What is a low carb diet?
Obesity is one of the main problems, especially in the US, leading to health problems such as Type 2 diabetes. One popular diet for weight management to control diabetes is known as the low carb diet. As the name already states, in a low carb diet, carbohydrates within the diet are restricted.
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There are different types of carbohydrate-restricted diets, some of which restrict carbohydrates to very low levels without restricting dietary protein or fat (for example, the Atkins-style diet). Others allow moderate carbohydrate consumption together with moderate protein and fat intake (for example, South Beach and Zone). Very-low-carb diets limit protein to moderate levels, which introduces ketosis without restricting fat or total calories.
The 2015 to 2020 Dietary Guidelines for America recommend three healthy eating patterns that including US healthy eating diet, a healthy Mediterranean diet, and the healthy vegetarian diet. The carbohydrates within a diet can be divided into simple and complex carbs. Simple carbohydrates such as sugar, white flour, and white rice have shown to increase the risk of type 2 diabetes.
To prevent type 2 diabetes mellitus, minimizing carbohydrate intake to total daily caloric consumption, as well as intake of foods with a low glycaemic index, is recommended. For patients with type 2 diabetes mellitus, the American Diabetes Association recommends a moderate intake of carbohydrates (44%-46% of total calories).
The carbohydrates should be taken from vegetables, legumes, fruits, low-fat dairy, and whole grains, with limited sugars from natural sources such as fruit. Brown rice and whole wheat products should replace white wheat products, and foods with added sugar should be avoided. Several studies have shown the effectiveness of a low carb diet for the purpose of weight loss. However, recent studies have emerged questioning whether this diet has any effects on patients with Type 2 diabetes.
What is Type 2 diabetes?
Worldwide, 382 million adults have diabetes. It is a major cause of death in patients under 60 years of age. Type 2 diabetes is characterized by insulin resistance. Overconsumption of food or certain types of food causes the pancreas to create extra insulin in order to keep blood glucose (or blood sugar) under control.
Eventually, the blood glucose reaches an uncontrollable level because the pancreas can no longer keep up with the demand. Being inactive or overweight increases the risk of Type 2 diabetes. Food consumption is, therefore, linked to the development of Type 2 diabetes.
Symptoms of diabetes can include thirst, increased toilet trips, and tiredness. However, diabetes also increases the risks of heart attack and stroke. Diabetes can also cause problems with blood vessels outside the heart and brain. Irreversible nerve damage, vision problems and blindness, and kidney disease are signs of long-term diabetes. Other than medication, diet is the most important way to prevent and control diabetes and its complications.
Effects of low carbohydrate diets for Type 2 diabetes
Several studies have been conducted to try to determine the most healthful diet for persons with type 2 diabetes. Results have been conflicting because the types of diets, methods, and study sizes have all varied. However, a pattern has emerged.
In 2005, Boden et al. studied the effects of a low carb diet on ten obese people with Type 2 diabetes. The study reported improved 24-hour blood glucose profiles. Furthermore, the hemoglobin A1c (HbA1c; blood glucose) decreased from 7.3% to 6.8 % in only two weeks.
These findings suggested that blood glucose levels were regulated by carbohydrate intake. Insulin sensitivity also improved due to an improved glucose-insulin ratio. This study was conducted for only two weeks, so the long-term effects of a low carb diet could not be determined.
More recently, Skytte et al. (2019) evaluated the dietary recommendations for people with type 2 diabetes. In the study, a carbohydrate-reduced high-protein (CRHP) diet was compared to iso-energetic conventional diabetes (CD) diet to see the effects on glycaemic control and selected cardiovascular risk markers during the six weeks.
Patients had the same number of calories on each diet but exchanged some of the carbohydrates from the CD diet for protein in the CRHP diet. The trial concluded that, compared to the CD diet, the CRHP diet reduced HbA1c levels and fasting glucose levels. The study shows that substituting carbohydrates with protein and fat without increasing calories for six weeks reduced HbA1c and liver fat content in people with type 2 diabetes.
Taken together, these two studies point to the likely benefits of replacing some carbohydrate intake with protein. However, this type of diet is very different from some popular high-protein and high-fat diets that encourage unrestricted or high levels of animal product intake. A 2019 review by Adeva-Andany et al. showed that increasing animal product intake and following a low-carb diet increases the risk of type 2 diabetes. Eating high-quality plant-based foods prevents diabetes.
It is generally accepted among healthcare providers and nutrition scientists that reducing simple carbohydrates in the diet (sweets, sodas, white bread, regular pasta) is important for improving diabetes outcomes and heart disease.
When decreasing carbohydrate intake and increasing protein intake, patients with diabetes should focus on plant sources whenever possible (beans, legumes, soy, nuts). Lean meats and fish can be healthful sources of protein and fat for those who eat animal products.
Impacts of low carbohydrate intake in patients with Type 2 diabetes
All studies confirmed that a low carbohydrate diet could be beneficial for the management of Type 2 diabetes. In some patients, it even helped to discontinue or reduce their medication.
Decreasing carbohydrate intake can be a healthful weight loss and diabetes control strategy, so long as carbohydrates are replaced with plant-based sources of protein and fat. Replacing carbohydrates with animal food sources increases the risk of diabetes and worsens glucose control for those with diabetes.
Boden, G., 2013. Effect of a Low-Carbohydrate Diet on Appetite, Blood Glucose Levels, and Insulin Resistance in Obese Patients with Type 2 Diabetes. https://www.ncbi.nlm.nih.gov/pubmed/15767618/.
Skytte, M. J. et al., 2019. A carbohydrate-reduced high-protein diet improves HbA1c and liver fat content in weight stable participants with type 2 diabetes: a randomised controlled trial. https://link.springer.com/article/10.1007%2Fs00125-019-4956-4.
Adeva-Andany M.M. 2019. Dietary habits contribute to define the risk of type 2 diabetes in humans. https://doi.org/10.1016/j.clnesp.2019.08.002.
Pallazola, V. et al., 2019. A Clinician's Guide to Healthy Eating for Cardiovascular Disease Prevention. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6713921/.