‘Healthy’ Foods May Be Better for Some People Than Others


People with diabetes have long known that the same food can affect different people’s blood sugar differently. Now, for the first time, there’s scientific proof: Scientists report that a food’s effect on blood sugar depends not just on the food, but on the person eating it.

Blood glucose, commonly known as blood sugar, may be best known for its association with diabetes, but it’s important for everyone to understand. Glucose travels through your bloodstream, delivering energy to every part of your body. Every time you eat, your blood sugar jumps up. Scientists call this jump a postprandial glycemic response (PPGR).

There are two popular methods for predicting how a given food will affect a person’s PPGR: the number of carbs in a food and the glycemic index. Both strategies assume that a food creates the same PPGR response no matter how it’s eaten—or who eats it.

That assumption may be flawed, says a team of researchers from Israel’s Weizmann Institute. Their new report, published last week in the journal Cell, argues that universal guidelines may actually be encouraging people to eat foods that make their blood sugar worse.

“Ascribing a single PPGR to each food … assumes that the response is solely an intrinsic property of the food,” study co-author Eran Segal told The Atlantic. “But there are very striking differences between people's responses to identical meals.” 

Segal and his colleagues recruited 800 healthy volunteers and gave them questionnaires on their eating habits and medical histories. The volunteers provided stool samples so the researchers could check out their gut bacteria. For one week, they tracked their meals and sleep using a mobile app, while a continuous glucose monitor measured their blood sugar. Everyone ate the same breakfast, but beyond that, what they ate was completely up to them. 

Studying people’s eating habits can be tricky, especially when the data is self-reported. People tend to slack or fudge the numbers when tracking their food. That wasn’t a problem for this experiment, Segal told The Atlantic. These volunteers were motivated: “They joined because we explained that we'd be able to tell them which of the foods they normally eat spike their glucose levels. They came because they wanted to know and we said that if they didn't log properly, we wouldn't be able to tell them.”

The results were dramatic, and completely unique to each volunteer. Foods that caused PPRG spikes in one person had little to no effect on another. The data showed that what and how much you eat matters, of course, but just how it matters varies immensely.

These results weren’t limited to high-carb junk food. One middle-aged woman was working hard to stick to a healthy diet that included lots of vegetables, including tomatoes. But data from her glucose monitor showed that her blood sugar spiked each time she ate tomatoes. The good-for-you produce wasn’t good for her at all.

The researchers’ next step was to turn their results into an algorithm. They recruited a fresh round of volunteers and provided each one with two customized meal plans: one “good” and one “bad.” Half of the meal plans came from nutrition experts, and the other half were generated by the algorithm.

Sure enough, the volunteers’ PPRGs improved during the “good” week—even though each person was eating something different. Even their gut bacteria changed for the better. This was true of both the man-made meal plans and those suggested by the computer; in fact, the algorithm's customized recommendations were slightly more effective than those made by the experts.

The researchers hope their results will inspire a new approach to nutrition and weight management. Co-author Eran Elinav said in a press release that the study "really enlightened us on how inaccurate we all were about one of the most basic concepts of our existence, which is how we eat and how we integrate nutrition into our daily life."

Our scientific and cultural approaches to obesity and diabetes may just be “really conceptually wrong,” he said. Scientists and medical professionals believe “we know how to treat these conditions, and it's just that people are not listening and are eating out of control," Segal said, "but maybe people are actually compliant and in many cases we were giving them the wrong advice."

Other researchers believe it may be too early to draw such strong conclusions, and note that Elinav, Segal, and their colleagues never directly compared their results to the glycemic index.

Still, these findings are making waves. The team won’t have any trouble finding volunteers for their next experiment; the wait list currently includes more than 4000 people.

Researchers Pore Over the Physics Behind the Layered Latte

The layered latte isn't the most widely known espresso drink on coffee-shop menus, but it is a scientific curiosity. Instead of a traditional latte, where steamed milk is poured into a shot (or several) of espresso, the layered latte is made by pouring the espresso into a glass of hot milk. The result is an Instagram-friendly drink that features a gradient of milky coffee colors from pure white on the bottom to dark brown on the top. The effect is odd enough that Princeton University researchers decided to explore the fluid dynamics that make it happen, as The New York Times reports.

