In the news: can a low-carbohydrate diet reduce your risk of type 2 diabetes?

Last Updated : 08 November 2022

Recent news stories reported that a low-carbohydrate diet could prevent type 2 diabetes, or even reverse the established disease. Carbohydrates are one of the three macronutrients in our diet (fat and protein being the others) and represent the most important source of energy for the body. In the past decades, the number of people living with type 2 diabetes has risen dramatically to over 422 million people worldwide. Could a low-carbohydrate diet be the next diet to keep this disease at bay? Before adopting such a diet on the basis of these new findings, here are a few things to keep in mind when reading the headlines. 

The study behind the headlines 

The study behind the news was carried out by researchers from Tulane University in the US.¹ Their aim was to explore the effect of a dietary intervention promoting a low-carbohydrate diet compared with a usual diet in haemoglobin A1c among adults with elevated untreated haemoglobin A1c over a period of 6 months.

Haemoglobin A1c (HbA1c), also known as ‘glycated’ haemoglobin, is a marker for diabetes control that can give clinicians an overall picture of what average blood sugar levels have been over a period of 2-3 months. In patients with type 2 diabetes, the body cannot effectively use insulin (a hormone produced by the pancreas which regulates blood glucose). As a result, glucose can’t get into the cells and blood sugar levels increase. Subsequently, this circulating glucose in your blood can stick to haemoglobin, a protein found in red blood cells. This will create so-called ‘glycated’ haemoglobin. As your blood glucose levels increase, more haemoglobin will be coated with glucose, and higher HbA1c levels will be observed. Patients with type 2 diabetes will thus show high HbA1c levels. If you’re at risk for type 2 diabetes, your target HbA1c levels should be <6% (i.e., <6% or 42 mmol/mol of red blood cells should have glucose-coated haemoglobin).  

The trial included 150 adults (average age of 59 years) with untreated type 2 diabetes or pre-diabetes (a condition where blood sugar levels are higher than usual, but not yet high enough to be diagnosed with type 2 diabetes). Their HbA1c levels ranged between 6.0-6.9% (levels of ≥7.0% indicate people that should be treated for type 2 diabetes). Participants were randomly assigned to follow 6 months of either a low-carbohydrate diet or their usual diet. The low-carbohydrate diet involved consuming <40 grams of carbohydrates per day for the first 3 months, with the option to increase this to <60 grams for the subsequent 3 months. Participants were given intensive dietary guidance and key supplemental foods to achieve this carbohydrate target and attended regular group or individual follow-up/counselling sessions. The usual diet group received written information with standard dietary advice according to the US dietary guidelines and optional monthly educational sessions on topics unrelated to diet.  

The main outcome of interest was HbA1c after the 6-month intervention. Before the intervention started, HbA1c levels were on average 6.16% in both groups. The results showed that after 6 months, the low-carbohydrate diet group reduced their HbA1c levels by 0.26%, whereas the usual diet group reduced their HbA1c level by only 0.04%. When comparing the groups directly to each other, those in the low-carbohydrate group saw HbA1c levels drop 0.23% more than the usual diet group. The low-carbohydrate group also experienced greater reductions in fasting blood sugar (a blood test done to measure the sugar levels of blood, usually done in the morning after a night of fasting. This gives an indication of short-term diabetic control) (-10.3 mg/dL difference) and body weight (-5.9 kg difference). The researchers suggest that a low-carbohydrate diet might be a useful approach to prevent or treat type 2 diabetes, but caution that more research is needed over the longer term and in different population groups.  

What to keep in mind when reading the study’s conclusions? 

• The trial provides no evidence that a low-carbohydrate diet improves diabetes outcomes. 

The study found a modest 0.23% reduction in HbA1c after 6 months, but the clinical significance of this is unclear. Furthermore, many outstanding questions remain. As the researchers say, they could not determine whether this reduction was a direct effect of consuming fewer carbohydrates or just reflected the greater caloric reduction and weight loss observed in this group. Excess body fat is considered the strongest risk factor for type 2 diabetes. Furthermore, changes in blood pressure, which is also a known risk factor for type 2 diabetes, did not significantly differ over the 6-month period between the low-carbohydrate group and the usual diet group. However, the researchers hypothesise that they may not have the appropriate statistical means to assess this effect. It is also unknown whether the observed effect could be a short-term measure, or whether carbohydrate restriction (if the true cause of effects) would need to be sustained in the long-term to make a meaningful difference to cardiometabolic health (i.e., the cluster of diseases including coronary heart disease, stroke, chronic kidney disease and diabetes). 

• Following a restricted, low-carb diet in the long-term may have adverse health effects. 

