Like the first smattering of grey hairs and the involuntarily "ooft" you mutter while getting out of a chair, forgetfulness has long been seen as an inevitable part of the midlife process.
But according to James Goodwin, a professor of physiology at Loughborough University and former chief scientist at Age UK, the scientific dogma that our brain cells decline with age, resulting in those moments where you can't remember what you went upstairs for, is one now being challenged by experts.
"Science now shows us that not only are we able to protect our brains from the ageing process, but we can continue to grow new brain cells throughout our entire life," he tells me.
According to Goodwin you can do all this with a few easy lifestyle tweaks, which he explains in his new book, Supercharge Your Brain. In it he explains that while the brain is our most vital organ, few of us understand how to keep it healthy, in the way we know how to anti-age our bones or hearts.
But the sooner you start the better. "Midlife is a great time to start thinking about brain health. Somebody in their 40s is at a much better starting point than somebody older. But it's never too late. There's always time to get a better brain."
1. Watch less TV – and move every day
What's the real standout in terms of brain anti-ageing, I ask Goodwin. "Exercise, which people dread hearing," he smiles.
Physical activity starts a process in the brain called neurogenesis – the growth of new cells. "A protein is released in muscles which flows to the brain to stimulate a growth factor called BDNF (brain-derived neurotropic factor). BDNF maintains existing brain cells, stimulates the growth of new ones, and promotes the formation of connections between them." All of which leads to what he calls "brain youthfulness".
Studies show exercise reduces our risk of mild cognitive decline through to Alzheimer's.
Thirty minutes of moderate to intensive movement every day is enough to reduce the risk and improve cognitive function and mood, and this can include walking the dog or with a friend.
However, regular exercise doesn't counter the effects of what Goodwin calls our "love affair with the chair". Prolonged sitting causes inflammation in the body which is "ageing for the brain," he says. This love affair started during the industrial revolution: "We've slowed down considerably. We sit down to eat, drive, watch TV and work, and lockdown has made this worse."
He suggests an "anti-sitting plan": "Walk rather than drive, watch less TV. Every 30 minutes I leave my desk and walk around, making coffee or talking to my dog."
Your four-week plan to get in shape for the end of lockdown
2. Talk to people as much as possible
Human brains are hard wired to socialise; Goodwin says social isolation is as bad for your brain as smoking or drinking too much alcohol. Research by the late American professor John T. Cacioppo found isolation even decreases the effectiveness of sleep, which leads to brain wear and tear.
Any social activity helps. "I've been phoning friends in lockdown, I email them, I use social media," he says. "When I walk the dog, I say hello to passers-by. A study on 'social prescribing' from Essex university found these small things can reduce the risks [of loneliness]."
There is a technology tipping point, however. "Loneliness is higher among young people with high social media use and among gamers too. So use technology to stay in touch with friends, but don't replace them with it."
3. Vary your diet and follow the 3/6 rule
Diet is hugely important for brain health – in particular, a varied one. "75 per cent of the processed foods we typically eat are based on five animals and 12 products," points out Goodwin. "Humans used to kill and eat pretty much anything and the diversity they enjoyed was immense – different types of meat and fish, nuts, seeds, fruits, berries, vegetables, roots. Now we are prisoners of our supermarket food rut and eat the same things on rotation."
The problem with eating this way is it deprives the brain of certain nutrients. "Vitamin B12 is only really found in animals or Marmite," says Goodwin, who recommends having a teaspoon of the latter each day.
Vitamin D is also limited in food sources and research shows there are receptors for vitamin D in parts of the brain responsible for learning and memory. A study from Cambridge University found people deficient in vitamin D performed worse in mental tests. The body makes it from sunlight but supplementation is advised during late autumn and winter.
The other one to watch is omega 3 and 6 fatty acids. The former is anti-inflammatory (and keeps the brain sharp), while the latter is inflammatory. "In hunter gatherer times we ate a good ratio of 3 and 6, but now we eat too much omega 6," says Goodwin, who explains that's because processed food is based on vegetable oils which is high in omega 6.
The solution? Eat more omega 3 with grass (not grain) fed beef, and cold water fatty fish like sardine and salmon. "Or my childhood staple – a teaspoon of cod liver oil every day," says Goodwin.
4. Practise hara hachi bu
All the research shows overeating is bad news for brains, with one US study finding that eating more than the recommended calories a day from mid-life onwards doubles your risk of memory loss later in life (the NHS recommends 2000 for women and 2500 for men).
"Overeating fills the brain with free radicals," says Goodwin. "There are five areas of the world where people live longer and have the sharpest minds and the thing they have in common is they don't overeat."
One of these places is Okinawa, a remote Japanese island, where a popular mantra is hara hachi bu, which translates as "Eat until you're 80 per cent full".
"Here in Britain we do the opposite," says Goodwin. "Paradoxically this is due to our hunter gatherer ancestors, who would feast when food was plentiful, and famine when it wasn't.
"This brings me to intermittent fasting, which is very good for brain health. I don't eat after 8pm or before 10am, which means I fast for 14 hours. By then, my brain has switched from using glucose to ketos for its energy and evidence shows this is good for brain performance."
Lastly, look after the bugs in your gut: "The trillions of bacteria in our guts affects our mood and thinking skills. Nourish it with whole, fibrous foods. This is why I eat a whole apple, core included, because that's where all the fibre is. I slice it horizontally and eat the entire fruit.
"This also keeps my teeth clean. A 2014 study found gingivitis, an inflammation of the gums, was linked to Alzheimer's. There's also conclusive evidence chewing gum improves memory by 35 per cent, and reduces the risk of Alzheimer's."
5. Have regular, loving, sex
In news, he says, that makes people sit up at dinner parties and take notice, Goodwin found frequent sex is very anti-ageing for a midlife brain, which releases the hormones dopamine, oxytocin and serotonin during orgasm. "During experiments with rats, news brain cells were produced when they were having regular sex."
In human trials looking at sexual activity among people aged over 50, those having regular sex had better memories – but the people who did really well were the ones in close relationships, who were found to also have improved verbal fluency, reasoning and mathematical skills. "Close, loving, relationship sex is good for the brain because of the neurogenesis effect, but also the social connection and bonding experience," he says.
6. Learn a new skill
Goodwin says while there's a widespread belief sudoku and brain games keep the brain young, there's little evidence they actually do. "What there is evidence for is that things like dancing, learning a language, playing a new card game or learning to juggle helps. Anything that involves concentration of effort over a period of time."
Goodwin says the complexity and brilliance of our brains is superior to anything created by artificial intelligence. "We talk about the wonders of A1 but our brains combine cold hard logic while still being driven by emotion and heart. When a computer learns to fall in love, juggle, or understand the depths of Shakespeare's poetry, I'll believe it's as good as our brains. That's why we've got to look after them."
Professor Goodwin is a director of the Brain Health Network