Written by Amy Brownstein, MS, RD
You may have heard that a low-carb diet is beneficial for managing diabetes. But does it really work, and can a low-carb diet reverse diabetes? The answer isn’t so simple.
A low-carb diet is effective for weight loss. And weight loss is correlated with a reduction in Hba1c and fasting glucose levels. Despite this association, it is still unclear whether a low-carb diet can reverse diabetes independent of the effects of weight loss on insulin resistance.
Read on to learn more about a low-carbohydrate diet and its effect on diabetes.
What Is a Low-Carbohyrate Diet?
There is no consistent definition of a low-carbohydrate diet. But consuming less than 130 grams of carbohydrates or 45% of calories per day generally constitutes a low-carb diet. Very low-carbohydrate or ketogenic diets restrict carbohydrate intake to less than 40-50 grams or 10% of calories per day. [1-3] In comparison, the United States Dietary Guidelines suggest carbohydrates should make up 45-65% of daily caloric intake (the equivalent of 225-325 grams of carbohydrates based on a 2000 kcal per day diet); protein should be 10-35% and fat 20-35% of daily calories. 
A low-carb diet generally limits grains, cereals, legumes, and other carbohydrate-containing foods, such as dairy, fruit, and starchy vegetables.
Why Are Low-Carb Diets Popular for Diabetes?
Low-carbohydrate diets for diabetes originated with traditional diabetes management, which relies on the glycemic index (a ranking of how quickly a food impacts blood sugar levels) and carbohydrate counting to help manage blood sugar. Foods high on the glycemic index — generally carbohydrates, in particular certain fruits and candy, pasta, bread, and rice — were to be reduced to support lowering blood sugar.
Research supports low-carbohydrate diets for reducing fasting blood sugar and HbA1c levels. In studies lasting up to six months, diets restricting carbohydrate intake to 50 - 130 grams per day appear effective for weight loss and reducing HbA1c. [5,6]
Diet Adherence and Nutrient Composition Matter for Reducing Type 2 Diabetes
When evaluating the effectiveness of a diet, consider two things: adherence and the replaced nutrients.
Adherence Assess the Long-term Sustainability of a Diet
Adherence evaluates whether you can maintain a particular eating pattern in the long term. The more restrictive a diet, the less likely it is to be followed for the long term, leading to weight regain and the return of medical conditions (such as type 2 diabetes). And yo-yo dieting further contributes to the risk of type 2 diabetes.  Dietary modifications need to be ongoing to lower your risk of type 2 diabetes or to put your diabetes into remission.
It Matters What You Replace Carbohydrates With
It is important to consider what a low-carb diet is being replaced with, meaning what foods are increased to compensate for the decrease in calories from carbohydrates. Substituting carbohydrates for animal-based proteins or fat is associated with an increased risk of mortality (death). Conversely, replacing carbohydrates with plant-based proteins — such as tofu, tempeh, or legumes — or fat (nuts and seeds) decreases the mortality risk. 
While healthy fats rich in omega-3 and omega-6 fatty acids — such as nuts and seeds — can be part of a nutritious and beneficial diabetes eating pattern, other fats (particularly saturated fat from animal products) contribute to insulin resistance and your risk of type 2 diabetes. Very low-carbohydrate diets, including the ketogenic diet, encourage a higher fat intake, usually in the form of animal products. This increases the risk of heart disease and can lead to insulin resistance, the root cause of type 2 diabetes.
Eating more animal-based protein is associated with a higher risk of all-cause mortality. And the saturated fat and types of amino acids in animal protein can contribute to insulin resistance.  Moreover, a high protein intake can affect kidney function, which may already be compromised due to preexisting hypertension (high blood pressure) or kidney disease.
Does a Low-Carb Diet Reverse Type 2 Diabetes?
A low-carb diet can reverse diabetes in the short term, but research shows that the beneficial effects of a low-carbohydrate diet may not last in the long term.
One large review found little to no difference in weight loss and HbA1c between people who followed a low-carbohydrate or balanced-carbohydrate diet in the short-term (3-8.5 months) and long-term (1-2 years). At the one to two-year mark, the average difference between these two groups was less than one kilogram (2.2 pounds). Likewise, no difference in HbA1c levels was observed between the two groups at the one to two-year mark. 
A recent systematic review assessing diabetes remission found a low-carbohydrate diet (less 130 grams of carbohydrates per day or less 26% total calories) was associated with a 32% increase in type 2 diabetes remission (defined as HbA1c < 6.5%) at six months compared to control diets (typically low-fat diets). Remission occurred in people following a low-carbohydrate diet regardless of diabetes medication, but the number of people achieving remission was smaller in the group without medication. However, by 12 months, weight loss and the improvements seen in insulin sensitivity and medication were diminished. 
That same systematic review conducted additional analyses on the benefits of very low-carbohydrate diets, defined as less than 40 grams of carbohydrates per day or less than 10% of total calories. Researchers learned that the very low-carb diets were less effective for weight loss than the low-carb diet, likely related to the difficulty associated with following this type of diet. What’s more, participants following the very low-carb diet reported a worsening quality of life at the 12-month mark. 
What Makes a Plant-Based Diet More Effective at Reversing Type 2 Diabetes?
Countless studies show the beneficial effects of a plant-based diet on improving insulin sensitivity. The success of a plant-based diet is due to its focus on diet quality instead of restricting certain macronutrients (protein, fats, and carbohydrates), which contributes to long-term diet adherence.
The switch in nutrient composition of a plant-based diet positively influences weight and other risk factors of type 2 diabetes independent of weight. Foods typical of a plant-based diet — such as whole grains, fruits, vegetables, and legumes — are high in fiber and low in fat, both of which influence insulin resistance and contribute to weight loss and maintenance. 
Fiber is the nondigestible part of plants. It reduces the glycemic index, so foods high in fiber are lower on the glycemic index. Increasing fiber intake is an effective intervention for treating diabetes: fiber can reduce fasting blood glucose levels and HbA1c. 
What’s more, a higher intake of plant proteins is associated with lower all-cause and cardiovascular mortality. [5,14] Replacing just 3% of total energy with plant protein instead of animal protein can significantly reduce the risk of all-cause mortality. 
Let's Recap: Can a Low-Carb Diet Reverse Type 2 Diabetes?
- Research shows that a low-carb diet can lower HbA1c and fasting glucose levels. But it is still being determined if these effects can be maintained in the long-term.
- A low-carbohydrate diet restricts many high-fiber foods, such as fruit, whole grains, and legumes. And fiber is beneficial for reversing type 2 diabetes.
- A plant-based diet improves insulin sensitivity due to its nutrient composition. A plant-based diet is lower in calories and fat and higher in fiber than a low-fat or low-carb diet.
- It is important to consider the long-term sustainability of a diet. Improving insulin sensitivity and achieving type 2 diabetes remission is an ongoing process.