Can a DNA test really tell you what you should and should not be eating?

The idea that genes play a role in our propensity to gain or shed the pounds is appealing. Photo / 123RF

Rightly or wrongly, I have always found carbs to be my friends. I frequently consume my own bodyweight in pasta at about 9pm after work, and when making sandwiches, no fewer than four slices will do. So when I took a DNA test to learn what kind of diet best suited my genetic make-up, the results came as a blow. I'd expected to be told I should follow a Mediterranean regime because, well, shouldn't everyone? Instead I was advised to switch to a low carb plan. It was sufficiently sobering to make me pause, my sandwich halfway to my lips.

This revelation about where I was erring gastronomically came courtesy of DNAfit, a genetic testing company with the catchline: "One test. A lifetime of change." The firm recently teamed up with YO! Sushi to offer a personalised eating plan tailored to one's DNA - calculated via a test tube of saliva, from which a large amount of data can be extracted. For instance, my results told me I am not lactose intolerant (which I already knew); that I have a normal sensitivity to alcohol (debatable, and probably depends who you ask); that I have a raised sensitivity to salt (might this explain my crisps addiction?) and a slow sensitivity to caffeine (surprising, given the speed with which I come up on coffee). A low carb diet plan was recommended because, apparently, I have a high sensitivity to carbohydrates.

"But I'm not obese," I protest to Amy Wells, lead dietician at DNAfit. "And neither is anyone in my family. So what does this actually mean?" "It relates to your insulin response," she explains. "So with your high carb sensitivity, you will see a higher insulin response to a high carb intake. In the long run, it increases your risk of type 2 diabetes."

This is not the news I've been hoping for, but since no-one in my family has ever been known to ration carbs, nor battle this illness, I remain sceptical about just how concerned I should be. "We're looking at environment in conjunction with genetics, so it's about the way your genes interact with the environment," says Wells. "If you're eating a high carb plan but the bulk is sugar or refined carbs, your risk would be higher than if you're eating a lot of starch. How physically active you are will also play a part." And the test is non-diagnostic, she stresses. "It's guiding you on what lifestyle adjustments to make."

The idea that genes play a role in our propensity to gain or shed the pounds is certainly appealing, and with new advances in genetic testing, growing numbers of companies are chasing the dream of 'nutrigenomics' - personalised nutrition plans which offer advice on foods to eat and those to avoid according to your DNA profile. Home testing kits can retail for as little as £59 (NZ$114). All the user has to do is take a swab from inside their inner cheek and send it to a lab to be analysed, completing a basic questionnaire.

But can such tests really deliver that holy grail - a diet that works? Another analysis, from MyDNA at Lifelab Testing, tells me I have a gene that helps me store fats for future energy needs, which is "great in times of famine. Not ideal for your beach bod goals." According to my report, people with my result have noticed they have a higher body mass index. Mine has always been low. The report does add that "if you're trim...your lifestyle is counteracting your genetics." As a sedentary worker with a serious pasta habit, I'm really not sure that it is. Further down the report, I learn I have another genetic variation that has no association with a higher BMI. Do the two therefore cancel each other out?

A sample of DNA can be taken at home via a swab of the inner cheek. Photo / 123RF

This month, a major new study raised questions over DNA diet plans. Research on 1,100 people by King's College London, and Massachusetts General Hospital and Stanford University in the US, found that genes only partly explain the fact that different people have different responses to the same foods. By looking at identical twins, who share the same genetic make-up, the researchers were able to see the limitations of the influence of genes in how we metabolise what we eat. It turns out that environmental factors, such as sleep, stress, exercise, the times at which we eat and the state of our gut microbiome, may all be just as important in determining how the body responds to food.

The findings came as a surprise, even to the scientists. "What we found by capitalising on the data from twins is less than 50 per cent of our glucose response is due to genetic factors, less than 30 per cent for insulin and less than 20 per cent for our triglyceride, or fat responses. So the large variation [in how people respond to food] is only partly explained by genetics," says Dr Sarah Berry, a senior lecturer in nutritional sciences at Kings who was involved with the study. "This is exciting because it shows it's not all in our genes. In fact, only a tiny bit is, so it's mostly modifiable. It's empowering, because how you're going to respond is not predefined."

In other words, today's early DNA diet plans don't hold the secret of easy weight loss. "Given that genetic factors play a relatively minor role in determining our responses to food, we need to be very cautious on putting too much weight on it," says Dr Berry.

That's not to say that personalised nutrition has no future. The research supports the idea that a one size fits all approach to diet doesn't work - but a DNA test may only provide part of the answer. The ultimate goal of those involved in her project is to create a more comprehensive home-based test to enable everyone to understand their own nutritional responses, integrating microbiome and other data. It is hoped this will become universally available from next year.

In the meantime, is it worth pursuing a DNA-based approach to diet? Dr Frances Elmslie, a consultant clinical geneticist at St George's University Hospitals NHS Foundation Trust, sounds a note of caution. "The science...is not powerful enough to predict independently of all the other risk factors [something like obesity or diabetes]," she says.

Some GPs are also sceptical. "I see it as this whole commodification of what's so essential about being human," says Dr Luke Kane, a London GP. "All you need to do to live a happy, healthy life is eat normal food, eat lots of vegetables and not too much unhealthy food. For something that's so complicated as the food that suits you or what kind of exercise you should do, no scientist or doctor would say there's [currently] enough information to give accurate advice.

"There's no quick fix or magic bullet. You just have to eat normal food that's not processed. We all know that, but we've lost sight of it."

Dr Keith Grimaldi, chief scientific officer at DNAfit, says the KCL study does highlight the need for dietary personalisation, however. "[Genetics] is an important part of the puzzle within the interaction between nature and nurture," he says.

"That's why we don't offer a 'DNA diet', but instead offer tools to help people personalise their exercise and nutrition choices. Crucially, we provide free dietician consultations alongside our reports, as well as access to coaching which educates customers on how to apply results to their lifestyles.

"The more pieces of the overall dietary puzzle we have, the better we can guide our customers to personalised actions. Based on the initial summary published from the Predict 1 [KCL] study, along with previous research, our genetic profile fits into this picture."

But Dr Berry is wary of fuelling an obsession with what we put in our mouths, and says "the whole denial thing" can be a problem. "Food is there to be enjoyed," she says. "It's a pleasure, a social event. I don't think you should deny yourself anything."