Unlike some lower animals that undergo molting, human hair does not seem to change much throughout the seasons. However, human hair actually goes through multiple cycles of growth and loss over a lifetime.
Each hair follicle has its own growth cycle. Some hairs are growing while others are in a resting or shedding phase. There is a dynamic balance maintained between hair loss and new hair growth, keeping the total number of hairs relatively constant. Hair growth and loss are primarily controlled by inherent biological rhythms, but they are also influenced by other factors.
Why is Human Hair Long?
As humans evolved, body hair, though almost omnipresent, became sparse and short. Compared to apes, human body hair is significantly reduced, yet scalp hair is lush and long. For instance, Asian hair tends to be longer, and it's not uncommon for Mongolian women's hair to exceed their height. According to a measurement on May 8, 2004, Xie Qiuping from Guangxi Zhuang Autonomous Region, China, held the world record for the longest hair at 5.627 meters.
The length of human hair may offer protection against external forces and climatic variations. However, this is not the primary reason for its length. Apes, for example, also have hair concentrated on their heads but none have long hair.
Observations in wild animals show that long hair on specific body parts is rare and is more common in domesticated animals. For example, domestic horses have long mane hair, while wild horses have short manes. Similarly, Persian cats have longer fur compared to wild cats.
This phenomenon can be explained by hormonal changes during domestication, affecting hair growth. Human hair growth can be analogously compared to domesticated animals, as humans can be seen as "socially domesticated" animals.
What is the Hair Growth Cycle?
It's known that if not cut, hair will grow longer and longer. The growth rate is about 0.27 to 0.4 millimeters per day. Based on this, hair grows approximately 1 centimeter per month and 10 to 14 centimeters per year. However, hair does not grow continuously but follows a growth cycle, divided into three phases: growth, regression, and resting. The growth phase lasts 2 to 6 years, regression for 2 to 3 weeks, and resting around 3 months. Of the approximately 100,000 hairs on a normal person’s head, 85% to 90% are in the growth phase, 1% in regression, and 9% to 14% in the resting phase. Hairs in the resting phase fall out during washing, combing, or scratching the scalp.
On average, a person loses 20 to 100 hairs per day, so there's no need to worry about hair growing excessively long. However, a few individuals have unusually long hair, sometimes exceeding their body height, due to a growth cycle of 15 to 20 years, which is 3 to 4 times longer than the average person. Hair reaching over 2 meters in length is rare, with a growth cycle of up to 25 years.
How Does Hair Grow?
Hair growth is inseparable from the hair follicle. In the growth phase, the follicle is active, with vigorous cell division at the base of the hair bulb, continuously supplying cells for hair and internal root sheath growth. As the growth phase nears its end, cell division ceases, and the hair stops growing, entering the regression phase.
During the resting phase, the hair and parts of the follicle degenerate and shrink, leading to hair shedding. Meanwhile, a new growth phase hair follicle forms nearby, and a new hair emerges.
Factors Influencing Hair Growth
Hair growth and loss are mainly controlled by the hair’s own growth cycle but are also affected by factors such as race, genetics, endocrine, illness, mental state, gender, age, and season.
- Race: There are obvious differences among races, not only in hair color but also in hair quantity and growth patterns. Baldness is common among Caucasians, less so in Chinese, and even rarer in Native Americans.
- Genetics: Within the same family, hair growth patterns are often similar. Male pattern baldness is closely linked to genetics.
- Endocrine: Androgens directly affect hair follicles, leading to male pattern baldness. Those lacking androgen stimulation, such as eunuchs, do not experience this. Estrogens counteract androgens, so women rarely become bald before menopause. Thyroid hormone deficiency leads to sparse hair, adrenal insufficiency to hair loss, and pituitary decline can cause total baldness.
- Mental State: Stress, fear, and anxiety can significantly increase hair loss. For instance, a condemned prisoner reportedlylost over 1000 hairs per day after sentencing.
- Vitamins: Long-term lack of Vitamin A can lead to thin hair; a deficiency in Vitamin B2 may result in increased seborrhea and hair loss; Vitamin B6 deficiency can cause abnormal sebum secretion. Contraceptive pills can accelerate metabolism and consume more Vitamin B6, which might be linked to diffuse hair loss in some women. Additionally, Vitamin B6 affects pigment metabolism, and its deficiency can lead to gray hair and poor growth. Pantothenic acid (Vitamin B3) deficiency can cause hair to turn white and grow poorly. Inositol, a part of the Vitamin B group, can prevent hair loss; biotin deficiency leads to hair loss; para-aminobenzoic acid, also a Vitamin B, helps maintain hair color and normal growth.
- Trace Elements: Studies on trace elements such as zinc, iron, molybdenum, calcium, lead, magnesium, manganese, and selenium in hair have shown significant differences in copper, iron, and manganese levels in individuals with hair loss, while calcium, magnesium, and selenium levels were higher. Copper deficiency can affect iron absorption and utilization, leading to anemia and agitation, which could trigger alopecia areata. Copper also influences the keratinization process of hair. Excess selenium can cause hair loss due to autoimmune reactions and increased scalp sebum secretion.
