Many people flocking to low-carb diets in an effort to shed pounds may be putting their health at risk, a new study suggests.

"Low-carb diets that replace carbohydrates with protein or fat are gaining widespread popularity as a health and weight-loss strategy," said Dr. Sara Seidelmann, lead author and a clinical and research fellow at Brigham and Women's Hospital in Boston, in a news release.

"However, our data suggests that animal-based low carbohydrate diets, which are prevalent in North America and Europe, might be associated with shorter overall lifespan and should be discouraged."

The observational study of data provided by more than 15,400 adults in the U.S., published in The Lancet Public Health journal, found that people who got less than 40 per cent of their calories from carbohydrates could expect to live four fewer years than those whose diet included a "moderate" amount of carbohydrates (50 to 55 per cent of total calories).

Eating too many carbohydrates was also unhealthy, the study found. People getting more than 70 per cent of their caloric intake from carbs had a one-year shorter life expectancy compared to the moderate carb eaters.

The results suggest there is a "sweet spot" — and it's in the middle of the two extremes says Andrew Mente, a nutrition epidemiologist at McMaster University in Hamilton, who reviewed the study and co-wrote an accompanying commentary.

There is a 'sweet spot' for carb consumption, says nutrition epidemiologist Andrew Mente of McMaster University. (Submitted by Andrew Mente)

"For weight loss, certainly lower carbohydrate diets have been shown to be beneficial," Mente told CBC News. "However the long-term effects are not as well known. And so we can get hints from studies like this as to what the long-term effects are."

"Focusing on a more moderate diet and avoiding very low carb, just like avoiding very high carb, would be the most appropriate," he said.

But just how unhealthy a low-carb diet is depends largely on what people are eating to replace those carbohydrates, the researchers found. People who ate few carbs but more protein and fat from animal sources had a higher mortality risk, whereas low-carb eaters who consumed "plant-derived" protein and fat, including from nuts and vegetables, had a lower mortality risk.

That's a key finding, said Richard Bazinet, an associate professor with the Department of Nutritional Sciences at the University of Toronto.

Richard Bazinet, an associate professor of nutritional sciences at the University of Toronto, says that if people are going to eat fewer carbohydrates, they need to choose healthy types of proteins and fats to replace them. (Craig Chivers/CBC)

"What makes nutrition so complicated is that when you remove something in your diet, you have to replace it. And so often we get excited: 'Oh, I've removed a bag of chips from my diet or I've removed a soft drink from my diet and I'm going low carb,"' he said.

"If you remove some junk food from your diet, you have to be very careful not to just replace it with another type of junk food. And you want to make healthy food choices in those replacements."

Bazinet, who was not involved in the study, said the findings are consistent with Canada's current nutritional recommendations for carbohydrate intake.

"It's a good example of a nutrition study that's not coming out and saying, 'Oh, we've been wrong for 20 years.' It's suggesting that the current policies are in line."

The data studied by the researchers came from dietary questionnaires completed by participants who had enrolled in a cardiovascular risk factors study between 1987 and 1989 in four American communities. They had several follow-up interviews over approximately 25 years to monitor their self-reported food intake.

The authors attempted to control many variables that could affect the results, but acknowledge the limitations of the study, including the fact that it was not a randomized trial. The results show an association between both low- and high-carb diets and increased risk of death, but can't conclude cause and effect.

The results of a 25-year diet study suggest low-carb diets can shorten your life expectancy by up to four years. 1:52

In addition, the study relied on participants to honestly report what they were eating and how much.

Despite these limitations, Bazinet said, it's a study that people should pay attention to — and at a time when the public is bombarded with often-conflicting nutritional advice, it reinforces a message many experts often repeat.

"Once again, it tells us that moderation seems to be, on average, the best approach we can take," he said.