摘要：Keeping your brain in good shape will not only stave off mental decline, but can also improve your relationships and boost your well-being – and it's never too late to make a difference

ONE sultry afternoon in 1862 in Luxor in Egypt, Edwin Smith was haggling with an antiquities dealer for an unknown papyrus. Though he suspected its importance, Smith couldn’t know it would turn out to be not just the earliest known medical text, at over 4000 years old, but the first ever documented mention of the brain. And what did it say about the most complex entity in the known universe? That it was “cranial offal”, to be unceremoniously trashed during embalming.

We have learned rather a lot about the brain since then. Even so, it is only in the past 25 years that learning how best to look after the stuff upstairs has become a major priority for researchers. It is easy to be resigned to the idea that as we get older, our brains wind down, memories decline and reactions slow. But a wealth of new research shows that it is never too late to improve our brain health – a concept that goes way beyond the absence of disease.

A long view of how, across some 2 million years, evolution has shaped the function of our brains is revealing new and unexpected ways to keep them healthy for longer.

In 2018, an international group of specialists forming the Global Council on Brain Healthidentified a surprisingly simple test to assess whether your brain is in good shape: whether you function well in daily life. This may even sound overly simplistic, but the group, for which I am a special adviser, found that the brain requires three vital functions to work together seamlessly: executive function, or our ability to think and reason; social cognition, which enables us to interact successfully with others; and emotion regulation, through which we generate our sense of well-being.

It is hard to overstate just how much the lifestyle choices we make matter in keeping these functions working well, both singly and together. A landmark study published in Nature in 2012 indicated that three-quarters of change in cognitive ability across our lives – as measured by evaluations of general intelligence – is determined by lifestyle factors and only 25 per cent by DNA. As our brains age, a range of physical processes take place, including shrinkage of regions associated with memory, perception, learning and attention. We now know that these processes begin to ramp up in our 20s and 30s, but there is some evidence that the underlying mechanisms of ageing start in the brain even in our earliest years.

The good news is, just as it is never too early to adopt habits that help slow – or even reverse – ageing, it is never too late either. A 2019 study, for instance, found that the adult human brain can produce new neurons until we are into our 90s. However, there is no silver bullet, no quick-fix brain game or easy diet to boost brain health. It is the cumulative effect of the little things we do every day that makes the difference. So, what are these little things? And which are the most important?

1. Go with your gut

The mismatch between the rapid changes that have shaped modern lifestyles and the slow, cumulative influence of evolution is perhaps best seen in our emerging understanding of the microbes in our gut. The 2019 discovery of 6000-year-old fossilised spittle in Denmark showed that, over time, we have lost millions of friendly bacterial species from our digestive systems. In recent years, countless studies have drawn connections between our gut bacteria and anxiety, depression, mood and even our thinking and behaviour. Now we are beginning to understand what forges these connections. Consider, for instance, that some 90 per cent of serotonin, a mood stabiliser, is produced in the gut and less than 10 per cent in the brain. Moreover, our gut bacteria are now firm suspects in many illnesses, including ALS, Alzheimer’s and Parkinson’s disease. For good brain health, we have to look after the contents of our colon.

That takes time. After birth, it takes three years to produce a stable community of gut microbes, and this involves ingesting a wide range of bacteria. Once your microflora is stabilised, interference with this hard-won balance can affect gut biochemistry. Based on analysis of more than 1000 human stool samples and detailed clinical questionnaires, one recent study identified 69 lifestyle factors that harm our microbiome. They include high BMI, lack of exercise, erratic eating habits, stress, dehydration, poor dental hygiene, jet lag and constantly changing intimate relationships.

What does the gut care about your love life? It turns out that a 10-second kiss can transfer as many as 80 million bacteria – some of which are likely to trigger an immune response such as damaging inflammation. In settled relationships, partners develop an equilibrium of gut bacteria over time.

To promote stable gut health, it also helps to feed your friendly bacteria by eating a wide array of plant-based foods. In the past decade or so, there has been a surge in research about potential ways of replenishing healthy gut bacteria – such as through faecal transplants or even “poop pills” – with the aim of treating conditions ranging from irritable bowel syndrome and obesity to autoimmune disorders. Most recently, researchers have started looking into how we might do this to improve brain health, by using our gut bacteria to help boost production of crucial hormones or neurotransmitters.

2. Watch what you eat

Modern life, which can involve long working hours, high levels of stress and hundreds of distractions, often means that we are constantly eating and snacking. But our brains evolved at a time when food was periodically scarce. For hunters back then, the shift from our bodies burning sugar in the form of glucose to burning energy released from our fat cells, known as ketosis, was inevitable. Today, most of us never reach this stage, unless we deliberately fast or adopt a low-carb diet.

