Eating for your genes

DNA tests may pinpoint who should eat what to lose weight—and why we crave certain foods

ZUMA/KEYSTONE

Few physicians have the cachet or the captive audience of Dr. Mehmet Oz. He’s written half a dozen books, and has been on the The Oprah Winfrey Show no less than 55 times. She nicknamed him “America’s doctor,” and every weekday he hosts his own radio and television talk shows. So, last month, when a segment of Dr. Oz was devoted to “exploring the perfect diet for your genes,” the nascent field of “nutrigenomics” was catapulted into pop science stardom.

“It’s totally sexy,” says Christopher Gardner, a Stanford University researcher who co-authored the study featured on Dr. Oz, which had not yet been published in a peer-reviewed journal as of late April. The study suggests that a simple genetic test (a mouth swab) may pinpoint for individuals exactly what kind of diet they should be on—low-fat, low-carb or balanced—to drop the maximum number of pounds. “Lots of people know they should go on a diet. Lots of people are pissed because they went on the same diet that their friend went on and their friend lost more weight than they did,” muses Gardner, who is also a professor at Stanford’s medical school. Hopefully, this study, he says, “will explain part of that.”

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Understanding the relationship between our DNA, dietary preferences and the impact that food has on us is at the core of nutrigenomics. “We study how our individual genetic differences—the variations and the sequence of a particular gene—can explain why some people respond differently than others,” says Ahmed El-Sohemy, the Canada Research Chair in Nutrigenomics, and a professor at the University of Toronto. “Ultimately, the goal is to develop better, personalized dietary advice for the individual. Because we know that the one-size-fits-all model is not the most effective.”

One thing does apply to practically everybody: 95 per cent of people who lose weight on a diet will gain it back within a few years; 41 per cent will get even heavier, according to Oz. Last year, a Nielsen survey found that 44 per cent of Canadians said they were trying to lose weight, mainly by cutting down on fat. At the same time, the Canadian Health Measures Survey showed that the average 45-year-old male is now 20 lb. heavier and has gained seven centimetres around his waist compared to his counterpart in 1981. Given these discouraging statistics, it’s no wonder that the “eat for your genes” ethos has been embraced. Soon after Oz’s show, the Massachusetts company that developed and sells the genetic test used in the Stanford study was inundated with calls from media, doctors and consumers. “There’s been a significant spike in sales,” says Interleukin Genetics CEO Lew Bender. “And it’s continuing.”

Gardner, who says he wasn’t paid by Interleukin, is reluctant to discuss the study until it’s accepted by a journal; he turned down an invitation to be on the Oz show (though his comments were used when preliminary findings were presented at an American Heart Association conference in March). But according to Bender, the study reveals that of the 138 women who were genetically tested, those on a diet matched to their genotype lost about 13 lb., compared to just two pounds among mismatched participants. There were improvements in metabolic factors such as cholesterol, triglyceride levels and waist circumference. As far as Bender is concerned, this study proves that the key to losing weight is reducing the right type of calories from your diet—fat, carbohydrates or protein, known as macronutrients—depending on your genes.

Macronutrients are the building blocks of our daily diet. Today, health authorities recommend that roughly half of the calories an adult consumes in one day come from carbohydrates, a third from fat, and the rest from protein. But as more genes are identified and understood in relation to obesity, it is becoming clear that not everyone processes food in the same way—so one individual may need more or less of a given macronutrient than another person.

Caffeine is a perfect example. A study led by El-Sohemy identified two types of a gene that differently affect how quickly a person processes caffeine. Those individuals with the slow version of the gene who drank two or three cups of coffee a day had a 36 per cent greater chance of heart attack—and four or more cups of coffee upped the risk to 64 per cent. People with the fast version of the gene had a 22 per cent lower risk of heart attack.

What makes some people crave coffee so much more than others? There is mounting evidence that we are genetically predisposed to prefer certain foods—which may explain why an individual, say, loves mashed potatoes but hates turnips. Another study by El-Sohemy looked at the gene GLUT2. People with a particular form of it consume more sugar on a daily basis than others. It seems that this version of the gene may prevent individuals from recognizing changes in their blood sugar levels—and signalling to the brain when to stop eating. Building on this, El-Sohemy is now researching why some people have a strong aversion to foods such as broccoli, even though their parents enjoy it. These broccoli haters, it appears, have inherited two copies of a gene that makes them overly sensitive to bitter taste.