The researchers selected the 40 grams per day carbohydrate target as this is said to have been demonstrated safe in prior studies and achieved cardiometabolic health effects. Over the 6-month trial, adverse effects were similar in both groups, though muscle cramps were more common in the low-carbohydrate (approximately 1 in 3) than the usual diet group (approximately 1 in 5). Yet, no assumptions can be drawn from these small numbers. However, consuming a variety of foods including high fibre carbohydrates, fruit and vegetables with low saturated fat and sugar is generally accepted as a healthy balanced diet. Dietary restriction may not provide adequate nutrition and may be associated with adverse health effects in the longer term. 

• The role of a low-carbohydrate diet in relation to standard diabetes management and dietary advice is unknown. 

This study specifically selected people with untreated diabetes or pre-diabetes. These conditions are usually managed initially with a balanced diet and physical activity recommendations to achieve a healthy weight and control blood sugar levels, with various medication options as indicated. How a low-carbohydrate diet would stand in comparison to, or supplementary to, these established approaches is unknown. Further research is needed.  

• The small, specific population sample may not be representative of other groups. 

The trial included 150 middle-aged adults from the US, who were majority female and 59% of Black ethnicity. The researchers observed some differences in effect between participants of different origins and between males and females. It’s not possible to draw any interpretation from this small sample, but it does highlight how there may be differences in the effect or experience of a low-carbohydrate diet among people of different ages, ethnicity, culture and environment.  

What do authorities say? 

• The WHO global report on diabetes (2016) highlights several approaches to prevent type 2 diabetes, such as exercising regularly, eating healthily, avoiding smoking, and controlling blood pressure and lipids.² Dietary recommendations include limiting your saturated fatty acid intake and free sugar intake to less than 10% of total energy intake (further reducing free sugars to 5% could have additional health benefits); and achieving adequate intakes of dietary fibre through the regular consumption of wholegrain cereals, legumes, fruits and vegetables. 
• The European Society of Cardiology (ESC) guidelines on diabetes and pre-diabetes (2019) highlight that lifestyle changes are key to prevent diabetes and its complications.³ They recommend calorie reduction if needed to reduce excess body weight in people with diabetes or pre-diabetes, alongside smoking cessation, and standard physical activity recommendations. The only specific diet recommended is the Mediterranean diet which may reduce the risk of cardiovascular events. ESC states that ‘the role of low-carbohydrate diet remains unclear.’ 
• UK guidelines from the National Institute for Health and Care Excellence (NICE) (2022) recommend ‘the same healthy eating advice as the general population’ including high-fibre, low-glycaemic-index carbohydrates, such as fruit, vegetables, whole grains and pulses, eating oily fish and limiting saturated fats.⁴ They suggest integrating dietary advice into a personalised diabetes plan, including individualised recommendations for carbohydrate intake. 
• Diabetes UK specifically states ‘there is no such thing as a special diet for people with type 2 diabetes’ and that there isn’t a ‘one-size fits all’ approach.⁵ They advise making healthier dietary choices more often, limiting treats and aiming for achievable goals such as blood sugar or weight targets. For weight loss specifically, a low-carbohydrate diet is suggested as one option (alongside others such as Mediterranean), though notably this is set at a higher 130 gram per day target, with advice to speak to your healthcare professionals and understand the potential effects.⁶ 

References 

1. Dorans, K. S., Bazzano, L. A., Qi, L., He, H., Chen, J., Appel, L. J., ... & He, J. (2022). Effects of a Low-Carbohydrate Dietary Intervention on Hemoglobin A1c: A Randomized Clinical Trial. JAMA Network Open, 5(10), e2238645-e2238645. 
2. World Health Organization. (2016). Global report on diabetes. World Health Organization. 
3. Cosentino, F., Grant, P. J., Aboyans, V., Bailey, C. J., Ceriello, A., Delgado, V., ... & Wheeler, D. C. (2020). 2019 ESC Guidelines on diabetes, pre-diabetes, and cardiovascular diseases developed in collaboration with the EASD. European heart journal, 41(2). 
4. National Institute for Health and Care Excellence (NICE). (2022). Type 2 diabetes in adults: management. Retrieved from https://www.nice.org.uk/guidance/ng28/chapter/Recommendations#dietary-advice-and-bariatric-surgery
5. Diabetes UK. (n.d.). I have type 2 diabetes – what can I eat? Retrieved from https://www.diabetes.org.uk/guide-to-diabetes/enjoy-food/eating-with-diabetes/i-have-type-2-diabetes 
6. Diabetes UK. (n.d.). Low-carb diet and meal plan. Retrieved from https://www.diabetes.org.uk/guide-to-diabetes/enjoy-food/eating-with-diabetes/meal-plans/low-carb