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Experts Debate Low-Carb Diets for People With Diabetes

It’s an ongoing debate in the diabetes world: Is it ideal to consume a very low carbohydrate diet, or is it better to go with moderate amounts of healthful carbs?

At the recent American Diabetes Association (ADA) 83rd Scientific Sessions, Carol F. Kirkpatrick, PhD, RDN, spoke first, arguing in favor of diets consisting of moderate, high-quality carbohydrates.

Dina Hafez Griauzde, MD, countered that very low carbohydrate diets are more beneficial for people with diabetes, primarily type 2 diabetes.

Both speakers based their arguments on published evidence but agreed in the end that discussion with patients about individual dietary preferences should play a major role in the ultimate decision.

Moderate Carbohydrate Eating Is Best

Kirkpatrick began by explaining that definitions of “low carb” vary in the literature, which makes comparisons between studies difficult. On the basis of a 2019 review that she co-authored, “moderate” carbohydrate consumption was defined as a diet in which 26% to 44% of total daily calories are from carbohydrates. “Low” carbohydrate consumption was defined as a diet in which 10% to 25% of calories were from carbohydrates. Consuming less than 10% was defined as a very low carbohydrate diet (ie, a ketogenic diet).

Across studies, she noted, the literature shows that within the first 6 months, weight loss is typically greater with carbohydrate-restricted diets than with higher-carbohydrate diets, but that by 1 year and beyond, weight loss is similar.

“That can be partly due to the difficulty in people maintaining that very severe dietary restriction, although…we can all acknowledge that it’s difficult for patients to adhere to any dietary pattern, so for sure by 12 months, the difference in the weight loss is gone between the two,” said Kirkpatrick, of Midwest Biomedical Research, Pocatello, Idaho.

In a recent meta-analysis of 35 trials that examined the dose-dependent effects of carbohydrate restriction for patients with type 2 diabetes, there was a significant decrease in weight as carbohydrates were reduced. But by 12 months (17 trials), the greatest weight reduction was seen at 35% carbohydrate intake.

“It may just be that people were able to adhere to that moderate intake better,” she explained.

Regarding lipids, in her 2019 review and in several meta-analyses since, the effects on low-density lipoprotein cholesterol (LDL-C) varied. For some patients, adhering to a low-carb diet led to reductions in LDL-C, especially if the participants also lost weight, whereas in other patients, a low-carb diet led to an increase in LDL-C.

Either way, a high intake of saturated fatty acids is key to an increase in LDL-C, Kirkpatrick noted. “So, it’s important that if a patient chooses to follow a very low carbohydrate diet or any kind of dietary pattern that restricts carbohydrate, that they replace the carbohydrate with unsaturated fat and not saturated fatty acid foods to avoid that increase in LDL-C.”

Generally, the evidence also shows that carbohydrate restriction typically leads to lower triglyceride levels and higher high-density lipoprotein (HDL) cholesterol levels. However, the same meta-analysis showed that the greatest reduction in LDL-C occurred at about 40% carbohydrate consumption.

Another recent meta-analysis showed that LDL-C rose significantly by an average 12.4 mg/dL with very low carb (3% to 30%) diets, but only slightly, by 0.4 mg/dL, with moderate carb (40% to 45%) intake.

Consuming very low carb diets did lead to greater reductions in triglycerides compared to consuming moderate carb diets (23.9 mg/dL vs 8.9 mg/dL).

“However, in terms of cardiovascular health, we are not entirely sure what that means…. We have to look at the overall results in the presence of both triglyceride lowering as well as LDL cholesterol,” Kirkpatrick noted.

Carbohydrate restriction did consistently lead to lower A1c levels by an average of 0.4, 0.6, and 1.0 percentage points at 6 months for diets of 40%, 30%, and 15% carbohydrate, respectively. However, by 12 months, the effect had waned to 0.15, 0.2, and 0.4 A1c percentage points.

“Again, carbohydrate restriction, especially severe, is difficult for people to adhere to, and moderate carbohydrate intake would allow our patients to consume an appropriate amount of carbohydrate and still achieve improved glycemic control,” Kirkpatrick said.

Two large randomized controlled trials ― PREDIMED and CORDIOPREV ― examined the effects of the Mediterranean diet on cardiovascular disease prevention. Both showed a decrease in cardiovascular events with the Mediterranean diet, which involves consuming moderate amounts of carbohydrates.

“The Mediterranean dietary pattern has the strongest evidence for benefit, and it’s moderate in carbohydrates,” she concluded.

Very Low Carbohydrate Eating Is Best

Griauzde was a last-minute replacement speaker for William S. Yancy, Jr, MD, of Duke University and presented his slides. She argued that consuming a very low carb diet improves glycemia and that it does not increase but possibly lowers cardiovascular risk.

