August 2016 Issue

Low-Carb Diets & Diabetes
By Constance Brown-Riggs, MSEd, RD, CDE, CDN
Today's Dietitian
Vol. 18 No. 8 P. 24

Research shows they're effective in managing blood glucose in many patients, but they may not work for everyone.

Toby Smithson, MSNW, RDN, LDN, CDE, was diagnosed with type 1 diabetes in the late 1960s when the exchange system was used to plan meals for people with diabetes. Smithson and her mother attended classes where she received an exchange booklet. "The exchange system was how I learned about meal planning," Smithson remembers. The exchange system organizes foods into lists by the amount of carbohydrate, protein, fat, and calories they contain. In the 1960s, experts were recommending that carbohydrate intake be limited to 40% of total calories—which was twice as much as previous recommendations.

Advice on meal planning approaches and recommended carbohydrate intake for people with diabetes has gone through several cycles. In the 1990s, experts recommended using carbohydrate counting for meal planning and individualizing carbohydrate content of meals. As a dietitian, certified diabetes educator, and an individual living with diabetes, Smithson stays abreast of the latest diabetes research. After reading several studies on the benefits of low-carbohydrate diets for people with diabetes, she decided to "play around" with the carbohydrate content of her own diet. Smithson found that she was always hungry on the low-carbohydrate diet and observed no significant change in her triglycerides or HDL cholesterol. This isn't to say that Smithson's usual diet was high in carbohydrate, as she averages about 135 g per day. Smithson says, "I'm a hard dietitian on myself." She's diligent with monitoring her carbohydrate intake, uses very little insulin, and is proud to say she controls her blood sugar well and has no diabetes complications.

The average person living with diabetes isn't a nutrition expert and may be easily confused by carbohydrate exchanges and the notion of carbohydrate counting. Many people with diabetes believe that total carbohydrate restriction is the only way to manage diabetes. Expert advice on recommended carbohydrate for people with diabetes has changed over time and remains a source of controversy. This article will discuss carbohydrate recommendations over the past century, provide a working definition of various carbohydrate levels, review evidence on safety and efficacy of low-carbohydrate diets, and provide strategies for counseling clients and patients about carbohydrate restriction.

Historical Perspective
Before the discovery of insulin therapy in the early 1920s, diets for people with diabetes were low in carbohydrate and high in fat. However, after the discovery of insulin therapy and oral hypoglycemic medications, recommendations for carbohydrate intake gradually increased.1 Between 1921 and 1950, diets for people with diabetes generally limited carbohydrate to about 20% of calories.2 In the early 1950s, the American Diabetes Association (ADA), the Academy of Nutrition and Dietetics (the Academy, formerly the American Dietetic Association), and the US Public Health Service joined forces to make nationally applicable nutrition recommendations, and the carbohydrate percentages increased to up to 43% of calories per day.3 By 1971 through 1986, there continued to be an increase in the recommended percentage of carbohydrate, reaching a high of 60% of total calories per day in 1986.3 In 2004, the ADA issued a position statement in agreement with the National Academy of Sciences Food and Nutrition Board's recommendation that carbohydrate intake shouldn't exceed 65% of total calories per day. Moreover, the ADA stated that low-carbohydrate diets with less than 130 g/day were not recommended.4 This position was maintained in the ADA's 2008 Nutrition Recommendations and Interventions for Diabetes.5 It wasn't until 2013, after a review of the available evidence, that the ADA changed course regarding carbohydrate intake, noting that there was no conclusive evidence of an ideal amount of carbohydrate intake for people with diabetes.6

Proposed Definitions
There's no standard definition for a low-carbohydrate diet, which may contribute to the controversy and confusion surrounding carbohydrate restriction for people with diabetes. Carrie S. Swift, MS, RD, CDE, BC-ADM, a dietitian at Kadlec Regional Medical Center in Richland, Washington, says the term "low-carbohydrate diet" is poorly defined in the literature and has varying meanings to people with diabetes and health care professionals alike. "Generally, I consider anything below the 130 g per day recommendation a low-carbohydrate diet," Swift says. In an attempt to eliminate the ambiguity, Richard D. Feinman, PhD, a professor of cell biology at the State University of New York Downstate Medical Center, and colleagues conducted a literature review of studies using various carbohydrate levels and proposed the following definitions:7

• very low-carbohydrate ketogenic diet: 20 to 50 g/day or <10% of total kcal;
• low-carbohydrate diet: <130 g/day or <26% of total kcal;
• moderate-carbohydrate diet: >130 g/day (based on a 2,000-kcal diet) or 26% to 45% of total kcal; and
• high-carbohydrate diet: >225 g/day (based on a 2,000-kcal diet) or >45% of total kcal.

