Low-Carbohydrate Diets for Managing Diabetes

Diabetes refers to a varied group of metabolic disorders characterized by chronic hyperglycemia (high blood glucose or sugar levels) resulting from genetic, lifestyle, and environmental factors. Type 1 and type 2 diabetes are the two major forms of the condition, with the latter accounting for more than 90% of all diabetic cases worldwide.¹ Type 1 diabetes develops secondary to the autoimmune destruction of pancreatic beta cells and a resultant insulin deficiency, whereas type 2 diabetes ensues due to insulin resistance, with or without abnormal insulin production. Nutrition has long been established as an important factor in the development, prevention, and management of diabetes.^2,3 It is against this backdrop that dietary intervention is integrated into the management guidelines of all diabetic individuals. A major independent risk factor for type 2 diabetes is obesity, associated with excessive caloric intake and high consumption of refined carbohydrates, free sugars, processed foods, and unhealthy fats.^3-5 In addition, carbohydrates are known to produce the highest increase in blood sugar levels and insulin demand.^6-8

Components of the diet, including macro- and micro-nutrients, influence weight, glucose and insulin homeostasis, insulin sensitivity, and the pathogenetic pathways of diabetes-related comorbidities such as cardiovascular disease.^2,9 In general, meal plans for diabetes should improve eating habits, metabolic status, overall health and quality of life, maintain a balanced diet while minimizing calorie intake, and address individual nutritional needs.¹⁰ Dietary interventions are a crucial and effective component of diabetes management, not only in maintaining glycemic control, but in preventing or managing its complications and minimizing the need for diabetes medications.^6,9,11 Several guidelines for a diabetes diet are available, with advice on the quality (type) and quantity of macronutrients to consume. In type 2 diabetes, dietary control mainly aims to reduce weight as most patients concomitantly have obesity.^1,10 In contrast, special attention is given to the quantity of carbohydrate intake in regard to insulin dose adjustments in those with type 1 diabetes.^6,12,10 Although macronutrient quantity in diabetes diet plans is given considerable attention, an individualized meal planning approach involving quality total nutritional intake, rather than one centering a single macronutrient, is recommended.^2,6,10 This article intends to highlight the definition, benefits, adverse outcomes, and limitations of low-carbohydrate diets in managing diabetes, describe the types of low-carbohydrate diets and compare them to other types of dietary interventions.

Low-carbohydrate diets for diabetes management

Carbohydrates, which constitute over 50% of daily caloric intake in the general population, are dietary macronutrients and the primary source of energy for the body, producing the greatest effect on blood sugar levels after meals.^6,7,10,13 They are present in varied types of foods and broadly classified as sugars, fibers, and starches. Both the quality and quantity of carbohydrates consumed have an influence on blood sugar levels.^8,10 In the setting of insulin resistance/deficiency, carbohydrate consumption leads to persistently elevated sugar levels.¹⁴ For this reason, the quantity and quality of this macronutrient has been a major focus in diabetes management.

A low-carbohydrate diet has been defined as a dietary intake of <130g of carbohydrates or less than 26% of total energy intake per day,^6,9,13 although this is not standardized.^10,15 Low-carbohydrate diets advocate for the consumption of carbohydrates that produce a minimal spike in sugar levels while decreasing the intake of refined carbohydrates and sugary foods which are calorie-dense.^8,16,17 As carbohydrate intake is reduced, proteins and fats are increased to compensate for this reduction; fats, mainly unsaturated, are preferred^6,8 but proteins may also be increased. Other carbohydrate-focused diets are described as very low (20–50g or 6–10% total energy intake per day), moderate (130–225g or 26–45% total energy intake per day), and high carbohydrate diets (>225g or >45% total energy intake per day).⁶ Very-low-carbohydrate diets are like low-carbohydrate diets, but the former aims to promote ketosis through further carbohydrate restriction.^16,19

Popular low-carbohydrate diets include the Atkins and South Beach diets.^13,19,20 The Atkins approach (considered a very-low-carbohydrate ketogenic diet¹⁹) is the most carbohydrate-restricted, while proteins and fats are higher in quantity. It comprises a two-week induction phase in which carbohydrates are limited to 20g/day and a maintenance phase in which they are reintroduced gradually to a daily intake that maintains optimal weight, up to 90g/day.^15,19 The induction phase eliminates sugar, bread, pasta, grains, legumes, and dairy products other than cheese, cream, and butter, as well as starchy vegetables and all fruits.^15,19 The South Beach diet is more balanced, permitting carbohydrates from vegetables and sometimes whole fruits and restricting intake of refined carbohydrates (e.g., bread and pasta) and sugar-sweetened products. The fat composition of this diet is largely unsaturated, unlike the Atkins diet which permits unlimited fat intake.¹⁹

