My big brother Joe has a famously bad memory. At a relatively youthful 38, he often forgets conversations I have had with him only a few days before. Regularly, he regales me with stories and anecdotes that I find very amusing, but only the first time round. A couple of months ago, I was recounting a tale of my own to my brother, when he gleefully reminded me that I had already told him the same story earlier that day. I have to say, alarm bells were a'ringing. In my mind, what seemed to be starting as a spot of absent-minded repetition was bound to end up as full-blown dementia down the track.

No one relishes the thought of losing their mental faculties as they age, but there is good evidence that we can do much to stop the rot. Plenty of research suggests that our diet has an important influence on brain function. Other studies show that exercise and natural substances might also be of benefit. It does appear that just a few simple lifestyle adjustments can really help us preserve our memory and brainpower well into old age.

The brain is to a degree dependent on the supply of certain nutrients for its proper function. Brain tissue is rich in healthy fats known as long-chain polyunsaturated fatty acids (PUFAs). In recent years, a lot of scientific attention has been focused on the role of PUFAs such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in brain function. More than one study has found that individuals with lower levels of EPA and DHA are at increased risk of mental impairment, such as dementia and even Alzheimer's disease. EPA and DHA are found in abundance in oily fish (eg tuna, salmon, trout, mackerel, herring and sardines), and research suggests that eating more of this can help to maintain brain function. In a Dutch study, men who ate a lot of fish were found to have half the rate of mental decline compared to occasional consumers. It really does seem as though fish is the original brain food.

The mental decline that can come with ageing is thought to be related to damaging molecules known as free radicals. They are quenched by antioxidant substances such as betacarotene and vitamins C and E. There is evidence that upping our intake of antioxidant nutrients might help maintain our mental sharpness. One study found that individuals consuming 2.1mg (3,500 IU) of betacarotene per day were half as likely to suffer cognitive impairment, disorientation or have difficulty solving problems compared to those taking 0.9mg (1,500 IU) or less per day. Another study found that higher levels of vitamin C in the body were associated with improvement in mood and intelligence and a reduction in everyday errors of memory and attention. Citrus fruits, kiwis, strawberries, broccoli and Brussels sprouts are rich in vitamin C, while deep-green and yellow-orange vegetables (eg carrots, spinach and sweet potato) are good sources of betacarotene.

Another link between nutrition and brain function concerns homocysteine. Raised levels of this amino acid have previously been linked to chronic diseases such as heart disease and osteoporosis. However, Welsh research presented last year found a direct relationship between homocysteine levels and cognitive decline, too. Folic acid, vitamin B12 and vitamin B6 have all been found to help reduce homocysteine levels in the body. While it is not known for sure whether lowering homocysteine levels will help brain function, keeping homocysteine in check does appear to make good sense. Folic acid can be found in green leafy vegetables, while liver, avocado, bananas and fish are rich in B6. Vitamin B12-containing foods include meat, fish and eggs.

Exercise also appears to impact on brain function. Research published last year in the Archives of Neurology found that regular vigorous exercise is associated with a significantly reduced risk of Alzheimer's disease, dementia and other forms of mental impairment. An earlier study published in the Journal of Internal Medicine found that walking as little as half a mile a day improved memory and reduced signs of dementia in the elderly.

For those inclined towards supplements, additional quantities of the nutrients mentioned above may offer long-term benefit. I generally recommend 5,000 to 10,000 IU of betacarotene, 1g of vitamin C and a B-complex supplement each day. Anyone looking for a little more in the way of mental edge might do well to take a supplement of Ginkgo biloba. This herb enhances the circulation and is believed to help improve delivery of oxygen and nutrients to the brain. In one study, individuals taking 120mg of Ginkgo biloba each day for a year were found to have improved short-term memory, vigilance and mood. Other studies show that, at a dose of 120 to 160mg per day, Ginkgo biloba can improve impaired memory and brain function related to ageing. Supplements can be found in good health-food stores.

Nutrition news

It is often assumed that calcium supplements can cause kidney stones. The most common form of kidney stone is made of calcium and a substance called oxalate. Because most kidney stones contain calcium, many doctors advise against taking calcium supplements, as this should, theoretically at least, increase risk of stone formation. Actually, research shows that the greater the amount of calcium in the diet, the lower the risk of kidney stones. How so?

It appears that calcium can bind to oxalate in the gut, thereby preventing its absorption into the body. As a result, levels of oxalate decline in the urine, and this seems to be a critical factor in preventing kidney stones. Calcium supplements seem to be beneficial for individuals with a history of kidney stones, but they should be taken with meals, not on an empty stomach.

Dear John...

I am prone to cold sores during the winter months. Do you know of any natural approaches that may help?
Tim Gregory, Swindon

A Cold sores are caused by the herpes simplex virus (HSV). Once it has infected the body, the virus lies dormant, but may reactivate and cause a cold sore at any time, especially when the immune system is weak or rundown, which usually happens during stress or illness. Cold sores typically last for seven to 10 days, though there is a lot that can be done to prevent or resolve attacks.

The HSV virus needs the amino acid arginine in order to multiply in the body. Arginine is found in high concentration in nuts (especially peanuts and cashews) and chocolate, and it would make sense for you to avoid these foods, especially at the first sign of an attack.

While arginine encourages growth of the HSV, another amino acid, lysine, inhibits it. Some studies have found that taking lysine in supplement form can reduce the frequency and severity of cold-sore attacks. I suggest you take 1g of lysine per day, and increase this to 1g three times a day during an acute attack. To this regime I would also add some vitamin C, as this nutrient has been shown to inactivate the HSV in the test tube. Take 1g of vitamin C a day, increasing to three times daily during an attack.

Finally, for topical relief I recommend vitamin E. Take a soft gelatine vitamin E capsule and soak a small piece of tissue with its contents. Apply this to the cold sore for a full 15 minutes. Do this twice during the day. Quite often, this approach can help cure a cold sore within a day or two.

· If you have any questions you would like Dr John Briffa to answer, or issues you would like him to address in this column, please email him at john.briffa@observer.co.uk or write to him c/o Life magazine, The Observer , 119 Farringdon Road, London EC1R 3ER. Please note that Dr Briffa cannot enter into personal correspondence.