- Diseases: Systemic diseases such as febrile illnesses, anemia, malnutrition, liver disease, and severe chronic wasting diseases often lead to thinning hair.
- Gender and Age: Women's hair grows faster than men's, and young people's hair grows faster than older people's, though the difference is not significant. With age, the reduction in the number of scalp hair follicles is more pronounced. Statistics show that the number of follicles per square centimeter is 615 for ages 20-30, 485 for 30-50, and 435 for 80-90.
- Other Factors: Hair growth is slightly faster in summer than in winter due to higher temperatures promoting metabolism. Hair grows faster during the day than at night. X-rays can control the sulfhydryl compounds in hair follicle matrix, causing temporary hair loss. Certain chemicals, like iron, can affect keratin formation, possibly interfering with cysteine in keratin composition, thus affecting hair growth. UV rays, drugs, trauma, chronic inflammation, skin diseases, and local massage stimulation can also impact hair growth and loss.
What is the Relationship Between Hair Growth and Protein?
It is understandable that healthy hair relies on overall health, which in turn depends on nutrition. If the scalp is considered a garden and hair as plants, then the scalp needs cultivation and hair needs nutrients.
Normal hair growth depends on: blood vessels in the hair papilla supplying nutrients to the hair; keratinocytes around the papilla secreting keratin and hard proteins to synthesize hair, ensuring lush growth; melanocytes in the matrix secreting melanin, forming pigment granules that fill the hair shaft, giving hair its black color. Clearly, if hair does not receive essential nutrients, it will become dry, sparse, fall out, and turn gray prematurely.
Hair is composed of 97% protein, and its growth requires sulfur-containing amino acids, which the body cannot synthesize and must be obtained from dietary protein. A daily protein intake of less than 50 grams can lead to severe protein deficiency, adversely affecting hair growth.
The most typical example of protein's effect on hair growth is observed in children from certain African tribes suffering from severe malnutrition. These children have significantly reduced hair that is dry, brittle, lacks luster, and is easily plucked, changing from normal black to pale red or white. When their nutritional status improves, their hair quickly darkens again. However, if nutrition becomes deficient again, the hair turns white rapidly, resulting in a zebra-like pattern of black and white on the same hair strand. This phenomenon is also seen in patients with ulcerative colitis or those who have undergone partial intestinal resection leading to protein deficiency.
What Foods Nourish Hair?
In addition to protein, hair growth requires iodine, vitamins, and trace elements, so foods rich in these nutrients should be consumed.
Hair color and luster are related to thyroid hormone secretion and trace elements. Seaweed and kelp, rich in iodine, enhance thyroid hormone secretion. These foods also contain calcium and iron, which can make hair black and shiny.
Spinach, tomatoes, potatoes, and persimmons contain copper, iron, and cobalt, essential for synthesizing hair's melanin.
Grains, peanuts, and mushrooms, rich in cysteine and methionine, are nutritious foods for hair care.
Vitamins A, B, C, D, E, and K are essential for nourishing hair. Various vegetables and foods contain these vitamins: carrots, tomatoes, and pumpkins are rich in Vitamin A; grains, bean sprouts, barley, and brewer's yeast are high in Vitamin B; kiwifruit, fresh dates, strawberries, oranges, cabbages, spinach, and mustard greens contain abundant Vitamin C; animal liver, milk, and cod liver oil are sources of Vitamin D; leafy vegetables, plant oils, brown rice, peanuts, and milk are rich in Vitamin E. These should be consumed appropriately.
Foods rich in iodine
Foods rich in copper, iron and cobalt
Foods rich in cysteine and methionine
Foods rich in vitamin A
Foods rich in vitamin B
Foods rich in vitamin D
Foods rich in vitamin E
Foods rich in vitamin C
How to Use Diet to Improve Hair Health?
A varied diet with controlled portions, a combination of meat and vegetables, and balanced nutrition are the basic dietary requirements for hair care.
Excessive consumption of sugar and fats can lead to the production of acids like lactic acid and pyruvic acid, increasing acidic toxins in the blood. This can make hair dry, yellow, and cause split ends. To counter this, foods rich in iodine and calcium such as seaweed, kelp, grains, milk, green leafy vegetables, and fruits, as well as iron-rich foods like animal liver, lean meat, and eggs, should be consumed to suppress blood acidity and maintain the body's acid-base balance, thereby preventing hair from becoming dry and yellow.
For dry, lusterless, and easily breakable hair, consuming plant oils and foods rich in protein, Vitamin A, and iodine, such as seaweed and animal products, is beneficial.
Zinc deficiency can affect protein synthesis and is one of the causes of sudden hair loss. Those with zinc deficiency should increase their intake of seafood, milk, beef, eggs, and seaweed.
For those experiencing premature graying, if not caused by genetics, it is advisable to eat foods rich in Vitamin B6, such as bananas, peanuts, chicken eggs, etc., to help reverse graying.
In summary, a balanced and nutritious diet plays a crucial role in maintaining healthy hair. Regular consumption of the right vitamins, minerals, and proteins not only nourishes the hair but also helps prevent common hair problems. Understanding the relationship between diet and hair health can lead to more effective and natural ways of hair care.