FRIDAY, May 8, 2015 (HealthDay News) -- A high-protein diet may backfire for people at risk for heart disease -- increasing the likelihood of weight gain and early death, a new study suggests.

Replacing carbohydrates and fats with protein is touted as a quick way to weight loss. But this long-term Spanish study of older adults found these high-protein diets -- think Atkins and South Beach, for example -- may be harmful.

When protein replaced carbohydrates, for instance, the eating plan was linked to a 90 percent greater risk of gaining more than 10 percent of body weight. It was also linked to a 59 percent higher risk of death from any cause, the researchers found.

When protein replaced fat, risk of death rose 66 percent, the researchers said.

"These results do not support the generalized use of high-protein diets as a good strategy for losing weight," said lead researcher Monica Bullo, of Pere Virgili Health Research Institute in Reus.

"Long-term efficacy and safety of these diets deserve more attention," she said.

However, the study only found an association between dietary protein, weight gain and death rates, not a cause-and-effect link.

Bullo isn't sure why high-protein diets may promote weight gain. But their connection to early death is a little clearer, she said, noting that high protein intake is related to kidney disease, changes in sugar and insulin metabolism, and changes in blood fat.

The researchers analyzed data from a government-funded trial of more than 7,000 men and women. Participants, all aged 55 and over without heart disease, filled out food questionnaires that assessed protein consumption for roughly five years. All had either type 2 diabetes or three or more of these risk factors: smoking, high blood pressure, poor cholesterol levels, overweight or obesity, or a family history of premature heart disease.

The study results were scheduled for presentation Friday at the European Congress on Obesity in Prague, Czech Republic. Research presented at meetings is usually considered preliminary until published in a peer-reviewed medical journal.

U.S. experts weren't surprised by the findings.

Dr. Gregg Fonarow, a professor of cardiology at the University of California, Los Angeles, said high-protein diets have generated interest as they may result in short-term weight loss.

"However, there is limited data as to whether there are longer-term health benefits or risks," Fonarow said.

Some studies have found that consuming a high-protein diet in middle age is associated with greater risk of early death, compared with a diet lower in protein, he said.

"However, additional rigorous studies are needed to determine whether these associations can be replicated," Fonarow said.

Samantha Heller, a senior clinical nutritionist at New York University Medical Center in New York City, said the quick weight loss seen with high-protein diets makes them appealing.

"High-protein diets are attractive because people feel they are given free rein to eat as much bacon, steak and pork as they like," she said. In addition, cutting carbohydrates initially causes quick weight loss, which is mostly water, she said.

But diets rich in animal protein have been linked to an increased risk of heart disease, colon cancer and diabetes, Heller said.

"Research suggests that the connections between a high intake of animal foods and health may be related to how the meats are processed, how we cook them and how our bodies use them," she said.

Consuming these high-protein diets also means eating fewer healthy foods, such as fruits, vegetables, whole grains and legumes, Heller said.

"Overall, the research is pretty clear that more plant-based diets offer a wide array of health benefits, including lower risks of many chronic diseases, better weight management and healthier hearts and brains," she said.

More information

For more on healthy diets, visit the U.S. Department of Agriculture.