Head hair comes in many colours, shapes and sizes, and hairstyles are often an expression of personal style or cultural identity.

Many different genes determine our hair texture, thickness and colour. But some people’s hair changes around the time of puberty, pregnancy or after chemotherapy.

So, what can cause hair to become curlier, thicker, thinner or grey?

Curly or straight? How hair follicle shape plays a role

Hair is made of keratin, a strong and insoluble protein. Each hair strand grows from its own hair follicle that extends deep into the skin.

Curly hair forms due to asymmetry of both the hair follicle and the keratin in the hair.

Follicles that produce curly hair are asymmetrical and curved and lie at an angle to the surface of the skin. This kinks the hair as it first grows.

The asymmetry of the hair follicle also causes the keratin to bunch up on one side of the hair strand. This pulls parts of the hair strand closer together into a curl, which maintains the curl as the hair continues to grow.

Follicles that are symmetrical, round and perpendicular to the skin surface produce straight hair.

Life changes, hair changes

Our hair undergoes repeated cycles throughout life, with different stages of growth and loss.

Each hair follicle contains stem cells, which multiply and grow into a hair strand.

Head hairs spend most of their time in the growth phase, which can last for several years. This is why head hair can grow so long.

Let’s look at the life of a single hair strand. After the growth phase is a transitional phase of about two weeks, where the hair strand stops growing. This is followed by a resting phase where the hair remains in the follicle for a few months before it naturally falls out.

The hair follicle remains in the skin and the stems cells grow a new hair to repeat the cycle.

Each hair on the scalp is replaced every three to five years.

Hormone changes during and after pregnancy alter the usual hair cycle

Many women notice their hair is thicker during pregnancy.

During pregnancy, high levels of oestrogen, progesterone and prolactin prolong the resting phase of the hair cycle. This means the hair stays in the hair follicle for longer, with less hair loss.

A drop in hormones a few months after delivery causes increased hair loss. This is due to all the hairs that remained in the resting phase during pregnancy falling out in a fairly synchronised way.

Hair can change around puberty, pregnancy or after chemotherapy

This is related to the genetics of hair shape, which is an example of incomplete dominance.

Incomplete dominance is when there is a middle version of a trait. For hair, we have curly hair and straight hair genes. But when someone has one curly hair gene and one straight hair gene, they can have wavy hair.

Hormonal changes that occur around puberty and pregnancy can affect the function of genes. This can cause the curly hair gene of someone with wavy hair to become more active. This can change their hair from wavy to curly.

Researchers have identified that activating specific genes can change hair in pigs from straight to curly.

Chemotherapy has very visible effects on hair. Chemotherapy kills rapidly dividing cells, including hair follicles, which causes hair loss. Chemotherapy can also have genetic effects that influence hair follicle shape. This can cause hair to regrow with a different shape for the first few cycles of hair regrowth.

Hormonal changes as we age also affect our hair

Throughout life, thyroid hormones are essential for production of keratin. Low levels of thyroid hormones can cause dry and brittle hair.

Oestrogen and androgens also regulate hair growth and loss, particularly as we age.

Balding in males is due to higher levels of androgens. In particular, high dihydrotestosterone (sometimes shortened to DHT), which is produced in the body from testosterone, has a role in male pattern baldness.

Some women experience female pattern hair loss. This is caused by a combination of genetic factors plus lower levels of oestrogen and higher androgens after menopause. The hair follicles become smaller and smaller until they no longer produce hairs.

Reduced function of the cells that produce melanin (the pigment that gives our hair colour) is what causes greying.

Theresa Larkin, Associate professor of Medical Sciences, University of Wollongong

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Still or sparkling? It’s a question you’ll commonly hear in a café or restaurant and you probably have a preference. But is there any difference for your health?

If you love the fizz, here’s why you don’t have to pass on the sparkling water.

What makes my water sparkle?

This article specifically focuses on comparing still filtered water to carbonated filtered water (called “sparkling water” or “unflavoured seltzer”). Soda water, mineral water, tonic water and flavoured water are similar, but not the same product.

The bubbles in sparkling water are created by adding carbon dioxide to filtered water. It reacts to produce carbonic acid, which makes sparkling water more acidic (a pH of about 3.5) than still (closer to neutral, with a pH around 6.5-8.5).

Which drink is healthiest?

Water is the best way to hydrate our bodies. Research shows when it comes to hydration, still and sparkling water are equally effective.

Some people believe water is healthier when it comes from a sealed bottle. But in Australia, tap water is monitored very carefully. Unlike bottled water, it also has the added benefit of fluoride, which can help protect young children against tooth decay and cavities.

Sparkling or still water is always better than artificially sweetened flavoured drinks or juices.

Isn’t soda water bad for my teeth and bones?

There’s no evidence sparkling water damages your bones. While drinking a lot of soft drinks is linked to increased fractures, this is largely due to their association with higher rates of obesity.

Sparkling water is more acidic than still water, and acidity can soften the teeth’s enamel. Usually this is not something to be too worried about, unless it is mixed with sugar or citrus, which has much higher levels of acidity and can harm teeth.

However, if you grind your teeth often, the softening could enhance the damage it causes. If you’re undertaking a home whitening process, sparkling water might discolour your teeth.

In most other cases, it would take a lot of sparkling water to pass by the teeth, for a long period of time, to cause any noticeable damage.

How does drinking water affect digestion?

There is a misconception drinking water (of any kind) with a meal is bad for digestion.

While theoretically water could dilute stomach acid (which breaks down food), the practice of drinking it doesn’t appear to have any negative effect. Your digestive system simply adapts to the consistency of the meal.

Some people do find that carbonated beverages cause some stomach upset. This is due to the build-up of gases, which can cause bloating, cramping and discomfort. For people with an overactive bladder, the acidity might also aggravate the urinary system.

Interestingly, the fizzy “buzz” you feel in your mouth from sparkling water fades the more you drink it.

Is cold water harder to digest?

You’ve chosen still or sparkling water. What about its temperature?

There are surprisingly few studies about the effect of drinking cold water compared to room temperature. There is some evidence colder water (at two degrees Celsius) might inhibit gastric contractions and slow down digestion. Ice water may constrict blood vessels and cause cramping.

However other research suggests drinking cold water might temporarily boost metabolism, as the body needs to expend energy to warm it up to body temperature. This effect is minimal and unlikely to lead to significant weight loss.

Which water wins?

The bottom line is water is essential, hydrates us and has countless other health benefits. Water, with carbonated bubbles or without, will always be the healthiest drink to choose.

And if you’re concerned about any impact to teeth enamel, one trick is to follow sparkling water with a glass of still. This helps rinse the teeth and return your mouth’s acidity back to normal.

Christian Moro, Associate Professor of Science & Medicine, Bond University and Charlotte Phelps, Senior Teaching Fellow, Medical Program, Bond University

This article is republished from The Conversation under a Creative Commons license. Read the original article.