Low-carbohydrate diets for diabetes management is a historical concept that, in recent years, has regained substantial interest. Both low- and very-low-carbohydrate diets have shown remarkable efficacy in reducing body weight, blood sugar levels, insulin resistance, insulin levels, medication requirements (including exogenous insulin), postprandial hyperglycemia, and blood and central organ lipids (triglycerides, low-density lipoproteins, hepatic and pancreatic fat, etc.).^{1,6,7,10,12,13,15,16,19,21} Much of the evidence for this relates to type 2 diabetes but studies encompassing type 1 diabetes have also been presented, albeit insufficient.^8,22-26 An important outcome of low-carbohydrate diets in diabetes is controlled glycemia through a reduction in glucose production; this in turn reduces insulin needs and medication dosing.^{6,23,24,27,28} Additionally, they promote weight loss, which subsequently has a positive impact on many aspects of diabetes including glycemic control, insulin resistance, and cardiometabolic risk.^{6,10,13,29,30} The decrease in weight is likely multifactorial; it may be a result of decreased intake of calorie-dense carbohydrates, reduced total calories, reduced hunger, and decreased insulin levels (hyperinsulinemia is associated with increased adiposity⁸).¹³ As low-carbohydrate diets constitute higher protein content, weight loss is also brought on by increased satiety, an effect associated with high dietary protein.^31-33 Low-carbohydrate diets can also alleviate insulin resistance.^13,34,35 This association can be primarily explained by fat reduction, mainly from central organs such as the liver. It may also be due to the body’s decreased need for insulin because of diminished postprandial hyperglycemia.¹³ Decreased insulin demand and lowered sugar levels confer further benefits; they potentially contribute to improved pancreatic beta-cell function by reducing the cells’ workload, allowing them to rest, and limiting their exposure to prolonged hyperglycemia.¹³ Another positive impact is reduced cardiovascular risk, associated with improvements in total and low-density lipoprotein cholesterol levels and an increase in high-density lipoprotein cholesterol.^6,18 Furthermore, low-carbohydrate diets may potentially lead to remission of type 2 diabetes (currently defined as sustained glycated hemoglobin (HbA1c) levels of <6.5% without antihyperglycemic medications or with metformin only).^13,15,19,36

Supporting evidence for low-carbohydrate diets in diabetes management

Many scientific studies demonstrate the benefits of low-carbohydrate diets in diabetes, endorsing their adoption in its management. They include studies assessing the effect of low-carbohydrate diets on weight loss, HbA1c (an indicator of glycemia), insulin sensitivity, medication requirements, lipid levels, and cardiovascular risk, and those comparing their efficacy to other dietary patterns. Low-carbohydrate diets have been successful in encouraging and maintaining weight loss^{6,10,13,29,30}; improving glycemic control^23,24,27,28; reducing diabetic medication needs^22,23,37-39; decreasing triglyceride levels^18,29,32,35; increasing high-density lipoproteins^29,40-42; and reducing cardiovascular risk.^5,43 Nevertheless, these studies are not without limitations, and researchers recommend meticulous interpretation of findings and additional research.^8,13 Regulatory bodies responsible for creating health policies and guidelines have asserted that low-carbohydrate diets are effective in managing diabetes.^17,44-47 They, however, maintain that a tailored approach is best as there is insufficient evidence to support a specific diet for all diabetic individuals. They also emphasize the need for further research in assessing the long-term efficacy and safety of a low-carbohydrate diet and its benefits in type 1 diabetes.

Certain eating patterns are more beneficial for specific outcomes in managing diabetes. The Mediterranean, Dietary Approaches to Stop Hypertension (DASH), plant-based (vegan or vegetarian), and low-fat diets are other meal plans that have demonstrated good efficacy in managing diabetes.^10,16 In comparison, studies suggest that low-carbohydrate diets are more likely to improve glycemic control, promote weight loss, and decrease lipid levels,^{8,13,16,18,19,29,48} while some studies have found no added advantage over other diets.^49,50 However, low-carbohydrate diets have shown greater potential to decrease medication requirements⁵¹ and produce remission of type 2 diabetes.¹³ The most commonly observed differences in efficacy are weight loss,^36,42,52-54 HbA1c,^{6,18,32,36,48,53} triglycerides,^18,32,36,52 high-density lipoproteins,^29,36.40,52 and medication requirements,^22,23,37-39 with most studies demonstrating the superiority of low-carbohydrate diets.^13,18 Greater reductions in diabetes medications substantiate the superior benefit of carbohydrate-restricted diets in glycemic control.¹³ In the long-term, low-carbohydrate approaches may provide no or only slight added benefit,^13,15,17,53 though other studies have found both short-term and long-term benefits.^18,30

