Any fit guy can distinguish between healthy foods and not-so-healthy foods. But what happens when you combine healthy and unhealthy—could the negative effects outweigh the positive ones?

That was the question posed by a new study published in the journal BMC Nutrition. In the study, researchers found out what happens when you mix something sugary with a protein-rich meal. The results are, shall we say, fattening.

Researchers rounded up 27 young adults of a healthy weight, and put them in two 24-hour studies—one where they were fed two 15% protein meals after a fast over the previous night, and the other with two 30% meals after the overnight fast. Each 500-calorie meal had 17g of fat, and one of the meals on each day was paired with a sugar-sweetened drink. Study subjects were housed in room calorimeter, a chamber that measures activity, oxygen, carbon dioxide, temp, and pressure to figure out energy expenditure and nutrient processing by the body.

The results showed that the meals with the sugary drink lowered fat oxidation (a process essential for breaking down fat), by 8%—with the 15% protein meal it decreased by about 7g, and with the 30% meal, about 13g. “We were surprised by the impact that the sugar-sweetened drinks had on metabolism when they were paired with higher-protein meals,” said study lead Shanon Casperson, Ph.D., research biologist at the USDA-Agricultural Research Service Grand Forks Human Nutrition Research Center. “This combination also increased study subjects' desire to eat savory and salty foods for four hours after eating.”

The study suggests that pairing a sugary drink—like fruit juice, soda, or the myriad beverages out there now with sugar added—with food can influence both the intake and the expenditure side of the energy balance equation. “On the intake side, the additional energy from the drink did not make people feel more sated,” said Casperson. “On the expenditure side, the additional calories were not expended and fat oxidation was reduced. The results provide further insight into the potential role of sugar-sweetened drinks—the largest single source of sugar in the American diet—in weight gain and obesity."

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