The authors based this on ADA recommendations, 2010 dietary guidelines, and National Health and Nutrition Examination Survey data. According to Feinman's findings, the average American diet is approximately 49% carbohydrate.

Safety and Efficacy
There's an emerging body of evidence showing the benefits and safety of carbohydrate restriction in people with diabetes. In addition to ADA's literature review resulting in the 2013 position statement on carbohydrate, nine randomized, controlled trials,7-15 one meta-analysis,16 and one retrospective study17 have shown efficacy and safety of low-carbohydrate and very low-carbohydrate ketogenic diets. Carbohydrate levels in these studies ranged from 20 g to 130 g per day.

One of the studies was the two-year Dietary Intervention Randomized Controlled Trial (DIRECT) published in 2013 in Diabetes Care. In the DIRECT study, Tirosh and colleagues studied the long-term effects of a low-carbohydrate, high-protein diet on individuals with and without type 2 diabetes. The intervention involved 318 individuals who were randomized to low-fat, Mediterranean, or low-carbohydrate diets. The two-year compliance was 85%, and the proportion of protein intake significantly increased to 22% of energy only in the low-carbohydrate diet (P <0.05 vs low fat and Mediterranean). Results of this study provide evidence that a low-carbohydrate diet is as safe as the Mediterranean or low-fat diet in preserving/improving renal function among moderately obese participants with or without type 2 diabetes.10

In another more recent one-year randomized controlled trial published in 2015, Tay and colleagues compared the long-term effects of a very low-carbohydrate, high-protein, low–saturated fat diet with a traditional high–unrefined carbohydrate, low-fat diet on markers of renal function in obese adults with type 2 diabetes but without overt kidney disease. Study participants were randomized to either a low-carbohydrate (14% energy as carbohydrate [CHO <50 g/day], 28% protein, 58% fat [<10% saturated fat]) or a high–unrefined carbohydrate (53% CHO, 17% protein, 30% fat [<10% saturated fat]), energy-matched, weight-loss diet combined with supervised exercise training (60 minutes three days per week) for 12 months. Compared with a traditional high-carbohydrate weight loss diet, results of this study suggested that consumption of a low-carbohydrate, high-protein diet doesn't adversely affect clinical markers of renal function in obese adults with type 2 diabetes and with no preexisting kidney disease.15

In a critical review of the literature, Feinman and 25 other doctors and researchers present 12 points of evidence supporting the use of low-carbohydrate diets as the first approach to treating type 2 diabetes and as the most effective adjunct to pharmacology in type 1 diabetes. "We had 26 authors because we wanted to include experts who had [practical] experience using the diet, scientists who understood the basis of the diet, and medical researchers who understood the diet and supported it as a great treatment," says Feinman, who was lead author of the review. The following summarizes the 12 points of evidence presented by the reviewers:7

• Hyperglycemia is the most salient feature of diabetes. Dietary carbohydrate restriction has the greatest effect on decreasing blood glucose levels.
• During the epidemics of obesity and type 2 diabetes, caloric increases have been due almost entirely to increased carbohydrates.
• Benefits of dietary carbohydrate restriction don't require weight loss.
• Although weight loss isn't required for benefit [in glycemic control], no dietary intervention is better than carbohydrate restriction for weight loss.
• Adherence to low-carbohydrate diets in people with type 2 diabetes is at least as good as adherence to any other dietary interventions and is frequently significantly better.
• Replacement of carbohydrate with protein is generally beneficial [to glycemic control].
• Dietary total and saturated fat don't correlate with risk of cardiovascular disease.
• Plasma saturated fatty acids are controlled by dietary carbohydrate more than by dietary lipids.
• The best predictor of microvascular and, to a lesser extent, macrovascular complications in patients with type 2 diabetes is glycemic control (HbA1c).
• Dietary carbohydrate restriction is the most effective method (other than starvation) of reducing serum triglycerides and increasing high-density lipoprotein.
• Patients with type 2 diabetes on carbohydrate-restricted diets reduce and frequently eliminate medication. People with type 1 usually require lower insulin doses.
• Intensive glucose lowering by dietary carbohydrate restriction has no side effects comparable with the effects of intensive pharmacologic treatment.

