- There is probably little to no difference in the weight lost by people following low-carbohydrate weight-reducing diets (also known as 'low-carb diets') compared to the weight lost by people following balanced-carbohydrate weight-reducing diets, for up to two years.
- Similarly, there is probably little to no difference between the diets for changes in heart disease risks, like diastolic blood pressure, glycosylated haemoglobin (HbA1c, a measure of blood sugar levels over 2-3 months) and LDL cholesterol (‘unhealthy’ cholesterol) up to two years.
- This was the case in people with and without type 2 diabetes.
What are low-carbohydrate and balanced-carbohydrate weight-reducing diets?
People spend lots of money on trying to lose weight using diets, products, foods and books, and continue to debate about which diets are effective and safe. So, examining the scientific evidence behind claims made is important. Low-carbohydrate diets are a broad category of weight-reducing diets that manipulate and restrict carbohydrates, protein and fat in diets. There are no consistent, widely-accepted definitions of these diets and different descriptions are used (e.g. 'low-carbohydrate, high-protein’, 'low-carbohydrate, high-fat', or ‘very low-carbohydrate’).
Low-carbohydrate diets are implemented in different ways, but they restrict grains, cereals and legumes, and other carbohydrate-containing foods; such as dairy, most fruit and certain vegetables. These foods are then typically replaced with foods higher in fat and protein; such as meats, eggs, cheese, butter, cream, oils. Some low-carbohydrate diets recommend eating as desired, while others recommend restricting the amount of energy eaten.
Balanced-carbohydrate diets contain more moderate amounts of carbohydrates, protein and fats, in line with current healthy eating advice from health authorities. When used for weight reduction, balanced diets recommend restricting the amount of energy eaten by guiding people to reduce their portion sizes and choose healthier foods (e.g. lean instead of fatty meat).
Low-carbohydrate weight-reducing diets are widely promoted, marketed and commercialised as being more effective for weight loss, and healthier, than 'balanced'-carbohydrate weight-reducing diets.
Professor Celeste Naude explains, "The weight lost by people on low-carbohydrate weight-reducing diets was similar to the weight lost by those on balanced-carbohydrate weight-reducing diets, for up to two years. Changes in heart disease risk factors were also similar in people following these diets for between one and two years. This was the case in people with and without type 2 diabetes.
"The longest of the trials lasted for two years, so we do not know if there are differences between the effects and safety of these diets beyond two years, which would be especially important for heart disease risk factors.
Most of the people in the trials did not have heart disease or related risks at the start of the studies, so we do not know if there are differences between the effects and safety of these diets in people with heart disease or risk factors, such as conditions that cause abnormal levels of fats in the blood.
Our review did not compare type or quality of carbohydrates, fats or proteins between the diets, or costs between the diets.”
What did the review authors want to find out?
They wanted to find out if low-carbohydrate weight-reducing diets were better for weight loss and heart disease risk factors than balanced-carbohydrate weight-reducing diets in adults who were overweight or living with obesity.
They wanted to find this out for people with and without type 2 diabetes.
What did they do?
They searched six electronic databases and trial registries for all trials* that compared low-carbohydrate weight-reducing diets with balanced-carbohydrate weight-reducing diets in adults who were overweight or living with obesity. The trials had to last for at least three months. The authors compared and summarised the results of the trials and rated the confidence in the combined evidence, based on factors such as study methods and sizes.
*A trial is a type of study in which participants are assigned randomly to two or more treatment groups. This is the best way to ensure similar groups of participants.
What did they find?
The authors found 61 trials involving 6925 people who were overweight or living with obesity. The biggest trial was in 419 people and the smallest was in 20 people. All except one of the trials were conducted in high-income countries worldwide, and nearly half were undertaken in the US (n=26).
Most trials were undertaken in people who did not have heart disease or risk factors (n = 36). Most people (n = 5118) did not have type 2 diabetes.
The average starting weight of people across the trials was 95 kg. Most studies lasted for six months or less (n = 37); and the longest studies (n = 6) lasted for two years.
Low-carbohydrate weight-reducing diets probably result in little to no difference in weight loss over the short term (trials lasting 3 to 8.5 months) and long term (trials lasting one to two years) compared to balanced-carbohydrate weight-reducing diets, in people with and without type 2 diabetes.
In the short term, the average difference in weight loss was about 1 kg and in the long term, the average difference was less than 1 kg.
People lost weight on both diets in some trials. The amount of weight lost on average varied greatly with both diets across the trials from less than 1 kg in some trials and up to about 12 kg in others in the short term and long term.
Similarly, low-carbohydrate weight-reducing diets probably result in little to no difference in diastolic blood pressure, glycosylated haemoglobin (HbA1c) and LDL cholesterol (‘unhealthy’ cholesterol) for up to two years.
The authors could not draw any conclusions about side effects reported by participants because very few trials reported these.
What are the limitations of the evidence?
The authors are moderately confident in the evidence. Confidence was lowered mainly because of concerns about how some the trials were conducted, which included that many trials did not report all their results. Further research may change these results.
How up to date is this evidence?
The evidence is up-to-date to June 2021.
What gaps did the authors identify?
They do not know if there are differences between the effects and safety of these diets beyond two years.
Since most of the people in the trials did not have heart disease or heart disease risks when they were recruited, the authors do not know if there are differences between the effects and safety of these diets in people with heart disease or risk factors, such as conditions that cause abnormal levels of fats in the blood.
What important related questions were not addressed in this review?
The author team did not compare type or quality of carbohydrates, fats or proteins between the diets. They also did not examine differences in costs between the diets.