Sunday, Sep 21, 2014

How carbohydrates make you crave for food

Written by New York Times | Posted: June 29, 2013 3:35 am

ANAHAD O’CONNOR

Are all calories created equal? A study suggests that in at least one important way,they may not be.

Sugary foods and drinks,white bread and other processed carbohydrates that are known to cause abrupt spikes and falls in blood sugar appear to stimulate parts of the brain involved in hunger,craving and reward,the new research shows. The findings,published in The American Journal of Clinical Nutrition,suggest that these so-called high-glycemic foods influence the brain in a way that might drive some people to overeat.

For those who are particularly susceptible to these effects,avoiding refined carbohydrates might reduce urges and potentially help control weight,said Dr David Ludwig,the lead author of the study and the director of the New Balance Foundation Obesity Prevention Center at Boston Children’s Hospital.

“This research suggests that based on their effects on brain metabolism,all calories are not alike,” he said. “Not everybody who eats processed carbohydrates develops uncontrollable food cravings. But for the person who has been struggling with weight in our modern food environment and unable to control their cravings,limiting refined carbohydrate may be a logical first step.”

In addition to raising blood sugar,foods that are sugary and highly caloric elicit pronounced responses in distinct areas of the brain involved in reward. Earlier imaging studies have shown,for example,that the main reward and pleasure center,the nucleus accumbens,lights up more intensely for a slice of chocolate cake than for blander foods like vegetables,and the activation tends to be greater in the brains of obese people than it is in those who are lean.

But do rich desserts have a select ability to change our longer-term eating habits?

To get a better idea,Dr Ludwig and his colleagues recruited a dozen obese men and then fed them milkshakes on two different occasions separated by several weeks. In each case,the milkshakes were nearly identical: flavored with milk and vanilla,and containing the same amount of calories,carbohydrates,protein and fat.

But on one occasion,the shakes were made with high-glycemic corn syrup; on the other,a source of low-glycemic carbohydrates was used. “These test meals were identical in appearance and tastiness,and we verified that our subjects had no preference for one or the other,” Dr Ludwig said.

As expected,blood sugar levels rose more quickly in response to the high-glycemic milkshake. But the researchers were especially interested in what happened several hours later,about the time most people are ready for the next meal.

What they found was that four hours after drinking the high-glycemic shake,blood sugar levels had plummeted into the hypoglycemic range,the subjects reported more hunger,and brain scans showed greater activation in parts of the brain that regulate cravings,reward and addictive behaviours. Although the subject pool was small,every subject showed the same response,and the differences in blood flow to these regions of the brain between the two conditions “was quite substantial,” Dr Ludwig said.

“Based on the strength and consistency of the response,” he added,“the likelihood that this was due to chance was less than one in a thousand.”

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