People who follow low-carb or low-fat diets may not live longer - unless they're also careful to avoid junk food and sweets, a U.S. study suggests. Researchers followed 37,233 adults for two decades starting when they were 50 years old, on average. During the study, 4,866 people died, or about 13% of participants.

Overall, mortality rates were similar for people who followed low-carb or low-fat diets and those who didn't, researchers report in JAMA Internal Medicine.

However, the risk of premature death did appear lower for people on these diets who consumed healthier foods like plant proteins, unsaturated fats and high-quality carbohydrates like vegetables, fruits, legumes and whole grains. In contrast, mortality was higher for people whose diets included lots of saturated fats and animal protein.

"The health benefits of a low-carb diet may not only depend on the types of protein and fat, but also the quality of carbohydrate remaining in the diet," said study leader Dr. Zhilei Shan of the Harvard T.H. Chan School of Public Health in Boston.

Among low-carb dieters, people who got the most calories from unhealthy foods were 16% more likely to die during the study than people with the healthiest diets.

With low-fat diets, people who got the most calories from unhealthy foods were 12% more likely to die.

The findings are drawn from responses to national dietary surveys conducted from 1999 to 2014. Participants were asked to recall everything they ate in the previous 24 hours, providing a snapshot of their eating habits.

During the study period, 849 people died from heart disease and 1,068 died of cancer. Several types of cancer and many cardiovascular diseases are associated with unhealthy diets.

The study wasn't designed to prove whether or how any specific eating habits might help people live longer, or have the opposite effect.

One limitation of the analysis is that researchers could only score participants' diet quality based on their recollection of a single day's food intake, and it's possible some people changed their eating habits over time.

It's not completely clear what happens in the body when people consume different types of carbs or fats that might impact longevity, said Kevin C. Maki, a researcher at Indiana University School of Public Health in Bloomington who wasn't involved in the study.

Eating lots of saturated fats, for example, might raise cholesterol, and consuming more unsaturated fats might help lower cholesterol, Maki said by email. High cholesterol is one risk factor for cardiovascular disease.

Beyond this, people who eat well may have other healthy habits that help them live longer.

"People who have a higher-quality diet tend to exercise more, have lower body weight, are less likely to smoke and drink alcohol to excess, and are more likely to undergo recommended health testing.

The study shows there can be both good and bad low-carb or low-fat diets, said Andrew Mente of McMaster University in Hamilton, Ontario, who wasn't involved in the study.

"It's more about selecting whole natural or minimally-processed foods, regardless of the amount of carbs or fat," Mente said by email.