For the first time, the U.S. government is urging people to avoid "highly processed" foods, which it says are driving diet-related diseases. But this recommendation puts many Americans in a predicament. Studies show many people want to reduce the amount of ultra-processed food in their diets, but they have trouble figuring out which foods fall into that category.
"I think advertising is really good at making people think foods are minimally processed when they're actually ultra-processed," says Alexandra DiFeliceantonio, who studies the neuroscience of food selection at Virginia Tech.
Ultra-processed foods are industrially manufactured products that contain ingredients rarely found in your kitchen, such as preservatives, artificial sweeteners, colorings, natural flavors and emulsifiers. Numerous studies have shown that these foods increase the risk of a host of health problems, including diabetes, heart disease, depression, and obesity.
"When people ask me about ultra-processed foods, they're often most confused about grains, carbohydrates, and starches," says Dr. Dariush Mozaffarian, who leads the Food is Medicine Institute at Tufts University. These foods include breads, crackers, pretzels, pea snaps, veggie straws, pastas and puffed rice or corn. "People want to know how to choose more healthful versions of these products," he says.
So Mozaffarian gives his patients two practical rules of thumb to follow when selecting grains and starches: the 10 to 1 test and the water test.
1. The 10 to 1 test
"A food should have at least one gram of fiber for every 10 grams of carbohydrate," Mozaffarian says. For example, if you're looking to buy a granola bar, examine the nutrient label. If there are 30 grams of total carbohydrate in the bar, then there should be at least three grams of fiber in it, too. If not, choose a different bar.
This test, he says, ensures foods aren't packed with only refined flours and sugars. "So there's a balance between refined starches, whole grains, bran, seeds and other other healthful ingredients," Mozaffarian explains.
And he says, the food should also pass "the water test."
2. The water test
Simply take the starchy food — say a chunk of bread, a cracker, pretzel or cereal — and put it in a glass of water. Let it sit in the water for three or four hours and see what happens.
Look specifically to see if the grain or starch dissolves or falls apart in the water, he says.
Minimally processed grains, such as whole wheat breads and steel cut oats, have the plant's cell wall still intact, which surrounds the chains of carbohydrates and forms a type of shield or barrier around them. The plant cell wall protects the carbs from dissolving in water.
So if the carbohydrate doesn't dissolve in the water, then it's likely a minimally processed food, Mozaffarian says. And it's a healthy choice, because, he says, the cell wall does something else important: it makes the grain hard to digest.
After you eat a carbohydrate, enzymes in your mouth and stomach break down the starch into simple sugars, which then enter your blood stream. In a way, Mozaffarian says, the water test models the process in your digestive tract.
Your enzymes can't work on carbs when they're protected by the cell wall. So you digest minimally processed grains much more slowly than ultra-processed ones. This slow digestion is good for you, Mozaffarain says. "It doesn't overwhelm your liver and the hormones that deal with metabolism." And in the long run, it reduces your risk of weight gain and diabetes.
This slow digestion also means the carbohydrate travels further into your gut, where it can feed the microbes in your large intestine, called the microbiome. You need a healthy microbiome to thrive.
On the other hand, ultra-processed grains and starches don't make it very far in your gut because of the way they're manufactured, says Dr. Meroë B. Morse, assistant professor at MD Anderson Cancer Center. Companies, in effect, predigest the grains, corn or potato, and in the process, they remove the plant's cell wall. "The grain or starch is ground down to its individual ingredients and then re-packaged and glued together," she says.
So the enzymes in your gut quickly break down the carbohydrates into simple sugars.
"These foods digest really quickly in your stomach, and can create a glucose spike," Morse explains. "And when you have a glucose spike, insulin levels tend to rise." Over time these spikes can contribute to insulin resistance and eventually diabetes, she says.
So when you're choosing grains, starches and other carbs, you want ones that will hold together both in your gut — and in a glass of water.
That's where the water test comes in.
A simple experiment
A few weeks ago, my 10-year-old daughter and I baked a loaf of whole wheat bread in our kitchen. And we decided to run it through the water test.
For comparison, we bought a French baguette from the grocery store, which contained preservatives, dextrose, wheat gluten, as well as "dough conditioner" and "crumb softener."
We took a chunk of both breads, put them into two glasses of water and waited roughly three hours. Then we examined each chunk.
The homemade whole wheat bread had absorbed some water, but it had stayed intact, and there were no signs of the starch dissolving into the water. The water remained clear. Ding, ding, ding! Our homemade whole wheat bread passed the water test with flying colors.
But the French baguette had transformed remarkably. "Oh, wow!" my daughter exclaimed, as she pulled the bread from the glass of water. "It's like a sponge, or slime, or Play-Doh!"
The baguette had absorbed an enormous amount of water and almost turned into a kitchen sponge, which you could wring out and use again. Plus, the water in the cup was cloudy and white because the starch had started to dissolve into it.
Bzzzt! The baguette failed the water test. That result confirmed that this bread is ultra-processed.
But the water test did something else for my daughter. It helped her to understand — and to see firsthand — how ultra-processed breads may look similar to homemade, but they act quite differently inside our bodies.
"It's kind of gross," she said, as she squeezed the chunk of ultraprocessed bread like a sponge. "Bread shouldn't be like that."
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