Imagine if your GP could predict your future by pricking your thumb. A routine check-up would tell you what diseases you’ll get in your lifetime and how to avoid them. You might even be told to eat strawberries to stave off wrinkles – and all from a simple blood test.

Since the completion of the Human Genome Project in 2003, which mapped our genes, a host of genetic tests have sprung up offering to predict everything from your risk of developing breast cancer to the likelihood of you losing your hair.

Now a number of companies are offering nutrigenetic tests, which look at your genes to calculate your risk of developing diet-related diseases such as cancer and type 2 diabetes. They then offer bespoke nutritional advice. But can we control our health by controlling our genes? It all sounds too good to be true – and as many of us don’t have the first idea about genetics, how do we know we’re not being bamboozled by impressive-sounding theories? Can it really be as simple as eating more celery to change your gene activity and lessen the risk of disease?

‘We are made up of proteins and our genes make our proteins,’ says Tim Spector, Professor of Genetic Epidemiology at King’s College London and author of Identically Different: Why You Can Change Your Genes

‘Although we can’t change the structure of genes it is now possible to change the way our genes work – like with new anti-cancer drugs. Chemicals produced by our body or diet can switch our genes on and off, like a dimmer switch on a light, to produce more or less of these proteins.’ Spector says food, exercise, stress and our emotions can all switch genes on or off.

‘If a pregnant woman takes folic acid to prevent birth defects, it’s also one of the strongest vitamins that switches off hundreds of genes that may have both good and bad effects on her and her baby in ways we don’t yet understand,’ he says. ‘This not only affects us but these altered genes can be passed down to the next two generations. The lives we’re leading now are influenced by our grandparents’ lives and the life we lead could influence  our grandchildren.’ 

The logical next step would seem to be genetic testing so we can find out how to tailor our lifestyle to suit our genes. But how reliable are such tests? According to  Spector, tests that claim to match diets, health plans and disease are based on pseudoscience.

‘This jargon is probably very persuasive for a non-scientist but is very misleading,’ he says. ‘The old idea that genes were our unalterable destiny is no longer true. We have made great genetic advances recently – but predicting an individual’s health risk for common diseases from single DNA tests is not one of them. Identical twins are like clones who have the same exact genes and yet they usually suffer from and die of different diseases. There’s a built-in randomness to humans that makes us all individual, which means for now we can’t rely on DNA tests alone.’ 

Dr Paula Saukko, reader in Social Science and Medicine at Loughborough University, has conducted research into the marketing claims of nutrigenetic companies. ‘We studied companies that claim that if you have a particular genotype then you should eat a specific diet’ she says. ‘Overall, we found nutrigenetic tests promise more than they can deliver. They often overplay the association between a particular gene variant and its ability to cause disease.’ 

As for looking after your health, Saukko says most tests give generic advice about avoiding smoking and fatty foods – but this nothing new. ‘The recommendation to eat garlic and soy for cardiovascular disease is common sense and so is eating fruit and vegetables and avoiding pesticides’ she says. ‘Companies also often don’t state that the predictive powers of many of these genes are modest or a work in progress. Although these tests might provide an insight, they’re just not at the advanced stages yet.’ 

Will we ever reach that stage? ‘I believe that, in the future, tests showing how our genes are modified by diet and lifestyle will become routine’ says Spector. But it looks like we still have some way to go.