In a new study in Nature Communications, Princeton engineering professor Howard Stone and his team explore just what creates the distinct horizontal layers pattern of layered latte. To find out, they injected warm, dyed water into a tank filled with warm salt water, mimicking the process of pouring low-density espresso into higher-density steamed milk.

Four different images of a latte forming layers over time
Xue et al., Nature Communications (2017)

According to the study, the layered look of the latte forms over the course of minutes, and can last for "tens of minutes, or even several hours" if the drink isn't stirred. When the espresso-like dyed water was injected into the salt brine, the downward jet of the dyed water floated up to the top of the tank, because the buoyant force of the low-density liquid encountering the higher-density brine forced it upward. The layers become more visible when the hot drink cools down.

The New York Times explains it succinctly:

When the liquids try to mix, layered patterns form as gradients in temperature cause a portion of the liquid to heat up, become lighter and rise, while another, denser portion sinks. This gives rise to convection cells that trap mixtures of similar densities within layers.

This structure can withstand gentle movement, such as a light stirring or sipping, and can stay stable for as long as a day or more. The layers don't disappear until the liquids cool down to room temperature.

But before you go trying to experiment with layering your own lattes, know that it can be trickier than the study—which refers to the process as "haphazardly pouring espresso into a glass of warm milk"—makes it sound. You may need to experiment several times with the speed and height of your pour and the ratio of espresso to milk before you get the look just right.

[h/t The New York Times]

Kirill Ignatyev, Flickr // CC BY-NC 2.0
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Why Some Cold Cuts Make Iridescent Meat Rainbows—and Why They're Still OK to Eat
Kirill Ignatyev, Flickr // CC BY-NC 2.0
Kirill Ignatyev, Flickr // CC BY-NC 2.0

We eat with our eyes first, and sometimes what we see on our plate turns us off a meal altogether. Take so-called “meat rainbows”: They happen when a slice of deli meat takes on an iridescent shimmer reminiscent of an oil puddle in a parking lot—a.k.a. not something you want on your sandwich. Despite giving a whole new meaning to the phrase "mystery meat," the odd discoloration is perfectly safe to eat, as physicist Dave McCowan at the University of Chicago explained for The Takeout.

The colorful sheen on a slice of roast beef or pastrami isn’t a sign of spoilage or chemical additives—it’s actually a result of the way the meat is cut. Slicing meat “against the grain” means cutting through, rather than parallel to, the bundles of fibers composing the meat’s musculature. This makes for a more tender bite, and it also leaves a grid of evenly-spaced meat fibers. In the right light, this surface lends itself to something called “diffraction.”

Diffraction occurs when light hits a repeating pattern of nooks and crannies. As the white light bounces off the grooves in the meat, it separates into a spectrum of distinct colors. Some of these colors are amplified, creating a mother-of-pearl appearance when viewed together. This is the same effect we see on the backs of CDs and DVDs.

Another possible culprit behind your rainbow meat is thin-film interference. This is sometimes present in meat with a thin layer of oily fat on the surface. The film affects the light passing through it in such a way that only some of the colors in the spectrum come through, hence the rainbow. This phenomenon produces a sheen closer to that of bubbles or oil slicks than laser discs.

Why do meat rainbows only seem to show up in deli slices, not raw cuts? The answer lies in the curing process. A cured ham is likely greasier than a raw pork cutlet, which makes thin-film interference more likely. The muscle fibers in cured and cooked meats are also more tightly packed together, producing the rigid grid necessary for diffraction.

Color also plays a role. Iridescent shimmers are easier to spot on darker meats like beef and some pork—so if you’re eating a slice of turkey from the deli, it could be covered in meat rainbows you don’t notice. We’ll let you decide if that’s a positive thing.

[h/t The Takeout]


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