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.

It’s that time of year when many people are getting started on their resolutions for the year ahead. Doing more physical activity is a popular and worthwhile goal.

If you’re hoping to be more active in 2024, perhaps you’ve invested in an activity tracker, or you’re considering buying one.

But what are the benefits of activity trackers? And will a basic tracker do the trick, or do you need a fancy one with lots of features? Let’s take a look.

Why use an activity tracker?

One of the most powerful predictors for being active is whether or not you are monitoring how active you are.

Most people have a vague idea of how active they are, but this is inaccurate a lot of the time. Once people consciously start to keep track of how much activity they do, they often realise it’s less than what they thought, and this motivates them to be more active.

You can self-monitor without an activity tracker (just by writing down what you do), but this method is hard to keep up in the long run and it’s also a lot less accurate compared to devices that track your every move 24/7.

By tracking steps or “activity minutes” you can ascertain whether or not you are meeting the physical activity guidelines (150 minutes of moderate to vigorous physical activity per week).

It also allows you to track how you’re progressing with any personal activity goals, and view your progress over time. All this would be difficult without an activity tracker.

Research has shown the most popular brands of activity trackers are generally reliable when it comes to tracking basic measures such as steps and activity minutes.

But wait, there’s more

Many activity trackers on the market nowadays track a range of other measures which their manufacturers promote as important in monitoring health and fitness. But is this really the case? Let’s look at some of these.

Resting heart rate

This is your heart rate at rest, which is normally somewhere between 60 and 100 beats per minute. Your resting heart rate will gradually go down as you become fitter, especially if you’re doing a lot of high-intensity exercise. Your risk of dying of any cause (all-cause mortality) is much lower when you have a low resting heart rate.

So, it is useful to keep an eye on your resting heart rate. Activity trackers are pretty good at tracking it, but you can also easily measure your heart rate by monitoring your pulse and using a stopwatch.

Heart rate during exercise

Activity trackers will also measure your heart rate when you’re active. To improve fitness efficiently, professional athletes focus on having their heart rate in certain “zones” when they’re exercising – so knowing their heart rate during exercise is important.

But if you just want to be more active and healthier, without a specific training goal in mind, you can exercise at a level that feels good to you and not worry about your heart rate during activity. The most important thing is that you’re being active.

Also, a dedicated heart rate monitor with a strap around your chest will do a much better job at measuring your actual heart rate compared to an activity tracker worn around your wrist.

Maximal heart rate

This is the hardest your heart could beat when you’re active, not something you could sustain very long. Your maximal heart rate is not influenced by how much exercise you do, or your fitness level.

Most activity trackers don’t measure it accurately anyway, so you might as well forget about this one.

VO₂max

Your muscles need oxygen to work. The more oxygen your body can process, the harder you can work, and therefore the fitter you are.

VO₂max is the volume (V) of oxygen (O₂) we could breathe maximally (max) over a one minute interval, expressed as millilitres of oxygen per kilogram of body weight per minute (ml/kg/min). Inactive women and men would have a VO₂max lower than 30 and 40 ml/kg/min, respectively. A reasonably good VO₂max would be mid thirties and higher for women and mid forties and higher for men.

VO₂max is another measure of fitness that correlates well with all-cause mortality: the higher it is, the lower your risk of dying.

For athletes, VO₂max is usually measured in a lab on a treadmill while wearing a mask that measures oxygen consumption. Activity trackers instead look at your running speed (using a GPS chip) and your heart rate and compare these measures to values from other people.

If you can run fast with a low heart rate your tracker will assume you are relatively fit, resulting in a higher VO₂max. These estimates are not very accurate as they are based on lots of assumptions. However, the error of the measurement is reasonably consistent. This means if your VO₂max is gradually increasing, you are likely to be getting fitter.

So what’s the take-home message? Focus on how many steps you take every day or the number of activity minutes you achieve. Even a basic activity tracker will measure these factors relatively accurately. There is no real need to track other measures and pay more for an activity tracker that records them, unless you are getting really serious about exercise.

Corneel Vandelanotte, Professorial Research Fellow: Physical Activity and Health, CQUniversity Australia

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