SACN report on lower carbohydrate diets for adults with type 2 diabetes

Low carbohydrate diets, particularly in relation to type 2 diabetes, have been a topic of increasing debate and discussion both in the media and scientific community in recent years. In recognition of this, Public Health England (PHE) requested SACN to perform a systematic assessment of the scientific evidence for low carbohydrate diets for adults with diabetes.  

Following a consultation on a draft report, which was published in March 2020, the final report on lower carbohydrate diets for people with type 2 diabetes (T2D) from the Scientific Advisory Committee on Nutrition (SACN) and Diabetes UK was published on 26 May 2021. The report looked at the effect of following a lower carbohydrate diet on a number of markers and clinical outcomes for people living with T2D, both in the shorter term (up to 6 months) and longer term (beyond 6 months). This report assesses the science on this topic. Any changes to advice for the public on low carbohydrate diets for people with type 2 diabetes will be considered by PHE and Diabetes UK, following this publication of the final report.

It was not possible to assess the impact of a ‘low’ compared to a ‘high’ carbohydrate diet due to variation in the definitions used across the studies included, as well as overlap in the reported carbohydrate intakes between diet groups. Instead, comparisons were made between ‘lower’ and ‘higher’ carbohydrate diets.

Based on 4 systematic reviews and meta-analyses, which included a total of 31 randomised controlled trials (RCTs), the Working Group concluded that:

• Evidence suggests beneficial effects of lower carbohydrate diets for some outcomes (HbA1c, fasting plasma glucose, serum triacylglycerol [triglyceride]) in the shorter term (up to 6 months).
• Since the shorter-term assessments did not report outcomes between 6 and 12 months it is uncertain if the suggested benefits are maintained beyond 6 months.
• There was no consistent evidence of reductions in body weight with lower carbohydrate diets – the primary outcome was weight loss after 12 months or more.
• Lower carbohydrate diets may allow reductions in diabetes medication (although interpretation is complicated by inconsistencies in reporting and measurement of changes in medication use).
• There were no differences between higher and lower carbohydrate diets on serum total cholesterol or LDL cholesterol, either in the shorter (≥3 to 6 months) or longer term (≥12 months).
• Evidence on HDL cholesterol was inconsistent in the shorter (≥3 to 6 months) and longer term (≥12 months).

The Working Group concluded that a lower carbohydrate diet can be recommended by clinicians as an effective short-term option (up to 6 months) for improving glycaemic control and serum triacylglycerol (triglyceride) concentrations for adults living with T2D and overweight (BMI between 25 and 30 kg/m²) or obesity (BMI of 30 kg/m² or higher). However, it was recommended that individuals choosing a low carbohydrate diet should include wholegrain or higher fibre foods, a variety of fruits and vegetables, and limit intakes of saturated fats, in line with current dietary advice for the general population.

The Working Group also highlighted that weight loss remains the primary goal for improving glycaemic control and reducing the risk of cardiovascular disease, as the majority of individuals with T2D are living with overweight or obesity. Health professionals should support evidence-based approaches to help individuals achieve long-term weight loss. For adults with T2D living with overweight and obesity who are taking diabetes medications, it was recommended that they receive advice and support from their healthcare team if changing to a low carbohydrate diet, so as to manage the risk of hypoglycaemia.

The majority of the participants in the RCTs included in the report were living with overweight or obesity, and so it is not known if the reported effects can be generalised to adults living with T2D with a healthy weight (BMI from 18.5 up to 25 kg/m²). Also, the Working Group did not assess the evidence on the effect of lower carbohydrate diets in the general population without T2D, and so it is not known if findings for lower carbohydrate diets in adults with T2D apply to the general adult population (without T2D). It was also not possible to assess whether the reported effects of lower carbohydrate diets apply to individuals of different ethnicities, since the majority of RCTs did not report ethnicity of participants.     

Details from the report

The purpose of the report was to review the evidence on ‘low’ carbohydrate diets compared to current UK government advice on carbohydrate intake for adults with T2D (that about 50% total energy should come from carbohydrates). Since SACN does not usually make recommendations relating to clinical conditions, a joint Working Group was established to consider this issue, including members of SACN and members nominated by Diabetes UK, the British Dietetic Association, the Royal College of Physicians and the Royal College of General Practitioners. The WG was jointly chaired by SACN and Diabetes UK.

