Before starting a ketogenic diet or any other highly restrictive diet, we strongly encourage you to consult your doctor or a member of your diabetes care team.

The ketogenic diet is attracting growing interest from people with type 2 diabetes because of its potential benefits for weight and blood sugar control. However, it is premature to recommend that people with diabetes adopt the ketogenic diet given the lack of knowledge about its medium- and long-term impact. Like other highly restrictive diets, the ketogenic diet is difficult to maintain for more than a few months and can lead to overeating in reaction to deprivation.

What is the ketogenic diet?

The ketogenic or keto diet, also called the “very low carb diet,” contains mainly fat (fatty acids, also known as lipids) and very few carbohydrates. Depending on the study, the carbohydrate content can vary between 20 g and 50 g per day. This is the level at which ketone bodies are produced. In comparison, the diet currently recommended for people with diabetes (the conventional diet) and presented as a balanced plate provides between 130 g and 225 g of carbohydrates per day! The conventional diet has enough carbohydrates to meet the brain’s requirements, estimated to be 130 g per day.

Table comparing a conventional and ketogenic diet

Conventional dietKetogenic diet
CharacteristicsVariety, moderation Meets the need for vitamins, minerals, fibre and other nutrients such as antioxidants Low in saturated fat and added sugarsLittle variety, some prohibited foods High in fat Low in vitamins and minerals, fibre No added sugars
CompositionHigh in carbohydrates: 130 g to 225 g per day Moderate in protein and fatVery low in carbohydrates: 20 g to 50 g per day High in fat Moderate or high in protein
FoodsVegetables and fruit Whole grain starches Meat, tofu, beans, eggs, fish, poultry, cheese Milk and alternatives Nuts and seeds Olive oil and canola oilRed meat, poultry, eggs, fish, cured meats, cheese Fat (oil, butter, cream, nuts) Low-carb vegetables only, such as green and leafy vegetables 1 serving per day of starchy foods 1 ½ servings per day of milk or milk alternatives No fruit or beans

This table highlights the major differences between the two types of diets. Notably, the conventional diet encourages variety and does not exclude any food group, which increases its nutritional value.

How the body reacts when carbohydrates are severely restricted

To function throughout the day, the body needs energy, which it draws from the food it ingests. Normally, carbohydrates are its main source of energy. In the ketogenic diet, fat takes over from glucose as a fuel. The brain is the exception because a natural barrier blocks access to fat. When carbohydrates are severely restricted, as in the ketogenic diet, the brain must therefore resort to an alternative source of energy, called “ketone bodies.”

During the first few days of a keto diet, the body responds by drawing energy from its reserves of glucose stored in the liver and muscles. The use of these reserves releases a significant amount of water, which is eliminated in the urine, resulting in temporary and rapid weight loss due to dehydration. Once the glucose reserves are depleted, the liver uses fatty acids (lipids) and proteins to provide energy for the body. The breakdown of fatty acids by the liver generates ketone bodies, which are used primarily to fuel the brain. Some of the ketone bodies are eliminated when breathing, which explains the fruity-smelling breath in people who follow this diet.

In addition to this unpleasant breath, some undesirable effects usually occur during the first few weeks after starting a ketogenic diet. The most commonly reported are fatigue, headache, dizziness, nausea, and muscle cramps. Hypoglycemia can also occur in some people with diabetes depending on their medication1. Constipation sets in quickly and persists because of a lack of fibre. This problem is aggravated by insufficient hydration.

Effectiveness of the ketogenic diet

Many benefits are attributed to the keto diet. However, many shortcomings in the studies reviewed limit the scope of the results obtained and make it difficult to evaluate this diet.

The main weaknesses of these studies are:

  • Great variability among the studies, in the amount of carbohydrates in the ketogenic diets tested; some so-called keto diets allow up to 125 g of carbohydrates per day!
  • Inconsistent composition of the ketogenic diet in terms of the type of fats and protein to be consumed, as well as the quantity of vegetables.
  • Some conventional diets tested did not meet recommendations and contained a significant amount of refined starchy foods and foods with added sugar.
  • Inability to verify whether the participants actually followed the assigned diet (ketogenic or conventional) until the end of the study.
  • Short-term studies, many biases, no distinction between diabetics and non-diabetics, no control group, small number of participants, etc.

