Hair growth begins before birth and continues throughout an individual's life. Hair on the head grows at a rate of about 1/2 inch per month, whereas hair elsewhere on the body takes longer to grow. The cycles of growth are affected by genes, hormones and environment.
Stages of Growth
Hair growth has three stages: anagen, catagen and telogen. These vary in length based on the location and type of hair. Typically, anagen is the longest phase. During anagen, actual growth of the hair takes place. In catagen, the hair follicle is resting and the hair is being keratinized, filled with the protein keratin. The final stage, telagen, is when the hair falls out and the root reforms to get ready for the next cycle. About 90 percent of hair is in the growth phase at any given time.
Types
There are a few different types of hair that differ in how they grow. The first hair any human has is lanugo, the fine hair that covers the fetus in the womb. Lanugo only goes through two growth cycles before falling out completely in the womb. Vellus hair is the thin hair found on most of the body, including the arms, torso and legs. It has repeating growth cycles that vary in length and can change into terminal hair at puberty. Terminal hair is the thick hair on the head and, after puberty, under the arms, in the pubic area and on men's faces. On the head, anagen lasts two to six years; catagen lasts two to three weeks; and telogen lasts three to four months.
Androgen
Some hair is androgen dependent, meaning that it requires hormones called androgens to control its growth. Two important androgens in hair growth are testosterone and DHT. These androgens bind to hair follicles in facial hair, pubic hair and other areas. Without working androgens and receptors, these areas will not grow hair. There are inherited genes that affect androgen and their receptors, such as the genes for male pattern baldness, which affect hair growth.
Nutrition and Growth
Proper hair growth relies on a healthy diet that includes specific vitamins and minerals. Nutrients that can affect hair growth include vitamin A, folic acid, vitamin B6, vitamin B12, biotin, copper, vitamin C, iron, zinc and protein. Deficiencies in these nutrients can cause hair growth to slow or stop temporarily. If the deficiency is corrected, hair growth will generally resume.
Hair Growth Disorders
Hirsutism is the overgrowth of hair on the body, usually occurring as male pattern growth on a female. This is a somewhat rare disorder, affecting up to 10 percent of women, but most cases are mild, such as the appearance of small amounts of facial hair on a woman's upper lip. The cause of severe hirsutism is believed to be an excess of androgens in the body.
Hair loss, alopecia, is another hair growth disorder that affects many people. The most common form of hair loss is male pattern baldness, but there are other forms, including the loss of hair from all over the body and female pattern baldness.
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