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Low carb diet for diabetes

The use of low-carbohydrate diets in the dietary management of diabetes is a contested field, and these diets remain popular in the community. We dig deeper into the topic.

What is a low carbohydrate diet?

One of the issues in the evidence base is there is no single definition of a low carbohydrate diet, although definitions have been proposed by Feinman and colleagues:

Very low carbohydrate 20-50g/day, <10% energy from carbohydrates (e.g. ketogenic diet).

Low carbohydrate diet <130g/day, <26% energy from carbohydrates

Moderate carbohydrate, 26-45% energy from carbohydrates

High carbohydrate diet, >45% energy from carbohydrates.

Interestingly, the average Australian diet consists of 45% energy from carbohydrates (2011-12 data) and would be considered ‘moderate’ using the definitions above. A high proportion (35%) of energy comes from discretionary foods.

Low and very low carbohydrate diets limit or exclude grain foods, starchy vegetables, legumes, most fruits and some dairy foods and encourage non-starchy vegetables, meats of all kinds (including fatty cuts), nuts, cheese, fats and oils (including butter and cream).

What professional guidance exists?

The Australian Diabetes Educator’s Association (ADEA) says low carbohydrate diets are one approach that may help to improve blood glucose levels and weight management for people with type 2 diabetes, at least in the short term, but they are not the only option and may not be best for long term health. Care is required to make healthy food choices within the carbohydrate restriction and work closely with their health care team to manage medication/insulin use.

Diabetes Australia’s position on low carbohydrate eating says it may be useful in lowering blood glucose levels, weight, blood cholesterol and blood pressure in the short-term (up to 6 months) in people with type 2 diabetes but further research is needed to establish the effectiveness and safety for people with type 1 diabetes. Personalised dietary advice from a dietitian is encouraged.

The American Diabetes Association (ADA) in their 2019 consensus statement says people with diabetes should be offered individualised MNT (Medical Nutrition Therapy), including the option of low carbohydrate diets. “For select adults with type 2 diabetes not meeting glycemic targets or where reducing antiglycemic medications is a priority, reducing overall carbohydrate intake with low- or very low-carbohydrate eating plans is a viable approach... Low-carbohydrate eating patterns, especially very low-carbohydrate (VLC) eating patterns, have been shown to reduce HbA1C and the need for antihyperglycemic medications.”

They caution against using them during pregnancy and breastfeeding and in people with renal disease or disordered eating.

The Diabetes UK Position Statement on low-carb diets (50-130g/day) for people with diabetes says low-carb diets are an effective short-term option for people with diabetes who have obesity or overweight but there is no consistent evidence they are any more effective than other approaches in the long term and should not be recommended for everyone. They also do not recommend this diet for people with type 1 diabetes due to a lack of evidence for safety or efficacy, or for children with diabetes due to the risks of poor growth, higher cardiovascular risk and psychological problems.

Diabetes Canada’s position (2020)on low (50-130g) or very low carbohydrate diets for people with type 1 and 2 diabetes says they may be effective for weight loss and improved glycemic control with a reduction in the need for antihyperglycemic therapies in the short term, but evidence for longer term benefits is limited and there are methodological limitations in the existing literature. If people choose to follow a very low carbohydrate diet should do so under supervision of a health care team including a dietitian to reduce the risk of adverse effects.

What do experienced practitioners think?

Robyn Barnes (APD) is based in the culturally and linguistically diverse and socio-economically disadvantaged area of Bankstown in Sydney’s Southwest and has over 20 years of clinical experience as a diabetes dietitian. She finds there are not many patients in her hospital choosing this approach, and a limited number sustain it for an extended period. She also has concerns about the nutritional adequacy of low-carb diets, such as micronutrient adequacy and obtaining optimal amounts of phytonutrients, and safety of high amounts of protein in some diets and the renal implications for people with diabetes. Robyn says there is evidence of benefit for the use of low carbohydrate diets in people with diabetes in studies of short duration. 

“Low carb diets might work in the short term to lose weight but the bigger challenge and the more important question is what eating pattern will help patients maintain the weight loss and stay healthy over the long term?”

Robyn says low carbohydrate diets are not going away and her approach is to be open and neutral about them when patients ask. “I’ve seen research that suggests if health professionals actively discourage such diets, patients simply disengage and will do it on their own anyway. I like to provide the information to help patients make an informed choice, and then if they decide to go ahead, I will support them to do it as safely as possible.”

Dr Kate Marsh, Advanced Accredited Practising Dietitian (AdvAPD) & Credentialled Diabetes Educator works on the more affluent north side of Sydney harbour (although she has clients all over Australia via tele-health) and has over 25 years of clinical experience. Her practice mostly consists of people with type 1 diabetes and gestational diabetes (GDM), but also women with PCOS (Polycystic Ovary Syndrome).

