Low Carbohydrate Diet

The Diabetes Canada guidelines recommend that in adults with diabetes, the daily energy should be split as:

• 45% to 60% from carbohydrate
• 15% to 20% from protein
• 20% to 35% from fat

Many patients with diabetes will adjust their macronutrient intake to improve glycemic control, reduce cardiovascular risk factors (e.g. hypertension, dyslipidemia), and reduce weight.  One of the most common eating patterns is a low carbohydrate diet.

What is a Low Carbohydrate Diet?

The American Diabetes Association published the Nutrition Therapy for Adults With Diabetes or Prediabetes: A Consensus Report. It reviews a variety of different eating patterns.  They state a low carbohydrate diet:

• Emphasizes vegetables low in carbohydrate (such as salad greens, broccoli, cauliflower, cucumber, cabbage, and others); fat from animal foods, oils, butter, and avocado; and protein in the form of meat, poultry, fish, shellfish, eggs, cheese, nuts, and seeds. Some plans include fruit (e.g., berries) and a greater array of non-starchy vegetables. Avoids starchy and sugary foods such as pasta, rice, potatoes, bread, and sweets. There is no consistent definition of “low” carbohydrate. A common definition is reducing carbohydrates to 26–45% of total calories.  Others state a low carbohydrate diet as < 130 g of digestible carbohydrates per day
• Reduces A1C, reduces weight, reduces triglycerides, increases HDL-C and lowers blood pressure
• In individuals with diabetes has demonstrated the most evidence for improving glycemia and may be applied in a variety of eating patterns that meet individual needs and preferences
• Is a viable approach for select adults with type 2 diabetes not meeting glycemic targets or where reducing antiglycemic medications is a priority,

Impact of Low Carbohydrates on Antihyperglycemic Agents

A review of diabetes medication adjustments for low carbohydrates was published in 2019 (Adapting diabetes medication for low carbohydrate management of type 2 diabetes: a practical guide). The authors have a few key points and recommendations for clinicians:

• When a patient starts on a low carbohydrate diet, there is commonly a substantial drop in blood glucose levels
• With these changes there can be adjustments that may be required as patients start these diets

Medications that increase the risk of hypoglycemia

• Sulfonylureas and megitinides – should be reduced or stopped when low carbohydrate diet is started.  An initial dosage reduction of at least 50% is typically appropriate, with further reductions according to blood glucose response
• Insulin – Practical expertise suggests a 50% reduction of daily insulin dose at initiation of the low carbohydrate diet is appropriate in most cases. In individuals whose A1C is markedly elevated, a smaller reduction of perhaps 30% may be appropriate, with further reductions over time. For individuals on a basal-bolus regimen it is preferential to reduce or stop bolus insulin. In individuals on a mixed insulin or basal insulin alone each dose can be reduced by 30–50% at the start of low carbohydrate diet. Monitoring is crucial for further adjustments

Medications that risk acidosis

• SGLT2 inhibitors – These carry a risk of causing ketoacidosis if a patient has significant insulin insufficiency, with any diet. SGLT2i-induced ketoacidosis may occur with a normal blood glucose, which heightens the risk of the life-threatening condition going unrecognized. In a community setting, for safety and simplicity, it may be appropriate for most patients to stop their SGLT2i when an low carbohydrate diet is initiated. This removes the SGLT2i-induced ketoacidosis risk, and additionally the effectiveness of the low carbohydrate diet means the benefit of a SGLT2i is diminished. (Note: a very low carbohydrate diet, typically <30–50 g carbohydrate/day, can produce a physiologically normal state of ketosis, which should not be confused with the pathological state of ketoacidosis.)

Medications that pose no increased risk

• Metformin – Safe to continue.  Up to 25% of patients have GI related adverse effects
• GLP-1 receptor agonists – These are safe to continue, with the beneficial actions of increased satiety and slowed gastric emptying, and possibly cardiovascular benefit.
• Thiazolidinediones – These are safe to continue from a short-term perspective. There are long-term safety issues and discontinuation be considered as soon as blood glucose levels allow
• DPP-4 inhibitors – These are safe to continue. However, clinical experience agrees that these seem to have little blood glucose-lowering effect in the context of an low carbohydrate diet

What does this mean for practice?

• Changes in dietary eating patterns can have a significant impact on glycemic control
• Low carbohydrate diet have been shown to have some positive effects in people with type 2 diabetes
• There are some adjustments to antihyperglycemic agents when patients start low carbohydrate diets