Knitted food by Jessica Dance

When a Double-Chocolate Brownie is Better for You Than Quinoa

A $299 microbiome test from DayTwo turns up some counterintuitive dietary advice.

Why do certain diets work well for some people but not others? Although several genetic tests try to answer that question and might help you craft ideal nutrition plans, your DNA reveals only part of the picture. A new generation of tests from DayTwo and Viome offer diet advice based on a more complete view: they look at your microbiome, the invisible world of bacteria that help you metabolize food, and, unlike your DNA, change constantly throughout your life.

These bugs are involved in the synthesis of vitamins and other compounds in food, and they even play a role in the digestion of gluten. Artificial sweeteners may not contain calories, but they do modify the bacteria in your gut, which may explain why some people continue to gain weight on diet soda. Everyone’s microbiome is different.

So how well do these new tests work?

Basic microbiome tests, long available from uBiome, American Gut, Thryve, and others, based on older “16S” sequencing, can tell you at a high level which kinds of microbes are present. It’s cheap and easy, but 16S can see only broad categories, the bacterial equivalent of, say, canines versus felines. But just as your life might depend on knowing the difference between a wolf and a Chihuahua, your body’s reaction to food often depends on distinctions that can be known only at the species level. The difference between a “good” microbe and a pathogen can be a single DNA base pair.

New tests use more precise “metagenomic” sequencing that can make those distinctions. And by combining that information with extensive research about how those species interact with particular foods, machine-learning algorithms can recommend what you should eat. (Disclosure: I am a former “citizen scientist in residence” at uBiome. But I have no current relationship with any of these companies; I’m just an enthusiast about the microbiome.)

I recently tested myself with DayTwo ($299) to see what it would recommend for me, and I was pleased that the advice was not always the standard “eat more vegetables” that you’ll get from other products claiming to help you eat healthily. DayTwo’s advice is much more specific and often refreshingly counterintuitive. It’s based on more than five years of highly cited research at Israel’s Weizmann Institute, showing, for example, that while people on average respond similarly to white bread versus whole grain sourdough bread, the differences between individuals can be huge: what’s good for one specific person may be bad for another.

In my case, whole grain breads all rate C-. French toast with challah bread: A.

The DayTwo test was pretty straightforward: you collect what comes out of your, ahem, gut, which involves mailing a sample from your time on the toilet. Unlike the other tests, which can analyze the DNA found in just a tiny swab from a stain on a piece of toilet paper, DayTwo requires more like a tablespoon. The extra amount is needed for DayTwo’s more comprehensive metagenomics sequencing.

Since you can get a microbiome test from other companies for under $100, does the additional metagenomic information from DayTwo justify its much higher price? Generally, I found the answer is yes.

About two months after I sent my sample, my iPhone lit up with my results in a handy app that gave me a personalized rating for most common foods, graded from A+ to C-. In my case, whole grain breads all rate C-. Slightly better are pasta and oatmeal, each ranked C+. Even “healthy” quinoa — a favorite of gluten-free diets — was a mere B-. Why? DayTwo’s algorithm can’t say precisely, but among the hundreds of thousands of gut microbe and meal combinations it was trained on, it finds that my microbiome doesn’t work well with these grains. They make my blood sugar rise too high.

So what kinds of bread are good for me? How about a butter croissant (B+) or cheese ravioli (A-)? The ultimate bread winner for me: French toast with challah bread (A). These recommendations are very different from the one-size-fits-all advice from the U.S. Department of Agriculture or the American Diabetes Association.

DayTwo’s app shows which bacteria (first and third screenshots) and nutritional factors (second and fourth screenshots) contribute to a food’s grade.

I was also pleased to learn that a Starbucks double chocolate brownie is an A- for me, while a 100-calorie pack of Snyder’s of Hanover pretzels gets a C-. That might go against general diet advice, but an algorithm determined that the thousands of bacterial species inside me tend to metabolize fatty foods in a way that results in healthier blood sugar levels than what I get from high-carb foods. Of course, that’s advice just for me; your mileage may vary.

Although the research behind DayTwo has been well-reviewed for more than five years, the app is new to the U.S., so the built-in food suggestions often seem skewed toward Middle Eastern eaters, perhaps the Israeli subjects who formed the original research cohort. That might explain why the app’s suggestions for me include lamb souvlaki with yogurt garlic dip for dinner (A+) and lamb kabob and a side of lentils (A) for lunch. They sound delicious, but to many American ears they might not have the ring of “pork ribs” or “ribeye steak,” which have the same A+ rating. Incidentally, DayTwo recently completed a $12 million financing round from, among others, Mayo Clinic, which announced it would be validating the research in the U.S., so I expect the menu to expand with more familiar fare.

Fortunately you’re not limited to the built-in menu choices. The app includes a “build a meal” function that lets you enter combinations of foods from a large database that includes packaged items from Trader Joe’s and Whole Foods.

There is much more to the product, such as a graphical rendering of where my microbiome fits on the spectrum of the rest of the population that eats a particular food. Since the microbiome changes constantly, this will help me see what is different when I do a retest and when I try Viome and other tests.

I’ve had my DayTwo results for only a few weeks, so it’s too soon to know what happens if I take the app’s advice over the long term. Thankfully I’m in good health and reasonably fit, but for now I’ll be eating more strawberries (A+) and blackberries (A-), and fewer apples (B-) and bananas (C+). And overall I’m looking forward to a future where each of us will insist on personalized nutritional information. We all have unique microbiomes, and an app like DayTwo lets us finally eat that way too.

Richard Sprague is a technology executive and quantified-self enthusiast who has worked at Apple, Microsoft, and other tech companies. He is now the U.S. CEO of an AI healthcare startup, Airdoc.