The terms of reference for the Working Group were to:

• Review the evidence on lower carbohydrate diets (alongside higher fat and/or higher protein) compared to current government advice for adults with T2D.
• Consider the impact, in adults with T2D, of lower compared with higher carbohydrate diets on markers and clinical outcomes of T2D, including any potential adverse effects.
• Make recommendations based on the review of the evidence.

Primary outcomes of interest were:

• Body weight (≥12 months)
• Glycated haemoglobin (HbA1c) (≥3 months)

Secondary outcomes of interest were:

• Body weight (≥3 to <12 months)
• Fasting plasma glucose (≥3 months)
• Serum total cholesterol
• Serum triacylglycerol (triglyceride)
• Serum low density lipoprotein (LDL) cholesterol
• Serum high density lipoprotein (HDL) cholesterol
• Serum total cholesterol:HDL cholesterol ratio (≥3 months)
• Medication use.

The evidence was graded as ‘adequate’, ‘moderate’, ‘limited’, ‘inconsistent’ or ‘insufficient’. Only outcomes where the evidence base was graded as ‘adequate’ or ‘moderate' were used to inform the recommendations.

Limitations

There were a number of limitations in the available evidence used for drawing the conclusions and recommendations in the report. These included:

• No agreed definition of a ‘low’ carbohydrate diet.
• Overlap in the reported mean carbohydrate intakes between lower (13 to 47% total energy) and higher (41 to 55% total energy) carbohydrate groups.
• Variation in the type and amount of macronutrient that replaced carbohydrate (fat and/or protein), and in the duration and intensity of advice, given to participants on following their prescribed diets.
• A lack of detail on the types of carbohydrate consumed (e.g., wholegrain, refined grain, free sugars, fibre) or consideration of how this could affect outcomes.
• Limited information on adherence to the prescribed carbohydrate intakes throughout the full duration of the study or consideration of how adherence to diets might impact outcomes.
• Inconsistent assessment and reporting of medication use.
• Shorter-term studies did not assess outcomes beyond 6 months and few longer-term studies assessed outcomes beyond 12 months.
• A ‘high’ or ‘unclear’ risk of bias in most of the RCTs included, which reduces confidence in the estimates of the effects of lower carbohydrate diets on the markers of T2D and clinical outcomes that were assessed.

Adverse events

In the shorter term (≥3 to 6 months), there was no evidence of any difference in adverse events between lower and higher carbohydrate intakes in adults with T2D. There was a lack of data from longer-term intervention studies to assess the implications of longer-term (≥12 months) restriction of carbohydrates in adults with T2D. For the ‘very low’ carbohydrate diets there was some evidence of symptoms such as headaches and nausea.

Summary of the strength of evidence for effects of lower vs. higher carbohydrate diets on markers and clinical outcomes of T2D

Research recommendations

The Working Group recommended that future research should:

• Develop and agree consistent international definitions for very low, low, moderate and high carbohydrate diets.
• Report medication usage in terms of quantitative details and analysis.
• Measure and report adherence to prescribed dietary interventions.
• Ensure robust study design in line with best international standards.

It was recommended that future research should consider the effects (and effectiveness) of lower carbohydrate diets for adults living with T2D from minority ethnic population groups and those with a healthy weight (BMI ≥18.5 to <25 kg/m²), and the health implications of lower carbohydrate diets followed over several years for adults living with T2D (including potential impacts on markers of nutritional status and clinical endpoints, such as diabetes complications and cardiovascular disease).

The Working Group also recommended future research should consider the following: the type of carbohydrate (e.g., wholegrain, refined grain, free sugars, fibre) being consumed; increasing the proportions and types of other macronutrients (e.g., fats and/or proteins) to compensate for reduced carbohydrate intakes in lower carbohydrate diet groups; lower carbohydrate diets independent of weight loss; lower carbohydrate diets compared with lower energy diets; adherence to prescribed diets.

The report and responses to the consultation are available here https://www.gov.uk/government/publications/sacn-report-lower-carbohydrate-diets-for-type-2-diabetes