What exactly constitutes a 'low-carb' diet?

The fascination with low-carb diets is not a passing trend. In the United States, these diets have seen their popularity double over the last decade.

Despite their widespread adoption, there remains a significant amount of confusion about what constitutes a low-carb lifestyle.

A recent scoping review in Critical Reviews in Food Science and Nutrition aims to clarify the definition and underscores the growing scientific consensus on this matter.

Scientific consensus on “low-carb” emerges

This comprehensive review analyzed over 500 clinical trials conducted between 2002 and 2022, revealing that a majority define a low-carb diet as one where carbohydrate intake is either 30% or less of total calories or below 100 grams of carbohydrates daily.

With more than half of these studies being randomized controlled trials and nearly a third government-funded, the evidence base for low-carb diets is robust and substantial.

Dr. Taylor Wallace, the principal investigator, expressed astonishment at the volume of clinical trials focusing on low-carb diets over the past two decades.

“The sheer volume of clinical trials on low-carb diets published over the last two decades was striking,” notes principal investigator Dr. 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.”

This statement challenges the notion that low-carb diets lack scientific validation.

Impact on weight and body composition

The review also spotlighted that 152 of the studies were designed to evaluate the impact of low-carb diets on weight or body composition.

These findings are particularly noteworthy because they often don’t factor into federal nutrition evidence review processes, such as the Dietary Reference Intake (DRI) guidelines for carbohydrates or the assessment of low-carbohydrate dietary patterns by the Dietary Guidelines for Americans.

These guidelines form the foundation for a range of public health and nutrition initiatives, from food labeling and federal nutrition programs to patient counseling and educational efforts.

Wallace pointed out the implications of excluding these studies from foundational dietary guidance tools, especially given the high rates of heavier people and obesity in the U.S.

He stated, “It leaves a lot of the scientific evidence on the table – given the high rates of obesity in this country.”

Need for a unified definition of “low-carbohydrate”

The review also identified discrepancies in the literature, with some studies defining low-carb diets as up to 50% of total calories from carbohydrates and others setting much lower daily thresholds for carbohydrate intake.

This variability underscores the need for a standardized definition of what constitutes a low-carb diet.

Wallace emphasized the urgency of reaching a consensus on this definition, considering the interest from both consumers and public health officials in the potential benefits of low-carbohydrate eating patterns.

He suggested that systematic reviews and dose-response meta-regressions using patient-level data on carbohydrate intake and health outcomes are crucial next steps.

These efforts aim to establish a clear, consistent, and widely accepted definition of “low-carbohydrate,” paving the way for more informed dietary choices and public health recommendations.

This recent review not only sheds light on the scientific basis for low-carb diets but also calls attention to the gaps in current dietary guidelines.

By moving towards a standardized definition of low-carbohydrate diets, researchers, healthcare providers, and policymakers can better address the nutritional needs and health challenges facing the population today.

The full study was published in the journal Critical Reviews in Food Science and Nutrition.

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