Beware ‘One Size Fits All’ Thinking on Carbohydrates and Cardiometabolic Health

When it comes to teasing out the effects of dietary carbohydrate intake on cardiometabolic health, the devil is very much in the details, and more details are needed for conclusions on some important questions. That’s an overarching message of a new evidence-based review of the topic published in Atherosclerosis (Epub March 27, 2021) by Cleveland Clinic cardiologist Leslie Cho, MD, and two European colleagues.

“Carbohydrates have been vilified in the media over the past two decades, often due to overbroad generalizations not supported by evidence,” says Dr. Cho, Co-Section Head of Preventive Cardiology and Rehabilitation. “The recent prominence of high-protein diets for weight loss has fueled further confusion, leading too many people to overlook the essential role carbohydrates play in a healthy diet.”

Analyzing a range of effects

Together with expert nutrition and health researchers from the Netherlands and Italy, Dr. Cho reviewed the latest evidence — almost exclusively from observational studies and meta-analyses of intervention studies — on the effect of fiber, whole grain and refined carbohydrates on the following:

• Weight
• Insulin resistance
• Diabetes
• Inflammation
• Lipids
• Long-term rates of major cardiac events
• All-cause mortality

They begin by noting that current understanding of carbohydrates’ health effects has been hindered by the traditional limitations of dietary recall studies and the fact that many studies to date have failed to differentiate among the various types of carbohydrates.

“Carbohydrates may be thought of in terms of complex versus simple and then further broken down into fiber, starches and sugar,” notes Dr. Cho, the paper’s corresponding author.

Accordingly, the paper proceeds to review the latest evidence from clinical studies in three main sections:

• Fiber and whole grain
• Refined carbohydrates
• Epidemiological studies of carbohydrates with mortality

A sampling of key conclusions

The review leads the authors to a number of conclusions, including the following:

• High consumption of dietary fiber and whole grains is associated with positive effects on metabolic health, whereas high consumption of sugar and refined carbohydrates has negative effects on cardiometabolic health.
• Carbohydrate type is also critical to clinical outcomes, with diets rich in fiber and whole grains associated with lower rates of mortality and cardiovascular events.
• Evidence consistently shows that low-fat and low-carbohydrate diets yield comparable effects on body weight, assuming comparable energy levels.
• The jury is still out on how a low-carbohydrate diet impacts diabetes control. Whereas short-term studies have demonstrated better fasting glucose and lower hemoglobin A1c, these benefits in diabetes control attenuated with longer follow-up.

“The quality of the carbohydrate dictates its effect on both cardiometabolic health and cardiovascular events,” observes Dr. Cho. “Moreover, individuals’ responses to dietary fiber interventions are likely determined by their initial microbial and/or metabolic phenotype. It’s becoming more and more clear that this is not a one-size-fits-all question.”

In fact, the review authors identify the impact of individual traits and lifestyle factors on the effectiveness of fiber interventions as a priority for future research. Another priority focus is the role played by regional and ethnic differences in various metabolic and clinical outcomes of the consumption of differing types of carbohydrates. And the authors note that because most data to date are from observational studies, definitive evidence on the benefits and adverse effects of various carbohydrate types is lacking.

New tech tools, and a near-term call to action

The review authors note that the emergence of mobile app-enabled patient-entered diet data should help bolster the number and quality of dietary and intervention studies to start filling these knowledge gaps. “A revolution in dietary research may be at hand, thanks to the power of patient-entered data along with evolving insights into the impact of the gut microbiome,” says Dr. Cho.

In the meantime, despite the considerable remaining uncertainties, she believes there’s enough clarity to make a broad recommendation for present action. “At a global level, most people consume less than 20 grams of fiber daily while consuming a large quantity of refined carbohydrates,” Dr. Cho notes. “The current evidence fully justifies additional measures to enhance the amount and variety of fiber consumption worldwide while simultaneously reducing intake of refined carbohydrates.”

The review paper is available here.