Despite the benefits of low-carbohydrate diets demonstrated in scientific publications, limitations exist. For instance, changing the number of carbohydrates requires an increase in fat and protein proportions, creating a challenge in the nutritional management of diabetes and possibly producing adverse outcomes.¹³ Another limitation is the difficulty in adhering to this diet long-term (often defined as >12 months in research settings), although no more than other dietary approaches.^6,13 Certain sources of quality carbohydrates are comparatively more expensive and may affect adherence to the meal plan.^12,13 Adherence to dietary interventions can be improved by continued support and monitoring of patients.^13,15,19 In addition, the restriction of certain types of carbohydrate-containing foods can lead to negative health impacts (discussed later).

Implementing low-carbohydrate diets for diabetes management

Optimal low-carbohydrate meal planning for diabetes should focus not only on the quantity of carbohydrates but on the quality too.¹⁴ High-quality carbohydrates include dairy foods (e.g., milk and yogurt), fruits, legumes, whole grains, and starchy vegetables, which also contain some amounts of protein, fat, fiber, vitamins, and minerals, whereas sugars and refined carbohydrates are considered less healthy^14,44; low-carbohydrate diets limit sugary and refined carbohydrates. Very-low-carbohydrate diets also restrict whole grains, fruit, fibers, and starchy vegetables in addition to sugars and refined carbohydrates.^8,18,35 Nevertheless, a fully balanced meal consisting of healthy fats, protein, and micronutrients should be consumed; supplementation should be given in highly-restricted diets.^18,35,41  Plant-based fats and proteins should be favored over animal sources.¹⁴ Most low-carbohydrate approaches begin with an induction phase in which carbohydrates are significantly restricted to induce weight loss, followed by a maintenance phase when they are gradually reintroduced until weight loss stops and optimal weight is maintained.³¹

Self-monitoring of blood sugar levels is an important part of managing sugar levels, and patient education is imperative. It is essential in providing feedback on glycemic control so that individuals can adjust their approaches to blood sugar management as needed.⁵⁵ In patients following a low-carbohydrate intervention, it will help prevent hypoglycemic events, especially while taking insulin or diabetes medications as dosing can be adjusted effectively.⁵⁶ In addition, it helps monitor how foods consumed affect blood sugar levels and encourages adherence to dietary and behavioral changes.⁵⁷ This can be done by using glucose testing strips and a glucometer. The frequency at which sugar levels are monitored varies from several times a day to daily to weekly; four times daily (postprandial, fasting, before meals, and before bed) is recommended in individuals taking insulin.⁵⁷ Continuous glucose monitoring (CGM) technologies are other means of monitoring sugar levels.⁵⁶

Other than dietary changes, healthcare providers should reinforce lifestyle behavioral changes including increased physical activity, cessation of smoking, and reduction of alcohol consumption. This will have a compounding effect on diabetes management. Close monitoring of patients following a low-carbohydrate diet is recommended to assess glucose levels, lipid profiles, renal function, protein intake, and medications for appropriate adjustments.¹⁹

Potential adverse effects of low-carbohydrate diets for diabetes management

Since the popularization of carbohydrate-restricted diets, theoretical concerns for adverse outcomes with long-term use have been raised,²¹ including the potential occurrence of hypoglycemia (low blood sugar), nutrient deficiencies, dehydration, constipation, growth and developmental delay in children, metabolic ketoacidosis, and cardiovascular, renal and skeletal risks.