“My clients on Continuous Glucose Monitoring (CGM) can be tempted to limit or avoid carbs to see the flattening of their glucose levels but I remind them that even people without diabetes have natural fluctuations and the aim is to choose better types of carbs and to match this well with their insulin rather than cutting them out,” says Dr Marsh.

She adds, “The women I see for PCOS all think they should be on a low carb diet. Many have tried a low carb diet by the time they see me, but few have been able to sustain this way of eating as they find it too restrictive. They could achieve the same result by eating fewer nutrient poor carbs (highly processed, low fibre, high in saturated fat) and eating more quality carbs like legumes, fruits, vegetables and wholegrains.”

Dr Marsh acknowledges that low carb diets are popular and even being recommended by some endocrinologists, and this led to the development of the Diabetes Australia Position Statement on low carbohydrate diets. She also acknowledges there is evidence they work in the short term for weight loss, but they don’t appear to be more effective than other energy-restricted diets after 12 months. It’s all about short term v s long term thinking. She is also concerned about the healthy foods that are restricted on low carbohydrate diets.

“Low carb diets go against all the evidence we’ve got about healthy eating patterns which suggests that a mostly plant-based diet that includes legumes, minimally processed wholegrains, fruits and a wide variety of vegetables helps to support a healthy microbiome, reduces chronic disease risk and is associated with longevity.”

“We need to address the underlying issue of insulin resistance, and eating more animal protein, particularly red and processed meats, and saturated fat worsens insulin resistance. Avoiding carbs only addresses the symptoms rather than the underlying cause of type 2 diabetes and PCOS. A higher animal protein intake can also exacerbate any underlying renal insufficiency in people with diabetes and there is some research to show that replacing red meat with soy protein can reduce the progression of renal disease in people with diabetes.”

What is the latest evidence?

A systematic review and meta-analysis of 23 randomised controlled trials (2021) on the efficacy and safety of low (<130g/day) and very low carbohydrate (<10% energy as carbohydrate) VLCD (very low calorie diets) for type 2 diabetes remission found low carbohydrate diets achieved higher rates of diabetes remission (HbA1<6.5% or fasting glucose <7.0mmol/L). At 12 months, remission data was sparse suggesting most studies were shorter term. The reduction of effect between 6 and 12 months was noted for weight loss, insulin sensitivity and triglycerides. The less strict low-carbohydrate diet was more effective than the VLCD at 6 months due to better adherence, although those who could adhere to the VLCD regime achieved clinically significant weight loss. Quality of life went down and LDL cholesterol went up at 12 months on the VLCD. The authors concluded there is moderate to low certainty evidence that adhering to a low carbohydrate diet for 6 months can induce remission of diabetes without adverse consequences, but there is continued debate about the efficacy, safety and dietary satisfaction of longer-term use of low-carbohydrate diets.

An umbrella review of published meta-analyses (2022) and systematic review of trials of diets for diabetes remission and weight management concluded hypocaloric diets in people with type 2 diabetes do not support any particular macronutrient profile or diet style over others. High quality evidence found Low-carbohydrate diets were no better for weight loss than higher-carbohydrate/low-fat diets. The evidence for ketogenic/very low carbohydrate and very low energy food-based diets for diabetes remission had serious risk of bias and low certainty. Very low energy diets and formula meal replacement appear most effective and provide less energy than food-based diets and are most effective for type 2 diabetes remission but most of the evidence is restricted to one year or less and longer-term research is needed to assess long term impacts.

The issue of compliance 

In our food culture following a low-carbohydrate diet can be challenging, especially using whole foods rather than highly processed products.

A narrative review on adherence to low-carbohydrate diets in patients with diabetes (2022) found although low carbohydrate (<130 g/day of carbohydrate) and very low carbohydrate, ketogenic diets (typically <50 g/day of carbohydrate) can be effective tools for managing diabetes given their beneficial effects on weight loss, glycemic control and lipid profile, these benefits can be limited by poor dietary adherence. Cultural, religious, and economic factors pose barriers to achieving compliance with either dietary approach.

In a meta-analysis of a very low carbohydrate ketogenic diet (VLCK) vs recommended diets in patients with type 2 diabetes found VLCK diets appear to control glycemia and decrease body weight for up to 6 months in people with diabetes and obesity. Reductions on triglycerides and diabetic medications and increases in HDL cholesterol continued until 12 months. The authors concluded there was still insufficient evidence to recommend VLCK diets and a major limitation to their use is lack of adherence to carbohydrate restriction.

Take out message

Low carbohydrate diets may give good short-term results for people with diabetes with weight to lose. They are hard to maintain over the longer term, and this may be why their effects wane over time. Individual support is important to optimise dietary choices and manage medication. Low carbohydrate diets restrict plant foods such as wholegrains, legumes and fruit associated with optimal health. There are unanswered questions about their safety and adequacy over the long term.

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