The New Perils of Carb-Loading

Anyone who has ever run a marathon – or trained with someone who has— knows the drill; the night before the big race, you chow down on carbohydrates. Known as “carbo-loading,” it's long been one of the tricks of the long distance runner's trade. By consuming large quantities of foods with low glycemic indices, like pasta, they can maximize the storage of glycogen (energy, for all intents and purposes) in their muscles.

But a brand new study threatens to throw some cold water on those hot pasta dinners. A team of researchers from Vanderbilt and the University of Alabama at Birmingham has just released a study that indicates carbo-loading can have an acute and detrimental effect on heart function. The work has been published in the Journal of the American College of Cardiology (JACC).

UAB studied 33 individuals who were given an acute carbohydrate load in the form of a 264-kilocalorie shake. They studied the subjects' blood levels for six hours looking for a number of things, chief among them whether this acute metabolic challenge could alter the heart's production of atrial natriuretic peptide (ANP). ANP is a hormone that helps the body get rid of excess salt and reduces blood pressure.

Researchers know through previous studies that obese people make less ANP, predisposing them to salt retention and hypertension.

If a high dietary carbohydrate load suppresses circulating ANP concentrations, this would be a disadvantage to obese individuals who have lower ANP levels to begin with, the authors wrote in the JACC study.

Before the participants began the study, they were normalized on a standard diet for a couple of days to remove any background dietary variability. The investigators observed that drinking the high-carb shake led to a 25 percent reduction in ANP in participants over the course of several hours.

"The carbohydrate load had a significant and notable effect on circulating ANP levels. Experimental studies suggest that it's not good to make less ANP," said senior author Thomas Wang, M.D., chief of the Division of Cardiovascular Medicine, adding that "carb-loading" may not be ideal for the body.

The investigators also looked at the mechanism for the decrease in ANP levels. They were able to reproduce the findings in mice, and found that the principal driver for the acute reduction in ANP appeared to be the increase in glucose.

"When you take in a high-carb shake a lot of things happen, including increases in glucose and insulin. However, the increase in glucose appears to be the main thing driving the drop in NP levels," Wang said.

UAB's Pankaj Arora, M.D., first author of the JACC study, helped to work out the novel chemical pathways that mediated the effect of glucose on heart cells.

This novel mechanism involved a molecule known as miR-425, which the research group has previously described as an inhibitor of ANP production. The glucose causes the cells to make more miR-425, and that, in turn, causes a reduction in ANP.