Resistant starch: Can you make the carbs you eat a little healthier?

Starchy carbohydrates are often viewed as dietary villains, but one type known as resistant starch has been getting a lot of attention as a possible ally for better blood sugar control and a healthier microbiome. Found in grains, root vegetables, and legumes, it has the health-boosting properties of a prebiotic fiber, with a lower impact on blood sugar than a typical starch. Even more intriguing: a simple cooking technique can boost the amount of resistant starch in potatoes, pasta, rice, oatmeal, and other starchy foods you eat. Here's what you need to know.

What is resistant starch?

To understand resistant starch benefits, it helps to know a little chemistry first. Starches are made of long chains of glucose (sugar) molecules, linked together by chemical bonds that your digestive enzymes break down into glucose. For "regular" starches, this takes place mostly in the small intestine, where glucose is quickly absorbed into the bloodstream, causing a swift rise in blood sugar.

But true to its name, resistant starch resists digestion in the small intestine - and "travels mostly intact to the colon," says Nancy Oliveira, a registered dietitian and manager of the Nutrition and Wellness Service at Harvard-affiliated Brigham and Women's Hospital. "That means the glucose tends to be absorbed more steadily, and blood sugar doesn't spike as high," she explains. Some research suggests increasing intake of resistant starch may lower blood sugar and insulin levels, especially in people living with diabetes or obesity.

While in the colon (large intestine), resistant starch also becomes food for the beneficial microorganisms in your microbiome (the population of microbes that inhabits your gut). In turn, these "good" bacteria produce butyrate and other short-chain fatty acids - anti-inflammatory compounds that help nourish the cells lining the gut and may also help regulate blood sugar and bolster immunity, among other health benefits.

Many of us fall short on resistant starch - and overall fiber intake

While there's no official dietary recommendation for resistant starch, many experts believe that Americans' intake - estimated in a 2020 study to be about 4 grams daily for a 2000-calorie diet - is probably low. The study's authors suggested 15 grams as an adequate daily intake goal. Resistant starch contributes to your daily fiber intake, which most people need to boost anyway, so choosing foods that naturally contain some resistant starch can help bring you closer to your daily fiber goals. Starchy foods tend to contain more resistant starch when they're unripe or uncooked; for example, green bananas have about a third more resistant starch than yellow ones (see "Food sources of resistant starch").

For context, the USDA recommends a daily goal of 25 grams of fiber for women and 38 grams for men before age 50 (21 and 30 grams, respectively, for women and men 50 and older). But most Americans only get between 10 and 15 grams per day.

A cooking trick to boost resistant starch

Another quirk about resistant starch: When you cook a starchy food, heat causes its starch granules to swell and become easier to digest. But if the cooked food is allowed to rest and chill (think potato or pasta salad, or overnight oats), the starch molecules reorganize into tighter structures through a process called retrogradation - in effect, converting some of the starch to resistant starch. The results vary widely, depending on the food and how it's cooked, chilled, and stored. For example, chilling a cooked russet potato could increase its resistant starch by 39%, compared with only an 18% increase from chilling a cooked red potato (see "Food sources of resistant starch)."

If you want to increase your intake of resistant starch, is it worth giving the cook-and-cool technique a try? Oliveira sees no harm, "as long as you're already making an effort to eat well and are committed to a healthy lifestyle." But she cautions against thinking of it as a green light to load up on potatoes, rice, or pasta. "That's the 'what can I get away with?' mentality we want to avoid."

Instead, she recommends, think of the cooking method as one tool among many to help manage your blood sugar. "The overall effect of this interesting hack is probably modest at best," she says. "It's much more important to focus on your whole eating pattern rather than a single food, and try to get a variety of plant foods daily." That ensures you'll get a full range of fiber types that all help to control your blood sugar and feed your microbiome - as well as the health-boosting phytonutrients that come in plant foods.

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