshape the hair (around a rod for curls, or flat for straightening). A neutralizing solution then locks the bonds in their new configuration.
This is a real structural change, not just styling. But it only affects the hair that’s already grown out. The follicle keeps producing hair with its original shape and texture. As new hair grows in, you’ll see your natural pattern at the roots. That’s why perms and relaxers need to be repeated every few months. They modify the strand permanently but have zero effect on the follicle producing it.
What Shaving and Cutting Don’t Do
Shaving does not make hair grow back thicker, darker, or faster. This is one of the most persistent myths about hair. When you shave, you cut the hair at the skin’s surface, creating a blunt tip. That blunt edge feels coarser and looks darker against the skin compared to a naturally tapered hair tip. But the strand’s actual diameter, color, and growth rate are unchanged. The follicle has no way of knowing the hair was cut.
What You Can Realistically Change
You can’t permanently alter your follicle shape, curl pattern, or growth direction. Those are genetically determined and, in the case of growth direction, set before birth. What you can influence falls into a few categories:
- Growth rate and cycle length: Correcting nutrient deficiencies (especially iron and vitamin D), managing stress, and treating scalp conditions can help your hair stay in its growth phase longer and shed less.
- Thickness and density: Topical treatments can slow or partially reverse hormonal thinning by extending the active growth phase. Scalp health plays a supporting role.
- Texture of existing hair: Chemical treatments can reshape strands that have already grown, but require maintenance as new growth comes in with its natural texture.
- Overall hair quality: Reducing scalp inflammation, avoiding excessive heat and chemical damage, and eating a balanced diet improve the strength and appearance of hair as it emerges.
The most effective changes come from removing what’s holding your hair back, whether that’s a nutritional gap, an unhealthy scalp, or untreated hormonal thinning, rather than trying to override your genetics.