Now-famous research on the herb cilantro also shows that some people may be genetically predisposed to dislike specific foods. Studies by Charles J. Wysocki at the Monell Chemical Senses Center in Philadelphia revealed that the most common complaint about cilantro is that it tastes soapy; fans describe it as fresh and citrusy. Wysocki suspects that people who dislike the herb may be missing a genetic component that would help them detect the chemicals that make it pleasant for others. He’s also found that identical twins (who share the same DNA) feel the same about cilantro, unlike fraternal twins.

This deluge of research on the genetic basis for dietary preferences and their impact on people is exactly as it sounds: exciting, overwhelming and scattered. There is still much more that’s unknown. Each finding raises questions about whether the results will be true for both genders, different ethnicities, and depending on each individual’s medical status. The Stanford study, for instance, included mostly Caucasian women. Interleukin’s Bender acknowledges the limitations of the company’s genetic test for diets. “Clearly we want to do further work with the existing test,” he says, “and create better versions going forward.”

To develop the test, Interleukin sifted through hundreds of genes associated with obesity that have been identified over the years. They whittled those down to 16 genes that had been studied in multiple, independent, large-scale clinical trials. Those were then downsized to five genes whose “biological function” (their code, and role in the body) are known. The Interleukin team referred to the Obesity Gene Map, a compendium of literature pioneered by Louise Pérusse at Université Laval, though he has no association with the company. “We have more evidence that genetics can affect the way we respond to diet,” Pérusse says. “On that principle it makes sense to identify a gene and find out what could be your best diet.” But Pérusse says there is one caveat: “The field is still evolving,” he warns, “So the question is, are these the best genes” to test for?

What’s more, experts say that genetics is only one part of the reason why some people struggle with weight. Gardner says environmental factors have an even bigger impact. While conducting the Stanford study, one woman had the full support of her spouse, but another was sabotaged by hers. One participant’s daughter died. He believes social support and emotional distress factors have an influence on a person’s success at losing weight. He’s heartened by the current food movement that has seen farmers’ markets flourish, and ignited debate about local and sustainable food. “If you go to the school or work cafeteria and all they serve is crap then it doesn’t matter if you know your genotype if all your food choices are poor.”

Like others studying nutrigenomics, Gardner is wary of foiled dieters thinking that “eating for your genes” will be the magic bullet—one that negates the need to cut out junk food or exercise. One woman who watched the Oz show wrote to Gardner pleading for advice. She had failed to lose weight using every diet on the market. “I wrote back, ‘If there are 10 diets available and you tried all 10, this isn’t going to make one of the 10 work now,’ ” says Gardner. “It must mean she’s got other issues. It’s not a perfect solution. It’s not, ‘Oh thank God, the obesity epidemic is over.’ ” Not yet, anyway.

EVEN BIGGER

The Canadian Health Measures Survey compared the physical characteristics and fitness levels of individuals in 1981 to the period between 2007 and 2009. Below, how the average 45-year-old man and woman has changed with time—and it’s not for the better in many ways.

MALE
FEMALE
1981
2007-2009
YEAR
1981
2007-2009
173 cm (5’8′)
175.3 cm (5’9″)
HEIGHT
161.5 cm (5’4”)
162.3 cm (5’4”)

77.4 kg
(171 lb.)

86.6 kg

(191 lb.)

WEIGHT
63.2 kg
68.4 kg

25.7kg/m2
- overweight

27.9 kg/m2
-overweight

BODY MASS

INDEX

24.1 kg/m2
- normal weight

25.8 kg/m
-overweight

90.6 cm (35.7”)
- low risk

97.0 cm (38.2”)
- increased risk

WAIST

76.3 cm (30.0”)
- low risk

83.4 cm (32.8”)
- increased risk

SOURCE: Shields M, Tremblay MS, Laviolette M, Craig CL, Janssen I, Connor Gorber S. Fitness of Canadian adults: results from the 2007-2009 Canadian Health Measures Survey.
Health Reports 2010; 21: 1-15.




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