She began by noting that prior to the discovery of insulin, very low carb diets had been consumed for over a century to prolong life for people with type 1 diabetes.

“We have long recognized the deleterious role of carbohydrate in type 1 diabetes management, and we have increasingly recognized that role in the management of type 2 diabetes,” said Griauzde, of the University of Michigan, in Ann Arbor.

In a small study that compared maintaining a very low carb diet for 2 weeks with maintaining a high carb diet for 2 weeks, total glucose areas under the curve were substantially lower (P < .05) during the low carb phase, while A1c levels dropped from 7.3% to 6.8% (P = .006).

“We don’t see those outcomes with meds,” Griauzde noted, adding, “A diet very low in carbohydrates is one of the most potent tools we have to help our patients achieve glycemic control.”

Griauzde said that the carbohydrate-insulin model provides an explanation for why dietary carbohydrates are particularly obesogenic and metabolically harmful. That model contrasts with the energy balance model, which suggests that all calories are equal.

The rationale of the carbohydrate-insulin model is that dietary carbohydrate ― either sugar or starch ― raises serum glucose and insulin levels. A carbohydrate-restricted diet therefore reduces the dietary contribution to serum glucose, which then results in lower insulin levels. Insulin is a potent stimulator of lipogenesis (fat storage), and it is a potent inhibitor of lipolysis (the burning of fat). By lowering insulin levels, stored body fat is burned, serum ketone levels increase, and body weight is lowered.

This model suggests that when insulin levels are chronically high because of excess carbohydrate consumption, circulating fuels are lowered, which leads to an increase in hunger and to overeating. This was demonstrated in a study that compared different levels of isocaloric glycemic index diets in 12 teenage boys with overweight or obesity. The higher carbohydrate meals led to higher glucose and insulin levels and more food consumption.

In a systematic review of 13 trials of restricted carbohydrate diets (<45% carbohydrates) for adults with diabetes, the degree of improvement in A1c level correlated with the degree of carbohydrate restriction over 2 to 26 weeks (P = .013).

And in a network meta-analysis of 56 trials that compared nine diets among a total of 4937 participants with type 2 diabetes, one conclusion was that “for reducing A1c, the low-carbohydrate diet was ranked as the best dietary approach (SUCRA: 84%), followed by the Mediterranean diet (80%), and Paleolithic diet (76%), compared to a control diet.”

Regarding the criticism that very low carbohydrate diets are high in saturated fat and therefore raise the risk of cardiovascular disease, Griauzde pointed to another meta-analysis of 21 prospective studies with more than 300,000 participants with 5 to 25 years of follow-up. In that analysis, the intake of saturated fat was not associated with an increased risk of cardiovascular disease or stroke.

Furthermore, a 12-week randomized controlled trial that involved 40 adults with overweight also suggested that a very low carb diet may be superior to a low fat diet in improving aspects of the metabolic syndrome, including body mass index, lipid levels, and insulin sensitivity. Small LDL particles, which are more atherogenic than larger LDL particles, also decreased despite a threefold increase in saturated fat intake.

Rebuttals: Overall Diet, Patient Preference Matter

During the rebuttals, Kirkpatrick pointed out that large LDL particles are also atherogenic. In addition, she noted that the studies that showed that saturated fat isn’t associated with cardiovascular disease didn’t consider the macronutrients that replaced the saturated fat.

“It really is about increasing consumption of foods that we know are associated with cardiovascular benefit, including plant-based foods that are high quality and not refined carbohydrates…and healthy protein sources…. Hopefully we can step away from just looking at macronutrients and look at the total amount of food that people are choosing to eat.”

Importantly, Kirkpatrick said, patients need to be asked about their current dietary patterns and preferences. “Interventions should be patient centered and sensitive to cultural differences. Personalized lifestyle interventions increase the likelihood of success.”

Griauzde pointed out that newer anti-obesity drugs can be added to any diet to decrease appetite and enhance adherence.

Griauzde also observed, “We can label a very low carbohydrate diet ‘extreme,’ but maybe from the patient’s perspective, it’s extreme to take 200 units of insulin a day. If you can give them the opportunity to discontinue use of the insulin by following a very low carbohydrate dietary pattern, that is the opportunity that our patients deserve to have.”

But overall, she agreed with Kirkpatrick about individualizing any dietary approach: “We will never know from any of the trials that have been done or that will be done in the future what diet is best for an individual patient…. Our job is to help our patients find the dietary approach that works best for them.”

Kirkpatrick is a clinical scientist with Midwest Biomedical Research, which has received funding from various food and pharmaceutical companies. She has not received any direct funding. Yancy is a consultant for The Simply Good Foods Co. Griauzde has disclosed no relevant financial relatinships.

American Diabetes Association (ADA) 83rd Scientific Sessions: Debate held June 23, 2023.

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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  • Posted on August 30, 2023