Sugary drinks and high protein meals make a highly unhealthy combination, new research suggests.
Consuming the two together impairs energy balance and causes the body to get fatter, say scientists.
Dr Shanon Casperson, from the US Department of Agriculture, said: "We found that about a third of the additional calories provided by the sugar-sweetened drinks were not expended, fat metabolism was reduced, and it took less energy to metabolise the meals.
"This decreased metabolic efficiency may 'prime' the body to store more fat."
A sugar-sweetened drink combined with a meal containing 15% protein reduced the amount of fat oxidised in the body by 7.2 grams on average, the study showed.
Fat oxidation kick starts the break down of fat molecules so less fat is stored.
Combining a sugary drink with a 30% protein meal reduced fat oxidation by 12.6 grams.
While the sugar provided more available energy for metabolism, this was cancelled out by the additional calories in the drink.
Dr Casperson added: "We were surprised by the impact that the sugar-sweetened drinks had on metabolism when they were paired with higher-protein meals.
"This combination also increased study subjects' desire to eat savoury and salty foods for four hours after eating."
For the study, the researchers recruited 27 healthy-weight adults with an average age of 23.
Participants were given meals containing either 15% or 30% protein and 17 grams of fat. The effects of consuming sugar or non-sugar sweetened drinks with the meals were compared, and the scientists measured how many calories the volunteers burned every minute.
Dr. Casperson said: "Our findings suggest that having a sugar-sweetened drink with a meal impacts both sides of the energy balance equation.
"On the intake side, the additional energy from the drink did not make people feel more sated.
"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."
The findings are reported in the journal BMC Nutrition.
Nutrition expert Dr Ian Johnson, from Quadram Institute Bioscience in Norwich - formerly the Institute of Food Research - said: "The results do suggest a mechanism whereby consumption of sugary drinks with meals might interfere with the body's ability to regulate energy intake, and eventually lead to weight gain.
"However, for this to happen, the observed effects would need to be maintained over an extended period of time, and the authors themselves are careful to point out that they have not yet shown this."