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A low-carbohydrate and low-fat diet was not linked with total mortality, researchers reported.

Looking at nearly 40,000 U.S. adults, those who consumed a diet comprised of low-carbs, but high total protein and fat, didn't see any reduced all-cause mortality risk (hazard ratio of 0.97, per 20-percentile increase in diet score 95% CI 0.93-1.00, P=0.06 for trend), according to Zhilei Shan, MD, PhD, of the Harvard T.H. Chan School of Public Health in Boston, and colleagues.

This same lack of association, as described in , was seen with a low-fat diet as well (HR of 0.97 per 20-percentile increase in diet score, 95% CI 0.93-1.02, P=0.34 for trend).

However, the authors reported that what did appear to bear a significant association on total mortality risk were the specific types and quality of macronutrients consumed.

For example, eating a "healthy" low-carbohydrate diet -- defined as low consumption of low-quality carbohydrates, but high consumption of plant protein and unsaturated fat -- was tied to a 9% reduction in all-cause mortality risk (HR 0.91, 95% CI 0.87-0.95, P<0.001 for trend). While consumption of an "unhealthy" low-carbohydrate diet -- comprised of few high-quality carbohydrates, high in animal protein, and high in saturated fats -- was associated with a 7% increased risk for death (HR 1.07, 95% CI 1.02-1.11, P=0.01 for trend).

Consumption of a "healthy" low-fat diet -- or a diet that was low in saturated fats, high in high-quality carbs, and also high in plant protein -- was associated with an 11% decreased risk for all-cause mortality (HR 0.89, 95% CI 0.85-0.93, P<0.001 for trend). Finally, people who ate an "unhealthy" low-fat diet -- one that was low in unsaturated fats, but high in low-quality, simple carbs and also high in animal protein -- was tied to a modest 6% increased risk for all-cause death (HR 1.06, 95% CI 1.01-1.12, P=0.04 for trend).

"Despite variance in macronutrient composition, [low-carb diets] and [low-fat diets] have shown similar associations with weight loss and metabolic biomarkers, with similar intensity of energy restriction and adherence to the intervention," Shan's group wrote. "However, the associations between 2 types of diets and long-term health outcomes were inconsistent."

"Our results were in accordance with previous studies that reported between overall LCD [low-carbohydrate diet] and health outcomes," they explained.

The prospective analysis drew upon data from the U.S. National Health and Nutrition Examination Surveys conducted from 1999 to 2014. After narrowing down the cohort to 37,233 adults 20 years or older, the researchers cross-examined 24-hour dietary recall data with all-cause mortality data from the National Death Index. During the near 300,000 person-years of follow-up, there were 4,866 deaths reported, 1,068 of which due to cancer and 849 due to heart disease.

People who consumed low-carb diets tended to be older, non-Hispanic white, and also be of higher socioeconomic status with a higher BMI. Those who ate a low-fat diet tended to be minorities, have a lower BMI, lower cholesterol levels, and have a lower intake of total energy.

One study limitation included the reliance of dietary recall data, which was only collected at one point in time.

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