Intermittent fasting has become a popular dietary approach to help people lose or manage their weight. It has also been promoted as a way to reset metabolism, control chronic disease, slow ageing and improve overall health.

Meanwhile, some research suggests intermittent fasting may offer a different way for the brain to access energy and provide protection against neurodegenerative diseases like Alzheimer’s disease.

This is not a new idea – the ancient Greeks believed fasting enhanced thinking. But what does the modern-day evidence say?

First, what is intermittent fasting?

Our diets – including calories consumed, macronutrient composition (the ratios of fats, protein and carbohydrates we eat) and when meals are consumed – are factors in our lifestyle we can change. People do this for cultural reasons, desired weight loss or potential health gains.

Intermittent fasting consists of short periods of calorie (energy) restriction where food intake is limited for 12 to 48 hours (usually 12 to 16 hours per day), followed by periods of normal food intake. The intermittent component means a re-occurrence of the pattern rather than a “one off” fast.

Food deprivation beyond 24 hours typically constitutes starvation. This is distinct from fasting due to its specific and potentially harmful biochemical alterations and nutrient deficiencies if continued for long periods.

4 ways fasting works and how it might affect the brain

The brain accounts for about 20% of the body’s energy consumption.

Here are four ways intermittent fasting can act on the body which could help explain its potential effects on the brain.

1. Ketosis

The goal of many intermittent fasting routines is to flip a “metabolic switch” to go from burning predominately carbohydrates to burning fat. This is called ketosis and typically occurs after 12–16 hours of fasting, when liver and glycogen stores are depleted. Ketones – chemicals produced by this metabolic process – become the preferred energy source for the brain.

Due to this being a slower metabolic process to produce energy and potential for lowering blood sugar levels, ketosis can cause symptoms of hunger, fatigue, nausea, low mood, irritability, constipation, headaches, and brain “fog”.

At the same time, as glucose metabolism in the brain declines with ageing, studies have shown ketones could provide an alternative energy source to preserve brain function and prevent age-related neurodegeneration disorders and cognitive decline.

Consistent with this, increasing ketones through supplementation or diet has been shown to improve cognition in adults with mild cognitive decline and those at risk of Alzheimer’s disease respectively.

2. Circadian syncing

Eating at times that don’t match our body’s natural daily rhythms can disrupt how our organs work. Studies in shift workers have suggested this might also make us more prone to chronic disease.

Time-restricted eating is when you eat your meals within a six to ten-hour window during the day when you’re most active. Time-restricted eating causes changes in expression of genes in tissue and helps the body during rest and activity.

A 2021 study of 883 adults in Italy indicated those who restricted their food intake to ten hours a day were less likely to have cognitive impairment compared to those eating without time restrictions.

3. Mitochondria

Intermittent fasting may provide brain protection through improving mitochondrial function, metabolism and reducing oxidants.

Mitochondria’s main role is to produce energy and they are crucial to brain health. Many age-related diseases are closely related to an energy supply and demand imbalance, likely attributed to mitochondrial dysfunction during ageing.

Rodent studies suggest alternate day fasting or reducing calories by up to 40% might protect or improve brain mitochondrial function. But not all studies support this theory.

4. The gut-brain axis

The gut and the brain communicate with each other via the body’s nervous systems. The brain can influence how the gut feels (think about how you get “butterflies” in your tummy when nervous) and the gut can affect mood, cognition and mental health.

In mice, intermittent fasting has shown promise for improving brain health by increasing survival and formation of neurons (nerve cells) in the hippocampus brain region, which is involved in memory, learning and emotion.

There’s no clear evidence on the effects of intermittent fasting on cognition in healthy adults. However one 2022 study interviewed 411 older adults and found lower meal frequency (less than three meals a day) was associated with reduced evidence of Alzheimer’s disease on brain imaging.

Some research has suggested calorie restriction may have a protective effect against Alzheimer’s disease by reducing oxidative stress and inflammation and promoting vascular health.

When we look at the effects of overall energy restriction (rather than intermittent fasting specifically) the evidence is mixed. Among people with mild cognitive impairment, one study showed cognitive improvement when participants followed a calorie restricted diet for 12 months.

Another study found a 25% calorie restriction was associated with slightly improved working memory in healthy adults. But a recent study, which looked at the impact of calorie restriction on spatial working memory, found no significant effect.

Bottom line

Studies in mice support a role for intermittent fasting in improving brain health and ageing, but few studies in humans exist, and the evidence we have is mixed.

Rapid weight loss associated with calorie restriction and intermittent fasting can lead to nutrient deficiencies, muscle loss, and decreased immune function, particularly in older adults whose nutritional needs may be higher.

Further, prolonged fasting or severe calorie restriction may pose risks such as fatigue, dizziness, and electrolyte imbalances, which could exacerbate existing health conditions.

