Low-Carbohydrate Diet Reduces A1c in Diabetes, Prediabetes

A 6-month low-carbohydrate diet may reduce A1c levels in people with prediabetes or diabetes, new research suggests.

Results from a randomized trial involving 150 adults with untreated A1c of 6.0% to 6.9% were published online October 26 in JAMA Network Open by Kirsten S. Dorans, ScD, assistant professor of epidemiology at Tulane University School of Public Health and Tropical Medicine, New Orleans, Louisiana, and colleagues.

The dietary intervention targeted a total net carbohydrate intake of less than 40 g for the first 3 months, then less than 60 g for the next 3 months, emphasizing high-fiber carbohydrates. The comparison group followed a usual diet. “I think the key message is that a low carbohydrate diet, if it can be maintained, might be a useful approach for preventing and treating type 2 diabetes, but more research is needed,” Dorans told Medscape Medical News.

She also noted, “Because of the design we can’t see the effect independent of calorie reduction or weight loss.”

Indeed, in a commentary via the Science Media Centre, Naveed Sattar, PhD, professor of metabolic medicine at the University of Glasgow, Scotland, UK, said: “We know weight loss leads to improvements in glucose, and so a low-carb diet, known to lower weight, will predictably lower A1c. What this study cannot show due to design is to what extent benefits are due to weight loss (likely most in my opinion) versus cutting carbs per se. That is the bigger question, though low-carb diets may not be as good for people in terms of cardiovascular risk and may not be sustainable over longer periods.”

However, Dorans explained, “We weren’t doing a strictly controlled feeding study, but more of a behavioral lifestyle intervention. We wanted to see what types of changes people would [undergo] by lowering their carbohydrate intake.”

A1c Dropped, Other Parameters Improved

The study participants, all adults aged 40-70 years, were recruited from the New Orleans, Louisiana, area via a mass mailing. Those identified to be at elevated risk for prediabetes or type 2 diabetes via pre-screening were invited for screening. Those with untreated A1c between 6.0% (the World Health Organization’s lower cutoff for prediabetes) and 6.9% (the upper bound of the American Diabetes Association’s recommended A1c target for people with diabetes) were selected.   

Of the 150 enrolled, 72% were women and 59% were Black. Overall mean A1c was 6.2%, and most (87%) had untreated A1c < 6.5% (ie, prediabetes).

The low-carbohydrate group received behavioral counseling, a handbook with dietary guidelines and recipes, and supplemental foods. In the first phase, called “go low,” individual sessions were held weekly for the first 4 weeks, followed by four small group sessions every other week and four telephone follow-ups. A second phase, “keep it low,” involved 3 monthly group sessions and three phone follow-ups.

In contrast, the usual diet group received written information with standard dietary advice and optional monthly educational sessions not related to diet.

This difference represents another limitation, noted Duane Mellor, registered dietitian and senior teaching fellow, Aston Medical School, Aston University, Birmingham, England, UK. “The groups were not given the same level of input, so the group of the low carbohydrate groups who were enrolled on courses called ‘go low’ and ‘keep it low’ were given a lot more support than the control group…Ideally both groups should have been given the same level of support.”

During follow-up, total calorie intake was lower in the intervention than usual diet group (net differences in change at 3 and 6 months were –389 kcal and –456 kcal, respectively). Intakes of total and net carbohydrates, added sugars, and sugar-sweetened beverages were all lower in the intervention group at follow-up, while percentages of calories from proteins and fats were higher.

The primary outcome, change in A1c from baseline to 6 months, was larger with the low-carbohydrate intervention group, with a drop of –0.26 percentage points, compared with 0.04 percentage points in the usual diet group, a significant 0.23-point difference. The 0.16-point difference at 3 months was also significant.

There were also significantly greater 6-month decreases in fasting plasma glucose (–8.4 mg/dL vs +1.9 mg/dL) and body weight (–6.4 kg vs –0.5 kg).

There were no significant differences between groups in changes in total-to-high-density lipoprotein cholesterol.

Assessed using continuous glucose monitoring, the intervention group spent 9.8% more time in range of 70-120 mg/dL than the usual diet group. At 6 months, 39 participants (53%) in the intervention group and 22 participants (32%) in the usual diet group had an A1c < 6.0%, a significant difference.

Directions of effects were consistent across subgroups of race and sex.

Rates of adverse events were similar between groups, although significantly more participants in the intervention group reported muscle cramps at 3 months (35% vs 19%; P = .03) and 6 months (34% vs 19%; P = .04).

The study was supported by the National Institute of General Medical Sciences and the Tulane University Carol Lavin Bernick Faculty Grant Program. Dorans has reported receiving grants from the National Heart, Lung, and Blood Institute outside the submitted work. The California Walnut Commission provided walnuts for the study. Shakes and bars were donated by The Simply Good Foods Company. Sugar replacement samples were provided by Swerve.

JAMA Netw Open. Published online October 26, 2022. Full text

Miriam E. Tucker is a freelance journalist based in the Washington, DC, area. She is a regular contributor to Medscape, with other work appearing in The Washington Post, NPR’s Shots blog, and Diabetes Forecast magazine. She is on Twitter: @MiriamETucker.

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