Forget Sudoku, crossword puzzles and computer games. If you really want to train your brain, then eat dark chocolate, have plenty of sex and follow the Scandinavian example of having cold meat for breakfast.

The growing numbers of people who are trying to strengthen their mental ability through 'brain training' should also avoid cannabis, watching soap operas, hanging out with serial complainers or pursuing fat-free diets, according to a new book on getting 'brain-fit'.

Many of the suggestions in Teach Yourself Training Your Brain are surprising, such as cuddling a baby, cheating at school, reading out loud and doing your university degree in business studies. Co-authors Terry Horne and Simon Wootton say their recommendations are based on and backed by the latest research by leading experts around the world.

'For decades we have thought that the cognitive capacity of our brains is genetically determined, whereas it's now clear that it's a lifestyle choice. What we eat and drink, how we learn at school and what type of moods we have are all crucial,' said Horne, a business lecturer at the University of Central Lancaster and an authority on thinking and learning.

'People can make lifestyle choices that will not only prevent what used to be seen as an inevitable decline in cognitive [ability] after the age of 17 - something that IQ tests seemed to bear out - but will constantly increase our cognitive capacity throughout our adult lives. Our far-reaching suggestions will empower people to develop their cognitive capacity or just let it decline.'

The book details the most up-to-date thinking in science about how diet, the environment, stress and other aspects of modern life affect our mental capacity and distils them in the form of chunks of advice as well as exercises.

It also points out that of the seven different chemical reactions that the brain undergoes in the sexual process, from contemplating intercourse to achieving orgasm, four help the thinking process. The ensuing increase in levels of the 'trust' hormone, oxytocin, for example, increases readiness to think of novel or risky solutions.

Similarly, the post-coital rise in serotonin levels aids both creative thinking and calm, logical decision-making.

The authors praise the plethora of useful elements in dark chocolate, such as magnesium and antioxidant flavonols, which benefit everything from the supply of oxygen to the brain to the reduction of the chances of brain damage through a stroke.

The book comes as more and more Britons are spending increasing sums on a fast-expanding market in books, DVDs, subscription websites and computer games which claim to supercharge people's memory to improve their performance at work, enhance their brain's ability to store information and ward off the effects of dementia.

Nintendo's Brain Age electronic brain training game, which includes memory, number and intelligence tests, has sold 10 million copies globally. In the United States, some insurance companies have begun giving older clients computer software programmes featuring such exercises to encourage them to stay as mentally sharp as possible. And some nursing homes there have set up 'brain gyms' for the same purpose.

Horne believes that many Britons need to make dramatic changes to their lifestyles to increase their cognitive capacity because of the switch to a 'knowledge economy' from one based on manual labour, and because of the fast-expanding number of over-65s. Happiness, confidence and an optimistic nature help the brain, he added.

'So mix with people who make you laugh, have a good sense of humour or who share the same interests as you, and avoid people who whinge, whine and complain, as people who are negative will make you depressed, which means you won't achieve things,' said Horne.

The book recommends that, instead of trying to find perfect happiness, people should instead seek a new concept the authors have called BLISS, which involves Body-based pleasure, Laughter, Involvement, Satisfaction and Sex, all of which enable the brain to think well.

Most people use only three to four per cent of their total supply of brain cells. But research among undergraduates has shown that those following the book's advice can experience a big enough increase in cognitive fitness to make the difference between getting a top job and a merely average one, said Horne.

Is this the recipe for a sharp mind?

Cheating

Children should not do homework on their own. Their minds will function better if they have help from parents or work with classmates.

Eating the right food

Have eggs, fish or cold meat at breakfast. Stick to protein-based foods at lunchtime such as oily fish with dark green vegetables.

Avoid bread, pasta or pizza and drink tea, ideally green or herbal, not coffee. Snack on nuts, not biscuits or other sweet things. Ideally, eat carbohydrates in the evenings only. Avoid caffeine, alcohol and red meat.

Speaking aloud

The old practice of getting pupils to speak in front of their class helps the brain because of the repetition involved. Adults can boost their memory by counting aloud to 99 in threes as fast as they can.