Hypoglycemia (low blood sugar): While anyone can experience low blood sugar, it becomes a potentially fatal condition in those taking antihyperglycemic medications. Extreme reduction of carbohydrate intake has the potential to result in low blood sugar when diabetic medication is administered.⁴¹ However, decreased insulin demand associated with low-carbohydrate diets minimizes the risk of this complication.⁸ To mitigate this risk, care providers must review the patient’s medications and make appropriate dose adjustments at the onset and during the course of treatment.^1,13,49

Nutrient deficiencies: Deficiencies in vitamins and minerals, such as thiamine, folate, vitamin C, magnesium, calcium, iron, and iodine, may occur when carbohydrate intake is substantially restricted.^8,35,58 However, a nutritionally balanced diet, which should be emphasized to patients, can be achieved with low-carbohydrate diets.⁴¹ In contrast, individuals on a very-low-carbohydrate diet may require supplementation.^18,35,41

Dehydration: Restricting carbohydrate intake leads to significant diuresis at the onset of the diet, which can result in dehydration.^16,31 This is because depletion of glycogen stores, which are utilized in the absence of glucose, is associated with renal water loss (2–4g of water are bound to 1g of glycogen).^31,58 It also results from ketosis-induced diuresis.⁵⁸

Constipation: Increased dietary intake of fibers has been used to treat constipation as they increase intestinal transit, and stool frequency and improve stool consistency.⁵⁹ Low-carbohydrate approaches with very low amounts of dietary fiber can thus lead to constipation.^31,41

Growth failure in children: Low-carbohydrate diets in children with diabetes are discouraged¹⁷ as nutrients and minerals essential for growth and development may be lacking in this dietary approach,^18,35 leading to failure of growth and developmental delay.^8,60

Metabolic ketoacidosis: As fats are the body’s main source of energy in low-carbohydrate diets, there is a risk of developing ketoacidosis.^15,35 It is a life-threatening complication that is more common in type 1 diabetes and results from excessive accumulation of ketones, which are produced from fatty acids in the liver when insulin is significantly deficient and glucose cannot be utilized for energy.^15,61 When pathological in concentration, ketones raise the blood pH, leading to metabolic acidosis (ketoacidosis). Ketoacidosis has been observed with very-low-carbohydrate diets; however, only a few cases have been reported.^8,35

Cardiovascular disease: Due to increased fat consumption in low-carbohydrate diets, a concern for adverse outcomes on cardiovascular health has arisen. Studies have reported negative cardiovascular implications of fats, mainly saturated fats, as they elevate the serum amounts of total and low-density lipoprotein cholesterol, a risk factor for cardiovascular disease.^8,34 Since low-carbohydrate diets lead to a reduction in triglycerides and increase high-density lipoprotein cholesterol, this risk is possibly counteracted.⁸ Still, unsaturated fats (generally from fish and plants), which are healthier, are promoted over saturated fats (most animal fats) to reduce this risk.^6,18

Kidney disease: Protein quantity in low-carbohydrate diets may be increased to compensate for decreased carbohydrate intake. High protein intake can have a negative impact on kidney health in diabetic patients with pre-existing renal dysfunction.^33,41 This can be avoided by assessing patients’ kidney function before commencing a low-carbohydrate, high-protein diet and monitoring renal function.¹⁸

Bone disease: Since some low-carbohydrate diets restrict the intake of dairy products, important sources of dietary calcium, a potential negative effect on bone health should be considered.^31,58 Additionally, ketosis, a feature of very-low-carbohydrate diets, can stimulate calcium mobilization from bones, leading to bone resorption.⁵⁸ This risk may be amplified by increased consumption of animal-based proteins, which promotes renal loss of calcium, although research findings are conflicting.⁵⁸

Conclusion

Diabetes remains a significant public health concern worldwide despite advances in its management. Dietary intervention makes up an important component of diabetes management. Low-carbohydrate diets in diabetes have been advocated for by current guidelines from regulatory bodies and professional groups. They are safe and effective in promoting weight loss, improving glycemic control, decreasing insulin demands, alleviating insulin resistance, and reducing or eliminating the need for medications in the short term. The limitations and potential adverse outcomes of a low-carbohydrate diet may deter patients from choosing this pattern of eating. Thus, healthcare providers are advised to equip patients with sufficient education on the benefits and potential consequences of a low-carbohydrate diet to facilitate informed decisions. There is no evidence for an ideal conclusive eating pattern that is expected to benefit all individuals with diabetes, and a specific diet should be presented to patients after consideration of individual nutritional needs, metabolic and overall health, and personal preferences.

Future research surrounding low-carbohydrate diets should include assessments of long-term impacts on relevant clinical outcomes and adherence; potential negative effects on health; their safety in patients with diabetes-related comorbid conditions; effects on diabetic parameters irrespective of weight loss and medication decreases; standardization of the classification of low-carbohydrate diets to eliminate inconsistencies in research findings; and comparisons between different carbohydrate-restricted diets with clear distinctions in carbohydrate quantity.^10,13,18,62 Additionally, studies with larger study samples and longer durations are required to draw more accurate conclusions.

References