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.

The holiday season can be a time of joy, celebration, and indulgence in delicious foods and meals. However, for many, it can also be an emotional and stressful period.

This stress can manifest in our eating habits, leading to what is known as emotional or stress eating.

There are certain foods we tend to eat more of when we’re stressed, and these can affect our health. What’s more, our food choices can influence our stress levels and make us feel worse. Here’s how.

Why we might eat more when stressed

The human stress response is a complex signalling network across the body and brain. Our nervous system then responds to physical and psychological events to maintain our health. Our stress response – which can be subtle or trigger a fight-or-flight response – is essential and part of daily life.

The stress response increases production of the hormones cortisol and insulin and the release of glucose (blood sugars) and brain chemicals to meet demand. Eating when we experience stress is a normal behaviour to meet a spike in energy needs.

But sometimes our relationship with food becomes strained in response to different types of stress. We might attach shame or guilt to overeating. And anxiety or insecurity can mean some people under-eat in stressful times.

Over time, people can start to associate eating with negative emotions – such as anger, sadness, fear or worry. This link can create behavioural cycles of emotional eating. “Emotional eaters” may go on to develop altered brain responses to the sight or smell of food.

What stress eating can do to the body

Stress eating can include binge eating, grazing, eating late at night, eating quickly or eating past the feeling of fullness. It can also involve craving or eating foods we don’t normally choose. For example, stressed people often reach for ultra-processed foods. While eating these foods is not necessarily a sign of stress, having them can activate the reward system in our brain to alleviate stress and create a pattern.

Short-term stress eating, such as across the holiday period, can lead to symptoms such as acid reflux and poor sleep – particularly when combined with drinking alcohol.

In the longer term, stress eating can lead to weight gain and obesity, increasing the risks of cancer, heart diseases and diabetes.

While stress eating may help reduce stress in the moment, long-term stress eating is linked with an increase in depressive symptoms and poor mental health.

What we eat can make us more or less stressed

The foods we choose can also influence our stress levels.

Diets high in refined carbohydrates and sugar (such as sugary drinks, sweets, crackers, cakes and most chocolates) can make blood sugar levels spike and then crash.

Diets high in unhealthy saturated and trans fats (processed foods, animal fats and commercially fried foods) can increase inflammatory responses.

Rapid changes in blood sugar and inflammation can increase anxiety and can change our mood.

Meanwhile, certain foods can improve the balance of neurotransmitters in the brain that regulate stress and mood.

Omega-3 fatty acids, found in fish and flaxseeds, are known to reduce inflammation and support brain health. Magnesium, found in leafy greens and nuts, helps regulate cortisol levels and the body’s stress response.

Vitamin Bs, found in whole grains, nuts, seeds, beans and animal products (mostly B12), help maintain a healthy nervous system and energy metabolism, improving mood and cognitive performance.

5 tips for the holiday table and beyond

Food is a big part of the festive season, and treating yourself to delicious treats can be part of the fun. Here are some tips for enjoying festive foods, while avoiding stress eating:

1. slow down: be mindful about the speed of your eating. Slow down, chew food well and put down your utensils after each bite

2. watch the clock: even if you’re eating more food than you normally would, sticking to the same timing of eating can help maintain your body’s response to the food. If you normally have an eight-hour eating window (the time between your first meal and last meal of the day) then stick to this even if you’re eating more

3. continue other health behaviours: even if we are eating more food or different food during the festive season, try to keep up other healthy behaviours, such as sleep and exercise

4. stay hydrated: make sure to drink plenty of fluids, especially water. This helps our body function and can help with feelings of hunger. When our brain gets the message something has entered the stomach (what we drink) this can provide a temporary reduction in feelings of hunger

5. don’t restrict: if we have a big day of eating, it can be tempting to restrict eating in the days before or after. But it is never a good idea to overly constrain food intake. It can lead to more overeating and worsen stress.

Plus 3 bonus tips to manage holiday stress

1. shift your thinking: try reframing festive stress. Instead of viewing it as “something bad”, see it as “providing the energy” to reach your goals, such as a family gathering or present shopping

2. be kind to yourself and others: practise an act of compassion for someone else or try talking to yourself as you would a friend. These actions can stimulate our brains and improve wellbeing

3. do something enjoyable: being absorbed in enjoyable activities – such as crafting, movement or even breathing exercises – can help our brains and bodies to return to a more relaxed state, feel steady and connected.

For support and more information about eating disorders, contact the Butterfly Foundation on 1800 33 4673 or Kids Helpline on 1800 551 800. If this article has raised issues for you, or if you’re concerned about someone you know, call Lifeline on 13 11 14. In an emergency, call 000.

Saman Khalesi, Senior Lecturer and Discipline Lead in Nutrition, School of Health, Medical and Applied Sciences, CQUniversity Australia; Charlotte Gupta, Senior Postdoctoral Research Fellow, Appleton Institute, HealthWise research group, CQUniversity Australia, and Talitha Best, Professor of Psychology, NeuroHealth Lab, Appleton Institute, CQUniversity Australia

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