Feb. 7, 2024 -- Atkins, keto, the Zone, the paleo diet – it seems like low-carb diets have been around forever, and they've only grown in popularity over the years. But despite their renown, it remains unclear what exactly "low carb" really means.
A study recently published in Critical Reviews in Food Science and Nutrition analyzed over 500 articles about low-carb diets and found that, among the scientific community, there remains a real discrepancy about what "low carb" means.
The study's principal investigator, Taylor Wallace, PhD, CEO of Think Healthy Group and an adjunct professor of nutrition and food studies at George Mason University, said that given the sheer amount of research that has been put into studying low-carb diets, it's striking that we don't have a clear definition.
The range of evaluated studies – all published between 2002 and 2022 – found that most trials defined low-carb as having 100 grams or less of carbohydrates daily. But many also had much stricter definitions, with carbs taking up only 40-60 grams per day.
The variability, Wallace said, should underline the fact that low-carb diets might not be right for everyone.
"There's a lot of data that shows that low-carb diets work in cases like diabetes and weight loss," said Wallace. "But I also think that there's a lot that we don't know, and we shouldn't overstep to 'Eat as much saturated fat as you want, it doesn't matter as long as you're on a low-carb diet.' That's a little bit out there for me."
The other issue Wallace pointed out was that most of the studies did not examine participants beyond the 6-month point, which doesn't shed light on how sustainable these meal plans are in the long term.
A systematic review and meta-analysis published by the the journal BMJin 2021, for example, found that for people with type 2 diabetes, low-carb diets increased remission rates among those who stuck to it for 6 months. But the same review found that by 12 months, the benefits of the diet had shrunk significantly and participants' LDL cholesterol levels had worsened.
The first recommended daily allowance, established by the Institute of Medicine in 2002, said that both children and adults should consume a minimum of 130 grams of carbs per day for brain function and health.
With the popularity of diets like keto and Atkins, which typically restrict carbs to less than the recommended daily allowance, focusing on long-term adherence is key to understanding overall health outcomes beyond the number of pounds lost on the scale.
"We just have to be mindful that we don't have the literature to say what could potentially be the long-term outcomes of adhering to a carbohydrate diet that's less than this 130 grams per day," said Katrina Hartog, a registered dietitian-nutritionist at New York University.
It's also important to keep safety concerns in mind, even if you do fall into the category of someone who could benefit from a low-carb diet, Hartog said. Nutritional deficiencies, disordered eating habits, decreases in lean muscle mass – these should all be essential parts of low-carb research. And these events can only be properly evaluated past the 6-month mark.
Wallace said that his study's findings should prompt further research. In particular, he'd like to see a systematic review to really synthesize the outcome evidence of the many studies we already have.
"If everybody's calling 'low carb' something different, then the research is never going to give us an answer because all the studies are different," said Wallace. "If we're going to move forward in this field, we have to decide what 'low-carb' is."
