What Does Low-Carb Mean? Study Reveals There’s No Clear Answer

WASHINGTON — New fad diets are constantly popping up, but one that continues to stick around is the low-carb diet. Keto, Atkins, Paleo, and South Beach are all different diets that share one thing in common: minimizing carbohydrate intake. With all of the different dieting types and conflicting messages, people are still confused about what it actually means to “eat low-carb.” Now, a new review looking at over 500 clinical trials shows that most define a low-carb diet as either limiting their intake to 30 percent or less of total caloric intake or consuming less than 100 grams daily. However, does this work for every dieter?

“The sheer volume of clinical trials on low-carb diets published over the last two decades was striking,” notes principal investigator Dr. Taylor Wallace in a media release. “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 includes results from 508 clinical studies published between 2002 and 2022. Over half of the studies were randomized controlled trials, and almost one-third were government-funded. They found that 152 of the studies were designed to assess a low-carbohydrate diet’s impact on weight or body composition.

Of note, these studies are typically excluded from consideration in federal nutrition evidence review processes like the Dietary Reference Intake (DRI) guidelines for carbohydrates and the Dietary Guidelines for Americans’ assessment of low-carb dietary patterns. These guidelines offer general guidance for things like 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.”

Moreover, the team found key gaps in the published data. Though most studies classified low-carbohydrate diets as either consuming 30 percent or less of total calories from carbs or limiting intake to 100 grams or less per day, there were some inconsistencies. Of the studies using percent of total calories as their definition, percentages ranged from anywhere from zero to 50 percent of total calories. Of those using the number of grams as their definition, many actually capped their limit at well below 100 grams.

“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.”

Healthy and unhealthy food
A review of over 500 dieting studies finds conflicting definitions of what a low-carb diet really means. (© M.studio – stock.adobe.com)

A Dietitian’s Take

Low-carb diets are all the rave, and the trend doesn’t seem to be slowing down any time soon. Most of the standard American Diet includes more highly refined, ultra-processed carbohydrates than anything else. In turn, we see the consequences in the form of exponentially increasing rates of insulin resistance, obesity, and diabetes. Although excessive amounts of simple carbohydrates are not the only reason for these outcomes, they are a big part of the equation for thousands of people who suffer from any of these three health concerns.

It’s no surprise that much of the research exploring low-carbohydrate diets has been done in relation to weight or body composition. However, this very real conversation that’s rooted in a degree of truth has transformed into a fad diet scheme.

It’s made people scared to eat carbs, deem them all as “bad,” and even go so far as to say that they all lead to weight gain and health issues. It severely lacks the necessary nuance, especially given that fruits, vegetables, and whole grains are all carbohydrate sources that offer full-spectrum nutrition.

Candies, sodas, and white breads are not the same. Most Americans aren’t even eating enough of the healthier types of carbs in the first place. Then once people hop onto the “low carb” trend, they often don’t even know how low is appropriate versus too low. Still, while a diet lower in carbohydrates may work for some people, as shown by select research, not everyone needs to eat that way. Further, discussions about low-carb diets should take place in an evidence-based way, recognizing that how we truly define “low-carb” isn’t even something that researchers have totally figured out.

The findings are published in the journal Critical Reviews in Food Science and Nutrition.

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