This metabolic switching could play a key role in helping to create new brain cells, or neurogenesis. Animal studies have shown a clear relationship between intermittent fasting and levels of brain-derived neurotrophic factor, a protein critical to neurogenesis, particularly in the hippocampus and other centres of learning and memory. Many researchers are now studying whether intermittent fasting can help slow the pace of ageing.

What we consume matters too (see “Go with your gut”). Today, 75 per cent of the world’s food is produced from only 12 plant and five animal species. This woefully homogeneous diet is in contrast to the wide array of nutrients we relied on throughout most of our evolution. In Western countries in particular, there’s no better example than our consumption of omega fatty acids. In our hunter-gatherer food, the ratio of omega-6 to omega-3 fats was about 1:1. The ratio is now 20:1, largely because of processed foods that contain vegetable oil. That’s a problem because some omega-6 fatty acids may promote excess inflammation, which can limit the creation of new brain cells and accelerate the death of existing ones. By contrast, many long-term studies show that omega-3 protects the brain. The interplay of the two is now being studied, but for brain health a good ratio to aim for seems to be 1:4 omega-6 to omega-3, which can be achieved by consuming a wide variety of oily fish, or foods like spinach or flax seeds.

Many other nutrients are important to brain health, but vitamin D is essential. Receptors for the vitamin are found widely in the brain and low levels of it are associated with poor mental performance and cognitive decline. To get enough vitamin D, we can try “sunbathing the brain”. But for most populations living above latitude 35 degrees north or below 35 degrees south – which includes all of the UK, most of New Zealand and North America from around Washington DC upwards – it is virtually impossible to generate enough via sunlight. Unfortunately, there’s no conclusive evidence that vitamin D supplements slow cognitive decline. The best dietary sources are fatty fish, eggs, butter, liver and fortified cereals.

A rule of thumb is that combining a greater variety of food and nutrient types is the best way to avoid deficiencies. That means opting for a mostly plant-based diet, as well as healthy fats, some dairy and a little fish and red meat, especially if you are over the age of 65.

3. Get moving

If there was a silver bullet to keep our brains young, it would be exercise – it slows down age-related changes and even reverses them. If we look at the five areas in the world identified as having the most centenarians, physical activity is a way of life. In Ikaria, Greece, Loma Linda, California, Nicoya, Costa Rica, Okinawa, Japan and Sardinia, Italy, dementia and cognitive decline are 75 per cent less frequent than throughout most of the Western world. The overwhelming evidence is that aerobic exercise has beneficial effects on the brain, including improved mood and thinking skills. Like intermittent fasting, exercise reduces inflammation, which can inhibit the growth of new brain cells. Exercise actually increases neurogenesis via the release of the critical protein mentioned earlier, brain-derived neurotrophic factor (BDNF).

So how much exercise should we do – and what kind? To raise your BDNF levels, you need at least 30 minutes of daily exercise, things like brisk walking or cycling. If you really want to max out your BDNF, you must push that up to vigorous exercise, things like jogging or high-intensity workouts.

However, the sobering news is that even if we exercise daily, prolonged sedentary behaviour may wipe out the benefits. As many as 13 per cent of Alzheimer’s disease cases globally are the result of inactivity. So the advice is simple: avoid sitting or adopt a more active sitting position and stand wherever and whenever possible. At the very least, get up out of your chair for 10 minutes in every hour.

4. Keep in touch

We are supremely social animals. It’s no wonder the past year of lockdowns and social distancing has been so difficult. And the research couldn’t be clearer. Both social isolation and loneliness are devastating to our health. People who are lonely are 50 per cent more at risk of dying prematurely than those who aren’t. In a vicious cycle, loneliness both triggers negative health and social behaviours and is also a consequence of them. Loneliness and isolation increase the risk for systemic inflammation, high blood pressure, diabetes, obesity, heart disease, heart attack and stroke, which all affect the brain either directly or through knock-on effects of blood vessel damage or reduced blood flow, for instance.

But the good news is, the opposite is also true. A wealth of research has shown that social contact decreases risk for this broad range of conditions, and that it can directly benefit the brain by improving memory formation and recall, and protecting against neurodegenerative diseases. It has also been shown that social engagement helps to maintain thinking skills throughout life, possibly by altering stress responses which lead to changes in gene expression.

Even a little social connection goes a long way. Passing interactions with shopkeepers, neighbours or fellow travellers can alleviate loneliness. Joining activities that generate a sense of belonging are also especially beneficial.

5. Learn a new skill

Brain-teasers, crosswords and computer games engage cognitive skills, including processing speed, working memory and reasoning. But the resulting benefits don’t seem to influence everyday mental abilities, slow down cognitive decline or reduce the risk of dementia.

But that doesn’t mean there aren’t things we can do to help keep our minds sharp. There is clear evidence that engaging in activities that psychologists call “cognitively stimulating” – meaning they require concentration and repeated practice – do make a difference to our brain health.

