• 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 (such as, '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.
What did we want to find out?
We 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.
We wanted to find this out for people with and without type 2 diabetes.
What did we do?
We 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. We compared and summarised the results of the trials and rated our 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 we find?
We 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 USA (26). Most trials (36) were undertaken in people who did not have heart disease or risk factors. Most people (5118 people) did not have type 2 diabetes. The average starting weight of people across the trials was 95 kg. Most studies (37) lasted for six months or less; and the longest studies (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.
We could not draw any conclusions about unwanted effects reported by participants because very few trials reported these.
What are the limitations of the evidence?
We are moderately confident in the evidence. Our 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.
There is probably little to no difference in weight reduction and changes in cardiovascular risk factors up to two years' follow-up, when overweight and obese participants without and with T2DM are randomised to either low-carbohydrate or balanced-carbohydrate weight-reducing diets.
Debates on effective and safe diets for managing obesity in adults are ongoing. Low-carbohydrate weight-reducing diets (also known as 'low-carb diets') continue to be widely promoted, marketed and commercialised as being more effective for weight loss, and healthier, than 'balanced'-carbohydrate weight-reducing diets.
To compare the effects of low-carbohydrate weight-reducing diets to weight-reducing diets with balanced ranges of carbohydrates, in relation to changes in weight and cardiovascular risk, in overweight and obese adults without and with type 2 diabetes mellitus (T2DM).
We searched MEDLINE (PubMed), Embase (Ovid), the Cochrane Central Register of Controlled Trials (CENTRAL), Web of Science Core Collection (Clarivate Analytics), ClinicalTrials.gov and WHO International Clinical Trials Registry Platform (ICTRP) up to 25 June 2021, and screened reference lists of included trials and relevant systematic reviews. Language or publication restrictions were not applied.
We included randomised controlled trials (RCTs) in adults (18 years+) who were overweight or living with obesity, without or with T2DM, and without or with cardiovascular conditions or risk factors. Trials had to compare low-carbohydrate weight-reducing diets to balanced-carbohydrate (45% to 65% of total energy (TE)) weight-reducing diets, have a weight-reducing phase of 2 weeks or longer and be explicitly implemented for the primary purpose of reducing weight, with or without advice to restrict energy intake.
Two review authors independently screened titles and abstracts and full‐text articles to determine eligibility; and independently extracted data, assessed risk of bias using RoB 2 and assessed the certainty of the evidence using GRADE. We stratified analyses by participants without and with T2DM, and by diets with weight-reducing phases only and those with weight-reducing phases followed by weight-maintenance phases. Primary outcomes were change in body weight (kg) and the number of participants per group with weight loss of at least 5%, assessed at short- (three months to < 12 months) and long-term (≥ 12 months) follow-up.
We included 61 parallel-arm RCTs that randomised 6925 participants to either low-carbohydrate or balanced-carbohydrate weight-reducing diets. All trials were conducted in high-income countries except for one in China. Most participants (n = 5118 randomised) did not have T2DM. Mean baseline weight across trials was 95 kg (range 66 to 132 kg). Participants with T2DM were older (mean 57 years, range 50 to 65) than those without T2DM (mean 45 years, range 22 to 62). Most trials included men and women (42/61; 3/19 men only; 16/19 women only), and people without baseline cardiovascular conditions, risk factors or events (36/61). Mean baseline diastolic blood pressure (DBP) and low-density lipoprotein (LDL) cholesterol across trials were within normal ranges. The longest weight-reducing phase of diets was two years in participants without and with T2DM. Evidence from studies with weight-reducing phases followed by weight-maintenance phases was limited.
Most trials investigated low-carbohydrate diets (> 50 g to 150 g per day or < 45% of TE; n = 42), followed by very low (≤ 50 g per day or < 10% of TE; n = 14), and then incremental increases from very low to low (n = 5). The most common diets compared were low-carbohydrate, balanced-fat (20 to 35% of TE) and high-protein (> 20% of TE) treatment diets versus control diets balanced for the three macronutrients (24/61). In most trials (45/61) the energy prescription or approach used to restrict energy intake was similar in both groups. We assessed the overall risk of bias of outcomes across trials as predominantly high, mostly from bias due to missing outcome data. Using GRADE, we assessed the certainty of evidence as moderate to very low across outcomes.
Participants without and with T2DM lost weight when following weight-reducing phases of both diets at the short (range: 12.2 to 0.33 kg) and long term (range: 13.1 to 1.7 kg).
In overweight and obese participants without T2DM: low-carbohydrate weight-reducing diets compared to balanced-carbohydrate weight-reducing diets (weight-reducing phases only) probably result in little to no difference in change in body weight over three to 8.5 months (mean difference (MD) −1.07 kg, (95% confidence interval (CI) −1.55 to −0.59, I2 = 51%, 3286 participants, 37 RCTs, moderate-certainty evidence) and over one to two years (MD −0.93 kg, 95% CI −1.81 to −0.04, I2 = 40%, 1805 participants, 14 RCTs, moderate-certainty evidence); as well as change in DBP and LDL cholesterol over one to two years. The evidence is very uncertain about whether there is a difference in the number of participants per group with weight loss of at least 5% at one year (risk ratio (RR) 1.11, 95% CI 0.94 to 1.31, I2 = 17%, 137 participants, 2 RCTs, very low-certainty evidence).
In overweight and obese participants with T2DM: low-carbohydrate weight-reducing diets compared to balanced-carbohydrate weight-reducing diets (weight-reducing phases only) probably result in little to no difference in change in body weight over three to six months (MD −1.26 kg, 95% CI −2.44 to −0.09, I2 = 47%, 1114 participants, 14 RCTs, moderate-certainty evidence) and over one to two years (MD −0.33 kg, 95% CI −2.13 to 1.46, I2 = 10%, 813 participants, 7 RCTs, moderate-certainty evidence); as well in change in DBP, HbA1c and LDL cholesterol over 1 to 2 years. The evidence is very uncertain about whether there is a difference in the number of participants per group with weight loss of at least 5% at one to two years (RR 0.90, 95% CI 0.68 to 1.20, I2 = 0%, 106 participants, 2 RCTs, very low-certainty evidence).
Evidence on participant-reported adverse effects was limited, and we could not draw any conclusions about these.