If you’re considering intermittent fasting, it’s best to seek advice from a health professional such as a dietitian who can provide guidance on structuring fasting periods, meal timing, and nutrient intake. This ensures intermittent fasting is approached in a safe, sustainable way, tailored to individual needs and goals.

Hayley O’Neill, Assistant Professor, Faculty of Health Sciences and Medicine, Bond University

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

Throughout recorded history, our hair and nails played an important role in signifying who we are and our social status. You could say, they separate the caveman from businessman.

It was no surprise then that many of us found a new level of appreciation for our hairdressers and nail artists during the COVID lockdowns. Even Taylor Swift reported she cut her own hair during lockdown.

So, what would happen if all this hair and nail grooming got too much for us and we decided to give it all up. Would our hair and nails just keep on growing?

The answer is yes. The hair on our head grows, on average, 1 centimeter per month, while our fingernails grow an average of just over 3 millimetres.

When left unchecked, our hair and nails can grow to impressive lengths. Aliia Nasyrova, known as the Ukrainian Rapunzel, holds the world record for the longest locks on a living woman, which measure an impressive 257.33 cm.

When it comes to record-breaking fingernails, Diana Armstrong from the United States holds that record at 1,306.58 cm.

Most of us, however, get regular haircuts and trim our nails – some with greater frequency than others. So why do some people’s hair and nails grow more quickly?

Remind me, what are they made out of?

Hair and nails are made mostly from keratin. Both grow from matrix cells below the skin and grow through different patterns of cell division.

Nails grow steadily from the matrix cells, which sit under the skin at the base of the nail. These cells divide, pushing the older cells forward. As they grow, the new cells slide along the nail bed – the flat area under the fingernail which looks pink because of its rich blood supply.

A hair also starts growing from the matrix cells, eventually forming the visible part of the hair – the shaft. The hair shaft grows from a root that sits under the skin and is wrapped in a sac known as the hair follicle.

This sac has a nerve supply (which is why it hurts to pull out a hair), oil-producing glands that lubricate the hair and a tiny muscle that makes your hair stand up when it’s cold.

At the follicle’s base is the hair bulb, which contains the all-important hair papilla that supplies blood to the follicle.

Matrix cells near the papilla divide to produce new hair cells, which then harden and form the hair shaft. As the new hair cells are made, the hair is pushed up above the skin and the hair grows.

But the papilla also plays an integral part in regulating hair growth cycles, as it sends signals to the stem cells to move to the base of the follicle and form a hair matrix. Matrix cells then get signals to divide and start a new growth phase.

Unlike nails, our hair grows in cycles

Scientists have identified four phases of hair growth, the:

1. anagen or growth phase, which lasts between two and eight years
2. catagen or transition phase, when growth slows down, lasting around two weeks
3. telogen or resting phase, when there is no growth at all. This usually lasts two to three months
4. exogen or shedding phase, when the hair falls out and is replaced by the new hair growing from the same follicle. This starts the process all over again.

Each follicle goes through this cycle 10–30 times in its lifespan.

If all of our hair follicles grew at the same rate and entered the same phases simultaneously, there would be times when we would all be bald. That doesn’t usually happen: at any given time, only one in ten hairs is in the resting phase.

While we lose about 100–150 hairs daily, the average person has 100,000 hairs on their head, so we barely notice this natural shedding.

So what affects the speed of growth?

Genetics is the most significant factor. While hair growth rates vary between individuals, they tend to be consistent among family members.

Nails are also influenced by genetics, as siblings, especially identical twins, tend to have similar nail growth rates.

But there are also other influences.

Age makes a difference to hair and nail growth, even in healthy people. Younger people generally have faster growth rates because of the slowing metabolism and cell division that comes with ageing.

Hormonal changes can have an impact. Pregnancy often accelerates hair and nail growth rates, while menopause and high levels of the stress hormone cortisol can slow growth rates.

Nutrition also changes hair and nail strength and growth rate. While hair and nails are made mostly of keratin, they also contain water, fats and various minerals. As hair and nails keep growing, these minerals need to be replaced.

That’s why a balanced diet that includes sufficient nutrients to support your hair and nails is essential for maintaining their health.

Nutrient deficiencies may contribute to hair loss and nail breakage by disrupting their growth cycle or weakening their structure. Iron and zinc deficiencies, for example, have both been linked to hair loss and brittle nails.

This may explain why thick hair and strong, well-groomed nails have long been associated with perception of good health and high status.

However, not all perceptions are true.

No, hair and nails don’t grow after death

A persistent myth that may relate to the legends of vampires is that hair and nails continue to grow after we die.

In reality, they only appear to do so. As the body dehydrates after death, the skin shrinks, making hair and nails seem longer.

Morticians are well aware of this phenomenon and some inject tissue filler into the deceased’s fingertips to minimise this effect.

So, it seems that living or dead, there is no escape from the never-ending task of caring for our hair and nails.

Michelle Moscova, Adjunct Associate Professor, Anatomy, UNSW Sydney

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