Feinman and colleagues concluded that the evidence represents "the best-documented, least controversial results. The insistence on long-term randomized controlled trials as the only kind of data that will be accepted is without precedent in science. The seriousness of diabetes requires that we evaluate all of the evidence that is available. The 12 points are sufficiently compelling that we feel that the burden of proof rests with those who are opposed."7

Most of the available research on low-carbohydrate diets is on individuals with type 2 diabetes. However, there are a growing number of people with type 1 diabetes following low-carbohydrate diets, particularly in the online community.18 A small retrospective study in which individuals with type 1 diabetes were instructed to consume 70 g to 90 g of carbohydrate per day for up to four years found a significant decrease in HbA1c, a dramatic reduction in hypoglycemic episodes, and improvement in lipid profiles in those with good adherence.18

Swift concedes that Feinman and colleagues present a convincing argument for low-carbohydrate diets, but she says in her experience that low carb doesn't work for everyone. "I don't think meal planning for diabetes management should be 'one-size-fits-all.' There's more than one way to achieve glycemic control. For instance, plant-based diets (eg, vegan) may work well, even though the percentage of calories from carbohydrates is greater than from a low-carbohydrate diet. Other meal planning approaches that I've found beneficial are the Mediterranean diet, DASH [Dietary Approaches to Stop Hypertension] diet, and the plate method [one-half the plate filled with nonstarchy vegetables, one-quarter filled with starch, and one-quarter filled with protein]. These approaches also have compelling data," she says.

The 2013 ADA nutrition recommendations for the management of adults with diabetes identify the Mediterranean, vegan, vegetarian, low-fat, low-carb, and DASH diets as eating patterns that are acceptable for diabetes management. According to the ADA nutrition recommendations, macronutrient distribution should be based on individualized assessment of current eating patterns, preferences, and metabolic goals. Further, the recommendations say there's no ideal percentage of calories from carbohydrate, protein, and fat for all people with diabetes. And the amount of dietary saturated fat, cholesterol, and trans fat recommended for people with diabetes is the same as that recommended for the general population.6

Research Scientist Maggie Powers, PhD, RD, CDE, at the International Diabetes Center at Park Nicollet in Minneapolis, and president of health care and education at the ADA, says the current, evidenced-based ADA nutrition recommendations advise that there's inconclusive evidence for an ideal amount of carbohydrate intake for people with diabetes. Powers recognizes that there are proponents on both sides of the question about carbohydrate intake goals. "Some say limit and others say higher carb. Overall, the best food plan is one that a person can follow," Powers says.

Counseling Clients and Patients
There's no standard meal plan or eating pattern that will work for all people with diabetes. Nutrition therapy must be individualized for each patient to be effective.6
Feinman says he isn't a low-carbohydrate advocate in the sense that he thinks everybody should follow a low-carbohydrate diet. "It [low carb] may not be for everybody, but it should be offered to everybody, and in our [the study authors'] opinion it should be offered as the first thing to try."

All agree that both the quantity and type of carbohydrate in a food influence blood glucose levels, and total amount of carbohydrate eaten is the primary predictor of glycemic response. Powers says "the best guidance is to use glucose pattern management to assess the glycemic impact of a particular food plan and discuss next steps if target goals aren't met. For some, this may be a decrease in carbohydrate, for some a redistribution of carbohydrate, and for others it may mean the addition or adjustment of activity or medication."

Smithson, a spokesperson for the Academy, and coauthor of Diabetes Meal Planning and Nutrition for Dummies, says, "I individualize the carbohydrate based on my client's food diary and usual eating pattern. After evaluating their self-monitoring blood glucose results, I make recommendations on where they should modify their diet."

Clients and patients also should be counseled on sources of carbohydrate and serving sizes. "I have found that many people underestimate their carbohydrate intake and may think they're following a low-carb diet when they may not be," Swift says. Mindful eating is another useful tool in improving glycemic control. "For some people, just increasing their awareness of what they are eating helps them make changes to improve glycemic control. Decreasing distracted eating and increasing mindful eating is a big part of the discussion I have with my clients," Swift says.

Many nutrition professionals feel that low-carbohydrate diets are unsustainable long term. Feinman says that Richard K. Bernstein, MD, another study author and a well-known proponent of low-carbohydrate diets, is an example of the diet's sustainability. Bernstein, who has type 1 diabetes, has followed a very low-carbohydrate diet (30 g per day) for more than 40 years. He maintains normal blood glucose and lipid levels and has no diabetes-related complications. "He calls it the law of small numbers. If you take in a small amount of carbohydrate, then you can match the carbohydrate with the appropriate amount of insulin in a way you can't with a large amount of carbohydrate," Feinman says.

Evolving Recommendations
Carbohydrate recommendations for successful management of people with type 1 and type 2 diabetes will continue to evolve and likely remain a source of controversy. Nutrition professionals must embrace the art and science of diabetes management. It requires an open-minded and evidence-based approach, combined with the skill to individualize nutrition recommendations.

— Constance Brown-Riggs, MSEd, RD, CDE, CDN, is past national spokesperson for the Academy of Nutrition and Dietetics, specializing in African American nutrition, and author of The African American Guide to Living Well With Diabetes and Eating Soulfully and Healthfully With Diabetes.

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