Low-carb diets can help people lose weight in the short term, numerous studies have found; but the long-term health effects of the diets are less clear.
In the new study, the researchers examined information from nearly 15,500 adults ages 45 to 64 from four communities in North Carolina, Mississippi, Minnesota and Maryland. At the start of the study in the late 1980s, and again six years later, participants completed surveys about the types of foods they ate and how often.
Participants were followed for about 25 years, during which 6,283 people died.
The researchers found that the link between carb intake and life expectancy was U-shaped; meaning that, during the study, both low- and high-carb diets were linked with a greater risk of death, while moderate-carb diets were linked with a lower risk of death.
The researchers then pulled data from seven other studies, as well as their own study, to perform a separate analysis involving more than 432,000 people in 20 countries. This analysis confirmed the researchers' earlier findings: Low- and high-carb diets were linked with a 20 percent increase in the risk of death during the study, compared with moderate-carb diets.
But the analysis also found that, with low-carb diets, what mattered was the source of proteins and fats. Diets that involved replacing carbs with proteins and fats from animal sources, including beef, lamb, pork, chicken and cheese, were linked with a greater risk of death. In contrast, diets that involved replacing carbs with proteins and fats from plant sources, such as vegetables, legumes and nuts, were tied to a lower risk of death.