The benefits of dancing and learning a new language have been particularly well studied, but a wide range of activities have shown gains, including learning to play a musical instrument or a new card game, or mastering complex new mental and physical skills, like tai chi or juggling. One recent study of 174 people between the ages of 60 and 79 compared people who pursued activities such as dance, walking or light toning exercise over six months. Only the dancers saw structural improvement in a brain region involved in transmitting signals to and from the hippocampus – the brain’s memory hub.

Again, there’s evidence that activities that require us to learn something novel stimulate the growth of new brain cells, prevent neuronal cell death and improve neuroplasticity, the brain’s ability to adapt and forge new connections. All of this helps to reduce the risk of cognitive decline and dementia.

6. Stay in rhythm

Our bodies are a humming jumble of rhythms, the control of which has been embedded in the brain over millions of years. Body temperature, blood pressure, metabolism and more all ebb and flow according to age-old patterns. But these days, those rhythms are being disrupted and the consequences for our health can be profound.

There is no more intractable health problem in modern life than sleeplessness. Insomnia, difficulty sleeping and sleep disorders are widespread. According to the US Centers for Disease Control and Prevention, at least one-third of people in the US get less than the recommended 7 hours of sleep each night. This chronic lack of sleep not only harms our general health, it also adversely affects learning, memory, attention, decision-making and mood. It is even a risk factor for dementia and cognitive decline.

Despite what you might have heard, it simply isn’t true that we need less sleep as we get older. That myth is born of the fact that, due to shifting circadian rhythms and other factors, as we get older, it gets harder to fall asleep. We also become lighter sleepers and get less sleep in a single session. That’s a problem because people over 60 still require 7 to 9 hours of sleep in a 24-hour period – though some of this can be met by napping.

There are many things we can do to help us sleep better, but in essence they amount to trying to keep a routine bedtime, avoiding caffeine late in the day and practising good sleep hygiene – sleeping in a dark, quiet room. If we don’t, disruptive patterns – such as jet lag, constantly varying bedtimes, late-night work and irregular habits of all kinds – will conspire to blunt the brain across a lifetime. It has been demonstrated time and again that people who routinely break their circadian rhythms are at raised risk for neurodegenerative and psychiatric disorders. Recent studies suggest this is because it throws our body’s many clocks out of sync, undermines the production of critical neurotransmitters and can even affect the way our brain cells process energy.

The odd eccentricity won’t do any permanent damage to your brain performance. But constant abuse will.

7. Do what makes you happy

Ranked equally with life and liberty, the pursuit of happiness has driven great thinkers for centuries. But it’s more than philosophy. There’s now real evidence that emotional well-being is critical to our brain health. Many of the tens of thousands of decisions we make each day are about seeking positive experiences and avoiding negative ones – a constant search for that feeling of well-being.

But how do we attain this magical state? The evidence shows that maintaining social relationships, staying active and having a sense of purpose in life all contribute to mental well-being. It has been found to reduce inflammation and biological markers of stress, and both improves cognitive function and reduces cognitive decline in later life. Finding a sense of purpose if you are struggling can be a challenge, but there are certainly steps you can take. The Global Council on Brain Health recommends we develop personal and work-related goals to cultivate a sense of purpose. This can be looking after friends or family, having an absorbing and demanding hobby or pastime, or striving for career goals.

There are other ways we can improve our sense of well-being too. It has also been shown that people who are better able to control negative thoughts and embrace positive thinking tend to have improved executive function, general brain health and longevity.

One of the keys to achieving this balance is managing stress. Many stress-reducing activities benefit brain health, including yoga, meditation, tai chi, art, music and the moderate consumption of alcohol. The message is that in seeking and achieving well-being, we have a huge amount of control over how we order our lives to make them more enjoyable, less stressful and more productive. In doing so, not only will we feel better, we will think better too.

Chew it over

Nearly two dozen studies worldwide point to an association between regular gum chewing and improvements in thinking and memory. The precise mechanisms aren’t fully understood, but one possible explanation is that chewing can increase blood flow, which in turn improves oxygen levels in the brain – and gum chewers benefit because they simply chew more.

Gum chewing may also exercise the part of the brain that controls voluntary muscle movement. One study out last year found that grey matter volume in this area is better developed in people with better “masticatory performance”.

Sex on the brain

What we know about sex and the brain is, in scientific terms, in its infancy. But it is dynamite. In animal studies, rewarding sexual encounters have been found to stimulate neurogenesis in the hippocampus, a centre for learning and memory. Moreover, in older rats, sexual activity rejuvenates the brain, elevating neurogenesis to levels seen in younger rats.

But what of humans? Not only will lifelong, regular, rewarding sexual activity improve feelings of well-being, but habitual sexual activity, especially with an emotionally close partner, confers benefits in high-level thinking skills, including memory and recall, mathematical performance, spatial awareness and verbal fluency.