Effect on weight

When there is a major restriction of calories, the ketogenic diet produces significant weight loss, comparable to other very restrictive diets, for a period of two to six months. In this case, the weight loss from the ketogenic diet appears to be caused by a decrease in the calories consumed, rather than by restricting carbohydrates. After one to two years, the weight loss observed is modest and similar to all these diets. The inability to maintain such diets beyond six months accounts for these results.

What about the effect on weight when the diets are not calorie restricted? Studies of overweight people with type 2 diabetes show that after one or two years, the ketogenic diet produces weight loss similar to the conventional diet. The decrease or loss of appetite associated with the presence of ketone bodies in the blood does not seem to persist over the long term. It has also not been possible to demonstrate that the intensive consumption of fat in the ketogenic diet would be more effective in increasing basal metabolism than the conventional diet.

In short, the ketogenic diet does not seem to be any more effective for weight loss in the long run than the conventional diet.

Effect on blood glucose control

One would expect to find a decrease in A1C (glycated hemoglobin) in a diabetic person who follows a ketogenic diet because the amount of carbohydrates consumed is minimal. Over a period of three to six months, studies have demonstrated a greater reduction in A1C on the ketogenic diet than on the conventional diet. However, this difference was no longer present after one year.

A decrease in diabetes medication has also been observed in the short term on the ketogenic diet, but the long-term results are contradictory.

Some recent studies suggest that the ketogenic diet could be more effective than the conventional diet in reducing meal-related blood sugar fluctuations, but these results remain to be confirmed by larger studies.

In short, the ketogenic diet seems to reduce A1C in the short term, but the results are inconclusive over the long term.

Effect on blood lipids and cardiovascular disease

The effect of the ketogenic diet on blood lipids (fatty acids) is unclear. Studies in people with type 2 diabetes comparing the ketogenic diet to the conventional diet have shown a similar effect on LDL-C (bad cholesterol) and HDL-C (good cholesterol) in the short and medium term.

However, the large amount of fat in the ketogenic diet and the lack of importance given to the type of fat consumed can have a significant impact on blood lipids. Thus, we often observe in the keto diet a predominance of foods rich in saturated animal fat, such as bacon, red meat, cream, and butter. Consumption of these foods is associated with increased LDL-C, a recognized risk factor for cardiovascular disease. It would be interesting if studies evaluated the ketogenic diet’s effect on blood lipids based on the different types of fat it allows in the diet.

Long-term effects of the ketogenic diet

To date, the long-term effects of the ketogenic diet are still unknown because the duration of the studies has not exceeded two years. However, several researchers are concerned about its potentially harmful impact on health. The low consumption of certain foods such as fruit, vegetables, and whole grain products, recognized to have a protective effect against certain cardiovascular diseases and cancer, could also lead to deficiencies in certain minerals, vitamins, and antioxidants in the medium term. In addition to chronic constipation, the lack of fibre in this diet could generate other negative effects on intestinal health by disrupting the microbiota (intestinal flora). This imbalance could reduce the effectiveness of the immune system.

Some precautions if you decide to follow a ketogenic diet

If you still decide to adopt a ketogenic diet, here are some ways to mitigate its potential unwanted side effects:

  • Measure your blood sugar more often if you are at risk of hypoglycemia1;
  • Take a multivitamin and dietary fibre supplements;
  • Hydrate well with water throughout the day;
  • Eat low-carb vegetables rather than completely eliminate them (e.g., peppers, broccoli, lettuce, cucumber, etc.);
  • Consume foods high in calcium, such as hard cheeses, yogurt, almonds, etc.
  • Choose the good fats for heart health, such as olive oil and canola oil, nuts and fatty fish.

In conclusion, unlike the “miracle” diets, the conventional diet, represented by the balanced plate model, offers people with diabetes and their families a varied diet, one that is sustainable over the long term and whose goal is to promote the joys of eating well. Mediterranean or vegetarian diets are also viable options. The choice of one diet over the other is a question of personal preference since all of them have demonstrated their beneficial effects on the health of people with type 2 diabetes.

1. People at risk of hypoglycemia: on insulin or taking insulin secretagogues, such as gliclazide (Diamicron® and Diamicron® MR), glimepiride (Amaryl®), glyburide (Diabeta®, Euglucon®), repaglinide (GlucoNorm®).