A new diet book slated to hit stores this summer, "The Carb Lovers Diet," promises that eating carbohydrates is the key to weight loss and long-term health. The so-called secret: Eat not just any old carbs but a certain type of carbohydrate called resistant starch.
FOR THE RECORD:
Carbohydrates: An article in Monday's Health section on nutritional properties of resistant starch misidentified professor Joanne Slavin of the University of Minnesota as Jennifer Slavin. —
The term "resistant starch" refers to starch that doesn't get fully digested in the small intestine, which is where refined carbohydrates, such as white bread and pasta, get broken down into glucose.
Because resistant starch, like fiber, avoids digestion in the small intestine, it travels on to the large intestine, where it feeds the bacteria that reside in the lower gut.
Like fiber, resistant starch also increases stool bulk and has a laxative effect. In fact, because it acts so much like fiber, food scientists classify it as such, says Jennifer Slavin, professor of food science and nutrition at the University of Minnesota in St. Paul.
Studies of human populations have shown that people whose diets are rich in fiber tend to eat less and also tend to weigh less, Slavin says. Still, it's not clear that resistant starch is better than other types of fiber, she adds.
In part, that's because the term resistant starch refers to a large category of carbohydrates with different activity in the body. Some pass through the intestines entirely undigested, whereas others get partially digested in the small intestine and partially broken down in the large intestine.
The purported benefits of resistant starch derive in part from their resistance to digestion, as well as their partial digestion in the lower gut, says Michael Keenan, a professor of human nutrition and food at Louisiana State University and the Louisiana State University Agricultural Center in Baton Rouge. Keenan has studied the effects of resistant starch on lab animals.
When lab rodents were fed a diet that consisted of as much as 25% of a type of resistant starch called RS2, they ended up with less body fat than rodents eating the same number of calories but no resistant starch, Keenan says. In addition to burning more fat, the rodents consuming the starch developed a different set of microflora, or bacteria, in their large intestine.
Keenan says this proliferation of bacteria in the gut could be a predictor of improved long-term health prospects: Human studies have indicated that obese people and people with Crohn's disease or ulcerative colitis have a different set of microflora than lean, healthy people. Still, the precise implications of the rodent experiment for humans remains unknown.
While it's clear that lab rats burn more fat when they eat resistant starch, there's no definitive evidence that humans who consume the starch will weigh less than humans who don't, says Janine Higgins, nutrition research director of the Clinical Translation Research Center at the University of Colorado, Denver.
There are four types of resistant starch. Two are fairly easy to find: One occurs naturally in whole grains and another forms in starchy foods such as white rice, pasta and potatoes after they are cooked and then cooled.
The other two types of resistant starch are ones that are modified in the lab to resist breakdown in the gut and are often sold as powders. The advantage of these, Higgins says, is that they can be sprinkled or baked into otherwise low-fiber foods, such as yogurt or ice cream. In other words, it's a convenient way to work a type of fiber into the diets of people who aren't otherwise consuming fruits, vegetables and whole grains.
Resistant starch powders can also be used as a substitute for flour in, say, white bread. The resulting slice of bread would provide fewer calories than the usual slice and could therefore help people trying to maintain a healthy weight, Higgins says.
The downside: Such products, including the flour-substitute Hi-maize and high-resistant-starch banana flour, can't be found in your average grocery store, but some can be purchased from online purveyors.
It's much easier, of course, to find foods naturally high in resistant starch. Slavin adds that not only whole grains but also many sources of complex carbohydrates, including beans, legumes, and some fruits and vegetables, contain resistant starch and make for a healthier overall diet too.
Higgins points out that while resistant starch may share the beneficial effects of fiber, consumers should be wary of promises of a weight loss miracle in the making.
"Fiber of any type makes you feel fuller longer and makes you eat less over time, but it's not the magic bullet," she says. "If you want to lose weight, you're going to have to restrict your diet. At the end of the day, it's about common sense: calories in minus calories out."