That doesn’t mean omega-3s aren’t important. “They’re an essential nutrient and there’s good evidence supplements can benefit heart health, particularly in people who eat very little fish,” says Manson. “Heart disease and stroke are closely related to cognitive decline so what’s good for the heart is usually good for the brain. But the benefits are not guaranteed.”

Scientists believe omega-3s may be more effective for brain health when consumed as part of a Mediterranean-style diet which is naturally rich in omega-3s from fish, nuts and dark green leafy vegetables.

The bottom line: Focus on getting omega-3s from your diet rather than relying on a supplement. “I recommend eating two or more servings of oily fish a week as part of a Mediterranean-style diet rich in vegetables, fruit, whole grains, nuts and healthy fats,” says Manson. “Dietary supplements should never be a substitute for a healthy diet.” If you don’t eat fish, or have very little, she recommends a daily supplement containing 1g of marine omega-3s, ideally with a ratio close to 50:50 of DHA.

Vitamin B12Vital if you’re deficient

If you’re struggling with brain fog or poor memory, one of the first things to check is your vitamin B12 level.

“Vitamin B12 deficiency is one of the few, potentially reversible causes of cognitive decline,” says Manson. “Treating a deficiency with supplements can improve symptoms.”

Our body needs vitamin B12 to keep nerve cells healthy and make neurotransmitters, the chemical messengers that allow brain cells to communicate. Chronic deficiency can lead to memory problems, depression and, eventually, brain atrophy.

B12 is found only in animal products or fortified foods, meaning vegetarians and vegans are at greater risk of deficiency. Our ability to absorb it also declines as we get older. According to NHS figures, around one in 20 people aged 65 to 74 and one in 10 people aged 75 and older have B12 or folate deficiency.

Yet, despite the link between low levels of B12 and cognitive decline, clinical studies, including those involving Alzheimer’s patients, haven’t found taking B12 supplements improves cognition in people whose levels are already adequate.

“Some people genuinely need extra B12,” says Manson. “That includes those with pernicious anaemia or who are taking medications such as metformin that reduce absorption. Otherwise, I wouldn’t routinely recommend high-dose vitamin B12 supplements.”

The bottom line: Don’t take B12 in the hope it will sharpen your memory. If you’re suffering symptoms or are at risk of deficiency, ask your GP about a blood test. “For most healthy people, a multivitamin provides sufficient B12,” says Manson. “A healthy diet including meat, fish, eggs, dairy and fortified foods should cover your daily needs. If you’re vegan or eat a largely plant-based diet, consider a B12 supplement, and discuss the dose with your GP.”

Vitamin DOnly if you’ve got low levels

One in six people in the UK have low levels of vitamin D – often called the sunshine vitamin because our bodies make it when sunlight hits the skin.

Not just important for healthy bones, vitamin D also protects our brain health. Low levels have been linked to poor memory and thinking skills and studies suggest deficiency may increase the risk of dementia.

A large trial of more than 1,600 people, in Neurology, found those with moderate vitamin D deficiency were more than 50 per cent more likely to develop Alzheimer’s or dementia. For those with severe deficiency, the risk was 125 per cent higher.

However, that doesn’t mean everyone benefits from taking a high-dose supplement. In the five-year VITAL trial of more than 25,000 people, Dr Manson and colleagues found taking 2,000 IU of vitamin D3 a day failed to significantly slow cognitive decline in adults whose vitamin D levels were already adequate.

“We saw benefits only in those who started out with lower levels,” she says. “That tells us the issue is correcting a deficiency rather than benefits from giving everyone extra vitamin D.”

The bottom line: The Government recommends UK adults take a daily 10mcg (400IU) vitamin D supplement during autumn and winter. “People at high risk of deficiency, such as those with osteoporosis, fat malabsorption or who get little sunlight exposure, may benefit from 1000-2000 IU a day,” says Manson. “For everyone else, the evidence for brain health benefits is much less convincing.”

GlucosamineBe cautious

Recent research has raised fresh questions about glucosamine, one of the most popular supplements for joint pain.

A large study from the University of Florida found people with mild cognitive impairment who took glucosamine were 28 per cent more likely to develop Alzheimer’s disease. Among those who already had Alzheimer’s, glucosamine users were 27 per cent more likely to die during the follow-up period.

While the exact reason isn’t yet clear, researchers believe glucosamine, an amino sugar, may disrupt glucose metabolism in the brain.

“While it’s important not to overinterpret a single study, the research raises legitimate concerns,” says Manson. “Given there’s never been consistently strong evidence that glucosamine improves joint problems, I wouldn’t recommend taking it routinely, particularly in people with existing cognitive impairment.”

The bottom line: If you’re taking glucosamine, discuss it with your doctor, advises Dr Manson. “If you’re not seeing any benefit for your joints, consider whether you need to continue it,” she says. “If you have cognitive impairment, it’s probably best to forgo it, unless under the supervision of your doctor.”

Don’t waste your money on these

Many popular brain supplements simply haven’t been tested well enough to justify recommending them, says Dr Manson.

Ginkgo biloba

This herbal remedy is promoted as a brain booster, with claims that it can improve blood flow to the brain and reduce oxidative stress. However, the evidence has proved disappointing. In a six-year trial of more than 3,000 older adults, taking 240mg of ginkgo a day failed to reduce the risk of dementia or cognitive decline. “Ginkgo has produced mixed and inconsistent results and it can increase bleeding risk, particularly in people taking aspirin or anticoagulant medication,” says Manson. “It’s not recommended by most health organisations.”

Curcumin

Curcumin, the active ingredient in turmeric, is prized for its anti-inflammatory and antioxidant properties. Some believe it may protect against Alzheimer’s, but human trials have been small and inconclusive. “I don’t think there’s compelling scientific evidence it slows cognitive decline,” says Manson.

Vitamin E

Once thought to protect against Alzheimer’s because of its antioxidant effects, vitamin E has not shown consistent benefits for preventing cognitive decline or dementia in clinical trials. High doses may also increase bleeding risk and interact with some medicines. “I wouldn’t recommend taking vitamin E specifically for brain health,” says Manson.

Brain-health blends

Pharmacy shelves are packed with products claiming to boost brain power. Yet few have been tested in rigorous clinical trials. “Research on these products is extremely limited,” says Manson. “In general, I wouldn’t recommend them over a standard multivitamin. Protect your wallet by avoiding expensive supplements that haven’t been properly tested.”

What really works

Ultimately, protecting your brain comes down to the fundamentals. “Dietary supplements can never replace a healthy lifestyle,” says Manson. “Eating a healthy diet, staying physically, socially and mentally active and controlling cardiovascular risk factors have far stronger evidence behind them than any supplement.”

“If someone asked me whether to spend £30 a month on supplements or on improving their lifestyle, I’d recommend investing in healthy habits every time,” she says. “Regular physical activity is the real magic bullet for your brain.”