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A new clinical trial shows that strictly limiting carbohydrates and eating more fat can help the body burn more calories.
The researchers found that among the 164 adults participating in a weight loss study, those placed in low carb and fat diet burned more calories a day, compared to those given high carbohydrate meals. On average, their bodies used 250 extra calories a day for 20 weeks.
The researchers estimated that over three years this would result in a 20-pound increase weightloss for a man of average size.
"This study refutes conventional thinking that only reducing calories is important," said Dr. David Ludwig, principal investigator. He is co-director of the New Balance Foundation's Center for Prevention of Obesity at the Boston Children's Hospital.
Instead, he said, the source of these calories can make the difference to whether your metabolism is "working with you or against you".
According to Ludwig, the results support a theory called "carbohydrate-insulin model". The principle is that diets high in processed carbohydrates significantly increase the level of insulin, which pushes the body to use fewer calories and store more in the form of fat.
"Our study suggests that you will do better if you focus on reducing refined carbohydrates, rather than just reducing calories," Ludwig said.
He and his colleagues reported the results online on November 14 in the BMJ.
Over the years, many studies have attempted to answer the question of whether weight loss is better or lower in carbohydrates. They often concluded that there was little difference.
But these studies, Ludwig said, have generally been behavioral studies where people can or can not stay with their diet.
So, his team conducted a "diet study" to carefully control what people were eating.
First of all, 234 overweight and obese adults were recruited for a break-in phase in order to lose about 12% of their weight in 10 weeks. Their diet was low in calories and contained a moderate amount of carbohydrates.
Of this group, 164 lost enough weight and moved on to the next phase. They were randomly assigned to a low carbohydrate, moderate carbohydrate or high carbohydrate diet for 20 weeks.
People on a low carb diet have 20% of their calories from carbohydrates like vegetables, fruits and beans; 60% of their calories came from fat, including meat, whole milk, cheese and nuts. The remaining 20% of calories come from protein.
The situation was reversed for people benefiting from the high carbohydrate diet: 60% of calories from carbohydrates and 20% of fat. The moderate plan divided the two nutrients equally, at 40/40.
After 20 weeks, the low carb group seemed to burn more calories – an average of 250 more per day than the high carbohydrate group and 111 more than the moderate carbohydrate group.
The researchers did not examine the effects on additional weight loss. Instead, each person's caloric intake was calibrated to maintain what she had already lost. Ludwig explained that the goal was to focus on the effects of different diets on the burning of calories.
According to Dr. Anastassia Amaro, medical director of Penn Metabolic Medicine at the University of Pennsylvania, "the study plan is very clever."
Amaro, who did not participate in the research, said that she already suggested that patients reduce their carbohydrate intake when they are trying to lose weight.
These results, she said, will give her more confidence in her advice.
However, said Amaro, the low carb diet used in this study is not ready for a "direct translation" in the real world. On the one hand, she explained, it's not clear if it's the lack of carbs that was key.
"It's also a high-fat diet," Amaro said. "Is it the lack of carbohydrates, the fat content, or both?"
What about the nutritional value of such a diet? Ludwig said it was good for health: allow fruits, legumes and an "unlimited" amount of vegetables, for example.
"What he does not have is cereals and added sugar," he said.
Ludwig acknowledged, however, that more research is needed to show if this approach is the best way to maintain weight loss. He and his colleagues recently launched a new clinical trial that will consist of a very low carbohydrate diet and a high carbohydrate but low sugar diet, and another diet high in carbohydrates and sugar.
And what about people who currently have a healthy weight? Would a low-carbohydrate, high-fat diet burn more calories?
It's a "good question," said Amaro – but this study can not answer it.
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