We are all unique. Put a thousand people on the same weight-loss diet, and you’ll see a range of outcomes. Some people will lose a lot, some will lose a little, and a few unlucky folks will even gain weight.

Science still can’t tell us why some people lose weight more easily than others. Maybe we each have our own ideal diet, and we just have to find it. Maybe the people who gain weight on the Mediterranean diet just need to give keto a try, and vice versa. In truth, few diets are successful long-term, but we all keep looking: “Maybe I just need to find the diet that’s right for me.”

Perhaps the key to who’s who lies in our DNA. Scientists are constantly finding links between genetics and nutrition. For example, Inuit people from Greenland have genes that may help their bodies process their culture’s traditional high-fat diet. Meanwhile, many of us have a variant of a gene called FTO that makes us more likely to be overweight. Studies have found, however, that diet and exercise can lessen the FTO gene’s effect on body weight.

The only problem with this approach: Scientists who study the genetics of nutrition think it’s premature to base nutritional advice on your DNA. But the sales pitch is just so tempting.

We live in the future, so of course you can get a genetic test to tell you which variant of the FTO gene you happen to have. And why stop at just one gene? Companies like Habit, Arivale, DNAFit, and even the calorie-counting app Lose It package genetic insights with coaching about what to eat. Costs range from $20 into the hundreds, plus a one-time fee to read your DNA, which is typically $80 or more.

The only problem with this approach: Scientists who study the genetics of nutrition think it’s premature to base nutritional advice on your DNA. That FTO gene, for example, has only been shown to make a few pounds’ difference in body weight.

But the sales pitch is just so tempting. “The one thing we’ve always gotten wrong is we’ve always taken a one-size-fits-all approach to food,” Habit founder Neil Grimmer laments in his TEDx talk. (Habit uses DNA results together with bloodwork to give nutrition advice and sell meal plans.) “I believe in 2017 it’s going to be the era of you. The era of personalization. The era where we’re going to look inside of ourselves to answer the question, ‘What foods are right for us?’”

But there are limits to what we can learn by looking inside ourselves. So far, the tools that allow us to examine our DNA, and the studies that help us interpret it, aren’t precise enough to say exactly what is going on.

How Nutritional Genetic Testing Works

The coded messages of our DNA are billions of letters (nucleotides) long, but personalized nutrition companies only care about a few of those letters—dozens to hundreds, depending on the product. You’ll find out which “variant” you have at each of those locations along your DNA strands. The locations themselves are known as single nucleotide polymorphisms, or SNPs (pronounced “snips”).

SNPs are a handy, well-studied shortcut for genetic variations. Scientists who are curious about the FTO gene, for example, have often looked at a single location on chromosome 16 known by the code name rs9939609. For any area of DNA a scientist might want to study, there is probably a known SNP in or near that area.

We have two copies of most of our chromosomes, one from each parent, so your genotype at a given SNP consists of a two-letter code representing the nucleotides on each chromosome. At rs9939609, I have one A and one T.

There are two main ways genetic testing companies can learn what SNP variants you have, and both start with you sending a vial of spit to a lab. If you send it to AncestryDNA or 23andMe, they’ll use a microchip-like device to test for hundreds of thousands of SNPs at a time. If you send it to Helix, the partner lab for many of the newer weight-loss services, they will actually read millions of letters from large swaths of your DNA, but keep the full data set private. Then you can authorize a partner company—say, DNAFit—to ask Helix for some of your SNPs.

Many of the companies that provide DNA-based dieting advice give you both options: You could authorize them to look at your Helix data, or you could upload a file from a genotyping service like 23andMe. Helix charges $80 for their one-time sequencing fee. 23andMe and Ancestry both currently offer genotyping, with the ability to download your data file, for $99.

I began researching this article when Lose It offered to let me try their new genetic testing feature for free, including covering the cost of Helix’s $80 sequencing fee. The service, dubbed EmbodyDNA, currently sells for $109. Later, I bought my own analysis from Arivale—currently $19—because I was curious to see how the two compared.

I’m listing current prices because all of these companies seem to be tinkering with their pricing, which changes from week to week. On a lucky day, you might even get your sequencing fee waived too.

The Results You’ll Get

“We’ve done a lot of interviews about embodyDNA, and a lot of the reporters were predisposed to some interesting [results],” Kevin McCoy of Lose It said to me during our interview. I had sent him screenshots of my results before the call. “And I think you are—you’re definitely more just predisposed to normal levels.” In other words, my results were boring.

EmbodyDNA is the name of Lose It’s genetic testing product. I can view my results in a special screen on the app, separate from the calorie-counting functions. There are 19 results in total, with five relating specifically to weight loss. Among the others, I’ve learned that I’m likely to have normal levels of vitamin D and iron, and that I’m not likely to be lactose intolerant. My DNA says I’m probably a supertaster, making me sensitive to bitter flavors, and that I’m a “slow” caffeine metabolizer, which means that I get to enjoy the effects of a strong cup of coffee for hours after I drink it. As far as I know, that’s all true.

Only two of my results felt surprising enough to be potentially useful. One said I was likely to lose weight with exercise. So is everybody, perhaps. But typically, exercise plays a very small role in weight loss for most people. “Your genes give you an extra edge in meeting your weight loss goals,” the app told me. It also said that 57 percent of the world’s population shares this trait, putting me in a slight majority.

The other surprising result was in the fitness section: “Your genes suggest you have more slow-twitch muscle fibers,” the screen said, “which can help sustain you in activities like distance running for a longer period of time without tiring.” According to this result, it sounds like I should opt for endurance activities over sprinting.

The problem is I know that conclusion is wrong. I get terrible results in long-distance races, and generally hate every minute of them. But I love to sprint, and my times are always surprisingly good for a person of my fitness level. There’s no doubt in my mind that I’m a sprinter.

So I looked up the science behind this. My result was based on just one SNP, rs1815739. A 2003 study in the American Journal of Human Genetics found that my variant was rare in elite Australian athletes specializing in power sports, but more common in their endurance-focused athletic counterparts.

There was nothing in the study to suggest that this gene matters for average folks, and certainly no reason to believe a recreational athlete like myself should switch sports because of it. Plenty of factors, biological and otherwise, contribute to athletic performance. Muscle fiber type may be one of them, but there’s also muscle size and shape, heart and lung function, psychology (what if you hate certain sports?), and much more. A 2014 review on the genetics of sports ability concluded that “few genes are consistently associated with elite athletic performance, and none are linked strongly enough to warrant their use in predicting athletic success.”

Recommended Stories