Many Americans are counting their carbs—but what counts as a 'low-carb' diet?

Interest in low-carbohydrate (or "low-carb") diets continues to grow. In fact, low-carb diets have doubled in popularity among US consumers over the last decade. Yet, despite the eating pattern's popularity, confusion persists around what exactly qualifies as a low-carb lifestyle.

Now, a scoping review published in Critical Reviews in Food Science and Nutrition sheds new light on the topic and suggests scientific consensus around the term is building. Of the more than 500 clinical trials reviewed, the majority defined a low-carb diet as either limiting carbohydrate intake to 30% or less or total calories or eating fewer than 100 grams of carbohydrates daily. The review included results from a total of 508 clinical studies published between 2002 and 2022, over half of which were randomized controlled trials.

"The sheer volume of clinical trials on low-carb diets published over the last two decades was striking," notes principal investigator Dr. Taylor Wallace. "Any perception that there is a lack of scientific evidence on low-carbohydrate eating patterns—or even a lack of government-funded evidence on the matter—clearly is not supported by the data."

The review also found that 152 of the studies included in the analysis were designed to assess a low-carbohydrate diet's effect on weight or body composition. Notably, these studies are often excluded from consideration in several federal nutrition evidence review processes, such as updates to the Dietary Reference Intake (DRI) guidelines for carbohydrates and the Dietary Guidelines for Americans' assessment of low-carbohydrate dietary patterns. These guidelines serve as the basis for numerous public health and nutrition activities, including food and nutrition labeling, federal nutrition programs, patient counseling and public health education initiatives.

"While it may not be surprising to learn that so many studies assessing the impact of low-carb nutrition interventions are focused on weight-related outcomes, it is important to understand that translates into a wealth of clinical data that has no bearing on some of the most foundational tools in U.S. dietary guidance," adds Wallace. "It leaves a lot of the scientific evidence on the table—given the high rates of overweight and obesity in this country."

Additionally, the investigators noted key gaps in the published literature. Though most studies classified low-carbohydrate diets as either consuming 30% or less of total calories from carbohydrates or limiting carb intake to fewer than 100 grams per day, the findings also revealed discrepancies. Of the studies using percent of total calories as their parameter for defining a low-carb diet, percentages ranged from zero to 50% of total calories from carbohydrates. Additionally, of the studies defining "low-carb" according to the number of grams of carbohydrates consumed daily, many used thresholds falling well below 100 grams per day.

"With both consumers and public health officials interested in understanding the potential benefits of low-carbohydrate eating patterns, arriving at a standardized consensus definition is non-negotiable and urgently needed," says Wallace. The researchers note systematic reviews and dose-response meta-regressions utilizing patient-level data on carbohydrate intake, status markers and health are key next steps to informing a clear, consistent and broadly adopted definition of the term "low-carbohydrate."